Addition reactions of mononuclear η3-allenyl/propargyl transition metal complexes: a new class of potent organometallic carbon electrophiles

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Coordination Chemistry Reviews 190 – 192 (1999) 1143 – 1168

Addition reactions of mononuclear

h

3

_{-allenyl/propargyl transition metal complexes:}

a new class of potent organometallic carbon

electrophiles

Jwu-Ting Chen *

Department of Chemistry, National Taiwan Uni6ersity, Taipei106, Taiwan, ROC

Contents

Abstract. . . 1144

1. Introduction . . . 1144

2. Mononuclearh3 -allenyl/propargyl transition metal complexes . . . 1145

2.1 Synthesis . . . 1145

2.2 NMR and structural characteristics . . . 1147

3. Addition reactions of_h3_{-allenyl/propargyl complexes . . . .} ₁₁₅₁

3.1 Reactions with Group VIA nucleophiles . . . 1151

3.1.1 Hydroxylation . . . 1151

3.1.2 Organochalcogenoxylation . . . 1152

3.1.3 Addition with weak nucleophiles . . . 1153

3.2 Reactions with Group VA nucleophiles . . . 1154

3.2.1 Amination and amidation . . . 1154

3.2.2 Reactions with tertiary amines and phosphines . . . 1156

3.3 Reactions with Group IVA nucleophiles . . . 1158

3.3.1 Reaction with carbanions . . . 1158

3.3.2 Aromatic electrophilic substitution . . . 1160

4. Mechanisms of nucleophilic addition of allenyl and/or propargyl complexes . . . 1161

4.1 Addition to metalh1_{-allenyl requires preceding coordination of nucleophile . . . .} ₁₁₆₁

4.2 Direct external nucleophilic attack at the h3 -allenyl/propargyl central carbon . . . 1162

4.2.1 NH addition versus OH addition. . . 1162

4.2.2 Aromatic CH addition . . . 1164 5. Concluding remarks . . . 1164 Acknowledgements . . . 1165 References . . . 1165 www.elsevier.com/locate/ccr * Tel.: + 886-2-2366-0352; fax: + 886-2-2363-6359.

E-mail address:[email protected] (J.-T. Chen)

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1144 J.-T. Chen/Coordination Chemistry Re_6iews190 – 192 (1999) 1143 – 1168

Abstract

The cationic mononuclear h3_{-allenyl/propargyl complexes exhibit new non-classical}

organometallic bonding ofs,p-mode and remarkable activity of potent carbon electrophiles. The organic moieties of the title species, general formula R%R¦CCCR, use all three adjacent unsaturated carbons to bond with metal in a unique coplanar manner. The C3skeletons thus

are severely distorted from the normal linear configuration to 15095°. The R%R¦CC distances are in the range of long double bonds; whereas CCR are between double and triple bonds, indicating the resonance structures of allenyl and propargyl. The cationic complexes withh3_{-allenyl/propargyl ligands can undergo feasible reactions of regioselective}

nucleophilic addition which provide a pragmatic synthetic access to new organometallic species of central-carbon-substitutedh3_-allyl,_h3_{-oxa- and} _h3_{-aza-trimethylenemethane. The}

broad spectrum of CNu bond formation encompasses hydroxylation, alkoxylation, phe-noxylation, acyloxylation, thioxylation, thio- and seleno-phephe-noxylation, amination, amida-tion, phosphinaamida-tion, etc. As to carbaamida-tion, the h3_{-allenyl/propargyl complexes react with}

carbanions to form h3_{-trimethylenemethane complexes. The electrophilicity of} _h3_-C 3H3

allows us to conduct regioselective carboncarbon coupling with the electron-rich aromatics to achieve aromatic electrophilic substitution. Although both h3_{-allenyl/propargyl and}

h1_{-allenyl complexes are essentially subject to similar nucleophilic addition, the former are}

distinctly more reactive and versatile. From the mechanistic viewpoint, the cationich3

-al-lenyl/propargyl complexes prefer to undergo external nucleophilic attack at the central carbon. In contrast, nucleophilic addition to the h1_{-allenyl complexes requires preceding}

Keywords:h3

-Allenyl/propargyl complexes; Nucleophilic addition; Aromatic electrophilic substitution

1. Introduction

Transition metal complexes with unsaturated hydrocarbons which arep-bonded

to the metal have long been the focus of organometallic chemistry. Those with non-classical organometallic interactions are of particular interest, for they often empower new activity to the organic moieties [1]. For example, organic allyls

(C3R5) often bond with metal in a h3-form rather than a conventionalh1-form in

organic chemistry. The chemistry involvingh3_{-allyl complexes has been extensively}

studied and proven to contribute immensely to organic synthesis [2].

Propargyl which may be conceived as the alkyne analog of allyl has a tautomeric

isomer of allenyl. Both propargyl and allenyl (C3R3) groups have three-carbon

skeletons and unsaturated hydrocarbyl functionalities. Such structural features are similar to the allyl group and are expected to have a rich chemistry with a metal. Indeed, transition metal complexes of allenyl and propargyl have drawn attention

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1145

J.-T. Chen/Coordination Chemistry Re_6iews190 – 192 (1999) 1143 – 1168

Organometallic propargyl and allenyl complexes have revealed their synthetic utility in many organic reactions [3,4]. However, mononuclear allenyl or propargyl metal

complexes with non-classical s,p-bonding were actually unexplored until recently.

