Aryl silanes regioselectivity

The use of silyl derivatives in Friedel-Crafts chemistry has not only improved the regioselectivity but extended the scope of these reactions. Substitution at the ipso position occurs with aryl silanes (eq 12). The ability of sUyl groups to stablize -carbenium ions ( -effect) affords acylated products with complete control of re-giochemistry (eq 13). ... 

Palladium-catalyzed hydrosilylation of styrene derivatives usually proceeds with high regioselectivity to produce benzylic silanes, 1-aryl-1-silyle thanes, because of the... 

Diastereoselective syntheses of dihydrobenzo[f>]furans have been accomplished by a rhodium-catalyzed regioselective and enantiospecific intermolecular allylic etherification of o-iodophenols as a key step, providing the corresponding aryl ally ether 122, which leads to a dihydrobenzo[b]furan by treatment of the intermediate aryl iodide with tris(trimethylsilyl)silane and triethylborane at room temperature in the presence of air <00JA5012>. 

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... 

Ellman used silyl chemistry for the direct linkage of aromatics onto the solid support by converting an aryl bromide to aryl lithium and reacting this with a silyl resin.90 It is the production of the silyl resin that is of interest in the context of this review, since an in situ Suzuki coupling was used to link the allyl silane to bromomethyl polystyrene resin (Scheme 40). 9-BBN is used to carry out the regioselective hydroboration, and this is linked to the resin with palladium catalysis in the usual way. After brief exposure of this... 

The first example was described in the mid-1980s independently by Jeffery, Hallberg, and others in their work on Heck coupling. The former demonstrated the beneficial role of silver salts in the formation of dienols from allylic alcohols and vinyl iodides, as well as the role of counterion or phase transfer conditions (Scheme 10.28).50 The latter found that silver salts accelerated the reaction between aryl iodide and allyl or vinyl silanes, reinforcing regioselectivity and avoiding desilylation (Scheme 10.29).51... 

R can be alkyl or aryl and can carry various functional groups such as the ester in 120. The vinyl bromide -121 is formed in 68% yield and is >95 5 E Z. However other transition metals may give the Z-vinyl silane as is the case with Et3SiH and an Ir(I) catalyst25 Yet other transition metals give the opposite regioselectivity as with Et3SiH and a Ru(I) catalyst.26 The best advice is to use conditions already determined by reliable workers. 

Heterocycle openings. a,p-Epoxy silanes fiimish P-hydroxy silanes due to the attack of cuprate reagents at the a-carbon. The regioselectivity for the reaction of methyl ( J-4,5-epoxypent-2-enoate with arylcopper is dependent on the presence or absence of BF,. The S 2 pathway giving the a-aryl esters becomes significant with the added Lewis acid. The Sf 2 products are observed in the reaction of epoxy vinyl sulfoxides with... 

Electron-rich aromatic substrates undergo direct trifluoromethylation catalyzed by a variety of Lewis acids, such as Zn salts, silanes, rhenium, or copper complexes. In the absence of an appropriate directing group, this kind of trifluoromethylation displays very low or no regioselectivity. More electron-poor arenes have to be activated first and are able to undergo trifluoromethylation in the form of aryl boronic acids or trifluoroborates (Scheme 7) [23-31]. 

Both y-mono and disubstituted allylphosphates react with silylboronates in the presence of a copperW-heterocyclic carbene catalyst and NaOH giving y-substituted allyl-silanes with high regioselectivities (5 275 2 ratios between 80/20 and >99/1) and high enantioselectivities (86 to 95%) ee Primary and secondary alkyl, aryl, halide, acetal, and ester groups are tolerated on C,. 

Nagao et al. studied the zirconocene-mediated coupling of unsymmetrical bis (alkynyl)silanes with bulky substituents on silicon atom. When unsymmetrical bis (alkynyl)silanes 1-30 was applied, upon hydrolysis, the silacyclobutene derivatives 1-32 could be synthesized regioselectively (Scheme 1.17) alkyl or alkenyl group was selectively located at a-position of silacyclobutene, while aryl group was selectively located at -position of zirconacyclobutene [54]. 

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