From Metal Silyls

Wilberg and coworkers studied the thermal and photolytic decomposition of metal silyls to account for the products formed they suggested, as shown in equations (60)-(63), that silylenes are involved as reaction intermediates. It is not necessary, however, that free silylenes be generated in these systems as they claimed. 

More recently Kumada and coworkers studied the thermal decomposition of disilanes such as SiHMCjSiHMCj at 90°C in the presence of either PtCl2(PEt3)2 or NiCl2(PEt3)2 as catalysts and observed reactions attributable to 

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P-Hydrogen eliminations and p-aryl eliminations from alkoxo and amido complexes are also known. Such eliminations have been shown to occur by migratory de-insertion pathways, as well as alternative p-hydride abstraction mechanisms. P-Hydrogen eliminations from metal-silyl complexes are rare because the silicon-carbon double bond in the product is weak. For similar reasons, p-hydrogen eliminations from metal-thiolate complexes are rare. 

The bulk of the TIPS group, introduced with TIPSCl (DMF, Im, 92% yield), directs metallation away from the silyl group as illustrated. ... 

Basically the same methods known from the synthesis of classical metal-silyl complexes can also be applied to the preparation of low valent Si compounds. The procedures given here are summarized with the focus on silylene complexes These are a) reactions of appropriate metal anions with halosilanes, which are the most important methods for the formation of M-Si bonds. Alternatively, silyl... 

The major synthetic routes to transition metal silyls fall into four main classes (1) salt elimination, (2) the mercurial route, a modification of (1), (3) elimination of a covalent molecule (Hj, HHal, or RjNH), and (4) oxidative addition or elimination. Additionally, (5) there are syntheses from Si—M precursors. Reactions (1), (2), and (4), but not (3), have precedence in C—M chemistry. Insertion reactions of Si(II) species (silylenes) have not yet been used to form Si—M bonds, although work may be stimulated by recent reports of MejSi 147) and FjSi (185). A new development has been the use of a strained silicon heterocycle as starting material (Section II,E,4). 

Insertion of unsaturated molecules into a transition metal-silyl bond has been suggested for the catalytic reactions related to hydrosilylation and silylcarbonylation. However, there is little direct evidence supporting such a process for unsaturated molecules to insert into a metal-silyl bond in organometallic complexes. " Thus, the fact that 108 is readily derived from 11 and 13 demonstrates the participation of this process in the catalytic cycle of silylformylation. 

In the reactions of azulene with trimethylsilylruthenium carbonyl complexes in refluxing heptane, there occurs a migration of organosili-con groups from metal to ligand to give complexes of silylated azulenes (94) ... 

Transition-metal catalyzed transfer of acylcarbenes to nitriles leads to 1,3-oxazoles via nitrile ylide intermediates123. The corresponding nitrile ylide chemistry derived from acyl(silyl)carbenes still awaits a closer look, but it has been shown that the rhodium-catalyzed decomposition of 198 in the presence of methyl cyanoformate and benzaldehyde provides 1,3-oxazole 221 (equation 71) exclusively120. This implies that the carbene moiety has been transferred only to the nitrile but not to the aldehyde. 

Alternatively, the reaction may proceed via 1,6-ring closure giving the isolated 2,3-disilanaphthalene 372109. It should be mentioned, however, that under the reaction conditions it is possible that silyl lithium compound 375 formed from the precursor of the silene 376 (obtained from metalation of 377) by a 1,3-trimethylsilyl shift adds to the intermediate silene 371 and that subsequent ring closure with elimination of lithium silanolate gives rise to the observed products 372 and 373 (equation 105). 

Banhidai, B. Schollkopf, U. (Dimethylcarba-moyljlifhium from DMF and LDA synthesis of a-hydroxy N,N-dimefhylcarboxamides. Angew. Chem. Int. Ed. 1973, 32, 836-837. Cunico, R. F. Carbamoylsilanes from the in situ metalation-silylation of a formamide. Tetrahedron Lett. 2001, 42,1423-1425. 

The crucial step of this new example of Si-C formation is a /1-hydrogen elimination from a silyl ligand followed by an insertion of rf-silene coordinated to the metal in the Ru-Si bond (Eq. 6) [ 14]. Only traces of hydrosilylation and de-hydrogenative silylation reactions are observed. 

In one group of reductive-elimination reactions, an HSiR3 molecule can be displaced from a silyl-substituted transition-metal hydride ML (H)(SiR3) by more efficient 7r-bonding ligands such as CO, PR3, C2H4, acetylenes or N2 which favor a lower oxidation state of M ... 

From metal hydrides (//i) or alkyls 40) and aryl or silyl azides. 

Chalk and Harrod provided the first mechanistic explanation for the transition metal catalyzed hydrosilation as early as in 1965. Their mechanism was derived from studies with Speier s catalyst and provided a general scheme, which could be used also for other transition metals. The catalytic cycle consists of an initial oxidative addition (see Oxidative Addition) of the Si-H bond, followed by coordination of the unsaturated molecule, a subsequent migratory insertion (see Insertion) into the metal-hydride bond and eventually a reductive elimination (see Reductive Elimination) (Scheme 3 lower cycle). The scheme provides an explanation for the observed Z-geometry in the hydrosilation of alkynes, which is a consequence of the syn-addition mechanism. The observation of silated alkenes as by-products in the hydrosilation of alkenes along with the lack of well-established stoichiometric examples of reductive elimination of aUcylsilanes from alkyl silyl metal complexes... 

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