N-Heterocyclic Carbenes in the Synthesis of Axially Chiral Square-Planar Platinum Complexes

Abstract

A new synthetic approach allows access to axially chiral square-planar platinum(II) complexes by combining an enantiomerically pure diphosphine (DuPHOS, BPE, or Chiraphos) and an unsymmetric NHC ligand. The same approach proVed to be highly stereoselectiVe when applied to the synthesis of a chiral triazolylidene-dppe mixed complex.

Cyclometalated N‐Heterocyclic Carbene‐Platinum Catalysts for the Enantioselective Cycloisomerization of Nitrogen‐Tethered 1,6‐Enynes

“…[13] Shortly after, our group disclosed that the Pt(II) complexes I displaying a bidentate phosphine and an NHC ligand (NHC = N-heterocyclic carbene), together with a labile iodide ligand, afford suitable pre-catalysts for the cycloisomerization of nitrogen-tethered 1,6-enynes ( Figure 1). [10] Enantioselective processes could be established by using (S,S)-Chiraphos as the chiral bidentate ligand, giving ees up to 74% in a model cycloisomerization reaction.…”

“…At this point, the specific design of the metallacyclic structures II was inspired by the synthetic approach used to access complexes I, that is a two-step sequence which involves oxidative addition of I 2 to a Pt(0)-NHC complex as the key step ( Figure 2). [10] We reasoned that the same oxidative addition/ ligand exchange sequence might be easily adapted to the synthesis of platinacyclic derivatives by taking advantage of an intramolecular oxidative addition step. The metallacyclic unit will be combined then with a monodentate phosphorus ligand to complete the coordination sphere of platinum.…”

“…They have been shown to proceed in the presence of various Pt(II), [4] Ir(I), [5] Rh(II) [6] and Au(I) [7] achiral pre-catalysts, while a very restricted range of enantioselective processes have been reported to date. In this context, we have disclosed recently Pt(II) complexes of the general formulae I [10] and II [11] as the first promising platinum-based chiral pre-catalysts for the cycloisomerization of 1,6-enynes bearing N-containing tethers ( Figure 1). Altough high enantioselectivity is regularly attained, the restricted substrate scope and overall moderate efficiency fully motivate the search for new and efficient catalytic systems.…”

“…Altough high enantioselectivity is regularly attained, the restricted substrate scope and overall moderate efficiency fully motivate the search for new and efficient catalytic systems. In this context, we have disclosed recently Pt(II) complexes of the general formulae I [10] and II [11] as the first promising platinum-based chiral pre-catalysts for the cycloisomerization of 1,6-enynes bearing N-containing tethers (Figure 1).…”

Cationic Pt(IV) Tp' carbenes

“…characterized, [1][2][3][4][5][6][7][8][9][10][11] but despite these numerous examples isolation of Pt(IV) carbenes remains rare. [1,[12][13][14][15][16][17][18][19][20][21][22][23][24] Such high oxidation state species have been postulated as intermediates in insertion reactions of diazo compounds and in elimination processes from platinumcycloalkanes.…”