ortho-Lithiated benzyl diorganophosphines [o-(R2PCH2)C6H4Li(Et2O)], R=Ph, Me. Synthesis, structural characterization, and reactions

(o-Lithiobenzyl)dimethylphosphine, [o-(Me2PCH2)C6H4Li(Et2O)]2 (7), is shown by crystal structure analysis to be dimeric, the two lithium atoms bridging the phenyl ortho-carbon atoms in a four-membered ring structure. A distorted tetrahedral coordination sphere at lithium is completed by coordination of one phosphino group and a molecule of diethylether (triclinic, space group P1̄, a=10.986(3), b=11.160(3), c=12.368(4) Å, α=86.04(2), β=89.51(2), γ=83.04(2)°, Z=2). [o-(Me2PCH2)C6H4Li(Et2O)]2 (7) cleanly reacts in donor solvents like diethylether to the thermodynamically more stable (α-lithiobenzyl)dimethylphosphine [Me2PCHLi(Et2O)C6H5]2 (9) with the same overall composition. Crystal structure analysis of the latter shows each lithium atom to be now bonded in an η2 fashion to the CbenzylCipso bond of one anion, the diethylether molecule, and to the phosphino group of the second anion thereby resulting in a dimeric structure with a central six-membered ring (triclinic, space group P1̄, a=8.482(3), b=9.388(3), c=9.482(3) Å, α=95.42(2), β=91.36(2), γ=104.20(2)°, Z=1). (o-Lithiobenzyl)diphenylphosphine undergoes the same reaction to the corresponding α-lithiobenzylphosphine upon addition of strong donors like N,N,N′,N′-tetramethylethylenediamine (tmeda). Upon addition of Cp2TiCl2, (o-lithiobenzyl)diphenylphosphine reacts to form 1,2-bis[o-(diphenylphosphino)phenyl]ethane, [o-(Ph2P)C6H4CH2]2 (10), the formation of which is thought to proceed by oxidation of the anion [o-(Ph2PCH2)C6H4]− by Cp2TiCl2 to the radical which undergoes a 1,3 shift of the diphenylphosphino group from the sp3 benzyl carbon atom to the sp2ortho-carbon atom of the phenyl ring to form the more stable benzyl radical. Dimerization of the latter ultimately leads to [o-(Ph2P)C6H4CH2]2 (10) whose molecular structure was determined in the solid state (monoclinic, space group P21/n, a=12.217(2), b=8.382(1), c=15.004(2) Å, β=90.81(1)°, Z=2). The same reaction occurs upon oxidation of (o-lithiobenzyl)diphenylphosphine with ferricinium tetrafluoroborate, [Cp2Fe]+BF4−, thus corroborating the proposed reaction sequence with the transition metal species acting solely as oxidizing agents.