Alkenes decarboxylative approach

The decarboxylative approach to the ylide formation generated cycloaddition products derived from cycloaddition of the ylide to the carbonyl moiety of the molecule, as opposed to the alkene as seen in previous examples. Kanemasa has reconciled this observation by consideration of the postulated transition state model of the reaction. It was assumed that the steric repulsion of the terminal olehnic substituent and the ylide would favor transition state 309 (Fig. 3.19). Additionally, nonstabilized azomethine ylides have a higher energy HOMO than stabilized ylides, and would therefore prefer the LUMO of the carbonyl than the lower lying alkene LUMO. Formation of fused hve-membered rings would also be kinetically favored over construction of six-membered ring (Scheme 3.103). 

A mild, but indirect, approach to oxidative decarboxylation involves a modification of the 0-acyl thiohydroxamate decarboxylative rearrangement (Section 5.4.6.1). An 0-acyl selenohydroxamate is photolyzed to give a noralkyl-2-pyridyl selenide which, after ozonolysis to the selenoxide, undergoes syn elimination to the alkene (equation IS). 

Fast Radical Scavenging and Oxidation. Rates of oxidative decarboxylation by Pb(OAc)4 of primary and secondary carboxylic acids to alkenes are enhanced in the presence of catalytic amounts of Cu(OAc)2 or Cu(OAc)2-H20. This effect is attributed to the fact that the rate of one-electron-transfer oxidation of alkyl radicals by Cu salts (eq 13) approaches a diffusion-controlled rate. Oxidative decarboxylation of carboxylic acids can also be carried out with (diacetoxyiodo)benzene in the presence of a catalytic amount of anhydrous Cu(OAc)2. ... 

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