Aryl/alkyl silanes

Table I. Structure-Activity of Aryl/Alkyl Silanes...

Reduction of aryl alkyl ketones with moderate to good (ee 30-80%) entantio-selectivity has been achieved using trialkoxysilanes in the presence of chiral quininium fluorides (or hydroxides) [20]. Greater selectivities were noted (ee >65%) when tris(trimethylsiloxy)silane was used. 

Comparison of all reported cleavages gives the apparent ordering H > Ph (or aryl) > alkyl in terms of tendency for groups to be cleaved from silicon, with the exception that at least one aryl substituent is retained on silicon in the anion VI. It is clear that reduction of aryl silanes leads to significant weakening of Si—R bonds that have appropriate symmetry for hyperconjugative interactions with the reduced n systems, a point that has been cited as evidence for the importance of n — type interactions in unsaturated silanes (86). 

Reduction The asymmetric reduction of a series of aryl alkyl ketones with quaternary ammonium fluorides and silanes was reported by Drew and Lawrence [55]. In these reactions, the best catalysts (e.g., 6f) were from the qui ni ne/quinidine series in fact, a fluoride salt prepared from cinchonine gave no induction. The use of trimethoxysilane resulted in faster rates but lower enantioselectivites when compared with tris(trimethoxy)silane. It is interesting that, with the... 

Perhaps the most direct method of synthesizing an acyl silane is by reaction of an acyl lithium, prepared by carbonylation of an alkyl lithium at —110°C, with a silicon electrophile, illustrated in Scheme 28106,107. Although this method is successful for a variety of alkyl acyl silanes in moderate yields, low temperatures must be used, and the method is not suitable for aryl acyl silanes. 

The asymmetric organocatalytic transformation of a ketone into an alcohol may be realized with the combination achiral silanexhiral phase-transfer catalyst, such a quaternary ammonium salt. The final alcohol is then recovered by an additional hydrolytic step. The asymmetric reduction of aryl alkyl ketones with silanes has been reported (ee-values up to 70%), the catalysts utilized being ammonium fluorides prepared from the quinine/quinidine series (e.g., 18 in Scheme 11.6) [19]. (For experimental details see Chapter 14.21.1). The more appropriated silanes were (Me3SiO)3SiH or (MeO)3SiH (some examples are... 

The next question is how side-chain substitutions and skeleton conformations affect the band structures of polysilanes. Side chains provide two interesting effects (7) band gap reduction caused by substitution of larger alkyl side chains and skeleton-side chain interaction (i.e., (j-tt mixing) in aryl poly silanes. This interaction was confirmed by UV photo spectroscopy (UPS) (8-9) and photoabsorption and luminescence measurements (iO, 11). Skeleton conformations are related to thermochromism (12-17). The ab initio... 

Apart from the reaction with halogen derivatives, use was also made, for the synthesis of silicon-containing cellulose esters, of the reaction of alcoholysis, with cellulose, of tetraalkoxysilanes or alkyl(aryl)trialkoxy-silanes and the amides of siliconic acids (64) ... 

However, with aryl(trialkyl)silanes, sometimes for electronic reasons, the reaction does not take place. In order to reduce the electron density on the silicon atom, alkyl groups were replaced by fluorine. The fluorine effect could be explained as follows i) the van der Waals radius of fluorine is roughly comparable to that of hydrogen and hence the fluorine-substituted silyl group is not so bulky than the trimethylsilyl one ii) electronegativity of fluorine favours the formation of the reactive pentacoordinated silicate, enhances the Lewis acidity of the silicate and... 

Alkylations. Molecules with acti ( compounds that are activated by Pd caial lated by aldehydes and allenes. By a th ester, an allene, and a vinylic bromide, the tutes a convenient preparation of a 2.4. Some aryl(hydro)silanes also undergo aryl... 

Pentasubstituted 2-fluoropyridines 66 were prepared by the reaction of perfluor-oalkene 65 with A-silyl-l-azaallyl anion 64, generated by coupling of a functional silane 62 and aryl/alkyl nitrile 63 using -BuLi in tetrahydrofurane" (Scheme 6.22). 

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). 

Other primary silanes, mainly functionalized aryl and alkyl silanes, were successfully polymerized and some examples are listed in Table 4. 

The cross-coupling of unactivated alkyl bromides and iodides with aryl(trimethoxy)silanes can be accomplished with PdBr2-P(f-Bu)2Me or air stable [HP(i-Bu)2Me]BF4-TBAF. Notably, the reactions occur under ambient conditions and with an array of compatible functional groups on the electrophile including ethers, acetals, nitriles, alkynes, esters, amides, and ketones (eq 41). Electron-rich and electron-deficient aryl(trimethoxy)silanes can be employed with the former providing higher yields. 

At the same time. Sun et al. developed another type of activators based on pipecolinic acids A 8 and A 9. These showed excellent yields (95%) as well as enantioselectivities (92%) and were able to reduce aryl-alkyl imines almost in the same manner as acetophenone derived ones. The first approach using C2-symmetrical activators, in particular tetraamide A 10 showed potential but resulted in lower yield and ee compared with A 8. A subsequent investigation of these symmetrical activators led to chiral sulfina-mides A 11 and A 12. The monosulfinamide A 11 forms in situ a dimer and therefore can chelate the silane as well as the bis-sulfinamide A 12 (Figure 32.7). ... 

Alkylation and arylation of organosilanes occur readily with alkyl and aryl alkaU metal compounds. Yields from these reactions are good but are iafluenced by steric requirements on both silane and metal compounds. There is Httie iaductive effect by the organic groups attached to siUcon, as measured by the yield of products (126,127). These reactions proceed more readily ia tetrahydrofuran and ethyl ether than ia ligroin or petroleum ether, where R and are alkyl or aryl and M is Li, Na, or K. 

Alkyl, aryl, alkoxy and halogen subsituted silanes are referred to by prefixing silane by the specific group present. The following are typical examples ... 

While there is clear evidence for complex formation between certain electron donor and electron acceptor monomers, the evidence for participation of such complexes in copolymerization is often less compelling. One of the most studied systems is S-.V1 Al I copolymerization/8 75 However, the models have been applied to many copolymerizations of donor-acceptor pairs. Acceptor monomers have substituents such as carboxy, anhydride, ester, amide, imide or nitrile on the double bond. Donor monomers have substituents such as alkyl, vinyl, aryl, ether, sulfide and silane. A partial list of donor and acceptor monomers is provided in Table 7.6.65.-... 

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