Precursor Cation Research Articles

Charge-remote fragmentation characteristics of functionalized alkanes in high-energy collision-induced dissociation

In this study we report on high-energy, collision-induced dissociation processes leading to charge-remote fragmentations, using three alkyl cations, namely n-hexadecylpyridinium, n-hexadecyltriphenylphosphonium and n-hexadecyltriethylammonium, each with and without (2)H(2)-labelling at the C(9) position of the hexadecyl chain. The characteristic patterns corresponding to the formal elimination of alkane elements were observed, and the (2)H(2)-labelling at C(9) clearly affected only one charge-remote fragment ion of the homologous series. However, in addition to the expected fragment ion containing only one deuterium atom, a significant ion retaining two deuterium atoms was observed. MS/MS/MS experiments demonstrated clearly that the latter ion showed partial deuteration around the charge site, the level of deuteration depending on the structure of the original precursor cation. These results can be interpreted in terms of two novel, distinct mechanisms, one of which involves an excited state in an aromatic ring. Mixed-site fragmentation (MSF) ions were also observed from the phosphonium and ammonium ion precursors. We believe that the observation of the MSF process occurring at an sp(2)-hybridized center in the phosphonium series has not been reported previously. It thus becomes apparent that high-energy collisions leading to charge-remote reactions in fact lead to a broad range of pathways. Copyright 2000 John Wiley & Sons, Ltd.

Free and Polymer-Bound Tricyclic Azaphosphatranes HP(RNCH(2)CH(2))(3)N(+): Procatalysts in Dehydrohalogenations and Debrominations with NaH.

The commercially available nonionic base P(CH(3)NCH(2)CH(2))(3)N (1a) was shown earlier to be superior to DBU as a stoichiometric reagent for the conversion of primary and secondary alkyl halides to alkenes (Arumugam, S.; Verkade, J. G. J. Org. Chem. 1997, 62, 4827). The precursor cation HP(CH(3)NCH(2)CH(2))(3)N(+) (2) to 1a, which is more stable and less expensive, is reported herein to be an efficient procatalyst for these reactions and also for the debromination of vicinal dibromides using NaH as a relatively inexpensive stoichiometric hydride source in CH(3)CN at room temperature. In dehydrohalogenations requiring more than ca. 10 h, the CH(2)CN(-) ion also acts as a base. By itself, NaH does not function well or at all under the same conditions. A catalytic cycle is proposed in which hydride deprotonates cation 2 liberating catalytic 1a. The cations HP(HNCH(2)CH(2))(3)N(+) (3) and HP[N(polymer)CH(2)CH(2)]N(CH(2)CH(2)NH)(2)(+) (4) are also shown to function as procatalysts for the efficient dehydrohalogenation of RX and for the debromination of vicinal dibromides. The preparation of the heterogeneous procatalyst 4(OTf) is also described.

Novel photochemical reactions of bicyclo[2.2.2]octa-2,5-diene radical cations

The radical cation of bicyclo[2.2.2]octa-2,5-diene produced in a γ-irradiated Freon matrix at 77 K shows a broad charge-resonance absorption band with λ max =680 nm. In contrast to earlier experiments in argon matrices, it was found that the primary photoreaction of this species is a cleavage to yield the recently described complex cation between benzene and ethylene whose λ max happens to coincide with that of the above precursor cation. This complex cation can in turn be converted eventually to 1,3,5,7-octatetraene radical cation

ESR and ENDOR studies of irradiated potassium dihydrogen phosphate

Irradiated single crystals of potassium dihydrogen phosphate were studied via ESR and ENDOR techniques in order to define phosphate group structures. A precursor cation species was produced at 4.2 K which converted upon warming to a radical previously reported for 77 K irradiation. The g values and proton ENDOR results allowed detailed models for both species to be constructed. Phosphorous ENDOR resonances were also detected and provided complete hyperfine splitting tensors for both species which reflected small internal phosphate group polarization densities.