Alkylation, geminal

ALKYLATION, w-ALLYLPALLADIUM COMPLEXES Palladium(II) chloride. ALKYLATION, GEMINAL Diphenylsulfonium cyclopropylide. 

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

Generally the most practical procedure for monoalkylation of steroidal ketones bearing geminal a-hydrogens is via formyl or oxalyl ketones. Some examples of the former method are described in this section. The alkylation of oxalyl ketones is described in the following section. 

The most common example is formation of dichlorocarbene by treatment of chloroform with a base (see Reaction 10-3) and geminal alkyl dihalides with Me3Sn but many other examples are known, such as... 

Allcock, H. R., Brennan, D. J., and Dunn, B. S., Synthesis of polyphosphazenes bearing geminal trimethylsilylmethylene and alkyl or phenyl side groups, Macromolecules. 22. 1534, 1989. 

The mono-alkylated species N3P3C15R is a good synthon for the generation of geminally substituted products by further reaction with Grignard reagents (Eq. 4) (150)... 

So in summary, the presence of a chiral centre in a molecule can render nearby geminal pairs of protons non-equivalent. Nearby is not an exact term and varies according to circumstance. Let s consider our molecule again, but this time, replace the -CH2- with an alkyl chain (Structure 6.2). 

SRN I reactions using related p-nitrophenyl or p-nitrocumyl systems41 as reductive alkylating agents have been studied by Komblum and co-workers these are well summarized in the reviews.39 At the same time, Russell discovered the S l reaction of geminal halonitroalkanes with stabilized carbanions (see Eq. 5.25).42 The products are readily converted into alkenes via elimination of nitro groups (see Section 7.3). 

Hyperconjugation does not seem to have much effect on alkene reactivity towards bromine, since (16) applies whatever the number of alkyl groups on the double bond. However, only cis-olefins are involved in this correlation. To include geminally substituted olefins, an additional term is necessary, as in (19) where d is unity for the pem-disubstituted compounds and zero for... 

The problem of retention of asymmetry of the formed free radical in the fast geminate recombination of radicals was studied by photolysis of the optically active azo-compound PhMeCH—N=NCH2Ph [88,89]. The radical pair of two alkyl radicals was initiated by the photolysis of the azo-compound in benzene in the presence of 2-nitroso-2-methylpropane as a free radical acceptor. The yield of the radical pair combination product was found to be 28%. This product PhMeEtCCH2Ph was found to be composed of 31% 5,5 -(-)(double retention), 48% meso (one inversion), and 21% R.R(+) double inversion. These results were interpreted in terms of the competition between recombination (kc), diffusion (kD), and rotation (kml) of one of the optically active radicals with respect to another. The analysis of these data gave kxo[Pg.126]

The reactivity of allyl radicals does, however, appear to be sufficient for intramolecular radical reactions. In a systematic study, Stork and Reynolds investigated the feasibility of allyl radical 5-exo cyclizations41. It was found that cyclization proceeds readily for a variety of systems, especially for those with geminal 3,3-diester substitution. Mixtures of c/s/fraws-cyclopentanes are formed as the major products, while 6-enclo cyclization is hardly observed42. Allyl radicals behave in this respect much like alkyl radicals43. Cyclization is not even hindered by the presence of substituents at the attacked carbon... 

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