Silyl benzyl ethers

Hydrosilylation occnrs by the reaction of triethoxysilane with ketones or aldehyde in the presence of CaO or hydroxyapatite (115). Benzaldehyde reacts with (C2H5)3SiH and PhMe2SiH to afford the corresponding silyl benzyl ether in the presence of KF/AI2O3 in 93% and 99% yields, respectively, at 303 K (112). 

To a mixture of vinyl bromide (40 mmol) and the catalyst dichloro-[(R)-Af,N-dimethyl-l-[(.S)-2-(diphenylphosphino)ferrocenyl]ethylamine]-palladium(n) (0.2 mmol) was added an ethereal solution of [a-(trimethyl-silyl)benzyl]magnesium bromide (0.6-1 m, 80 mmol) at —78 °C. The mixture was stirred at 30 °C for 4 days, and then cooled to 0 °C and hydrolysed with dilute aqueous HC1 (3 m). The organic layer was separated, and the aqueous layer was re-extracted with ether. The combined organic extracts were washed with saturated sodium hydrogen carbonate solution and water, and dried. Concentration and distillation gave the chiral allylsilane (79%, 66% ee), b.p. 55°C/0.4mmHg. 

The use of iodotrimethylsilane for this purpose provides an effective alternative to known methods. Thus the reaction of primary and secondary methyl ethers with iodotrimethylsilane in chloroform or acetonitrile at 25—60° for 2—64 hours affords the corresponding trimethylsilyl ethers in high yield. The alcohols may be liberated from the trimethylsilyl ethers by methanolysis. The mechanism of the ether cleavage is presumed to involve initial formation of a trimethylsilyl oxonium ion which is converted to the silyl ether by nucleophilic attack of iodide at the methyl group. tert-Butyl, trityl, and benzyl ethers of primary and secondary alcohols are rapidly converted to trimethylsilyl ethers by the action of iodotrimethylsilane, probably via heterolysis of silyl oxonium ion intermediates. The cleavage of aryl methyl ethers to aryl trimethylsilyl ethers may also be effected more slowly by reaction with iodotrimethylsilane at 25—50° in chloroform or sulfolane for 12-125 hours, with iodotrimethylsilane at 100—110° in the absence of solvent, " and with iodotrimethylsilane generated in situ from iodine and trimcthylphenylsilane at 100°. ... 

Treatment of the elimination product 107 with triethylamine resulted in smooth isomerization of the olefin, to afford the a,p-unsaturated ketone 108. Ally lie oxidation of 108 then generated the secondary alcohol 109 in 72 % yield. The acetonide and silyl ether functions of 109 were cleaved in one reaction to afford a tetraol intermediate that was regioselectively acylated at the secondary alcohol functions, to provide the triacetate 110 in high yield (89 %). Hydrogenolysis of the benzyl ether... 

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

Hydrogenation of silyl enol ethers with the DIOP catalyst followed by hydrolysis [Eq. (52)] has yielded a route to optically active alcohols with low optical purities, 7% ee NMDPP (12) and MePhPR (R = n-Pr, Et, benzyl) systems were less effective (299). 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

The cleavage of benzyl ethers using hydrobromic acid is promoted by tetra-n-butylammonium bromide [38]. Selective cleavage of aryl silyl ethers can be effected in the presence of aliphatic silyl ethers using solid sodium hydroxide with tetra-n-butyl-ammonium hydrogen sulphate [39]. 

Secondary benzylic bromides, allylic bromides, and a-chloro ethers can undergo analogous reactions with the use of ZnBr2 as the catalyst.1 2 Primary iodides react with silyl enol ethers in the presence of Ag02CCF3.3... 

The concept of a diastereoselective Friedel-Crafts alkylation of a-chiral benzyl alcohols was first examined by Bach and coworkers [62, 63]. The initial protocol required stoichiometric amounts of strong Brpnsted acids like HBF4 and was followed by a more valuable methodology in which catalytic amounts of AuC L were employed for the diastereoselective functionalization of chiral benzyl alcohols [64], Beside arenes, allyl silanes, 2,4-pentanediones and silyl enol ethers have been used as nucleophiles. Depending on the diastereodiscriminating group and on the catalyst (Brpnsted or Lewis acid), the authors observed either the syn or the anti diastereoisomer as the major product. 

Scheme 24 Bi(OTf)3-catalyzed diastereoselective benzylation of silyl enol ethers...

On the basis of this pioneering work, Rubenbauer and Bach developed a Bi(OTf)3-catalyzed highly diastereoselective benzylation of silyl enol ethers [65]. Various cyclic and acyclic silyl enol ethers were amenable to this protocol (Scheme 24). Various a-substituted benzyl acetates were tested with terf-butyl-substituted silyl enol ether 31a, and the use of only 1 mol% of Bi(OTf)3 was enough to obtain the desired benzylated ketones 32 in high yields and with excellent diastereoselectivities (up to 95 5). Whereas a-nitro- (30a), ot-cyano- (30b) and a-methylester-substituted (30d) benzyl acetates gave the anti diastereoisomer as the major product, the phosphonate-substituted benzyl acetate (30c) exclusively resulted in the syn isomer (Scheme 24). 

During the reaction of p-methoxy benzyl alcohol with silyl enol ether 31b, dibenzyl ether 33 was observed as a by-product, which disappeared after prolonged reaction time. In fact, if 33 was used as alkylating reagent, the silyl enol ether 31b was benzylated and the desired cyclopentanone 32e was obtained in a similar yield (Scheme 25). 

In addition to the alkylation of benzyl alcohols with silyl enol ethers, the hydroxyl group could be removed in a reduction employing triethylsilane Et3SiH as the reductant. With 1 mol% of Bi(OTf)3 as the catalyst, the desired (5-arylester 34 could be isolated in 75% yield (Scheme 26). 

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