Metal h1_{-propargyl can transform into the thermodynamically more stable}

h1_{-allenyl tautomer [5]. The mechanistic studies of this process of tautomerization}

indicate the involvement of a 1,3-metal shift. Comparing such a metal shift with the

h1_-to-_h3_-to-_h1 _{motion of allyl [6], it would be chemically logical to expect the}

h3_{-bonding interaction between the metal and the allenyl or propargyl ligands.}

Recent development in metal h3_{-allenyl/propargyl chemistry has filled such a}

category and opened a new page in organometallic chemistry. We choose to use the

nomenclature of h3_{-allenyl/propargyl, because the organic ligands with a general}

C3R3 formula are shown to bond with metal through all three carbon atoms and

possess the resonance structures between allenyl and propargyl. Such a bonding

mode of h3_-C

3R3 groups remarkably enhances their electrophilic character,

espe-cially at the central carbon, and makes them behave as potent organometallic

carbon electrophiles [7]. In fact, the cationic mononuclear h3_{-allenyl/propargyl}

complexes are prone to external attack by a variety of hard and soft nucleophiles. A couple of previous reviews have covered the synthesis and some results of early investigation for these title complexes [3g,8]. This article is prompted by the rapid development of their reactivity. We will concentrate more on the reaction scope of nucleophilic addition and trace the literature up to 1998.

2. Mononuclearh3_{-allenyl/propargyl transition metal complexes} 2.1. Synthesis

Different synthetic routes have been developed to prepare the mononuclear

h3_{-allenyl/propargyl complexes. The transition metals involved, include Os, Ru, Fe,}

W, Mo, Re, Zr, Ti, Pt, and Pd. In 1985, Werner et al. discovered an osmium

complex [Os(PMe₃)₄(h3_-PhCCC_CHPh)](PF

6), resulting from oxidative coupling of

two alkynyl groups [9a]. The first dozen examples of the isolableh3

-allenyl/propar-gyl complexes all contain ah3_{-butenynyl moiety (RCCC}_{CR%R¦) which is generally}

formed by incorporating two ‘ynes’ [9]. The typical reactions comprise the alkyne coupling (Eq. 1), alkyne-vinylidene coupling (Eq. 2), as well as enynyl rearrange-ment (Eq. 3), etc.

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The first unsubstituted h3_{-allenyl/propargyl complexes [(Me}

nC6H6 − n)Mo(CO)2

-(h3_-HCCCH

2)]+ (1) were reported by Krivykh and his coworkers in 1991 [10a]. A

one-pot reaction containing propargyl alcohol, HBF4·Et2O, and the

coordination-unsaturated molybdenum complex derived from (MenC6H6 − n)Mo(CO)3 under

UV irradiation provided a compound with a h3_-HCCCH

2 ligand (Eq. 4). Similar

reactions with tungsten and rhenium complexes also yield the corresponding

cationic h3_{-allenyl/propargyl products [(Mesityl)-W(CO)}

2(h3-HCCCH2)]+ and

[Cp*Re(CO)₂(h3_-HCCCH

2)]+ [10b]. The related complexes of the latter rhenium

cation, [Cp*Re(CO)₂(h3_-RCCCHR_%)]+ _{(R = H R%=Me 2a, Et 2b,}t_{Bu 2d; R = Me}

R%=Me 2e, Et 2f), were acquired by Casey and his coworkers mainly via an

alternative route from the coordinated alkynes in the formula of RCCCH2R% by

hydride abstraction (Eq. 5) [11].

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(5) Meanwhile, our laboratory succeeded in preparing the cationic platinum and

palladium complexes containing unsubstituted h3_{-allenyl/propargyl} _ligand,

[M(PPh3)2(h3-HCCCH2)]+ (M = Pt 3a, Pd 4a) (Eq. 6) [12]. The synthetic strategy

was to open a coordination site from the h1_{-allenyl(halo) complexes}

trans-M(PPh₃)₂(X)(h1_-CHCCH

2) (X = Cl, Br) by halide abstraction. Wojcicki and

Kurosawa also reported the substituted derivatives [M(PPh₃)₂(h3_-RCCCH

2)]+

(M = Pt. R = Ph 3b, Me 3c; M = Pd. R = Ph 4b) using the same synthetic

method-ology from eitherh1_{-allenyl or} _h1_{-propargyl species (Eq. 6) [13]. Kurosawa could}

extend this method to achieve the synthesis of neutral h3_{-allenyl/propargyl}

palla-dium complexes Pd(X)(PPh3)(h3-RCCCH2) (R"H 5) by eliminating a phosphine

ligand (Eq. 7) [14].

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1147

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Stang and his coworkers synthesized other substitutedh3_{-allenyl/propargyl}

plat-inum complexes by adding alkynyl(phenyl) iodonium salts to a d10 _ethylene

complex Pt(PPh3)2(C2H4) (Eq. 8) [15]. Other synthetic methods such as reactions of

propargyl ether with Lewis acids (Eqs. 9, 10) [9n,13a], transmetallation by propar-gyl Grignard nucleophiles (Eq. 11) [16], and reactions of proparpropar-gyl halide with organotitanium (Eq. 12) [17] can accomplish the preparation for various title species as well. (8) (9) (10) (11) (12)

2.2. NMR and structural characteristics

Although there have beenh3_{-allenyl/propargyl species proposed on the basis of}

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com-1148 J.-T. Chen/Coordination Chemistry Re_6iews190 – 192 (1999) 1143 – 1168

plexes is not quite straightforward. This is because these complexes not only have severely distorted non-classical configuration, but often also contain substituents and more than one quaternary carbon. Detailed comparison of the NMR spectro-scopic data has been done in the previous reviews [8a]. A few new species are added and shown in Table 1.

