Deprotonation Pathway in the Reaction of Me6Si2 with MeLi

(1)

Deprotonation Pathway in the Reaction of Me

6

Si

2

with

MeLi

Ling-Kang Liu*

,†,‡

_{and Lung-Shiang Luh}

†

Institute of Chemistry, Academia Sinica, Nankang, Taipei, Taiwan 11529, ROC,

and Department of Chemistry, National Taiwan University, Taipei, Taiwan 10767, ROC

Received October 13, 1999

Summary: A mixture of 3 mol of Me6Si2

and 1 mol of

MeLi in the presence of P(O)(NMe2)3

produced Me3SiLi

as the initial product. This then deprotonated excess Me

6

-Si2

and started an unprecedented transformation

lead-ing to (Me3Si)2(SiMe2H)CLi (3), whose quench by

[(η

5

_{-C5H5)Fe(CO)2PPh3}

+

] produced

{

η

4

-exo-[(Me3Si)2C-(SiMe

2

H)]C

5

H

5

}

Fe(CO)

2

(PPh

3

) (2e). In the literature the

reaction of Me6Si2

and MeLi gives Me3SiLi and/or

Me3-SiSiMe2Li (4). The lithium compound 3 is the major

product when a previously unnoticed deprotonation

pathway is enhanced by use of an excess of Me6Si2

and

a longer reaction time.

Organosilanes have been used to enhance reactivity

and selectivity in chemical transformations.

1

_{In one}

synthetic approach, the addition of silyl anions to a

variety of organic electrophiles results in formation of

the needed Si-C bond.

2

_{Among such silyl anions, R}

3

-SiLi can be generated in situ by reaction of R

3

SiCl with

Li, of (R

3

Si)

2

Hg with Li, or of R

3

SiSiR

3

with R

′

Li.

3

We

have used the electrophile [(η

5

_-C

5

H

5

)Fe(CO)

2

PPh

3+

] (1)

to quench the generated silyl anions. The results

revealed an unprecedented conversion of Me

3

SiLi to

(Me

3

Si)

2

(SiMe

2

H)CLi and gave retro-chemical evidence

for a deprotonation pathway in the Me

6

Si

2

reaction with

MeLi.

The R

3

SiLi anions were generated by reaction of the

respective disilane with MeLi. The resulting solutions

were cooled to -78 °C and transferred dropwise by

cannula to a solution of 1:1 (η

5

_-C

5

H

5

)Fe(CO)

2

I/PPh

3

in

THF at -78 °C, the practical equivalent to [1][I], after

chemical initiation with a trace of lithiated reagent,

4

which in the present cases is the first few drops of R

3

-SiLi. The color of the solution changed gradually from

black to orange-red during the addition of R

3

SiLi,

sometimes with formation of a yellow precipitate that

redissolved as the reaction proceeded. For the silyl

anions with at least one aryl group, the Cp ring

silylation products (η

4

_-exo-R

3

SiC

5

H

5

)Fe(CO)

2

(PPh

3

) (R

3

) Ph

3

(2a, 51%), MePh

2

(2b, 36%), Me

2

Ph (2c, 45%))

were isolated as major products after column

chroma-tography.

5

_{Thus, the Si-based nucleophiles add at the}

Cp ring of 1, similar to what occurred with C-based

nucleophiles

6

_{and different from O-based nucleophiles}

* To whom correspondence should be addressed. E-mail: liuu@ chem.sinica.edu.tw.

†_{Academia Sinica.}

‡_{National Taiwan University.}

(1) (a) Fleming, I. In Comprehensive Organic Chemistry; Barton, E., Ollis, W. E., Eds.; Pergamon Press: Oxford, U.K., 1979. (b) Colvin, E. Silicon in Organic Synthesis; Butterworth: Boston, 1981. (c) Weber, W. P. Silicon Reagents in Organic Synthesis; Springer-Verlag: New York, 1983. (d) Patai, S., Rappoport, Z., Eds. The Chemistry of Organic Silicon Compounds; Wiley: New York, 1989. (e) Hwu, J. R.; Wang, N. Chem. Rev. 1989, 89, 1599. (f) Rappoport, Z.; Apeloig, Y., Eds. The Chemistry of Organic Silicon Compounds; Wiley: New York, 1998; Vol. 2.

(2) Oshima, K. In Advances in Metal-Organic Chemistry; Liebeskind, L. S., Ed.; JAI Press: London, 1991; Vol. 2, pp 101-141.

