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 sho wn by crystal structure analysis to be dimeric, the two lithium atoms bridg ing the phenyl ortho-carbon atoms in a four-membered ring structure. A dist orted tetrahedral coordination sphere at lithium is completed by coordinati on of one phosphino group and a molecule of diethylether (triclinic, space group P (1) over bar, a=10.986(3), b=11.160(3), c=12.368(4) Angstrom, alpha =86.04(2), beta =89.51(2) gamma =83.04(2)degrees, Z=2). [o-(Me2PCH2)C6H4Li (Et2O)](2) (7) cleanly reacts in donor solvents like diethylether to the th ermodynamically more stable (alpha -lithiobenzyl)dimethylphosphine [Me2PCHL i(Et2O)C6H5](2) (9) with the same overall composition. Crystal structure an alysis of the latter shows each lithium atom to be now bonded in an eta (2) fashion to the C-benzyl-C-ipso bond of one anion, the diethylether molecul e, and to the phosphino group of the second anion thereby resulting in a di meric structure with a central six-membered ring (triclinic, space group P (1) over bar, a=8.482(3), b=9.388(3), c=9.482(3) Angstrom, alpha =95.42(2), beta =91.36(2), gamma =104.20(2)degrees, Z=1). (o-Lithiobenzyl)diphenylpho sphine undergoes the same reaction to the corresponding alpha -lithiobenzyl phosphine upon addition of strong donors like N,N,N',N'-tetramethylethylene diamine (tmeda). Up on addition of Cp2TiCl2, (o-lithiobenzyl)diphenylphosph ine reacts to form 1,2-bis[o-(diphenylphosphino)phenyl] ethane, [o-(Ph2P)C6 H4CH2](2) (10), the formation of which is thought to proceed by oxidation o f the anion [o-(Ph2PCH2)C6H4](-) by Cp2TiCl2 to the radical which undergoes a 1,3 shift of the diphenylphosphino group from the sp(3) benzyl carbon at om to the sp(2) ortho-carbon atom of the phenyl ring to form the more stabl e benzyl radical. Dimerization of the latter ultimately leads to [o-(Ph2P)C 6H,CH,I, (10) whose molecular structure was determined in the solid state ( monoclinic, space group P2(1)/n, a=12.217(2), b=8.382(1), c=15.004(2) Angst rom, beta =90.81(1)degrees, Z=2). The same reaction occurs upon oxidation o f (o-lithiobenzyl)diphenylphosphine with ferricinium tetrafluoroborate, [Cp 2Fe]+BF4- thus corroborating the proposed reaction sequence with the transi tion metal species acting solely as oxidizing agents. (C) 2001 Elsevier Sci ence B.V. All rights reserved.