Phosphonium bis(glycolates) and phosphinoglycolates: Synthesis, solvolysis, oxidation to (thio)phosphinoylglycolates and use as ligands in Ni-catalyzed ethylene oligomerization

Secondary phosphines, glyoxylic acid hydrate and amines react to form organoammonium phosphonium bis(glycolates) 1a–d. In CD3OD solution, diphenylphosphonium bis(glycolates) undergo reversible solvolysis to phosphinoglycolates 2a,b and acetalic glyoxylic species. The P-dialkyl species 1c avoids this and maintains the phosphonium bis(glycolate) structure in CD3OD (cHex2P) or undergoes further solvolysis with partial formation of R2PH (R=tBu). Condensation to phosphinoglycines, e.g. 3b, observed for primary amines, does not take place with N-secondary amines at room temperature. Heating leads to condensation but is followed by decarboxylation as shown for the conversion of 2a to 4a. Because of the kinetic lability, the phosphonium compounds 1a–d are sensitive to oxidation by air, H2O2, or sulfur. The resulting phosphinoyl and thiophosphinoyl glycolates and glycolic acids 5–8 are kinetically stable. Precatalyst solutions formed from 1a, c, d and Ni(COD)2 in THF developed moderate to good activity in the oligo- or polymerization of ethylene to linear products containing methyl and vinyl end groups. Activity and molecular weights increased with the +I-effect of the P-substitutents. The solution structures of the novel compounds were elucidated by multinuclear NMR spectroscopy. For 7a a crystal structure analysis is also presented.