Solvent-controlled lithiation of PC–N-heterocycles: Synthesis of mono- and bis(trimethylsilyl)-tert-butyl-dihydrobenzazaphospholes – A new type of highly bulky and basic phosphine ligands

The influence of solvents on the lithiation of N -methyl-1,3-benzazaphospholes is reported; these are accessible via catalytic phosphonylation of 2-bromoanilines, subsequent reduction to 2-phosphinoanilines and acid-catalysed disproportionative ring closure with excess paraformaldehyde. Reactions with t BuLi in polar solvents (THF, Et 2 O), particularly in the presence of t BuOK, lead to 2-lithiobenzazaphospholes (CH-lithiation) whereas hydrocarbons favour “normal” (hexane) or “inverse” (toluene) addition at the P C bond. Reactive Li-species were trapped by ClSiMe 3 , present during the lithiation in hydrocarbons, and give rise to 2- and 3-trimethylsilyl-dihydro-1,3-benzazaphospholes, respectively. In hexane, via preferred lithiation of the primary adduct, the 2,2′-bis(trimethylsilyl)-dihydro-1,3-benzazaphosphole is the main product. 5-Methyl-1,3-benzazaphosphole, with NH function, reacts in toluene in the “normal” mode to 3- tert -butyl-1,2-bis(trimethylsilyl)-5-methyl-dihydrobenzazaphosphole. The sterically demanding tert -butyl and trimethylsilyl groups are arranged in anti -position as shown by crystal structure analyses, the second 2-SiMe 3 group in gauche position. The P - tert -butyl-2,2′-bis(trimethylsilyl)-dihydrobenzazaphospholes represent a new type of sterically congested dialkylaryl phosphine ligands with increased basicity by the +I-effect of the silyl groups and +M-effect of the basic nitrogen in o -position. t BuLi reacts with aromatic –P CH–N-heterocycles dependent on solvent and additives by CH-lithiation and/or normal or invers addition at the P C bond.x. • The reactivity of t BuLi towards aromatic –P CH–N-heterocycles is solvent-controlled. • Selective CH-lithiation at –P CH–NMe– species is achieved in THF or Et 2 O/KO t Bu. • In hexane sterically congested t BuP-C(SiMe 3 ) 2 -NMe ligands are formed with excess t BuLi/ClSiMe 3 . • In toluene t BuLi/ClSiMe 3 react with –P CH–NMe– preferably to –Me 3 SiP-CH t Bu-NMe species. • t BuLi/ClSiMe 3 and –P CH–NH– give preferably – t BuP-CH(SiMe 3 )-N(SiMe 3 ) species.