Base-catalyzed H-D exchange

Of the studies which bear more particularly on electronic effects we mention first a study of the base-catalyzed H-D exchange in the 3-arylsulfonylbicyclo[2.2.1]hept-5-ene-2-carboxylic acids, cis (33), (35) and trans (34), and of the base-catalyzed inversion of the arylsulfonyl group in cis compounds (33) and (35)191. R comprised several meta- or para-... 

One commonly used procedure for studying the steric stability of the carbanion is comparison of the rate of base-catalyzed H/D exchange reaction (kel) on the chiral carbon with the rate of racemization (fcrac). By this comparison, inversion, racemization or... 

TABLE 16. Base-catalyzed H/D exchange reactions of sulfonyl compounds 70-73... 

In the base-catalyzed H/D exchange of the four-membered cyclic sulfone 85, the rate of exchange is slightly higher than that of racemization and the carbanion formed was also considered to be planar101. 

Some representative data on base-catalyzed H/D exchange reactions of several optically active sulfonyl compounds 70-73 are listed in Table 16. In all these reactions, 10. 

H/D exchange of H and Hg protons of sulfone 86 and estimated the difference in the free energies of activation for 79a and 79b to be < 1.2 kcal mol , based on the kjk value of 3 0.5. In the base-catalyzed H/D exchange of 87, kjk = 1.6, where k and k are the rate constants of H/D exchange of H, and H, respectively. Based on the small kjk value. Brown and colleagues suggested that if the carbanion is pyramidal, the steric stabilities of 79a and 79b are almost identical. Meanwhile, based on their C-NMR study Chassaing and Marquet proposed that the hybridization of the carbon atom of the sulfonyl carbanion, PhSOjCHj , would be between sp and sp . 

The 6-methyl group of the pyranylidene complex 69b is even more acidic than the acetyl group and readily undergoes base-catalyzed H/D exchange reactions as well as condensation reactions with aldehydes. Thus, a route to styryl pyranylidene complexes 79 other than that shown in Scheme 25... 

Trinitroso derivative 27 underwent in CD30D/D20 solution fast base-catalyzed H/D exchange on the whole set of methylene hydrogens, and nitroso groups can be subsequently removed by reduction with Ni/Al alloy <2002TL771>. 

Studies of the base-catalyzed H-D exchange of benzylcyclopropane (163) and cyclopropylacetophenone (164) and analogous compounds did not reveal any large kinetic effects due to cyclopropyl stabilization of the resulting anions. 

Around 1960, it was already known that unsaturated heterocyclic azolium cations reacted in a base-catalyzed H,D-exchange reaction. Hoping that the delocalization of the six 7t-electrons in such derivatives might stabilize the intermediately formed carbene species, Wanzlick attempted to prepare the free carbene 7 by deprotonation of tetraphenylimidazolium perchlorate with KOt-Bu (Equation (1.1)). Again, he could not isolate free 7 but its intermediate formation was demonstrated indirectly by identification of some of its reaction prodncts with water or with [Hg(OAc)2]. Almost three decades later, Ardu-engo sncceeded in the preparation of free 7 by the deprotonation method originally snggested by Wanzlick. 

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