Selective Homogeneous Palladium(0)-Catalyzed Hydrogenation of Alkynes to (Z)-Alkenes

Abstract

The selective semihydrogenation of alkynes to (Z)-alkenes seems to be a simple reaction, yet this type of conversion remains a desirable synthetic tool. A variety of catalysts is available for the conversion of alkynes into (Z)-alkenes; the best known and most efficient ones are heterogeneous catalysts such as the Lindlar catalyst, [1] nickel boride, [2] the ªP2Niº catalyst, [3] and palladium immobilized on a clay.[4]Especially the Lindlar catalyst suffers from a number of major problems in the selective cis-hydrogenation of alkynes, notably, partial isomerization of the (Z)-alkene product to the (E)-alkene, shift of the double bond, overreduction to the alkane, and problems with reproducibility. Only a few examples are known of homogeneous catalyst systems that show good selectivity for a wider range of substrates, for example, rhodium- [5] and Cr(CO) 3 -catalyzed hydrogenation.[6]For palladium, however, few homogeneous catalyst systems are known, and these involve palladium(ii) complexes. [7,8] The only homogeneous palladium(0) catalyst is the complex [Pd 2 (dppm) 3 ], which shows low activity in the hydrogenation of propyne and 2-butyne.[9]Here we report the first example of a homogeneous palladium(0) catalyst bearing a bidentate nitrogen ligand that is able to homogeneously hydrogenate a wide variety of alkynes to the corresponding (Z)-alkenes with very high selectivity. Additionally, enynes are selectively converted into dienes. As part of our continuing interest in carbon ± element (e.g., CÀC, CÀH, CÀN, CÀX) bond-forming reactions, [10] we employed zerovalent palladium precatalysts I, which contain the rigid bidentate nitrogen ligand bis(arylimino)acenaphthene (bian). These have previously been isolated with an electrondeficient alkene as the ancillary ligand; [11] however, analogues of I with electron-rich alkenes are not stable. We reasoned that since simple alkenes are expected to be readily substituted by alkynes, if alkynes were hydrogenated in the presence of molecular hydrogen, subsequent fast substitution by excess alkyne could give rise to a viable catalytic cycle (Scheme 1). Scheme 1. Proposed catalytic cycle for hydrogenation of alkynes.We conducted hydrogenation reactions of alkynes at 20 8C and 1 bar hydrogen pressure with initial sample concentrations of about 0.5 m in THF and 1 mol % of I a as precatalyst. With 3-hexyne (1) and 4-octyne (3), high stereoselectivity (b 99.5 %) for the formation of (Z)-3-hexene and (Z)-4-octyne was observed, and hydrogenation proceeded with excellent yield. Various other alkynes were then subjected to these hydrogenation conditions (Table 1).The stereoselectivity for the formation of the (Z)-alkene is generally excellent and in many cases superior to that obtained with Lindlar [1] or P2Ni catalysts [3] and comparable to or in several cases better than those of known homogeneous systems.[5±7] Generally, quantitative conversion of the alkyne to the desired (Z)-alkene takes place, without concomitant alkane formation. The selectivity is highest (Z/E b 99/1) for s...

Ruthenium‐Catalyzed E‐Selective Partial Hydrogenation of Alkynes under Transfer‐Hydrogenation Conditions using Paraformaldehyde as Hydrogen Source

“…[17] Reduction of alkynes selectively to Zalkenes can be carried out using the well-known Lindlar catalyst, [18] and even base metal catalysts. [19][20] Various transition metal catalysts [5,10,13,15,[36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] have been developed so far which in combination with different transfer hydrogenation sources [21][22][23][24][25][26][27][28][29][30][31] or dihydrogen gas [32][33][34][35] can be used for the stereoselective semi-hydrogenation (reduction) of alkynes to olefins. In this regard, selective formation of E-alkenes is more challenging and less well-studied.…”

A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes

“…19,20,21,22,23 . Heterogeneous as well as homogeneous metal catalysts for this purpose are known 24,25,26,27,28 , however, metal-free catalytic hydrogenation of unactivated alkynes into alkenes has not been reported previously.…”

Modulation of Metal Species as Early Control Point for Ni-catalyzed Stereodivergent Semihydrogenation of Alkynes with Water

“…Ideally, several issues should be addressed: (a) independent catalytic cycles should be initiated by simple adjustment of the reaction factor(s) to enable high yield and stereoselectivity; 8-10 (b) nonprecious metals and ligands without toxic additives would be more favorable; [14][15][16][17][18] (c) water is the rst choice of the hydrogen donor for TH process; 10,[19][20][21][22] (d) alkynes with various substituents should be hydrogenated in high yield and stereoselectivity in mild conditions, and over-reduction to saturated alkanes need to be avoided. 23,24 Based on our interest in Ni-catalyzed reaction and alkyne transformations, [25][26][27] We launched a project with nickel catalysts to address the above challenges. After laborious trials, we realized an unprecedented Ni-catalyzed stereodivergent TH of alkynes with water in a novel controlling mode, in which the key to the success of modulation is the judicious inclusion of the base.…”