Silane cr-complexes

Only few examples of silane cr-complexes are known for the titanium triad and most of the work has been done on titanium itself. The first examples were... 

The compound [Ti(H)(SiXR2)(PMe3)Cp2] (99) is an isolobal analog of the ni-obocene compounds 83 with IHI, and is therefore expected to have the IHI too. By contrast, the compound [Ti(ri2-H2SiPh2)(PMe3)Cp2] (20) discussed above is a stretched silane cr-complex, i.e. it has an electronic structure intermediate between Ti(IV) and Ti(II).66... 

It is obvious that studying interligand Si-H interactions has reached a great extent of sophistication. At least three classes of nonclassical Si-H bonding can be identified. These are the electron-deficient residual Si-H interactions in silane cr-complexes and agostic complexes, electron-rich IHI MH SiX, and the more recent multicenter H Si interactions, which are the subject of current debate and have features common to both IHI and a-complexes. This surprising diversity stems from the special role the substituent at silicon can play in tuning the extent of Si-H interaction, and from the propensity of silicon to be hypervalent. 

In an NMR tube reaction, H NMR spectra recorded from —80 to 25 °C showed a broad signal at —9.23 ppm presumably due to an H2 complex formed as an intermediate. After reaction was complete, NMR showed a triplet at — 16.75 ppm corresponding to the silane cr-complex (from excess unreacted silane). Manganese carbonyl species such as MnlCOlslCHg) and [Mn(CO)4Br]2 are also pre-catalysts for silane alcoholysis and may operate via a similar pathway. Pentacarbonyl chromium(O) silane complexes also catalyze alcoholysis of silanes. 

Silane a-complexes of the Group 6 metals are among the best studied. Three families of compounds are particularly noteworthy. These are the half-sandwich arene complexes [Cr(ri -HSiMe2H)(CO)2(C6Me6)] (27), the above-mentioned Kubas s complexes and the pentacarbonyl derivatives [M(t -HSiR3)(CO)5]. ... 

Weak adducts of alkanes and metal derivatives (the alkane molecules play the role of token ligands in these complexes) have been detected and even isolated using a number of methods [14], These complexes are unstable at room temperature. Matrix isolation is one of the best established methods for the stabilization and characterization of intermediates. Complexes of alkanes with metal atoms and ions have been detected in the gas phase. All these adducts belong to the larger class of cr-complexes, which has been defined as complexes where the donor is a o-bond [14c], Dihydrogen and silane complexes are also from this class. 

TriaUcyltin substituents are also powerful ip5o-directing groups. The overaU electronic effects are similar to those in silanes, but the tin substituent is a better electron donor. The electron density at carbon is increased, as is the stabilization of S-carbocation character. Acidic cleavage of arylstannanes is formulated as an electrophilic aromatic substitution proceeding through an ipjo-oriented cr-complex. 

Adhesion promotion Chromium complexes, silanes, titanates, zirconium aluminates Al, Cr, Si, Ti, Zr... 

Yamamoto has proposed a mechanism for the palladium-catalyzed cyclization/hydrosilylation of enynes that accounts for the selective delivery of the silane to the more substituted C=C bond. Initial conversion of [(77 -C3H5)Pd(GOD)] [PF6] to a cationic palladium hydride species followed by complexation of the diyne could form the cationic diynylpalladium hydride intermediate Ib (Scheme 2). Hydrometallation of the less-substituted alkyne would form the palladium alkenyl alkyne complex Ilb that could undergo intramolecular carbometallation to form the palladium dienyl complex Illb. Silylative cleavage of the Pd-G bond, perhaps via cr-bond metathesis, would then release the silylated diene with regeneration of a palladium hydride species (Scheme 2). 

Yttrium-catalyzed enyne cyclization/hydrosilylation was proposed to occur via cr-bond metathesis of the Y-G bond of pre-catalyst Cp 2YMe(THF) with the Si-H bond of the silane to form the yttrium hydride complex Ig (Scheme 8). Hydrometallation of the C=G bond of the enyne coupled with complexation of the pendant G=G bond could form the alkenylyttrium alkyl complex Ilg. Subsequent / -migratory insertion of the alkene moiety into the Y-C bond of Ilg could form cyclopentylmethyl complex Illg. Silylation of the resulting Y-C bond via cr-bond metathesis could release the silylated cycloalkane and regenerate the active yttrium hydride catalyst. Predominant formation of the /ra //j--cyclopentane presumably results from preferential orientation of the allylic substituent in a pseudo-equatorial position in a chairlike transition state for intramolecular carbometallation (Ilg —IHg). 

Titanium-catalyzed cyclization/hydrosilylation of 6-hepten-2-one was proposed to occur via / -migratory insertion of the G=G bond into the titanium-carbon bond of the 77 -ketone olefin complex c/iatr-lj to form titanacycle cis-ll] (Scheme 16). cr-Bond metathesis of the Ti-O bond of cis- iij with the Si-H bond of the silane followed by G-H reductive elimination would release the silylated cyclopentanol and regenerate the Ti(0) catalyst. Under stoichiometric conditions, each of the steps that converts the enone to the titanacycle is reversible, leading to selective formation of the more stable m-fused metallacycle." For this reason, the diastereoselective cyclization of 6-hepten-2-one under catalytic conditions was proposed to occur via non-selective, reversible formation of 77 -ketotitanium olefin complexes chair-1) and boat-1), followed by preferential cyclization of chair-1) to form cis-11) (Scheme 16). 

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. 

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