Dehydrogenative coupling of silanes

With the stable donor adducts of silylene complexes, valuable model compounds are now available for reactive intermediates which otherwise cannot be observed directly. For example, a side reaction occurring in the hydrosilation process [61 -63], is the dehydrogenative coupling of silanes to disilanes. This reaction could be explained in terms of a silylene transfer reaction with a coordinated silylene as the key intermediate. 

Recent investigations have been concerned with the reactivities observed with secondary silanes R2SiH2. In these cases, a dehydrogenative coupling of silanes to disilanes is observed as a side reaction of the hydrosilation. However, the hydrosilation can be totally suppressed if the olefins are omitted. The key intermediate in the coupling reaction has been identified as a silylene complex (sect. 2.5.4). 

The dehydrogenative coupling of silanes does not stop at the stage of disilanes in the coordination sphere of early transition metals like Zr and Hf, but chain polymers of low molecular weight are formed. As reactive intermediates in this reaction, silylene complexes are also assumed. However, alternative mechanisms have been discussed (sect. 2.5.4). 

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]. 

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. 

C. Phosphine-alanes as bifunctional cocatalysts for the Ni-catalyzed dehydrogenative coupling of silanes... 

In the last years transition metal-silyl complexes have received special attention for several reasons [1, 2], On the one hand, they are assumed to be important intermediates in catalytic processes [2] (transition metal-catalyzed hydrosilylation reaction, dehydrogenative coupling of silanes to polysilanes, etc.), on the other metal-substituted silanes show special properties, which can be tuned systematically by judicious choice of the metal and its ligands [3] Furthermore, silylenes (silanediyls) are stabilized by unsaturated transition metal fragments leading to metal-silicon double-bonds [4]. In the light of a possible application in MOCVD processes some of these complexes are of interest as potential single-source precursors for the manufacture of thin silicide films [5]. 

Dehydrogenative coupling of silanes catalyzed by phosphine-alane Ni complexes... 

Scheme 4 Dehydrogenative coupling of silanes. TOF = turnover frequency...

Figure 19 Active species proposed to account for dehydrogenative coupling of silanes and its Lewis base adduct...

Metal-assisted dehydrogenative coupling of silanes with nucleophiles can also lead to the silicon-heteroatom bond formation. Thus, in the presence of catalytic amount of (ri -benzene)(R -HSiHPh2)Cr(CO)2 124 diphenylsilane 125 reacts smoothly with either aniline or thiophenol to afford the products 126 or 127 respectively (Scheme 34) [60]. This process represents a rare example of arene-chro-mium complex catalysis, which proceeds without breaking arene-metal bonds. 

Figure 1. A mechanism for dehydrogenative coupling of silanes, based on a transition-metal silylene complex as an intamediate.

In summary, we have shown that d metal-silicon a-bonds can be quite active toward a-bond metathesis. Investigation of this reaction chemistry has identified a likely mechanism for the dehydrogenative coupling of silanes as catalyzed by early (dO)... 

Harrison DJ, Edwards DR, McDonald R, Rosenberg L (2008) Toward selective functionalization of oligosilanes borane-catalysed dehydrogenative coupling of silanes with thiols. Dalton Trans 3401... 

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