The complexes containing h3_-HCCCH

2 are of the prototype, which provide

particularly useful diagnostic NMR data [10a,12,19]. In the 31_{P-NMR spectra}

of [M(PPh3)2(h3-HCCCH2)](BF4) (M = Pt 3a and Pd 4a), two different

phos-phine signals afford evidence for unsymmetric h3_-HCCCH

2. Comparably,

[(C6H3Me3)Mo(CO)2(h3-HCCCH2)]+ (1) has distinct 13C-NMR data for two

car-bonyls. The1_{H-NMR spectra of 3a shows two resonances with an integration ratio}

of 2:1 atd 2.91 and 4.60. Similar to the h1_{-allenyl complexes, the CH resonance of}

3a is more downfield than that of CH2. But, the comparable values of JHPtfor CH6

(27.2 Hz) and for CH6 2(30.8 Hz) indicate that both carbon termina are attached to

the metal. The small values of 4_J

HH (2.2 – 2.4 Hz) are similar to those of h1

-CH₂CCH [5] but smaller than those of h1_-CH_CCH

2 (6.4 Hz) [8a,19]. The

magnetic equivalency of the CH2 in complexes 3, 4, and 5 is consistent with their

coplanar coordination sphere of P2MC3. In contrast, the two methylene hydrogens

in complexes 1 and 2 are non-equivalent since they lack the symmetric molecular

plane. The geminal coupling constants for CH2 are relatively large (ca. 10 Hz) in

complexes with a piano-stool geometry.

The 1_H-coupled 13_{C-NMR data are most revealing. In the spectrum of 3a, a}

resonance at d 101.6 with 2_J

CH of 29 Hz is assigned to the quaternary central

carbon. It is clearly distinguishable from theb-carbon of h1_{-allenyl. A doublet at}_d

90.8 with a large1_J

CH value (246 Hz) is attributed to the propargyl terminus CH

as in form A. A triplet at d 51.8 with 1_J

CH= 171 Hz, which is too large to be

explained by common sp3_{carbon but is close to the C}

tofh3-allyl, fits anh3-allenyl

resonance of structure B.

Thep-attachment of the C3R3 moiety is also supported by the coupling constants

J_CPt: 54 – 63 for Cc, 93 – 114 for CH2, and 126 – 137 Hz for CH. Such a highly

strained C3 framework is indicated by the large J_PPt values (3743 – 3810 and

4179 – 4285 Hz) which are comparable to those in the h2_{-alkene and} h2_-alkyne

complexes [2b]. The chemical shift of theh3_{-allenyl/propargyl central carbon, which}

apparently varies with metal, is another sign for a MCc bonding interaction.

X-ray crystallography affords the technique for authentic identification of the

mononuclearh3_{-allenyl/propargyl complexes [10a,13b,14a,15]. The most}

character-istic feature of theh3_-C

3R3 ligands is their coplanar unsymmetrical propensity. In

comparison with other known tricarbon organometals, theh3_{-allyl [20] and}

metal-lacyclobutane [21] complexes have folded structures. The metallacyclobutene [22] and deprotiometallacyclobutadiene complexes [23] are planar, nevertheless, with no bonding between the metal and the central carbon.

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1149 J .-T . Chen / Coordination Chemistry Re 6 iews 190 – 192 (1999) 1143 – 1168 Table 1

Characteristic13_{C-NMR and structural data of}_h3_{-allenyl/propargyl complexes}

ÚCtCcCt%c(°) Ccb₍_d) _M_C _Ref. tcMCt% (A,) MCcc(A, ) CcCtcCcCt% (A,) Compound [9a] 1.39 2.21 150 2.15 – [Os(PMe3)4(h3-PhCCCCHPh)]+ 2.39 1.29 [9b] – 148.7 [Ru(Cyttp)(CCPh)2a 2.200 2.191 1.379 2.258 1.249 (h3_-PhCCC_CHPh)]+ 1.416 154.3 [9f] – [RuCl(Cyttp)(h3_-PhCCC_CHPh)]+ a _2.040 _2.229 1.220 (synmer) 2.558 [9f] 2.169 1.396 148.2 2.084 [RuCl(Cyttp)(h3_-PhCCC_CHPh)]+ a _– 2.319 1.248 (antimer) 1.379 148.7 [9f] – [Ru(Cyttp)(CCPh) 2.200 2.191 2.258 1.249 (h3_-PhCCC_CHPh)]+_(mer)a [9f] [(C5Me5)W(CO)2 45.8 2.244 2.296 1.369 145.0 2.328 1.27

(_h3_-t_BuCCC_CMet_Bu)]+

154.0 – 2.144 2.234 1.392 [9h] [Ru(PP3) 1.247 (h3_-TMSCCC_CHCH 2TMS)]+ 2.485 – 57.8 – – – [9i] [Ru(PP3)(h3-PhCCCCHPh)]+ 150.9 ca. 115 2.135 2.244 1.39 [9j] [Ru(PPh(OEt)2)4 1.23 (h3_-TolCCC_CHTol)]+ _2.430 1.398 157.1 [9k] 110.0 [Ru(Cl)(PPh3)2(CO) 2.282 2.192 (h3_-TMSCCC_CHCH 2TMS)]+ 2.366 1.250 147.4 – 2.170 2.233 1.371 [9n] [Ru(PPh3)2(CO)2 -2.320 1.244 (h3_-PhCCC_CHPh)]+ [9m] 2.094 1.398 149.0 1.987 [Fe(dmpe)2(h3-PhCCCCHPh)]+ 51 2.305 1.243 1.321 144.8 [9m] – [Fe(depe)2(h 3_-MeCCC_CHMe)]+ _2.048 _2.113 2.196 1.296 [10a] [(C6Me5H)Mo(CO)2(h3-HCCCH2)]+ 69.9 2.319 2.282 1.380 150.9 2.340 1.236

