Interaction of alkynes with palladium POCOP-pincer hydride complexes and its unexpected relation to palladium-catalyzed hydrogenation of alkynes

Abstract

Pe = cyclopentyl) have been synthesized from [2,6-(R 2 PO) 2 C 6 H 3 ]PdCl (1a-c) and LiAlH 4 or LiBEt 3 H. These hydride complexes react with phenylacetylene to afford H 2 , [2,6-(R 2 PO) 2 C 6 H 3 ]PdCuCPh (3a-c) and a small amount of styrene. When the R groups are isopropyl groups, a second palladium species is generated, and it has been identified as an alkenyl complex (E)-[2,6-( i Pr 2 PO) 2 C 6 H 3 ]PdCHvCHPh (4b). Mechanistic studies have shown that decomposition of these palladium pincer complexes and related palladium methyl complexes [2,6-(R 2 PO) 2 C 6 H 3 ]PdCH 3 (5a-c) occurs at room temperature in the presence of H 2 (1 atm or lower), resulting in the leaching of palladium particles. These particles have been shown to catalyze the hydrogenation of phenylacetylene and diphenylacetylene to their alkene and alkane products. A mechanism for the formation of palladium particles has been proposed. The structures of 1a, 1c, 2a, 2c, 3a, 4b and 5b have been studied by X-ray crystallography.Scheme 1 † Electronic supplementary information (ESI) available: Details of DLS experiments. CCDC 965919-965925 for 1a, 1c, 2a, 2c, 3a, 4b and 5b. For ESI and crystallographic data in CIF or other electronic format see

Pincer‐Type Complexes for Catalytic (De)Hydrogenation and Transfer (De)Hydrogenation Reactions: Recent Progress

“…Moreover, alkyne hydrogenation was accomplished by using palladium POCOP pincer complexes 78. At a high catalyst loading of 20 mol %, full conversion of phenylacetylene into ethylbenzene with quantitative yield was achieved.…”

Catalytic dehydrogenative borylation of terminal alkynes by POCOP-supported palladium complexes

“…Alkynylboronates are widely applicable in organic synthesis and their use has recently been reviewed. [12][13][14][15] We chose to focus on the aryl/bisphosphinite ligand family 16 because of its simple synthesis and the relative ease with which we can vary the size of the phosphine substituents. 10 A slower DHBTA catalysis with a similar scope was reported by Tsuchimoto et al using Zn(OTf ) 2 /pyridine.…”

“…15 However, it was found that the easily synthesized 2b 19 could exchange an acetate group for the hydride of HBpin to form 2a in situ within seconds. 15 However, it was found that the easily synthesized 2b 19 could exchange an acetate group for the hydride of HBpin to form 2a in situ within seconds.…”

“…15 Many pincer-ligated palladium complexes have been shown to leach soluble Pd(0) into solution. When 2a was treated with 1 eq.…”

“…15,19,24 5c was synthesized through the same method as complex 2c. Complexes 4a, 5b, 6b, 7b, 8b were synthesized according to literature procedures and precedent and were used as precatalysts (Scheme 4).…”