Reaction with Benzyl Bromide

Kornblum et al. (1963) demonstrated that O- vs. C-alkylation (24) of yS-naphthoxide ion (an ambident ion) is markedly solvent dependent. For example, the reaction with benzyl bromide conducted in dimethylformamide gave 97% O-alkylated product, whereas in water 81% C-alkylated product resulted. The difference is attributed to changes in the solvation of ambident anions. The course of the reaction is also influenced by water concentration in... 

A further attempt has been made to develop a predictive model for chirality transfer achieved through alkylation reactions of ester enolates which feature chiral auxiliaries. " Hippurate esters (30) derived from (lI , 25 )-trani-2-(p-substituted phenyl)cyclohexanols were found, on reaction with benzyl bromide, to give (31) with predominantly the S configuration at the alkylation centre but with no correlation between the degree of stereoselectivity (20-98%) and the electron density on the aromatic ring. 

The metallation of 3-methyl-4//-5,6-dihydro-l,2-oxazine has been shown to take place at the methyl group with hindered bases and at the methylene group with unhindered bases (81JA5916). Deprotonation of (753) with lithium dimethylamide at -65 °C followed by reaction with benzyl bromide gave (754) in 85% yield. This product was converted to enone (755) by reaction first with triethyloxonium tetrafluoroborate to produce an oxoiminium salt. The salt was stirred with trimethylamine and the resulting a,/3-unsaturated imine hydrolyzed with wet silica gel to the enone (Scheme 174). The lithiated derivative of (753) serves as a synthon for the unknown a-anion of methyl vinyl ketone. 

The 2,3-dihydrobenzo[6]selenophene (113) yields the oxide (114) on treatment with ozone. The oxide may be ring opened by treatment with sodium hydride and the product of the ring opening can be alkylated by reaction with benzyl bromide. Thermal rearrangement of the oxide yields a 15 85 mixture of compounds (115) and (116) (Scheme 15) (76JOC2503). 

Benzyl 4, 6 -0-benzylidene-P-lactoside with five free hydroxyl groups was converted to the dibutylstannylene intermediate by azeotropic removal of water from its mixture with 2.5 molar equiv. dibutyltin oxide in benzene, the reaction with benzyl bromide in the presence of tetrabutylammonium bromide then gave the 2,3 -di-0-benzyl derivative in 52% yield [139]. When the 3, 4 -0-isopropylidene analog was treated with 1.2 molar equiv. only, the 2-O-benzyl derivative was the main product [150]. 

Treatment of benzyl 3, 4 -0-isopropylidene-P-lactoside with 3 molar equiv. of bis(tributyl)tin oxide and subsequent reaction with benzyl bromide gave a complex mixture of products. When benzylation was carried out in the presence of tetrabutylammonium bromide or TV-methylimidazole, 50% of the 2-O-benzyl derivative was obtained after some hours. After 7 days, the 2,6,6 -tri-0-benzyl derivative was the main product (38%) [150]. 

Schiff base 52 in one-pot under mild phase-transfer conditions. For example, the initial treatment of a toluene solution of 52 and (S,S)-32e (1 mol%) with allyl bromide (1 equiv.) and CsOHH20 at —10 °C, and the subsequent reaction with benzyl bromide (1.2 equiv.) at 0 °C, resulted in formation of the double alkylation product 53 in 80% yield with 98% ee after hydrolysis. Notably, in the double alkylation of 52 by the addition of the halides in reverse order, the absolute configuration of the product 53 was confirmed to be opposite, indicating intervention of the chiral ammonium enolate 54 at the second alkylation stage (Scheme 4.17) [50]. 

Path B in Fig. 2 is the convergent method. It is the outside-inward method, proposed independently by Miller and Neenan [9] and by Hawker and Frechet [10]. This method is well suited when the branch point is an aromatic ring. As an example of the convergent process we show in Scheme 3 the preparation of poly(benzyl ether) dendrimers. The phenol functionality of 2,5-dihydroxyben-zyl alcohol is first protected by Williamson reaction with benzyl bromide to provide the first generation dendron [G-l]-OH. The benzyl alcohol in [G-l]-OH is then converted to the benzyl bromide form [G-1]-Br. This in turn reacts with... 