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1150 J .-T . Chen / Coordination Chemistry Re 6 iews 190 – 192 (1999) 1143 – 1168 Table 1 (Continued) Compound Ccb₍_d) _M_C Ref. tcMCt% (A,) MCcc(A, ) CcCtcCcCt% (A,) ÚCtCcCt%c(°) [11b] – – – – 56.7 [(C5Me5)Re(CO)2(h 3_-MeCCCH 2)] + – – [11b] 57.8 [(C5Me5)Re(CO)2(h3-EtCCCH2)]+ – – [11b] [(C5Me5)Re(CO)2(h3-MeCCCHMe)]+ 59.8 – – – – – 60.8 – – – [11b] [(C5Me5)Re(CO)2(h3-EtCCCHMe)]+ 1.38 153 [11b] 60.5 [(C5Me5)Re(CO)2(h3-tBuCCCH2)]+ 2.305 2.239 1.26 2.345 – 100.6 – – – Unpublished [Pd(PPh3)2(h3-HCCCH2)]+ – – [12] 101.6 [Pt(PPh3)2(h3-HCCCH2)]+ – – – – [13b] 91.2 [Pd(PPh3)2(h3-MeCCCH2)]+ – – [13c] 1.38 154 2.143 [Pd(PPh3)2(h3-PhCCCH2)]+ 94.6 2.162 2.334 1.22 [13a] 97.3 152 [Pt(PPh3)2(h3-PhCCCH2)]+ 2.186 2.150 1.39 2.273 1.23 1.38 151.6 [14a] – Pd(C6F5)(PPh3)(h3-tBuCCCH2) 2.156 2.116 2.238 1.244 [15] [Pt(PPh3)2(h3-tBuCCCHMe)]+ 94.2 2.243 2.140 1.390 154.1 2.265 1.266 113.8 – – – [13c] [Pd(PPh3)2(h 3_-Me 3SiCCCH2)] + _– [16] 2.438 1.344 155.4 2.361 (C5H5)2Zr(Me)(h3-PhCCCH2) 98.8 2.658 1.259 1.41 152 [9n] 297.1

[(CO)4Re[h3-(CO)5ReCCCRe 2.24 2.27

1.28 (C5Me5)(NO)(PPh3)]+ 2.43 a_{Cyttp = PhP[CH} 2)3P(c-C6H11)2]2. b 13_{C-NMR chemical shift.} c

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1151

Theh3_-C

3R3skeleton configuration is severely bent to 145155°. All the known

title compounds have one CcCtdistance in the region between short double bonds

and long triple bonds (1.22 – 1.29 A, ), and the other in the range of long double

bonds (1.32 – 1.41 A, ), clearly showing the resonance structure between allenyl and

propargyl. The MCcbond distances (2.11 – 2.44 A, ) are between those of the two

MCtbonds. The MCR2bonds (2.23 – 2.56 A, ) are longer than those of the MCR%

bonds (1.98 – 2.32 A, ) which are well correlated with the corresponding J_CPtvalues.

Important structural data are also listed in Table 1.

3. Addition reactions of h3_{-allenyl/propargyl complexes}

3.1. Reactions with Group VIA nucleophiles

3.1.1. Hydroxylation

Although the development of diverse syntheses for mononuclear h3_-allenyl/

propargyl complexes has been rapid, reactivity studies for such species are only now appearing. In a general sense, these complexes are susceptible to nucleophilic addition mainly at the central carbon of the organic ligands, but also at the metal

centers in a few cases. Complexes with the unsubstituted h3_-C

3H3 ligand are the

most reactive and demonstrate an extraordinarily broad reaction scope.

The molybdenum complex 1 shows activity toward water addition to give a

h3_{-2-hydroxyallyl product [(Me}

6C6)Mo(CO)2(h3-CH2C(OH)CH2)]+ (6) (Eq. 13). In

the platinum system, although solid 3a may be preserved in a dry nitrogen atmosphere, it deteriorates rapidly in air. The stoichiometric reactions of 3a with

water at 25°C instantaneously produce a h3_{-2-hydroxyallyl cation [Pt(PPh}

3)2(h3

-CH₂C(OH)CH₂)]+ _{(7a) and a diplatina-}_h6_{-diallyl ether dication {[(PPh}

3)2Pt(h3

-C₃H₄)]₂O}2 + _{(8a) with relative yields of 4:1. When the same reaction was carried}

out below 0°C, the relative yields of 7a versus 8a change to 1:4. This is presumably due to the mobility and solubility of water in methylene chloride which drop markedly below the freezing point. The hydroxyallyl complex therefore, first formed is allowed to react with 3a (vide supra) [19a]. A deliberate reaction containing equimolar amounts of 3a and 7a under strictly anhydrous conditions indeed affords 8a quantitatively. The reaction of 3b and a large excess of water also produces diplatinadially-ether 8b [19b]. However, the corresponding hydroxyallyl complex 7b was not detected, presumably it would rapidly react with available 3b.

Deprotonation of 7a or treatment of 8a or 8b with OH− _{or OR}− _{will lead to the}

formation of h3_{-oxatrimethylenemethane (h}3_{-OTMM) complexes, Pt(PPh}

3)2(h3

(10)

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Similar hydroxylation of metalh1_{-allenyls is also possible, although it is still not}

clear whether the formation ofh3_{-allenyl/propargyl intermediates take place during}

the reaction course. Addition of water to Pt(Cl)(PPh3)2-(h1-CHCCH2) is much

slower than to 3a. However, the octahedral iridium complex

(OC-6-42)-Ir(Cl)2(PPh3)2(CO)(h1-CHCCH2) is inert to water addition under similar

condi-tions. By contrast, the labile iridium triflate derivative, (OC-6-42)-Ir(Cl)(PPh3)2

-(OTf)(CO)(h1_-CHCCH

2) (10) readily reacts with water to give {Ir(Cl)(PPh3)2

(CO)-[h3_-CH

2C(OH)CH2]}(OTf) (11) and Ir(Cl)(PPh3)2(CO)[h3-CH2C(O)CH2] (12) (Eq.

16), indicating that the pre-coordination of water has to be crucial [25]. Further-more, the addition rates to complex 10 are significantly accelerated when excess

water is used. Curiously, the h3_{-allenyl/propargyl palladium complexes 4 are}

completely inert to water.

