Enantiomerically pure phosphaalkene-oxazolines (PhAk-Ox): synthesis, scope and copolymerization with styrene

The design of a synthetic route to a class of enantiomerically pure phosphaalkene-oxazolines (PhAk-Ox) is presented. The condensation of a lithium silylphosphide and a ketone (the phospha-Peterson reaction) was used as the P=C bond-forming step. Attempted condensation of PhC(=O)Ox (Ox = CNOCH(iPr)CH(2)) and MesP(SiMe(3))Li gave the unusual heterocycle (MesP)(2)C(Ph)=CN-(S)-CH(iPr)CH(2)O (3). However, PhAk-Ox (S,E)-MesP=C(Ph)CMe(2)Ox (1 a) was successfully prepared by treating MesP(SiMe(3))Li with PhC(=O)CMe(2)Ox (52 %). To demonstrate the modularity and tunability of the phospha-Peterson synthesis several other phosphaalkene-oxazolines were prepared in an analogous manner to 1 a: TripP=C(Ph)CMe(2)Ox (1 b; Trip = 2,4,6-triisopropylphenyl), 2-iPrC(6)H(4)P=C(Ph)CMe(2)Ox (1 c), 2-tBuC(6)H(4)P=C(Ph)CMe(2)Ox (1 d), MesP=C(4-MeOC(6)H(4))CMe(2)Ox (1 e), MesP=C(Ph)C(CH(2))(4)Ox (1 f), and MesP=C(3,5-(CF(3))(2)C(6)H(3))C(CH(2))(4)Ox (1 g). To evaluate the PhAk-Ox compounds as prospective precursors to chiral phosphine polymers, monomer 1 a and styrene were subjected to radical-initiated copolymerization conditions to afford [{MesPC(Ph)(CMe(2)Ox)}(x){CH(2)CHPh}(y)](n) (9 a: x = 0.13n, y = 0.87n; GPC: M(w) = 7400 g mol(-1) , PDI = 1.15).