Benzazaphospholine-2-carboxylic acids: Synthesis, structure and properties of heterocyclic phosphanyl amino acids

1,3-Dialkyl-1,3-benzazaphospholine-2-carboxylic acids 2a,b can be conveniently prepared by metalation and alkylation of N-methyl- and N-neopentyl-o-phosphanylaniline in liquid ammonia and cyclocondensation of the resulting N,P-disecondary phosphanylanilines 1a,b with glyoxylic acid hydrate (GAH) in ether. The primary neopentylphosphanylaniline reacts with two equivalents of GAH and forms a phosphanylbis(amino acid) 3 with toluidine. α-Branched P-substituents induce strongly preferred formation of trans-diastereoisomers with R,R- and S,S-configuration at P and C2, as shown by a crystal structure analysis of 2a, whereas a P-neopentyl (P-Np) group gives rise to trans/cis-diastereoisomeric mixtures. The trans-configuration exhibits the P lone-pair in cis-position to the COOH group, suitable for formation of five-membered chelate rings, as in diphenylphosphanylacetate nickel catalysts for ethylene oligomerization. Screening of 2a,b/Ni(COD)2 solutions in THF by a batch procedure indeed showed formation of catalysts for conversion of ethylene to linear oligomers and waxy low-molecular weight polymers. The conversion depends strongly on the size of the N-alkyl group, being slow and limited for the N-Me catalyst 2a/Ni and much faster and more complete for the N-Np-substituted catalysts 2b/Ni and 2c/Ni (N-Np, P-tBu). Comparison of 2b/Ni with 2c/Ni shows that the more bulky P-substituent further increases the catalyst activity.

Graphical abstract1,3-Benzazaphospholine-2-carboxylic acids were prepared via P-alkylation of primary o-phosphanylanilines and cyclization with glyoxylic acid hydrate. The bulkier P-substituted heterocycles prefer trans-configuration, which in combination with bulkier N-alkyl groups allows formation of efficient ethylene oligomerisation catalysts with Ni(COD)2.Figure optionsDownload full-size imageDownload as PowerPoint slide