Bulky N-substituted 1,3-benzazaphospholes: Access via Pd-catalyzed C-N and C-P cross coupling, lithiation, and conversion to novel P=C-PtBu2 hybrid ligands

Abstract

The syntheses of novel bulky N-substituted 1,3-benzazaphospholes are presented, together with their reactions with tert-butyllithium and coupling with tBu(2)PCI to novel PP-hybrid ligands that combine the highly basic and bulky di-tert-butylphosphanyl group with g-acidic low-coordinated phosphorus. The syntheses start with the preparation of new N-seconclary 2-bromoanilines 1 by reduction of N-acyl 2-bromoanilides or more generally by Pd-catalyzed selective monoamination of o-dibromobenzene, followed by Pd-catalyzed C-P coupling with P(OEt)(3) to the respective 2-anilino-phosphonates 2. The next steps are reduction to 2-phosphanylanilines 3 and condensation with Me2NCH(OMe)(2), which leads via phosphaalkenes 4 to the corresponding N-substituted benzazaphospholes 5. The reaction with tBuLi depends on the steric demand of the N substituent. Methyl, neopentyl-, and mesityl-derivatives were converted to P=C Li species 6 and coupled with tBu(2)PCI to novel P=C-PtBU2 ligands 7, whereas N-adamany and N-2,6-diisopropylphenyl-derivatives prefer addition of tBuLi at the P=C bond to form dihydroderivatives. The chemical shifts of the low-coordinated phosphorus of 5 and 7 were found to reflect electronic and steric effects of the N substituents. The comparison of the crystal structures of N-neopentyl-1,3-benzazaphospholes 5 and 7 gives evidence of steric repulsion between the adjacent di-tert-butyl and neopentyl groups by the preferred anti orientation of the P-tert-butyl groups and moderate deviations of C2 and P3 of 7b from the ring plane.