Heterolytic P-Fragmentation

Water Elimination, Heterolytic P-Fragmentation and Formation of Radical Cations 118... 

Heterolytic Fragmentation A Class of Organic Reactions C. A. Grob and P. W. Schiess, Angew. Chem., Int. Ed. Engl, 1967, 6, 1-15. 

The Wharton fragmentation was used as a key step in an approach toward the total synthesis of xenicanes by H. Pfander et al. ° Two optically active substituted frans-cyclononenes were synthesized starting from (-)-Hajos-Parrish ketone. First, the bicyclic 1,3-diol was protected regioselectively on the less sterically hindered hydroxyl group with p-toluenesulfonyl chloride in quantitative yield. Next, the monosulfonate ester was exposed to dimsylsodium in DMSO, which is a strong base, to initiate the desired heterolytic fragmentation. 

Although the protonated TMH or TMD complexes as formulated above have so far never been observed directly, and are just postulated intermediates, there exists experimental evidence that the dinitrosyl hydride complexes posses a strong basic site at the atom. For example, we were able to characterize A—ON-ReH(NO)(P Pr3)2 adducts, A representing a Lewis acid like BF [52] or even the cationic metal fragment [Re(NO)2(P Pr3)2] itself [50]. The formation of these adducts not only illustrates the basicity of the nitrosyl group, but also the strong Lewis acidity of the 16e cation. We also found that the bimetallic complex shows a similar reactivity as the free cationic species itself. Further evidence for the proposed heterolytic splitting was obtained when the reaction with H2 or D2 is performed in the presence of an external base (Scheme 5). 

Su, Z., Mariano, P.S., Falvey, D.E., Yoon, U.C., and Oh, S.W., Dynamics of aniUnium radical a-heterolytic fragmentation processes. Electrofugal group, substituent and medium effects on desilylation, decarboxylation and retro-aldol cleavage pathways, /. Am. Chem. Soc., 120, 10676, 1998. 

See, for example, Newcomb, M., Homer, J. H., Whitted, P. O., Crich, D., Huang, X., Yao, Q., and Zipse, H., P-Phosphatoxyalkyl radical reactions competing phosphate migration and phosphoric acid elimination from a radical cation-phosphate anion pair formed by heterolytic fragmentation, /. Am. Chem. Soc. 121,10685, 1999 (and references cited). 

Su, Z., Mariano, P.S., Falvey, D.E., Yoon, U.C,. and Oh, S.W, Dynamics of anihnium radical a-heterolytic fragmentation processes. Electrofugal group, substituent and medium effects on desi-lylation, decarboxylation and retro-Aldol cleavage pathways, /. Am. Chem. Soc., 120, 10676,1998. Hasegawa, E., Xu, W, Mariano, P.S., Yoon, U.C., and Kim, J.U., Electron transfer induced photoadditions of the silylamine EtjNCHjTMS to a,P-unsaturated cyclohexenones. Dual reaction pathways based on ion pair selective cation radical chemistry, /. Am. Chem. Soc., 110, 8099, 1988. Yoon, U.C., Kim, J.U., Hasegawa, E., and Mariano, P.S., Electron transfer photochemistry of a-silylamine cyclohexenone systems. Medium effects on reaction pathways followed, /. Am. Chem. Soc., 109, 4421,1987. 

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