Scheme 3. Bond dissociation energies (BDE) of 2 b·THF, 3 a·THF, and 3 b·3 THF by DFT calculations (B3LYP-d/def2-TZVPP) with COSMO [17] (e = 7.6).

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.

Oxidation of Li/X phosphinidenoid complex 2, obtained via selective deprotonation from the P-H precursor 1, with [Ph(3)C]BF(4) 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)(3)C]BF(4) 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(SiMe(3))(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.

The binding of 2,2-diphenyloxirane to Cp2TiCl is studied on the electronic level by magnetic resonance spectroscopy and quantum chemical calculations. The complexation of 2,2-diphenyloxirane is accompanied by dissociation of the chloride ligand, and thus, the epoxide binds to the cationic titanocene(III) complex. The titanocene(III)-epoxide species persists only for short periods of time (<5 min) even at 243 K, indicating that the ring-opening reaction is exothermic. A short-lived paramagnetic titanocene(IV)-epoxide radical species has not been directly observed. However, by a combination of isotope labeling and spin-trapping, evidence for the existence of such a species has been unequivocally demonstrated. The observation of a titanocene(III)-epoxide complex is unprecedented and provides direct evidence for inner-sphere electron transfer between epoxides and titanocenes, responsible for the high regioselectivity of ring-opening.

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