Pim Schipper

Single crystal ESR study of x‐irradiated thiophosphate O,O,O‐triester of methyl β‐D‐ribopyranoside: Electron capture by the thiophosphate moiety

X‐irradiation of a single crystal of thiophosphate O,O,O‐triester of methyl β‐D‐ribopyranoside at 77 K produces a σ phosphoranyl radical anion (2) by electron capture among secondary radicals resulting from bond scission. The ESR parameters of (2) in correlation with the x‐ray analysis of its precursor establish a TBP structure with the unpaired electron in apical position. The results suggest that in irradiated nucleic acid polymers, electrons can be trapped by phosphorus giving rise to a phosphorus‐centered radical which subsequently induces bond break by α or β scission generating thermodynamically more stable sugar and base radicals.

Single-crystal esr study of x-irradiated 2-chloro-2,2'-spirobis (1,3,2-benzodioxaphosphole): phosphorus in an octahedral geometry with the unpaired electron and chlorine in axial positions

X-irradiation of a single crystal of 2-chloro-2,2-spirobis(1,3,2-benzodioxaphosphole) yields the oriented PVI radical anion formed by electron capture. Directions of maximum values of the magnetic hyperfine tensors of 31P, 35Cl, and 37Cl are coincident and oriented along the P—Cl linkage, indicating an octahedral structure with the unpaired electron and chlorine in axial positions.

Cope interconversions of the 9-methoxy-endo-tricyclo[4.2.1.02,5]nona-3,7-dienyl cation and its [4.3.0.02,5] tautomer. Evidence of enhanced ionic stability by delocalization of neighboring sigma bonds

Dissolution of 9-chloro-9-methoxy-endo-tricyclo[4.2.1.02.5]nona-3,7-diene (7b, c) in liquid SO2 gives rise to a dissociation equilibrium which involves the rearranged 7-methoxy-endo-tricyclo[4.3.0.02.5]nona-3,9-dienyl cation (11). This allylic ion was formed quantitatively by dissolution of (7b, c) in FSO3H–SO2 or AlCl3–CH2Cl2. Its structure was established by 1H and 13C n.m.r. spectroscopy. Quenching of the ionic solutions with methanol afforded the starting acetal (7a). In contrast, reaction of (11) with pyridine produced its corresponding pyridinium salt (17a). Ionization of (7b, c) under short-life conditions, i.e. reaction with pyridine in CH2Cl2, proceeds without skeletal rearrangement. The structure of the reaction products of the reference compounds (8)–(10) appeared to be independent of the conditions used. In all cases the unrearranged structures were observed. The ready skeletal rearrangement of (7b, c) under acidic conditions is ascribed to the involvement of a homoaromatic Cope transition state. From the enhanced reaction rate of (7b, c) relative to (8b, c)–(10b, c) it is concluded that the σ bond involved in the Cope rearrangement provides stabilization of the incipient cation in (7). This kind of stabilization is also present in the rearranged cation (11), as indicated by its enhanced stability relative to the corresponding allylic ion (3) and by comparison of their 1H and 13C n.m.r. data.

Formation of the 7-triphenylphosphonio-norbornen-7-ylium dication: a non-classical dication

While the norbornen-7-ylium cation dissolved in liquid SO2 does not undergo 7-substitution with PPh3, the analogous reaction with the 7-methoxynorbornen-7-ylium cation produces the 7-methoxynorbornen-7-yl-triphenylphosphonium cation; the intermediate 7-triphenylphosphonio-norbornen-7-ylium dication could be detected.