(3) (a) Tamao, K.; Kawachi, A. Adv. Organomet. Chem. 1998, 38, 1 and references cited herein. (b) Wiberg, E.; Stecher, O.; Andrascheck, H. J.; Kreubichler, L.; Staude, E. Angew. Chem., Int. Ed. Engl. 1963, 2, 507. (c) Fujita, M.; Hiyama, T. J. Synth. Org. Chem. Jpn. 1984, 42, 293. (d) Vyazankin, N. S.; Razuvaev, G. A.; Gladyshev, E. A.; Korneva, S. P. J. Organomet. Chem. 1967, 7, 353. (e) Gladyshev, E. N.; Fedorova, E. A.; Yuntila, L. A.; Razuvaev, G. A.; Vyazankin, N. S. J. Organomet. Chem. 1975, 96, 169.

(4) Gipson, S. L.; Liu, L.-K.; Soliz, R. U. J. Organomet. Chem. 1996, 526, 393.

(5) Manipulations were carried out under N2with dry degassed reagents. Preparation of 2a (typical): a 100 mL two-necked flask was charged with Ph3SiCl (5 mmol), and fine-cut Li wire (20 mmol) and then THF (30 mL) was added. The solution became turbid after stirring for several minutes and the color changed gradually from yellow to brown to black. After it was stirred for 6 h, the resulting solution was cooled to -78 °C and filtered through a pad of Celite. The filtrate was added dropwise to a mixture of (η5_-C

5H5)Fe(CO)2I (3 mmol) and PPh3 (3 mmol) in THF (100 mL), also at -78 °C. The color of solution changed from black to orange-red during the addition, accompanied by the formation of a yellow precipitate, which redissolved when the addition was completed. The reaction mixture was quenched with H2O (200 mL) and extracted with Et2O (100 mL× 2) after it was stirred overnight. The organic layers were combined, dried over MgSO4, and then evaporated to dryness under vacuum. The oily residue was purified by SiO2column chromatography with 1/15-20 EtOAc/hexane as eluent to give yellow-orange 2a (51%). IR (CH2Cl2): νCO1962 (s), 1903 (s) cm-1.1_{H NMR (C}

6D6): δ 2.71 (b, 2H), 3.90 (b, 1H), 5.10 (b, 2H), 6.96-7.53 (m, 30H). 31_{P NMR (C}

6D6): δ 72.1 (s). 29Si NMR (C6D6): δ -22.6 (s). FAB MS: m/z 698 (M+). Anal. Calcd for C43H35 -FeO2PSi: C, 73.92; H, 5.05. Found: C, 73.60; H, 4.99. 2b: yield 36%. IR (CH2Cl2): νCO1963 (s), 1902 (s) cm-1.1H NMR (C6D6): δ 0.32 (s, 3H), 2.57 (b, 2H), 3.51 (b, 1H), 5.18 (b, 2H), 6.97-7.52 (m, 25H).31_P NMR (C6D6): δ 71.6 (s).29Si NMR (C6D6): δ -17.3 (d, JPSi) 10.0 Hz). FAB MS: m/z 636 (M+_{). Anal. Calcd for C}

38H33FeO2PSi: C, 71.70; H, 5.23. Found: C, 71.58; H, 5.22. Orange side product (η5_-C

5H5 )Fe(CO)C-(O)SiMePh2(PPh3): yield 12%. IR (CH2Cl2): νCO1906 (s), 1574 (m) cm-1_.1_{H NMR (C} 6D6): δ 1.22 (s, 3H), 4.07 (s, 5H), 6.96-7.77 (m, 25H). 31_{P NMR (C} 6D6): δ 75.7 (s).29Si NMR (C6D6): δ -36.5(s). Anal. Calcd for C38H33FeO2PSi: C, 71.70; H, 5.23. Found: C, 71.87; H, 5.15. 2c: yield 45%. IR (CH2Cl2): νCO1960 (s), 1901 (s) cm-1.1H NMR (C6D6): δ -0.04 (s, 6H), 2.46 (b, 2H), 3.02 (b, 1H), 5.18 (b, 2H), 6.95-7.48 (m, 20H).31_{P NMR (C}

6D6): δ 71.8 (s).29Si NMR (C6D6): δ -11.5 (d, JPSi) 8.0 Hz). FAB MS: m/z 574 (M+_{). Anal. Calcd for C}

33H31FeO2PSi: C, 69.00; H, 5.44. Found: C, 69.16; H, 5.33.