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3.1.2. Organochalcogenoxylation

The reaction of Eq. 15 suggests that cationich3_{-allenyl/propargyl complexes are}

subject to alkoxylation. Indeed, complexes 3a and 3b react not only with a variety of alcohols but also poor nucleophiles of phenol derivatives to give,

respective-ly, the central-carbon-substituted alkoxy- and phenoxy-h3_-allyl _complexes

{Pt(PPh3)2[h3-CHRC(OR%)CH2]}+ (13) [13b,19,26]. For synthetic purposes, excess

nucleophiles, especially for the weak ones, are often needed to suppress water

addition. Treating 13 with strong base such as RO− _{also gives}_h3_{-OTMM (Eq. 17)}

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1153

(17) To extend the bond-forming scope to other chalcogen atoms, the construction of CS and CSe bonds between 3a and thiol, thiophenol, or selenophenol succeeds to

yield thioxy- and selenoxy-allyl complexes {Pt(PPh3)2-[h3-CH2C(ER%)CH2]}+ (E =

S 14, Se 15), respectively (Eq. 18). These reactions which readily overwhelm water addition do not need excess nucleophiles. In fact, extra REH would lead to the

formation of cis-Pt(ER)₂(PPh₃)₂ as by-products.

(18) According to the competitive experiments, addition of either PhSeH or PhSH to

3a overwhelmingly exceeds methoxylation even when a 20-fold excess of methanol

is used. PhSeH is more reactive than PhSH. The reaction of 3a with p-HSC6H4OH

results in exclusive CS bond-formation, leading to {Pt(PPh3)2[h3

-CH2C(SC6H4OH)CH2]}+ (16) (Eq. 19). All these results suggest that the

nucleo-philicity of the added substance is essential to the addition reactions of the

h3_{-allenyl/propargyl complexes. On the other hand, the relative rates of adding}

R%OH to 3a are MeOH:EtOH:i

PrOH:t

BuOH:PhOH = 70:40:23:4:1, indicating the influence of steric effect (vide supra).

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3.1.3. Addition with weak nucleophiles

The feasible phenoxylation of 3a indicates that the reactivity of h3_-allenyl/

propargyl toward addition might not be solely decided by the nucleophilicity of the added species. The addition reactions of 3a are also attained with carboxylic acids

RCO2H, ArCO2H and CX3CO2H (X = F, Cl). The resulting acyloxylation of

h3_{-allenyl/propargyl} _yields _new _acyloxyallyl _complexes _{Pt(PPh

3)2[h3

-CH2C(OCOR%)CH2]}+ (17) (Eq. 20) [27]. Electrophilic addition of organic halides

toh3_{-OTMM complexes 9 affords an alternative methodology for the synthesis of}

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transition metal allyl species which incline to suffer nucleophilic attack at the terminal carbon, alkoxy- and acyloxyallyl complexes are subject to nucleophilic substitution at the central carbon (Eqs. 22 – 24), presumably undergoing an addi-tion – eliminaaddi-tion mechanism. Such a reactivity certainly allows one to provide new features to organic synthesis [44].

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(22 – 24)

3.2. Reactions with Group VA nucleophiles

3.2.1. Amination and amidation

Regioselective addition of ammonia, primary or secondary amines, aniline and its

derivatives to cationich3_{-allenyl/propargyl complexes of Pt or Pd 3 is also feasible.}

Similar reactions of neutral h1_{-allenyl complexes of Pt or Pd are successful but}

relatively sluggish. The common products are cationic complexes of N-protonated,

N-alkylated, and N-arylated h3_{-azatrimethylene-methane} _(h3_-NTMM)

{M(PPh3)2[h3-CH2C(NRR%)CH2]}+ (M = Pt 18, Pd 19), wherein the broad

spec-trum of attempted substituents comprise R = H R%=H, Me, Et,i

Pr,t

Bu, c-C6H11,

Ph, CH₂CH₂OH, R = R%=Et, c-C₃H₆ (azetidine), Ph, R = Me R%=Ph (Eq. 25)

[28]. Hydrazine undergoes double addition with 3a to give metallapyrazoline

[(PPh3)2Pt(CH2CMeNN

¸¹¹¹¹¹¹¹º

H2)]+ (20) (Eq. 26) [29].

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1155

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Addition reactions of amides R%NH− _{to 3 provide the synthesis of neutral}

h3_{-NTMM complexes Pt(PPh}

3)2[h3-CHRC(NR%)CH2] (M = Pt 21, Pd 22). The

h3_{-NTMM complexes can also be prepared by reaction of} _h3_{-allenyl/propargyl}

complexes with primary amine, followed by deprotonation [8b,19b,28]. As to weak

amines such as TsNH2 and PhSO2NH2 which are not reactive towards 3, their

corresponding amides may achieve addition. Protonation of the h3_-NTMM

com-plexes compensates to afford the products of amination (Eq. 27). Another

alterna-tive synthetic route for the tosylate derivaalterna-tive, Pt(PPh₃)₂-[h3_-CHRC(NTs)CH

2] is

via the reaction of h3_{-OTMM and TsNCO [44a]. The} _h3_{-NTMM complexes are}

isoelectronic with theh3_{-OTMM complexes, and make a class of new}

organometal-lic species which may be envisaged to have intermediary structural characteristics

betweenh3_{-2-amidoallyl and} _h2_{-metallacyclobutanimine.}

(27)

The reactions of h1_{-allenyl complexes with amide have been further examined by}

treating trans-Pt(PPh₃)₂(OAc)(h1_-CHCCH

2) with PhSO2NH−. A trans-h1

-allenyl-(amido) complex trans-Pt(PPh3)2(NHSO2Ph)(h1-CHCCH2) (23) was found to form

first and transform subsequently into Pt(PPh3)2(h3-CH2C(NSO2Ph)CH2] (Eq. 28).

Adding diphenylphosphinoethane (dppe) to trans-Pt(PPh3)2(Cl)(h1-CHCCH2) first

gives a cis-h1_{-allenyl(amido) complex cis-Pt(dppe)(NHSO}

2Ph)(h1-CHCCH2) (24)

which then produces cis-Pt(dppe)-[h3_-CH

2C(NSO2Ph)CH2] upon heating (Eq. 29).

