Trichlorosilyl ligand

Figure 4. Geometry of model A showing the steric interaction between the trichlorosilyl ligand and the hydrogen at the 3 position of the pyrazole ring that results in a slight deviation from the intrinsic square planar coordination geometry at Pd.

The deviation from the square planar geometry at the Pd center in complexes 3 can be attributed wholly to steric factors. Calculations have shown that there is a moderate electronic preference favouring the planar coordination amounting to approximately 4.5 kcal/mol. However, steric interactions between the bulky trichlorosilyl ligands with both phenyl substituents of the phosphine and pyrazole ring result in a distortion away from the ideal square planar geometry. We further have found that the framework of the specific chelate backbone positions the pyrazole ring such that these interactions are enhanced. 

All three osmium atoms have four carbonyl ligands. The two osmium atoms with trichlorosilyl ligands are assigned central atom locants 1 and 2, as these ligands are the first that are not evenly distributed. Symmetry in the structure means that the locants 1 and 2 can... 

The hydrosilylation of butadiene proceeds with palladium compounds even in the absence of phosphines. Other ligands, such as glyoxime, benzonitrile, and 1,5-cyclooctadiene, can be used as effective ligands for the hydrosilylation of butadiene (65, 67). The reaction of trichlorosilane and dichlorosilane with isoprene proceeded regioselectively and stereo-selectively to give Z-l-trichlorosilyl-2-methyl-2-butene (67) (65, 66, 68). No reaction of trimethylsilane with isoprene took place, and this shows the lower reactivity of trialkylsilane. 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

The palladium-catalyzed asymmetric hydrosilylation of styrenes has been applied to the catalytic asymmetric synthesis of l-aryl-l,2-diols from arylacetylenes (Scheme 6).46 Thus, ( )-l-aryl-2-(trichlorosilyl)ethenes, which are readily generated by platinum-catalyzed hydrosilylation of arylacetylenes, were treated with trichlorosilane and the palladium catalyst coordinated with MOP ligand 12f to give 1 -aryl-1,2-bis(silyl)ethanes, oxidation of which produced the enantiomerically enriched (95-98% ee) 1,2-diols. 

The coordination state of the silyl enol ether in the transition state strongly influences the diastereoselectivity (synlanti). If a ligand is sterically demanding, like phosphoramide 33, a boat-like transition state with a pentacoordinated silicate is formed and affords the syn product in the reaction of trichlorosilyl enol ether with benzaldehyde. In contrast, the less hindered ligand 34 gave the anti product through a chair-like transition state with a hexacoordinated silicate (Scheme 25). 

Boron enolates bearing menthol-derived chiral ligands have been found to exhibit excellent diastereo- and enantio-control on reaction with aldehydes34 and imines.35 Highly diastereo- and enantio-selective aldol additions of geometrically defined trichlorosilyl ketone enolates (31) and (32) have been achieved by promoting the reactions with chiral Lewis bases, of which (,S., S )-(33) proved to be the most effective.36 Moderate enantiomeric excesses have been achieved by using chiral ammo alcohols as catalysts for the Baylis-Hillman condensation of aldehydes with methyl vinyl ketone the unexpected pressure effect on the reaction has been rationalized.37... 

Hydrosilylation of styrene with trichlorosilane in the presence of a heterogeneous polymer-supported palladium(III) catalyst containing the same chiral ferrocenyl ligand gives 1-phenyl-l-(trichlorosilyl)ethane with only 15% ee8b. 

Bis(trichlorosilyl)nickel(II) complex 4, having an r/ -arene hgand,is prepared by reaction of hexachloro disilane with highly reactive, vaporized nickel in the presence of toluene (Eq. 2) [12]. Worthy of note is that the arene ligand is displaced by three molecules of carbon monoxide to give 5 [13]. 

Johannsen and co-workers found that the phosphoramidite of axially chiral BINOL (IH-9) is an effective chiral ligand, and they obtained 32 with 99 % ee in 87 % yield from styrene. Also they prepared 1-trichlorosilyl-l-phenylpropane (35) with 98 % ee in 91 % yield by the hydrosilylation of /S-methylstyrene (34) with this ligand, and chiral 1-phenyl-1-propanol was prepared from 35 [21]. Furthermore, they claimed that arylmonophosphinoferrocene was an efficient ligand. In particular, the p-MeO-Ph-MOPF (VIII-8) they synthesized was the most effective ligand for asymmetric hydrosilylation of styrene, and ultrafast asymmetric hydrosilylation occurred with TOF exceeding 180000 h [22]. 

In the Pd-catalyzed hydrosilation of silyl-substituted ethylenes with HSiCls, the a//3 silation ratio was shown to depend significantly on the nature of the silyl substituent, as shown in Scheme 6. " The use of Ph2PCH2PPh2(dpm) was reported to be a useful ligand for the preparation of l,l-bis(trichlorosilyl)ethane.t ... 

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