(6) (a) Liu, L.-K.; Luh, L.-S. Organometallics 1994, 13, 2816. (b) Luh, L.-S.; Liu, L.-K. Bull. Inst. Chem., Acad. Sin. 1994, 41, 39. (c) Liu, L.-K.; Luh, L.-S.; Chao, P.-C.; Fu, Y.-T. Bull. Inst. Chem., Acad. Sin.

1995, 42, 1. (d) Luh, L.-S.; Eke, U. B.; Liu, L.-K. Organometallics 1995,

14, 440.

374 Organometallics 2000, 19, 374-376

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(2)

which react at the CO ligand.

7

_{The single-crystal X-ray}

structure of 2b confirmed that the MePh

2

Si group is exo

to Fe (Figure 1),

8

_{indicative of a direct silyl attack.}

Compounds 2a-c were desilylated upon acidification

with HBF

4

(aq) or HCl(aq) to re-form the cationic

com-pound [1][X] (X ) BF

4

, Cl), as shown by IR and

1

H and

31

_{P NMR spectroscopic studies.}

Me

3

SiLi was prepared by treating Me

6

Si

2

with MeLi

in the presence of P(O)(NMe

2

)

3

for 15 min at 0 °C.

9

The

reagent solution was then quenched with 1:1 (η

5

_-C

5

H

5

)-Fe(CO)

2

I/PPh

3

at -78 °C in THF, with the expectation

that

{

η

4

_-exo-(Me

3

Si)C

5

H

5

}

Fe(CO)

2

(PPh

3

) (2d) would be

formed. IR monitoring of the reaction solution indicated

a quantitative formation of an η

4

_{product. However,}

after column chromatography, only a low-yield product

(<10%) was obtained with spectroscopic data as follows

(cf. 2a-c): IR ν

CO

stretching bands at 1967 (s) and 1908

(s) cm

-1

, a

31

_{P NMR resonance (C}

6

D

6

) at δ 76.1 (s), and

1

_{H NMR resonances (C}

6

D

6

) at δ 2.58 (b, 2H), 3.65 (b,

1H), and 5.03 (b, 2H). Nevertheless, the proton

integra-tion for silyl-Me was not correct, nor was the elemental

analysis. A single-crystal X-ray analysis confirmed the

structure as being

{

η

4

_-exo-[(Me

3

Si)

2

C(SiMe

2

H)]C

5

H

5

}

-Fe(CO)

2

(PPh

3

) (2e), which is actually a Cp ring

alkyla-tion product (Figure 2).

10

It seemed possible that 2d might have been produced

as the initial product but was desilylated or decomposed,

since 2a-c are acid-sensitive. However, the generation

of Me

3

SiLi by the literature procedure

9

must have also

produced, at the same time, a small amount of (Me

3

-Si)

2

(SiMe

2

H)CLi (3) before the quench (vide infra). We

suggest that the in situ generation of deep red Me

3

SiLi,

from MeLi and Me

6

Si

2

, is facile because a strong

nucleophilic base easily breaks the Si-Si bond.

11

_When

the reaction is carried out on a larger scale, the C-Si

bond in Me

6

Si

2

is often observed to be cleaved in a side

reaction, which leads to the formation of Me

3

SiSiMe

2

Li

(4)

12

_{(Scheme 1).}

13

_{Obviously, the negative charge on}

the silicon atom is stabilized by the second silicon atom

in the R-position.

14

_{Still another pathway that has not}

(7) Liu, L.-K.; Eke, U. B.; Mesubi, M. A. Organometallics 1995, 14, 3958.

(8) Crystal data for 2b: C38H33FeO2PSi, triclinic, P1h, a ) 9.619(1) Å, b ) 10.424(1) Å, c ) 16.566(3) Å, R ) 87.22(1)°, β ) 77.06(1)°, γ ) 82.19(1)°, V ) 1603.5(3) Å3_{, Z ) 2, D}

calcd) 1.318 g/cm3, 4739 reflections (I > 2.0 σ(I)), 389 parameters, R ) 0.031, Rw) 0.039, GOF ) 1.92.

(9) Still, W. C. J. Org. Chem. 1976, 41, 3063.

