Amidine functionalized phosphines: tuneable ligands for transition metals

Abstract

Attachment of racemic 1,3,5,7-tetramethyl-2,4,6-trioxa-8-phosphatricyclo[3.3.1.1]decane (α,β-CgP) to (1R,5S)-1,8,8-trimethyl-2,4-diazabicyclo[3.2.1]oct-2-ene gave a diastereomeric mixture of a novel amidine-phosphine ligand, α,β-CgPAm. The phosphination was completely selective for the 4-position of the bicyclic amidine and there was no subsequent 1,2-migration of the α,β-CgP group. Methylation of the non-phosphinated nitrogen gave the amidinium salt [α,β-CgPAmMe]BF as a diastereomeric mixture. The donating ability of α,β-CgPAm and [α,β-CgPAmMe] has been assessed through the synthesis and characterization of appropriate Rh(i), Au(i) and Pt(ii) complexes. As expected α,β-CgPAm is a better net donor than the cationic derivative as shown by the magnitude of the νCO stretches in the IR spectra of the [Rh(L)(CO)(acac)] complexes and through determination of the relative energies of the HOMO and LUMO orbitals for both ligands by DFT. Attempts to resolve [Au(α,β-CgPAmMe)Cl]BF, [Pt(α,β-CgPAmMe)Cl], [Rh(α,β-CgPAmMe)(acac)(CO)]BF and [Rh(α,β-CgPAmH)(acac)(CO)]BF by fractional crystallization were unsuccessful as diastereomeric mixtures were obtained in every case; the structures of the last three complexes have been determined by single-crystal X-ray techniques and compared with related literature complexes.

Asymmetric Cationic Phosphines: Synthesis, Coordination Chemistry, and Reactivity

“…Yield = 32 mg. The crystals were suitable for structural analysis by single-crystal X-ray techniques 31 After stirring for 1 hr, the CH2Cl2 was removed in vacuo and the solid residue dissolved in THF (4 ml). Solid KHMDS (1.2 equivs) was added to the stirred solution and the mixture left overnight at RT.…”

“…[28][29][30] Our preliminary report detailed the synthesis of the ligand as a diastereomeric mixture as our original attempts at resolution were unsuccessful. 31 We have since discovered that, in contrast to slow evaporation at the bench, crystallisation from hot THF under N2 is a viable route to the isolation of pure [α-CgPAmMe]BF4 which can be employed as a ligand itself or as a potential precursor to NHC-phosphines. The present paper discusses aspects of the coordination chemistry of the cationic phosphine and attempts at accessing NHC derivatives thereof.…”

“…We − and Wilhelm − have already demonstrated the synthetic utility of amidinium species derived from 1,3-diamino-1,2,2-trimethylcyclopentane which made this the obvious candidate for the cationic fragment. The need for a robust, inherently chiral PR 2 group capable of supporting a stable N–P bond to the amidinium led to the selection of 1,3,5,7-tetramethyl-2,4,6-trioxa-8-phosphatricyclo­[3.3.1.1 , ]­decane or Cagephos (CgP) as it satisfied all of the desired criteria. − Our preliminary report detailed the synthesis of the ligand as a diastereomeric mixture as our original attempts at resolution were unsuccessful . We have since discovered that, in contrast to slow evaporation at the bench, crystallization from hot THF under N 2 is a viable route to the isolation of pure [α-CgPAmMe]­BF 4 which can be employed as a ligand itself or as a potential precursor to NHC-phosphines.…”

Twisting the arm: structural constraints in bicyclic expanded-ring N-heterocyclic carbenes

“…5: Yield = 42%. 1 H NMR (400 MHz, CDCl 3 , 298 K) δ = 7.15 (2H, ArH, m) 7.05 (4H,ArH,d,7.3 Hz), 3.85 (4H, CH 2 , m), 2.30 (12H, CH 3 , s), 2.25 (4H, CH 2 , m) ppm. 13 C{ 1…”

Recent Advances in the Construction of Phosphorus‐Substituted Heterocycles, 2009–2019

“…Indolyl‐based phosphines have proved to be effective in promoting metal‐catalyzed cross‐coupling reactions . Tetrahydropyrimidines ( 27 ) bearing a phosphoryl group are highly promising ligands for transition metal complexes …”