Benzyl cations crowded

A comprehensive review (over 200 references) of the Yukawa-Tsuno relationship in cationic systems, primarily benzylic, has appeared. When the relationship is applied to the solvolysis rates and gas-phase stabilities of stericaUy crowded benzylic cations (methyl, 2-methylphenyl, and /-butyl groups at the carbocation centre) the reduced coplanarity shows up as an r parameter of <1. Extensive ab initio calculations... 

Nitrile-stabilized anions are so nucleophilic that they will react with alkyl halides rather well even when a crowded quaternary centre (a carbon bearing no H atoms) is being formed. In this example the strong base, sodium hydride, was used to deprotonate the branched nitrile completely and benzyl chloride was the electrophile. The greater reactivity of benzylic electrophiles compensates for the poorer leaving group. In DMF, the anion is particularly reactive because it is not solvated (DMF solvates only the Na+ cation). 

We should consider both S l and 5 2 mechanisms for the substitution. We might have some preference for S l because of the steric hindrance in the molecule (Sn2 reactions are more sterically demanding because we need to crowd more groups around a single carbon atom) and the fact that the cation is benzylic and hence stabilized by resonance. The product is approximately 10 % optically pure. This means we have 55 % of one enantiomer and 45 % of the other. If we assume that the inverted product comes from an 8 2 reaction, and the race-mized product from an S l reaction, we would conclude that our reaction is 90 % 8 1 and 10 % 8 2. However, we should also consider the possibility that the reaction is entirely 8 1, but that there is a small ion pair effect. 

Other compounds characterized and analyzed by XRD, with other complementary techniques used, include the sterically-crowded triarylphos-phines with formyl and benzoyl groups, benzyl(triphenyl)phosphonium dichloroiodate, the A,A-bis(terphenyl)aminophosphenium cation, dithiophosphinate ligands, and mercury(II) complexes of a-keto stabilized phosphorus ylides. Also, the crystal and molecular structure of 2-(4-chlorophenyl)-5,7-dimethoxyquinolin-4-yl phenyl bis(2-chloroethyl)phos-phoramidate (32) has been determined by NMR, IR, and XRD analysis. The crystal is triclinic, PI space group, a = 9.5188 A, )= 12.856 A, c= 13.250 A, F= 1412.0 A, and Z = 2 (at 291K). The crystal packing... 

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