Asymmetric Platinum Group Metal-Catalyzed Carbonyl-Ene Reactions:  Carbon−Carbon Bond Formation versus Isomerization

Abstract

A comparative study of the carbonyl-ene reaction between a range of 1,1'-disubstituted or trisubstituted alkenes and ethyl trifluoropyruvate catalyzed by Lewis acid-platinum group metal complexes of the type [M{(R)-BINAP}]2+ (M = Pt, Pd, Ni; BINAP is 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) revealed subtle but significant differences in their reactivity. For instance, the palladium-based Lewis acid [Pd{(R)-BINAP}]2+ catalyzes the ene reaction between methylene cycloalkane to afford the expected alpha-hydroxy ester in good yield and excellent diastereo- and enantioselectivity. In contrast, under the same conditions, the corresponding [M{(R)-BINAP}]2+ (M = Pt, Ni) catalyzes isomerization of methylene cycloalkane and the ene reaction of the resulting mixture of methylene cycloalkane and 1-methylcycloalkene at similar rates to afford a range of -hydroxy esters in high regioselectivity, good diastereoselectivity, and good to excellent enantioselectivity. In addition, [Pt{(R)-BINAP}]2+ also catalyzes postreaction isomerization of the ene product as well as consecutive ene reactions to afford a double carbonyl-ene product. The sense of asymmetric induction has been established by single-crystal X-ray crystallography, and a stereochemical model consistent with the formation of (S)-configured -hydroxy ester has been proposed; the same model also accounts for the observed exo-diastereoselectivity as well as the level of diastereoselectivity.

[2,2′-Bis(diphenylphosphanyl)-1,1′-binaphthyl-κ2 P,P′]dichloridoplatinum(II) acetonitrile trisolvate

“…Angles closer to the ideal of 90 are seen between cis-disposed phosphorus and chlorine atoms. The bond lengths involving the Pt metal centre are similar to those in the enantiopure (R-BINAP)PtCl 2 (Doherty et al, 2006); however, deviations are observed in several of the angles. In a single unit cell, four molecules can be found ( Fig.…”

“…Although less common than complexes of the second-row metal Pd, atropisomers of (BINAP)PtCl 2 ( Fig. 2) have found use in catalytic reactions such as enantioselective Baeyer-Villiger oxidations of cyclic ketones with hydrogen peroxide (Strukul et al, 1997) and as precatalysts for asymmetric carbonyl-ene reactions (Doherty et al, 2006). Enantiomeric complexes of the formula L 2 PtCl 2 including (BINAP)PtCl 2 data reports have also been examined for their cytotoxic activity against cancer cell lines and their ability to bind to the human telomeric sequence folded in the G-quadruplex structure (Bombard et al, 2010).…”

“…While the structure of {(R)-BINAP}PtCl 2 has been described as a dichloromethane solvate in the orthorhombic space group P2 1 2 1 2 1 (Doherty et al, 2006), the corresponding racemate (racBINAP)PtCl 2 has yet to be structurally char-acterized. We report here the solid-state crystal structure of (racBINAP)PtCl 2 determined via single-crystal X-ray diffraction and discuss its structural properties.…”

The first organocatalytic carbonyl-ene reaction: isomerisation-free C-C bond formations catalysed by H-bonding thio-ureas

“…This type of ene reaction has recently been explored by other groups using Pd, Ni and Pt catalysts. [ 7 , 8 ] The importance of such reactions is derived from the ever-increasing occurrence of trifluoromethyl substituents in drugs and biologically active compounds. [ 9 , 10 ] Given that trifluoropyruvate ene reactions seemingly have a low activation barrier, and as part of our ongoing interest in hydrogen bond mediated catalysis,[ 11 ] we have investigated thioureas as H-bonding additives for organocatalytic carbonyl ene reactions and report these results here.…”

Fe(BF4)2-Catalyzed Inter- and Intramolecular Carbonyl-Ene Reaction of Trifluoropyruvate

“…Secondly, it was reported that a marked effect of the counterion existed on the conversion of Lewis acid-catalyzed reaction between methylene cyclohexane and ethyl trifluoropyruvate. 26 Anhydrous Fe(BF4)2, generated from FeCl2 and AgBF4, led to higher yields than Fe(BF4)2•6H2O. Furthermore, a bidentate coordination can be postulated such that in the Cu II catalyzed carbonyl-ene reaction of ethyl glyoxylate disclosed by Evans.…”