Validation of a recording system for computer pointing device activity

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Journal of Organometallic Chemistry 572 (1999) 291 – 293

Priority communication

Organosulfur compounds as promoters in the Mo(CO)

6

-catalyzed ring

opening metathesis polymerization of norbornenes

Chung-Wai Shiau, Jitendra A. Sattigeri, Clifton K.-F. Shen, Tien-Yau Luh *

Department of Chemistry, National Taiwan Uni6ersity, Taipei106, Taiwan, ROC Received 15 July 1998; received in revised form 8 September 1998

Abstract

Thioethers, dithioacetals and thioketones, but not mercaptans, are active promoters for the Group 6 metal carbonyl-catalyzed ring opening metathesis polymerization (ROMP) of norbornenes. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Norbornenes; Organosulfur compounds; Promoters

Group 6 metal carbonyls are thiophilic reagents to promote homolytic cleavage of the carbon – sulfur bond [1,2]. A sulfur-containing metallic product is suggested [2]. Some of these metallic intermediates are highly reactive [2,3], and it is envisaged that such species may serve as an active catalyst for further reactions. Recently, haloalkanes are found to be promoters for the Mo(CO)6

-mediated metathesis reactions under photolytic condi-tions [4]. Phenols [5] and various Lewis acids [6] are also known to facilitate Group 6 metal carbonyl-catalyzed metathesis reactions. The authors felt that the sulfur-con-taining species generated in situ from the Group 6 metal carbonyl-mediated desulfurization reaction may also serve as an efficient olefin metathesis catalyst.

At the beginning of this investigation, a chlorobenzene solution of 1a with 0.3 mol% each of Mo(CO)6 and

2,2-diphenyldithiolane was heated under reflux for 10 min (entry 1). Rubber-like solid 2a was formed immedi-ately and the Mn is 45 K (PDI = 3.1). The NMR

spectrum of this polymer is consistent with the corre-sponding ring opening metathesis polymerization (ROMP) of 1a prepared by Grubbs’s ruthenium carbene catalyst [7]. Table 1 summarizes representative results of the ROMP of 1 under different conditions. It is notewor-thy that the reaction does not proceed in the absence of the organosulfur compound. The reaction can proceed in the dark, no irradiation being required under these conditions.

* Corresponding author. Fax: + 886-2-2364-4971.

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C.-W. Shiau et al./Journal of Organometallic Chemistry572 (1999) 291 – 293

292

Table 1

Organosulphur compound-promoted M(CO)6-catalyzed ROMP of 1

Mn/104(PDI) 2 (%yield) Reaction time Solvent M(C0)6 Promoter 1 Entry E/Z 4a a 1 Mo PhCl 10 min a (95) 4.5 (3.1) 1/1 30 min a (72) 4.8 (2.4) 1/1 2 a 4a W PhCl 5.0 (2.7) 1/2 3 a 4a Mo THF 15 h a (30) 5 h a (45) 9.7 (2.1) 4 a 4b Mo THF 1/3 5 a 3a Mo THF 5 h a (25) 1.2 (5.6) 1/1.5 11.9 (1.9) a (78) 20 min PhCl 1/1 Mo 3b a 6 THF 5 h 7 a 3b Mo a (23) 6.2 (3.3) 1/2 a 5 Mo PhCl 8 20 min a (85) 8.9 (2.7) 1/1 1/2 3.3 (6.4) a (32) a 10 h 9 5 Mo THF a 6 Mo PhCl 10 30 min a (74) 5.3 (3.1) 1/1 11 a 6 Mo THF 10 h a (59) 15.2 (1.7) 1/2.5 b (43) 4c b 12 Mo PhCCl3 4 h 14.5 (2.2) 1/1

As can be seen from Table 1, thioethers 3, dithioac-etals 4 as well as thioketones 5 were active promoters for the Mo(CO)₆-catalyzed ROMP of 1. Lawesson reagent 6, which contains phosphorussulfur double bond, was also an active promoter. W(CO)6 appeared

to be less reactive, with a longer reaction time being required (entry 2). The reaction can also be carried out in ethereal solvents. Although a higher temperature will shorten the reaction time and increase the yield of 2, better Z-selectivity was slightly favored when the reac-tion was carried out in refluxing THF (entries 3 – 5, 7, 9 and 11).

Substituted norbornene such as 1b was less reactive. Elevated temperature (refluxing chlorobenzene or trichlorotoluene) was required for a smooth transfor-mation (entry 12). When a solution of 1b was heated with 1 mol% of Mo(CO)₆ and 4a in chlorobenzene under reflux for 1 h followed by addition of one equivalent of 1a and refluxed for an additional 5 h, a block copolymer 7 (Mn= 1.1 × 105and PDI = 2.3) was

obtained and the ratio of the constituent monomeric moieties rising from 1b and 1a was 1.3: 1.

Mercaptans were inactive for these reactions. It is known that the carbon – sulfur bond in mercaptans can be reduced via a radical mechanism under these condi-tions and the SH moiety provides the hydrogen source for the radical abstraction [1]. It was felt that other hydrogen source that can be abstracted by a radical may also prohibit the reaction. Indeed, when the reac-tion was carried out in diglyme under reflux, no ROMP of 1a was observed. Presumably, the CH bonds in diglyme can undergo radical abstraction under these conditions [1]. Elemental sulfur was ineffective for this catalytic process.

Although the actual mode of the reaction remains unclear, a sulfur containing species is speculated. Treat-ment of 4a with Mo(CO)6 in refluxing chlorobenzene

yielded, in addition to tetraphenylethylene, an insoluble and very air-sensitive black species 8 that exhibited broad absorptions for the carbonyl moieties (1945 –

2000 cm− 1_{) in the IR region. When 8 was treated with}

NaOH followed by acidification, hydrogen sulfide was liberated. It is noteworthy that 8 can also serve as a catalyst for the ROMP of norbornene. For example, 2 was obtained in excellent yield with a similar M_n (ca. 45 000) when 1a was treated with 8 in refluxing chlorobenzene for 10 min.

In summary, the authors have depicted for the first time that an organosulfur compound can serve as a promoter for the Group 6 metal carbonyl-catalyzed ROMP of norbornenes. Investigations on the nature of this catalytic system and extension to other cyclic olefinic substances are in progress.

Acknowledgements

The authors thank the National Science Council of the Republic of China for financial support.

References

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[3] T.-Y. Luh, L.L. Yeung, unpublished results.

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[5] (a) D. Villemin, P. Cadiot, Tetrahedron Lett. 23 (1982) 5139. (b) N. Kaneta, K. Hikichi, S.-I. Asaka, M. Uemura, M. Mori, Chem. Lett. (1995) 1055.

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[6] For a recent review see: K.J. Ivin, J.C. Mol, Olefin Metathesis, Metathesis Polymerization, Academic Press, San Diego, 1997, Chapter 13.

[7] (a) S. Nguyen, L.K. Johnson, R.H. Grubbs, J. Am. Chem. Soc. 114 (1992) 3974. (b) R.R. Schrock, Acc. Chem. Res. 23 (1990) 158.