(10) Preparation of 2e. A solution of Me3SiSiMe3 (5 mmol) in anhydrous P(O)(NMe2)3(4 mL) was cooled to 0 °C. MeLi (4 mmol) was added via syringe, and the resulting deep-red solution was stirred for 15 min. THF (30 mL) was added, and the solution was cooled to -78 °C. The solution was transferred dropwise via cannula into the mixture of (η5_-C

5H5)Fe(CO)2I (3 mmol) and PPh3(3 mmol) in THF (100 mL) at -78 °C. The color of solution gradually changed from black to orange, accompanied by the formation of a yellow precipitate that redissolved when the addition was completed. The solution was warmed to room temperature and stirred overnight before the mixture was quenched with H2O (200 mL) and extracted with Et2O (100 mL × 2). The combined organic layers were dried over MgSO4and then evaporated to dryness under vacuum. The oil-like residue was purified by SiO2 column chromatography with 1:12 (v/v) EtOAc/hexane as eluent to give yellow-orange 2e (6%). Improved procedure: The treatment of Me3SiSiMe3(15 mmol) in anhydrous P(O)(NMe2)3(12 mL) with MeLi (5 mmol) and stirring for 2 h, (otherwise the same procedure as above) resulted in 2e (60%). IR (CH2Cl2): νCO1967(s), 1908(s) cm-1.1H NMR (C6D6): δ 0.14 (s, 18H), 0.21 (d,3JHH= 4 Hz, 6H), 2.58 (b, 2H), 3.65 (b, 1H), 4.30 (hept,3_J HH= 4 Hz, 1H), 5.03 (b, 2H), 6.98-7.53 (m, 15H). 31_{P NMR (C} 6D6): δ 76.1(s).29SiNMR (C6D6): δ -15.9 (s), -16.5 (s). FAB MS m/z: 656 (M+). Anal. Calcd for C34H45FeO2PSi3: C, 62.18; H, 6.90. Found: C, 61.99; H, 6.81. Crystal data of 2e: C34H45FeO2PSi3, triclinic P1h, a ) 11.283(2) Å, b ) 12.591(2) Å, c ) 14.276(3) Å, a ) 66.63(1)°, β ) 72.17(2)°, γ ) 76.70(1)°, V ) 1758.5(5) Å3_{, Z ) 2, D}

calcd ) 1.240 g/cm3_{, 5238 reflections (I > 2.5σ(I)), 374 parameters, R ) 0.033,} Rw) 0.045, GOF ) 2.31.

(11) Marschner, C. Eur. J. Inorg. Chem. 1998, 221.

(12) (a) Hudrlik, P. F.; Waugh, M. A.; Hudrlik, A. M. J. Organomet. Chem. 1984, 271, 69. (b) Nadler, E. B.; Rappoport, Z. Tetrahedron Lett.

1990, 31, 555. (c) Allred, A. L.; Smart, R. T.; Van Beek, D. A., Jr.

Organometallics 1992, 11, 4225. (d) Hwu, J. R.; Wetzel, J. M.; Lee, J. S.; Butcher, R. J. J. Organomet. Chem. 1993, 453, 21. (e) Krohn, K.; Khanbabaee, K. Angew. Chem., Int. Ed. Engl. 1994, 33, 99.

(13) It is believed that the C-based nucleophile MeLi is transformed to the Si-based nucleophile Me3SiLi before other second-stage reactions. As an indirect clue, the formation of 4 was only observed in large-quantity preparations.

(14) Armitage, D. A. In Comprehensive Organometallic Chemistry: The Synthesis, Reactions and Structures of Organometallic Compounds; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1982; Vol. 2, pp 1-203.

Figure 1. Molecular plot of 2b.

Figure 2. Molecular plot of 2e.

Communications

Organometallics, Vol. 19, No. 4, 2000

375

(3)

yet been reported for Me

6

Si

2

is deprotonation of a

methyl substituent, which results in a lithiated

carban-ion Me

3

SiSiMe

2

CH

2

Li (5) (Scheme 2). The R-SiMe

2

group stabilizes the polar C-Li bond. The β-SiMe

3

group, however, destabilizes the C-Li bond (a β-Si atom

normally stabilizes a carbonium ion).

14

_Thus,

intramo-lecular Me

3

Si group migration

15

from the silicon atom

to the carbon atom results, followed by a 1,2-proton

shift. This gives the lithiated carbanion Me

3

Si(SiMe

2

H)-CHLi (6), which is isomeric with 5 and is stabilized by

two R-silyl groups. An extra 1 equiv of Me

6

Si

2

is

attacked by 6 to cleave the Si-Si bond and regenerate

Me

3

SiLi, which deprotonates the (Me

3

Si)

2

(SiMe

2

H)CH

thus formed to give the final lithiated species, 3, which

is stabilized by three R-silicon atoms. The nucleophilic

alkylation of the Cp ring of 1 by 3 affords 2e.