(28)

(14)

In the octahedral iridium systems, the reactions of

(OC-6-42)-Ir(Cl)-(PPh3)2(OTf)(CO)(h1-CHCCH2) (10) and amino compounds such as NH3,

NH₂NH₂, MeNH₂, EtNH₂, i_PrNH

2, and PhCH2NH2, PhSO2NH−, C6H5N or

MeCN, PhCH₂CN give rise to substitution, leading to

{OC-6-42-Ir(Cl)-(PPh₃)₂(L)(CO)(h1_-CHCCH

2)}(OTf) (25) (Eq. 30). On the other hand, the aniline

derivatives XC₆H₄NH₂ (X = F, NO2, MeO, H, Me) achieve the addition to 10,

yielding arylated h3_{-NTMM products {Ir(Cl)(PPh}

3)2(CO)[h3-CH2C(NHC6H4

-X)CH2)}(OTf) (26) (Eq. 31) [30]. This also indicates that the coordination of

nucleophiles is a prerequisite for the addition to theh1_{-allenyl complexes. Besides,}

NH bond activation by metal has to be crucial to addition too.

(30)

(31)

3.2.2. Reactions with tertiary amines and phosphines

The reaction between 10 and PPh3 results in CP coupling, leading to an

iridacyclobutene {(Cl)(PPh₃)₂(CO)Ir[CH¸¹¹¹¹¹¹¹¹º₂C(PPh₃)CH]}(OTf) (27) (Eq. 32) as sole

product [31]. Similar reactions are also observed forh3_{-allenyl/propargyl rhenium}

and platinum complexes 2 and 3, but not for trans-Pt(PPh3)2(X)(h1-CHCCH2) and

(OC-6-42)-Ir(Cl)2(PPh3)2(CO)(h1-CHCCH2).

(32)

The reactions of [Cp*Re(CO)2(h3-H2CCCR)]+ (R = H, Me,

t

Bu) and phosphines

yield rhenacyclobutene adducts {Cp*(CO)2Re[CH2C(PR%3)C

¸¹¹¹¹¹¹¹¹º

R]}+ (R%=Me, Ph 28)

(Eq. 33) [32]. However, the reaction of NEt3 with 2 leads to anh2-allene complex

{Cp*Re(CO)2[h2-H2CCCR(NEt3)]}+ (29) by adding the amino group at a carbon

terminus (Eq. 34), indicating regiochemistry for nucleophilic addition at the central carbon is not the only possibility. Similar reactivity also occurs with the

molybde-num h3_{-allenyl/propargyl analog 1, with which mixed products of phosphinoallyl}

and allene complexes are observed [10a]. The h3_{-allenyl/propargyl platinum}

(15)

1157

{(PPh3)2Pt[CH2C(Nu)C ¸¹¹¹¹¹¹¹º

H]}+ _{(Nu = NEt}

3 30 PPh3 31) exclusively, at − 40°C (Eq.

34). Reactions of the platinum system are sensitive to elevated temperature or excess nucleophile which would destroy the products. The presence of acid trans-forms metallacyclobutenes into allyl species from metallacyclobutenes (Eq. 35) [31].

(33)

(34)

(35) Reactivity with pyridine shows some mechanistic similarity between 2 and 3. The platinum complex 3a can undergo nucleophilic attack by pyridine either at the

central carbon to give {(PPh3)2Pt[CH2C(Py)C

¸¹¹¹¹¹¹¹º

H]}+ _{(32) or at the metal center to}

give cis-[Pt(PPh3)2(Py)(h1-CHCCH2)]+ (33) (Eq. 36). Complex 32 is the kinetic

product and 33 is thermodynamically more favored. Both products may lose

pyridine to revert to 3a which can be hydrolyzed to the hydroxyallyl,h3_-O-TMM,

andh6_{-diallyl ether complexes if moisture is present. To rhenium complexes 2b and}

2c, the pyridine derivatives also kinetically add to the central carbon to give

rhenacyclobutene complexes (34, 36) at low temperature. Upon warming, rhenacy-clobutenes dissociates pyridine which, however, readded at the terminal site to give either an allene (35) or an alkyne complex (37), respectively (Eqs. 37, 38).

(16)

(37, 38)

3.3. Reactions with Group IVA nucleophiles

3.3.1. Reaction with carbanions

Successful addition of electron donors involving the elements in the lower

periodic table to the h3_{-allenyl/propargyl complexes indicate that such new}

organometallic species are reactive to soft as well as hard nucleophiles. The endeavor for CC bond formation with allenyl and/or propargyl complexes is not only a logical extension of reaction scope, but also intriguing for organic synthetic purposes.

Casey and coworkers led in this realm, with the reactions of 2 and acetylides, organic cuprates, or carbanions, from which rhenacyclobutenes again were the products [11,32]. Similar reactions of the palladium and platinum complexes with

carbanions exhibit distinguishable results. Synthesis of zwitterionich3

-trimethylen-emethane (h3_{-TMM) complexes of Pd and Pt M(PPh}

3)2(h3-CHRC(CE1E2)CH2)

(E1, E2= CN, CO2Me, SO2Ph, etc.; M = Pt R = H, Ph 38; M = Pd R = H 39) has

been accomplished by adding carbanions Na[CH(E1)(E2)] to either theh1-allenyl or

h3_{-allenyl/propargyl complexes (Eq. 39) [8b,33]. The}h3_{-TMM complexes which are}

similar to the aforementioned h3_{-OTMM and} h3_{-NTMM complexes exhibit a}

resonance structure between zwitterionic h3_{-allyl and alkenic metallacyclobutane}

forms. Spectral evidence for such structures were first acquired mainly using NMR techniques, and then single-crystal X-ray crystallography offered unequivocal confirmation.

