N‐Heterocyclic Carbene Non‐Innocence in the Catalytic Hydrophosphination of Alkynes

Abstract

Studies on alkyne hydrophosphination employing nickel‐NHC catalysts (NHC=N‐heterocyclic carbene) revealed that the free N‐alkyl substituted NHCs themselves were catalytically active. DFT calculations showed the mechanism involves the NHC acting as a Brønsted base to form an imidazolium phosphide species which then undergoes rate‐limiting nucleophilic attack at the terminal alkyne carbon. This mechanism explains the preference seen experimentally for reactions with aryl substituted phosphines and alkynes, while the rearrangements of the alkenyl anion formed upon P−C bond formation account for the observation of both Z‐ and E‐regioisomers of the products.

Reductive Elimination at Carbon under Steric Control

“…Having shown the influence of the sterics on the reductive elimination process, we wondered if we could show its implications in catalysis. Recently, some of us reported that N -alkyl substituted N -heterocyclic carbenes NHCs can promote the anti-Markovninov hydrophosphination of terminal alkynes, whereas Et CAAC fails to catalyze this reaction. Capitalizing on our findings, we found that the bulkier Menth CAAC 2 (10 mol %) allows for the anti-Markovninov hydrophosphination of phenyl acetylene with diphenylphosphine in full conversion after 16 h at 60 °C (Scheme ).…”

Reactivity studies and structural outcomes of a bulky dialkylaluminium amide in the presence of the N-heterocyclic carbene, ItBu

“…Finally, ItBu acts itself as a base in the deprotonation of diphenylacetonitrile with no contribution from the metallic component. The catalytic role of NHC Brønsted basicity in hydrophosphination has recently been elucidated 54 and further progress in non-metallic catalysis constitutes an interesting future endeavour for us.…”

Catalytic Hydrophosphination of Allenes Using an Iron(II) β-Diketiminate Complex

“…This regiochemistry/NMR chemical shift has been reported by several others. 10 21 Work from Bookham, 22 which uses diphenylacetylene as a substrate, 23 reports the Z -product (( Z )-(1,2-diphenylvinyl)diphenylphosphine) at –7.8 ppm and the E -product (( E )-(1,2-diphenylvinyl)diphenylphosphine) at +8.7 ppm. Bookham’s work is cited by Westerhausen as having comparable data to ( E )-diphenyl(1-phenylprop-1-en-2-yl)phosphane, for which a crystal structure is reported.…”