In striking contrast, when the reaction was carried out in methanol or benzene containing a small amount of a base such as aqueous NH3 or triethylamine (TEA) (Scheme 32), the N-2-C-3 interchange product, 4-phenylthiazole (52), was obtained in 90% yield along with a small quantity of the deprotonated photocleavage product (57), which was trapped by reaction with benzyl bromide to yield the ( /Z)-benzyl-thioether (58) <98JOC5592>. Conversion of 55-5d to 52-5d (Scheme 33) under these conditions confirmed that the transposition occurred via the... 

The same alkyl thiocarbamates 95 can be prepared in 35-72% yield by reaction with disulfides and with carbon disulfide followed by alkylation with alkyl halides100. When carbonyl sulfide was used as electrophile, lithium /V,/V-dialkyldioxamates were formed, which after reaction with benzyl bromide gave thiooxamates 96 in 37-56% yield100. 

SCHEME 83. Li Enolate (LiEn) aggregates and LiHMDS mixed aggregate in their reaction with benzyl bromide (rate/[BnBr][Li2EnHMDS] = kuixed + M/ aggiLiHMDS])256... 

For GC analysis, the salts of the lowest molecular weight acids present in ozonation products subjected to base-promoted hydrolysis have been converted to their benzyl esters by reaction with benzyl bromide (Bonnet et al. 1989). The salts of all acids produced have commonly been converted to the free acids, usually with the aid of a cation exchange resin. The acids have then been converted to methyl esters by reaction with diazomethane (Bonnet et al. 1989) or, more often, have been converted to trimethylsilyl (TMS) esters (Matsumoto et al. 1986, Taneda et al. 1989, Habu et al. 1990). Trimethylsilylation has the major advantage that alcoholic and phenolic hydroxyl groups are simultaneously converted to TMS ethers, thus greatly facilitating GC analysis. 

Benzyl and p-nitrobenzyl esters are usually made by classical methods such as reaction of benzyl alcohol with an acid chloride in the presence of pyridine or with a carboxylic acid in the presence of a carbodiimide (see general esterification methods). We have already shown the conversion of the cesium salt of a carboxylic acid to its benzyl ester by reaction with benzyl bromide [Scheme 6.40] ... 

Cyclizations by formation of carbon—selenium bonds represent a modern method with a high synthetic potential in the chemistry of cyclophanes. Selenocyanates such as 16 are accessible usually in excellent yields through the reaction of bromides with KSeCN [27], The reaction with benzylic bromides under reductive conditions using the dilution principle results in good to excellent yields of [3.3]di-selenacyclophanes which can be deselenized photochemically, pyrolytically (without previous oxidation), or by reaction with arynes, Stevens rearrangement and subsequent reaction with Raney nickel. [2.2]Metacyclophane (18), for example, is accessible in 47% total yield by using this sequence of reactions starting with... 

Potassium diphenylarsenide-2-dioxane reacts with (—)-menthyl chloride in boiling tetrahydrofuran to give (lS,2S,5i )-( + )-neomenthyldiphenylarsine, (S)-( + )-175 (neo-menars). The arsine was obtained as colourless needles in ca 40% yield after recrystallization from warm methanol (Table 9). The neomenthylarsine, upon reaction with benzyl bromide in benzene, affords the epimeric (li ,2S,5i )-benzylmenthylarsonium bromide, (i H-f-)-176, which, upon reduction with lithium aluminium hydride, affords (l/t,2S,5K)-diphenylmenthylarsine, (J )-( — )-177 (menars) (equation 25). 

This would account for the observation that the polymethylene contains less carbon-14 than expected if only the preceding mechanism was operating. In reactions of the ylid with other alkyl bromides, proton abstraction occurs in preference to substitution. Reaction with cyclo-hexylbromide gives a 92% yield of cyclohexene by a (3-elimination. Reaction with benzyl bromide gives a 58% yield of stilbene, probably by a-elimination, with only a small amount of 2-phenylethyltrimethyl-ammonium bromide by substitution 158>. 

When the complex is treated with organic halides or acyl halides, dimethyl-aminocarbonylation proceeds smoothly. For example, reaction with benzyl bromide gave N,N-dimethylphenylacetamide in 64.5 %.96L... 

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