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

• Published in: Inorganic chemistry Vol. 47; no. 15; pp. 6900 - 6912
• Main Authors: Aluri, Bhaskar Reddy, Kindermann, Markus K, Jones, Peter G, Dix, Ina, Heinicke, Joachim
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 04.08.2008
• Subjects: Aniline Compounds - chemistry; Aza Compounds - chemical synthesis; Aza Compounds - chemistry; Carbon - chemistry; Catalysis; Ligands; Lithium - chemistry; Nitrogen - chemistry; Palladium - chemistry; Phosphorus - chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic800430f

The syntheses of novel bulky N-substituted 1,3-benzazaphospholes are presented, together with their reactions with tert-butyllithium and coupling with tBu2PCl to novel P,P′-hybrid ligands that combine the highly basic and bulky di-tert-butylphosphanyl group with π-acidic low-coordinated phosphorus. The syntheses start with the preparation of new N-secondary 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 tBu2PCl to novel PC−PtBu2 ligands 7, whereas N-adamantyl 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.