16

As the characteristic deep red color of Me

3

SiLi in

solution is clearly observed, the base effecting

deproto-nation in Scheme 2 must be Me

3

SiLi. Thus, it takes

overall 3 mol of Me

6

Si

2

in order for 1 mol of MeLi to

produce 1 mol of 3; therefore, an increase in

stoichio-metric ratio between Me

6

Si

2

and MeLi should favor the

deprotonation pathway. The isolated yield of 2e was

improved to 60% when a 3:1 mixture of Me

6

Si

2

/MeLi

was allowed to react for a longer time (2 h), resulting

in a color change from deep red to orange before the

quench. To our knowledge, this is the first example of

the transformation of a Me anion to a silyl anion and

then back to a carbanion, starting with a simple

disilane. The present deprotonation pathway in the

reaction of Me

6

Si

2

with MeLi is intermolecular. The

known intramolecular transfer of the organolithium

function in 1-Me

3

Si-8-Li-C

10

H

6

to form 1-Me

2

SiCH

2

Li-C

10

H

717

is a similar process (C

10

H

6

) 1,8-disubstituted

naphthalene skeleton; C

10

H

7

) 1-substituted

naphtha-lene skeleton).

The speculative mechanism shown in Scheme 2 was

tested with different organic electrophiles in order to

provide evidence that 3 actually is formed under the

reaction conditions. When a 3:1 mixture of Me

6

Si

2

/MeLi

was quenched with Me

3

SiCl, for instance, the expected

(Me

3

Si)

3

CSiMe

2

H

18

could be isolated (ca. 30%, not

optimized) (

29

_{Si NMR (C}

6

D

6

) δ -16.4 (SiMe

3

) and -16.1

(SiMe

2

H);

1

H NMR (C

6

D

6

) δ 0.24 (s, SiMe

3

, 27H), 0.29

(d,

2

_J

HH

) 4.0 Hz, SiMe

2

H, 6H), 4.31 (hept,

2

J

HH

) 4.0

Hz, SiMe

2

H, 1H)). Spectroscopic evidence for the

forma-tion of Me

3

SiH also was obtained. In a sealed NMR tube

experiment, a 3:1 mixture of Me

6

Si

2

/MeLi with

P(O)-(NMe

2

)

3

in d

8

-THF gave

1

H NMR peaks at δ 4.61 (hept,

2

_J

HH

) 4.0 Hz), assigned to the unique SiH of 3, and at

δ 4.00 (decatet,

2

_J

HH

) 4.0 Hz), assigned to the unique

SiH of HSiMe

3

, in the correct molar ratios. The

corre-sponding

29

_{Si NMR data were δ -28.5 (SiMe}

3

) and

-27.9 (SiMe

2

H) for 3 and δ -16.1 for Me

3

SiH.

In conclusion, the reaction of Me

6

Si

2

and MeLi results

in Me

3

SiLi (and 4), plus the previously unnoticed 3. The

latter is the more important product when excess Me

6

-Si

2

is used.

Acknowledgment. Thanks are due to the National

Science Council of the ROC for financial support, to Mr.

Yuh-Sheng Wen for X-ray data collection, and to Prof.

Jih-Ru R. Hwu and Prof. Hiromi Tobita for fruitful

discussions.

Supporting Information Available: Details of the single-crystal structure analyses for 2b,e including tables of posi-tional parameters and bond lengths and angles. This material is available free of charge via the Internet at http://pubs.acs.org. OM9908175

(15) (a) Brook, A. G.; Bassindale, A. R. In Rearrangements in Ground and Excited States; De Mayo, P., Ed.; Academic Press: New York, 1980; Vol. 2, pp 149-227. (b) Eisch, J. J.; Tsai, M.-R. J. Organomet. Chem.

1982, 225, 5.

(16) Occasionally{η4_-exo-[(Me

3Si)CH(SiMe2H)]C5H5}Fe(CO)2(PPh3), the speculative Cp-ring alkylation product of 6 and 1, could be detected in trace amount in the1_{H NMR spectrum of 2e. The pure complex} has not been isolated for complete characterization.

(17) Wroczynski, R. J.; Baum, M. W.; Kost, D.; Mislow, K.; Vick, S. C.; Seyferth, D. J. Organomet. Chem. 1979, 170, C29.

(18) Dua, S. S.; Eaborn, C.; Happer, D. A. R.; Hopper, S. P.; Safa, K. D.; Walton, D. R. M. J. Organomet. Chem. 1979, 178, 75.