(17)

1159

The zwitterionic character of h3_{-TMM complexes is also supported by their}

chemical behavior. Protonation or alkylation of 38 or 39 leads to the

central-car-bon-substituted h3_{-hydrocarbylallyl} _cations _{M(PPh

3)2[h3-CHRC(CR%E1-E2

)-CH₂]}+ _{(R = H, Ph; R%=H, Me, Et; M=Pt 40, Pd 41) (Eq. 40). When carbon}

enolates react with palladium h3_{-allenyl/propargyl or palladium} _h1_-allenyl

com-plexes, hydrofurans were produced via the formation of Pd-h3_{-TMM. Such results}

afford direct evidence that Pd-h3_{-TMM are the key intermediates in Pd-catalyzed}

addition – cyclization reactions of propargyl carbonates and carbon nucleophiles (Eq. 41) [34].

(40)

(41)

In addition, zwitterionic Pd-h3_{-TMM complexes have also been proposed as}

fleeting intermediates in Pd-catalyzed [3 + 2] cycloaddition reactions (Eq. 42) [35]. Trost et al. have endeavored to design a variety of organic bifunctional conjunctive

reagents, reacting with Pd(0) to provide Pd-h3_{-TMM complexes in situ, but no}

success in isolating these species [36]. Theoretical and mechanistic studies suggest

that Pd-h3_{-TMM species should be energetic and may exhibit distinguishable}

chemistry from the better-studiedh4_{-TMM complexes [37]. Although the}

EWG-sta-bilized Pd-h3_{-TMM are not as reactive as the unsubstituted one, they still undergo}

coupling with strong olefinic electrophiles such as tetracyanoethylene (TCNE) or maleic anhydride (MA) to form the highly substituted products of [3 + 2]

cycload-dition (Eqs. 43, 44). A similar reaction between Pt-h3_{-TMM and TCNE was also}

reported (33b). These cyclopentanoids can not be obtained by the catalytic process,

for goodp-acids as TCNE and MA would react with Pd(0) before Pd-h3_{-TMM is}

formed.

(42)

(18)

3.3.2. Aromatic electrophilic substitution

Preliminary studies have shown that the cationic h3_{-allenyl/propargyl platinum}

complex 3a can curiously react with alkane (Eq. 45), alkyne (Eq. 46), and silane

(Eq. 47) to yield various h3_{-allyl products (42 – 44) [13]. Successful expedition for}

the addition reactions of 3a with soft nucleophiles has led to research into other

electron-rich systems. One of the fundamental strategies to achieve CC coupling

between unsaturated hydrocarbons is via Friedel – Crafts reactions [38]. In the typical examples, a Lewis acid is required to create a carbon electrophile which results in aromatic substitution. To use organometallic electrophiles for such a purpose is attractive because the metal may activate the ligated organic moiety and

facilitate the carboncarbon bond formation in a new manner [39,40]. The h3

-al-lenyl/propargyl complexes conduct the addition of the aromatic CH bonds across

the CCH bond of the C3H3 moiety [41].

(45)

(46)

(47) Hydropyrrolylation of 3a is accomplished by directly reacting 3a with pyrrole or N-methylpyrrole in a nitrogen atmosphere. As the electrophilic attack on pyrrole

normally occurs at the 2-position, the CC coupling takes place exclusively between

the central carbon of C3H3 and the 2-pyrrolyl carbon, thereby generating the

central-carbon-substituted h3_{-2-pyrrolylallyl} _complexes _{Pt(PPh

3)2[h3-CH2

C(2-C4H3NR)CH2]}+ (45) (Eq. 48). In the reaction of 3a with indole, the insertion of

C3H3into the 3-indolyl CH bond leads to the formation of {Pt(PPh3)2[h3-CH2

C(3-indolyl)CH2]}+ (46) (Eq. 49). The addition of 3-methylindole to 3a yields

forma-tion of {Pt(PPh3)2[h3-CH2C(2-3-methylindolyl)CH2]}(BF4) (47) at a relatively slow

rate (Eq. 50).

(48)

(19)

1161

(50) As to the aryl system, complex 3a can undergo addition of the aryl CH bonds

of PhNMe2, dimethoxy-, or trimethoxybenzene to form 2-h3-arylallyl complexes

{Pt(PPh3)2[h3-CH2C(Ar)CH2]}(BF4) (Ar = 4-Me2NC6H4 48, 2,4-(MeO)2C6H3 49,

2,4,6-(MeO)3C6H2 50) (Eq. 51) with regioselectivity of conventional electrophilic

aromatic substitution. The reactions between 3a and the less electron-rich arenes such as benzene, toluene, xylene, or anisole, etc. do not take place under the same

conditions. Nucleophilic addition ofn_Bu

4NBH4, NaSPh, or Na[CH(SO2Ph)2] at the

terminal carbon of 50 gives CH₂C[2,4,6-(MeO)₃C₆H₂]CH₂Nu (51) (Eq. 52). With

respect to the arenes, the overall transformation achieves vinylation of arenes, which provides the same kind of products as result from the Heck or Stille reactions of vinylic arylation [42].

