Diaryl- silane

Reversible control over the helix sense in polysilanes was achieved in the case of poly(diaryl-silanes)128 as well as in (co)polymers of ((S)-3,7-dimethyloctyl)(3-methylbutyl)silane,129 which both showed helix reversal upon heating. For the latter polymer (Figure 12a), it was calculated that the potential curve has a double-well ( W j shape (Figure 12b) with a slight preference for the M-helix over the P-helix. CD spectroscopy indeed revealed that above the transition temperature the ordered (low-entropy) M-helical conformation becomes less stable than the entropically more favored P-helical state (Figure 12c,d). 

The SiH2 group. There is a small further fall in the SiH stretching frequency when a second alkyl group is introduced. Dialkyl silanes absorb between 2138—2117 cm", and diaryl silanes between 2147—2130 cm" [17, 25, 29, 32, 33]. The frequency rises sharply if halogens are substituted for the alkyl groups. Difluoro-silane absorbs at 2246 cm" and methyl chlorosilane [17] at 2200 cm". The deformation frequency is less affected by the substituents, and in alkyl or aryl silanes it falls in the 940—928 cm" range close to the SiHs deformations. Replacement of one methyl by fluorine raises this to 975 cm". Methyl chlorosilane absorbs at 960 cm". ... 

The one-pot twofold Heck coupling of 2-pyridyldimethyl(vinyl)silane 42 was also carried out with two different aryl iodides to afford 2, 2 -diaryl(vinyl)silanes 43 in good yields (Scheme 15)." " A coordination of the pyridyl group to... 

Hydrolysis of a reactive silane derivative under the proper conditions first produces a silanol, but except in the case of a few that are exceptionally stable, this product is not isolated, since the acid or alkaline reagents that are normally present cause most silanols to condense rapidly to form siloxanes. The product generally isolated when a dialkyl-or diaryl-substituted silane is hydrolyzed is, of course, a mixture of cyclic polysiloxanes, (R2SiO)n, from which high polymers (often elastomeric) are obtained by catalytic rearrangement and polymerization reactions. 

A Japanese group have reported an unusual reaction, mediated by lanthanide metals, involving the deoxygenative acylation of diaryl ketones with aryl acyl silanes, to give 1,1-diaryl acetophenones (Scheme 79)189. 

More impressive results were obtained for the asymmetric hydrosilylation of ketones over the insolubilized catalyst (34). Acetophenone, for example, was hydrosilylated with phenylnaphthylsilane and the intermediate was hydrolyzed with HC1 to give (-)-S-phenylmethylcarbinol (58% ee). Similar results were obtained with the homogeneous DIOP catalyst. Using diaryl or arylalkyl silanes, yields from 52 to 100% and optical purities from 7 to 59% were realized (Table V). 

Aryl tellurium trichlorides are easily converted to symmetrical or unsymmetrical diaryl tellurium dihalides via reactions with organometallic compounds (see p. 549). Trimethyl(aryl)silanes transfer only the aryl group to tellurium1. 

As indicated with a strong electron-withdrawing group such as CF3, the reaction gave lower yields of products. When the coupling reaction of aryl(aIkyl)difluoro-silanes and aryl iodides is carried out under an atmospheric pressure of carbon monoxide, unsymmetrical fluorinated diaryl ketones were obtained (refs. 35, 36) ... 

Activity of the Wilkinson catalyst [RhCUPPhsls] in hydrosilylation of carbonyl compounds was already reported by Ojima and co-workers in 1972 (201). The catalyst is highly active in the hydrosilylation of a variety of diaryl, aryl alkyl, and dialkyl ketones, aryl and alkyl aldehydes, a,jS-unsaturated ketones, and esters. Stereoselectivity of hydrosilylation of cyclic ketones depends onbuUd-ness and electronic nature of silanes used. The hydrosilylation of a,/9-unsaturated carbonyl compounds in the presence of [RhCKPPhsls] and other rhodium complexes shows a specific dependence of l,2-/l,4-addition selectivity on the nature of the silane used so that monohydrosilanes undergo 1,4-addition whereas di- and trihydrosilanes give selectively 1,2-addition product (Scheme 29). 

One of the earliest methods for preparing aromatic boronic acids involved the reaction between diaryl mercury compounds and boron trichloride [198]. As organomer-curial compounds are to be avoided for safety and environmental reasons, this old method has remained unpopular. In this respect, trialkylaryl silanes and stannanes are more suitable and both can be transmetallated efficiently with a hard boron halide such as boron tribromide [199]. The apparent thermodynamic drive for this reaction is the higher stability of B-C and Si(Sn)-Br bonds of product compared to the respective B-Br and Si(Sn)-C bonds of substrates. Using this method, relatively simple arylboronic acids can be made following an aqueous acidic workup to hydrolyze the arylboron dibromide product [193]. For example, some boronic acids were synthesized more conveniently from the trimethylsilyl derivative than by a standard ortho-metallation procedure (entry 11, Table 1.3). 

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