Polymerization dehydrogenative silane

Figure 3. Proposed mechanism for dehydrogenative silane polymerization by zirconocene and hafnocene catalysis.

Further studies quickly revealed that the rapid dehydrogenative coupling of primary organosilanes to oligomers and the slower coupling of secondary silanes to dimers can be effected under ambient conditions with compounds of the type CP2MR2 (M = Ti, R = alkyl M = Zr, R = alkyl or H)(11,12,13). None of the other metallocenes, metallocene alkyls, or metallocene hydrides of groups 4, 5 or 6 have shown any measurable activity for polymerization... 

The properties of siloxide as ancillary ligand in the system TM-O-SiRs can be effectively utilized in molecular catalysis, but predominantly by early transition metal complexes. Mono- and di-substituted branched siloxy ligands (e.g., incompletely condensed silsesquioxanes) have been employed as more advanced models of the silanol sites on silica surface for catalytically active centers of early TM (Ti, W, V) that could be effectively used in polymerization [5], metathesis [6] and epoxidation [7] of alkenes as well as dehydrogenative coupling of silanes [8]. 

Dehydrogenative Coupling. Transition-metal catalyzed polymerization of silanes appears to hold promise as a viable route to polysilanes. A number of transition-metal complexes have been investigated, with titanium and zirconium complexes being the most promising (105—108). Only primary silanes are active toward polymerization, and molecular weights are rather low. The dehydrogenative polymerization is depicted in reaction 11, where Cp = cyclopentadienyl ... 

Besides the cr-bond metathesis mechanism proposed by Tilley23 for the dehydrogenative coupling of silanes, a Zr(II) pathway25 and a silylene mechanism26 have been proposed based on the nature of the products. The dehydrogenative polymerization of 1,2,3-trimethyltrisilane or of a mixture of diastereomers of 1,2,3,4-tetramethyltetrasilane showed evidence that, besides Tilley s mechanism, a further mechanism is present. The product formation can be explained by a silylene mechanism where the silylenes are formed by a-elimination from the silyl complexes by a new type of /J-elimination which involves Si—Si bond cleavage (/F-bond elimination) as described in Scheme 727. 

Optically active poly(silyl ether)s were also synthesized by asymmetric induction on the silicon atom from chiral diols or a chiral Rh catalyst [178]. In the dehydrogenative polycondensations of bis(silane)s and several chiral diols, the highest ee value of the silicon atom was 13.7%. In the polymerization of a bis(silane) and an achiral diol, however, the use of only 5 mol% of RhCl[(R)-BINAP] resulted in induction of 39.8% ee of the silicon atom (Scheme 45). 

Tilley, T. D. Woo, H.-G. Catalytic Dehydrogenative Polymerization of Silanes to Polysilanes by Zirconocene and Hafnocene Catalysts. A New polymerization Mechanism, (H. F. Harrod, R. M. Laine, Eds.) Inorganic and Organometallic Oligomers and Polymers, Kluwer Academic Publishers, Netherlands, 1991, p. 3. 

Some mechanistic material is included in an annual review of silicon chemistry. A novel mechanism for the dehydrogenative polymerization of silanes... 

The same Y complex has been shown to be an efficient catalyst for the annulation and subsequent silylation of substituted 1,5- and 1,6-dienes (Scheme 12.81) [179]. Significantly, this process took precedence over dehydrogenative polymerization of the silanes, which was reported to be a remarkably facile process. The reaction tolerated a number of functional groups and proceeded in excellent yields and diastereoselectivities. 

CATALYTIC DEHYDROGENATIVE POLYMERIZATION OF SILANES TO POLYSILANES BY ZIRCONOCENE AND HAFNOCENE CATALYSTS. A NEW POLYMERIZATION MECHANISM. 

A second class of catalysts was discovered by Harrod and coworkers [8], who showed that titanocene and zirconocene derivatives dehydrogenate primary silanes (RSiH3) to poly silanes with 10-20 monomer units, in high yield. These catalysts therefore appear to be more promising with respect to production of polymers, and have revived interest in coordination polymerization routes to polysilanes. 

In general, d silyl complexes are reactive toward primary and secondary silanes. We have found that a number of metallocene derivatives of the type Cp2M(SiR3)R (Cp = Cp, Cp s TlS-CsMes M = Zr, Hf R = Me, Ph, SiMes R = Cl, alkyl, silyl) are catalyst precursors for the dehydrogenative polymerization of silanes. An example of such a polymerization is shown in equation 3. The polymer s molecular weight... 

Catalytic Dehydrogenative Polymerization of Silanes to Polysilanes by Zirconocene and Hafnocene Catalysts. A New Polymerization Mechanism. 

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