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Oxidation of Li/X phosphinidenoid complex 2, obtained via selective deprotonation from the P-H precursor 1, with [Ph3C]BF4 led to the formation of two P-F substituted diorganophosphane complexes 6, 7; the latter tautomer 7 formed via H-shift from 6. In contrast, oxidation of 2 with [( p-Tol)3C]BF4 led to three major and one minor intermediates at low temperature, which we tentatively assign to two pairs of P-C atropisomers 10 a, a′ and 10 c, c′ and which differ by the relative orientations of their CH(SiMe3)2 and W(CO)5 groups. Conversion of all isomers led finally to complex 11 having a ligand with a long PC bond to the central trityl* carbon atom, firmly established by single-crystal X-ray analysis. DFT calculations at the B3LYP/def2-TZVPP//BP86/def2-TZVP level of theory on real molecular entities revealed the structures of the in situ formed combined singlet diradicals ( 4+ 5 and 5+ 9) and the nature of intermediates on the way to the final product, complex 11. Remarkable is that all isomers of 11 possess relative energies in the narrow energy regime of about 20 kcal mol−1. A preliminary study revealed that complex 11 undergoes selective PC bond cleavage at 75 °C in toluene solution. [ABSTRACT FROM AUTHOR]

The mechanism of catalytic 4-exo cyclizations without gem-dialkyl substitution was investigated by a comparison of cyclic voltammetry, EPR, and computational studies with previously published synthetic results. The most active catalyst is a super-unsaturated 13-electron titanocene(III) complex that is formed by supramolecular activation through hydrogen bonding. The template catalyst binds radicals via a two-point binding that is mandatory for the success of the 4-exo cyclization. The computational investigations revealed that formation of the observed trans-cyclobutane product is not possible from the most stable substrate radical. Instead, the most stable product is formed with the lowest energy of activation from a disfavored substrate in a Curtin-Hammett related scenario., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)