Mechanistic Studies on Platinum(II) Catalyzed Hydroarylation of Alkynes

Abstract

The dicationic acetylene platinum(II) complex [ P t ( P N P ) ( C 2 H 2 ) ] ( B F 4 ) 2 (PNP = 2,6-bis-(diphenylphosphinomethyl)pyridine) was generated in situ by ligand substitution from the ethylene complex [Pt(PNP)-(C 2 H 4 )](BF 4 ) 2 and was reacted with a series of arenes at low temperature. Only electron-rich arenes added across the coordinated C−C triple bond and gave the corresponding arylalkenyl complexes (E)-[Pt(PNP)(CHCHAr)]BF 4 (Ar = C 6 Me 5 , C 6 H 2 Me 3 -2,4,6, C 6 H 3 Me 2 -2,6, C 6 H 3 Me 2 -2,4). A slow E− Z isomerization of the arylalkenyl complexes was observed. Single-crystal X-ray structure analyses were obtained for both E and Z isomers of the pentamethylbenzene derivative. The E isomers of [Pt(PNP)(CHCHAr)]BF 4 (Ar = C 6 Me 5 , C 6 H 2 Me 3 -2,4,6) reacted with excess HBF 4 •Et 2 O to give the corresponding arylalkene complexes [Pt(PNP)(CH 2 CHAr)](BF 4 ) 2 , whereas the Z isomers did not undergo immediate protonolysis. Using (E)-[Pt(PNP)(CDCDC 6 Me 5 )]BF 4 it was shown that the stereochemistry of the C−C double bond in the protonolysis product depends on the nature of the acid anion HX (X − = Cl − , BF 4 − ). The catalytic hydroarylation was studied in solution by NMR spectroscopy. The reaction studies provide a more refined view of the individual steps proposed for the Friedel−Crafts type mechanism of the Pt II -catalyzed intermolecular hydroarylation of alkynes.

Enhancing the catalytic properties of well-defined electrophilic platinum complexes

“…Hahn et al reported that dicationic Pt(II) complexes activate alkynes once bound to the metal centre, to the point that they can undergo nucleophilic addition of electron-rich arenes under very mild reaction conditions through a Friedel-Crafts type process (as in Scheme 1). 31 Under acidic conditions it is possible to cleave the generated [Pt]-C bond releasing the hydroarylated product and regenerating the catalyst (Scheme 9). This last step is ratelimiting since it is known that protonolysis of dicationic [Pt]-C bonds is a rather difficult process.…”

Acetylene in Organic Synthesis: Recent Progress and New Uses

“…Hydroarylation of alkynes provides a valuable route to various alkenes, although chemo-, regio and stereoselectivities of this reaction may be compromised by collateral formation of diaryl alkanes, dialkenyl arenes, and aryl dienes. An in situ generated dicationic complex [Pt(2,6-bis(dephinylphosphinomethyl)pyridine)C 2 H 2 ](BF 4 ) 2 151 reacts with arenes [ 184 ]. Only electron-rich arenes are incorporated into arylalkenyl products 152 ( Scheme 79 ).…”

The chemistry of the carbon-transition metal double and triple bond: Annual survey covering the year 2014

“…formation of alkynylpalladium complex 1218), followed by reductive elimination [1165]. Facile E/Z isomerization of alkenylplatinum complexes was attributed to resonance contribution from a carbene complex-carbocation form [1166]. A computational study of nickel-catalyzed [2+2+1] cycloaddition of a carborane and two silylalkyne units favored an alkyne insertion mechanism over a vinylidene mechanism [1167].…”