(51)

(52)

4. Mechanisms of nucleophilic addition of allenyl and/or propargyl complexes

4.1. Addition to metal h1_{-allenyl requires preceding coordination of nucleophile}

Wojcicki and Kurosawa have shown that the h3_{-allenyl/propargyl complexes}

may release theirp-interaction to form either h1_{-allenyl or}_h1_{-propargyl complexes}

[5c,13a,14b] (Eqs. 53, 54). Theh1_{-allenyl complexes are also subject to nucleophilic}

addition. The formation of (h1_{-allenyl)amido platinum complexes 23 and 24 and}

their transformation into theh3_{-NTMM products support the view that}

hydroam-ination of the h1_{-allenyl complexes likely involves a preceding amide coordination}

step [28]. The octahedral h1_{-allenyl iridium 10 undergoes substitution of a variety}

of amino compounds for the triflate ligand [30]. However, the reactions of 10 and aniline derivatives give products of hydroanilination. Detailed studies found that when equimolar amounts of 10 and aniline react at 243 K, an aniline-ligated

(20)

complex [OC-6-42-Ir(Cl)(PPh3)2(NH2Ph)(CO)(h1-CHCCH2)]-(OTf) (25an) was first

formed. Adding benzylamine (one equivalent) followed by raising the reaction temperature to 273 K resulted in a mixture of the benzylamine-coordinated

complex (25bz) and the N-phenylated h3_{-NTMM complex 26 in 1:1.3 relative}

yields. Complex 25bz did not cause the formation of corresponding N-benzylated

h3_{-NTMM product. However, the mixing of equimolar amounts of aniline and}

25bz at 25°C also resulted in the formation of 26 to 96% yields, basically at the

expense of 25bz. Apparently, ligand substitution is reversible, and the equilibrium favors the better electron-donating 25bz more than 25an (Eq. 55).

(53, 54)

(55) The reaction course from 25an to 26 is not explicit. The reactivity discrepancy

between 25an and 25bz suggests that the feasibility of NH bond cleavage is crucial,

in addition to the nucleophilicity of added groups [43]. Similar phenomena were also displayed by water and alcohol which have relatively low basicity and nucleophilicity, but are readily added to 10, whereas ammonia, amide, as well as amines of good nucleophilcity and basicity are inert to addition. One may conceive that coordination of the amino group allows the metal to play a prominent role to

the activation of the NH bond as well as the formation of the CN bond, and

thereby conveys selectivity into such a chemical transformation.

4.2. Direct external nucleophilic attack at theh3_{-allenyl/propargyl central carbon}

4.2.1. NH addition 6ersus OH addition

None of the reactions of NH addition and OH addition have shown the detection of metallacyclobutene intermediate. However, protonation of the isolated

(21)

1163

metallacyclobutenes leads to the formation ofh3_{-allyl complexes [22f,31]. Wojcicki}

with his phenyl-substituted h3_{-allenyl/propargyl complexes could observe that}

alkoxylation and amination showed different stereochemistry in the resulting

h3_{-allyl complexes, a syn-product for the former (Eq. 56) and an anti-product for}

the latter reaction (Eq. 57) [13b]. The same stereochemistry of addition was

observed by Casey and his coworkers in the reaction of D₂O with [C₅Me₅

-(CO)2Re(h3-HCCCH2)]+. Clean formation of 2-hydroxyallyl {C5Me5(CO)2Re[h3

-HDCC(OD)CH2]}+ (53) with regio- and stereospecific isotope-addition was

isolated (Eq. 58). The added deuterium on the allyl terminal carbon only appears at the anti-position.

(56)

(57)

(58) The anti-product from amination may be explained by nucleophilic attack of

amine at the central carbon ofh3_-RCCCH

2with an essentially synchronous proton

transfer to the CR carbon (Eq. 59). In contrast, water or alcohol addition

undergoes nucleophilic attack at theh3_{-allenyl/propargyl central carbon to form a}

neutral metallacyclobutene intermediate I. The following protonation at metal will give a (hydrido)metallacyclobutene II. Transfer of the proton to the CR carbon

away from the alkoxy group would afford the anti-labeling h3_{-allyl product (Eq.}

60). The relative rates for the addition of R%OH to 3a indicate the mild influence of

a steric effect which may be elucidated by protonation at metal with use of ROH₂+_.

(22)

(60)

4.2.2. Aromatic CH addition

The reactions shown in Eqs. 46 – 51 reveal regiochemistry of electrophilic aro-matic substitution. To seek further evidence for such a mechanism, a crossover

labeling experiment has been carried out. A sample of 1,3,5-(MeO)3C6D3 − nHn

(n = 0, 1) with d2:d3= 22:78 was mixed with an equimolar amount of

1,3,5-(MeO)3C6H3. The resulting 1,3,5-trimethoxybenzene with a labeling distribution of

d0:d2:d3= 50:11:39 was reacted with 3a followed by treatment with NaSPh. The

NMR spectra showed the formation of 2-CH2C[2,4,6-(MeO)3C6H2]CH2(SPh) (51)

with deuterium appearing at the aryl ring and the allyl terminal carbon but without stereospecificity. The mass spectroscopy provides the deuterium distribution of intermolecular hydrogen scrambling. A mechanism involving electrophilic addition (intermediate III) and formation of metallacyclobutene IV which undergoes exter-nal protonation to yield the allyl product may explain these reactions (Eq. 61).

(61) Comparison with the Friedel – Craft aromatic substitution reactions which

re-quire an external Lewis acid promoter, the unique bonding of the h3_-allenyl/

propargyl complexes intrinsically confers sufficient electrophilicity at the central

carbon of the ligands. As a result, theh3_{-allenyl/propargyl ligand reveals}

remark-ably high reactivity as well as subtle chemical selectivity towards various aromatics.

5. Concluding remarks

The cationic mononuclearh3_{-allenyl/propargyl complexes exhibit new}

(23)

1165

which appeal to direct external nucleophilic attack at the central carbon. The reactions of nucleophilic addition of the title complexes provide pragmatic synthetic

access to new organometallic species of various central-carbon-substitutedh3_-allyls,

h3_{-heterotrimethylenemethanes,} _{g-substituted h}3_{-trimethylenemethanes,} _h2_-allenes,

and metallacyclobutenes. The h3_{-allenyl/propargyl ligands may release the}

p-interaction to transform into either h1_{-allenyl or} _h1_{-propargyl complexes. The}

linearh1_{-allenyl complexes are also subject to regioselective nucleophilic addition at}

the central carbon of the ligand; however, this requires the preceding coordination of nucleophiles.

Acknowledgements

The collaboration of the coworkers and the financial support from the National Science Council, Taiwan, Republic of China are gratefully acknowledged.

References

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(24)

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