P/O ligand systems: Synthesis and reactivity of primary and secondary o-phosphinophenols

Several organometallic reagents such as lithium 2-lithio 4-methylphenolate 1 intermediates formed by orthometallation of o-bromoaryloxy-phosphorus(V)- 2 or -phosphorus(III)-derivatives 3 with magnesium and sodium, respectively, as well as O-methoxymethyl-protected o-lithio-4-methylphenol 4 were used to synthesize suitable precursors 5,6,9,10 of primary and secondary o-phosphinophenols. The P–C bond formation involved coupling with ClPR(NMe₂), CIPR(O)(OEt) or an intramolecular carbanionic O → C shift of the P-substituent. Reduction with LiAlH₄, in the cases of phosphonous or phosphinous acid amides after alcoholysis (to 7,8,11 ), produced primary and secondary o-phosphinophenols 12 , respectively, or O-protected derivatives 13 . o-Phosphinophenols 12 are easily protonated at the phosphorus atom, supported by a P⁺-H … O hydrogen bridge. Metallation ( 14 ), acylation, and silylation ( 16,17 ) take place preferably at the hyxdroxy group and alkylation at the phosphorus atom. Alkylation of 12 and 14 was found to be slow, but C,O-dilithiated species 15 react to give P-secondary ( 12b,d,e, ) or P-tertiary products ( 20,21 ). Cyclization of 15a with Me₂SiCl₂ affords the 2,3-dihydro-1,3,2-benzoxaphosphasilol 22 , cyclocondensation of 12c with RP(NMe₂)₂ or ClP(NMe₂)₂ furnishes 2,3-dihydro-1,2,3- benzodiphospholes 23 and 24 . A phosphiniden-phosphoran 25 is detected in the reaction between 12a and P(NMe₂)₃. © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8: 383–396, 1997