Kinetic and Spectroscopic Studies of the [Palladium(Ar-bian)]-Catalyzed Semi-Hydrogenation of 4-Octyne

Abstract

The kinetics of the stereoselective semi-hydrogenation of 4-octyne in THF by the highly active catalyst [Pd{(m,m'-(CF(3))(2)C(6)H(3))-bian}(ma)] (2) (bian = bis(imino)acenaphthene; ma = maleic anhydride) has been investigated. The rate law under hydrogen-rich conditions is described by r = k[4-octyne](0.65)[Pd][H(2)], showing first order in palladium and dihydrogen and a broken order in substrate. Parahydrogen studies have shown that a pairwise transfer of hydrogen atoms occurs in the rate-limiting step. In agreement with recent theoretical results, the proposed mechanism consists of the consecutive steps: alkyne coordination, heterolytic dihydrogen activation (hydrogenolysis of one Pd-N bond), subsequent hydro-palladation of the alkyne, followed by addition of N-H to palladium, reductive coupling of vinyl and hydride and, finally, substitution of the product alkene by the alkyne substrate. Under hydrogen-limiting conditions, side reactions occur, that is, formation of catalytically inactive palladacycles by oxidative alkyne coupling. Furthermore, it has been shown that (Z)-oct-4-ene is the primary reaction product, from which the minor product (E)-oct-4-ene is formed by an H(2)-assisted, palladium-catalyzed isomerization reaction.

Mechanism of Pd(NHC)-Catalyzed Transfer Hydrogenation of Alkynes

“…The latter is seen in alkyne hydrogenations with Pd(Ar-Bian)(dmfu). 4 In the current case, we have neither observed maleic anhydride nor succinic anhydride in the GC of aliquots of the reaction, nor in the NMR-spectrum of the product, so we postulate that maleic anhydride remains coordinated to the complex during reaction (vide infra).…”

“…4 In that system the optimum concentration of alkyne was 0.32 M; more electron-poor alkynes give more stable metallacycles, so the concentration above which palladacycle formation effectively competes with hydrogenation will be lower for aromatic alkynes than for aliphatic alkynes. 23 Attempts to isolate these palladacyclic species from the reaction mixture were not successful.…”

“…In previous research we have shown that during alkyne hydrogenation with Pd 0 (Ar-BIAN)(alkene)-complexes as catalysts, the alkene is hydrogenated off to form the active catalyst 4 We wondered whether this would also be the case for our current NHC-based pre-catalysts 1 or 3. However, by 1 H-NMR this cannot be verified, because the hydrogens of non-coordinated maleic anhydride would be obscured by the aromatic hydrogens of the mesityl-substituent on the carbene ligand.…”

“…Despite the usefulness of this reaction, it has been studied far less extensively than the similar hydrogenation of carbon-carbon double bonds. 1,2 Several homogeneous Pd-complexes that catalyze this reaction have been reported by us [3][4][5] and others 6 , and examples of other metal-complexes that are able to catalyze this reaction are known based on Ru-, 7,8 Rh-, 9 or Fe-complexes. 10 However, this process is often plagued by overreduction after full conversion, leading to saturated compounds.…”

“…1,11 Some examples of chemo-and stereoselective alkyne semihydrogenation have been reported, but very few studies have addressed the mechanism of the reaction or explained the observed selectivity. 4,12,13 For the development of better catalytic systems it is of vital importance to understand the mechanism, enabling a directed search for new catalysts.…”