Allylic bond cleavage

Care has to be taken when elements of changing valence are encountered, e.g., S in sulfoxides and sulfones or P in phosphates. In such a case, Eqs. 6.9 or 6.10 have to be used, whereas Eq. 6.11 yields erroneous results. 

Note It is good practice to apply the r + J algorithm as soon as the empirical formula of a molecular ion or a fragment ion seems to be apparent. This yields valuable information on possible substructures. 

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There are few reports of the transition metal complex-catalyzed isomerization of S-allyl sulfides and sulfones. This is clearly a consequence of the very strong coordinating ability of sulfur atoms and the resulting tendency for S-C (allyl) bond cleavage. In the case of a bulky substituent being present at the sulfur atom, the isomerization to 1-propenyl derivatives is successful (Eqs. 12.14 and 12.15) [20]. 

The two-electron reductions of [Fe(CO)2(NO)(>/3-C3H4R)] (R = Cl, Br, or Me) (248) and [Co(CO)2L(>/3-allyl)] (L = phosphine or phosphite) (249) result in metal-allyl bond cleavage. For the cobalt complexes there is a linear correlation between Elj2 and the half neutralization potential of L. The rhodium complex [Rh P(OMe)3 2(fj3-allyl)] reacts with AgPF6 to give [Rh P(OMe)3 2(f 4-C6Hi0)] in which the two C3 fragments have been oxidatively coupled (250). 

Since nonaromatic compounds do not usually exhibit strong peaks corresponding to cleavage of an allylic bond, the locations of double bonds in alkenes is difficult. However, in specific cases such as myrcene, which has a doubly allylic bond, cleavage may be highly favored. An intense C5H9+ ion (miz 69) is formed from myrcene (Equation 2.39), but it is not formed from the isomeric compound allo-ocimene (Equation 2.40). 

Alternatively, the two processes could be independent however, it is not unreasonable that bond formation is favorable to relieve the strain of one trigonal carbon in the three-membered ring and to avoid the generation of a third trigonal center as well as the potential for anti-aromatic destabilization were allylic bond cleavage to occur - compare to Chapter 7, Section 4.1, hexadiene 3,3-shift. 

Interestingly, the allylation of a stabilized carbon nucleophile has been found to be reversible. Complete isomerization of dimethyl methylmalonate, involving bis-allylic C—C bond cleavage, from a secondary carbon 38 to a primary carbon 39 was observed by treatment with a Pd catalyst for 24 h. The C—C bond cleavage of a monoaliylic system proceeds slowly[40]. 

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

Another application of [Bu4N][Ph3SiF2] 826 involves the silicon-carbon bond cleavage of allyl-, benzyl-, and alkynylsilane derivatives 827-829.826 Subsequent reactions of the generated carbanions with electrophiles (Scheme 112) and alkyl halides (Scheme 113) provide high yields of carbon-carbon coupled products. 

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