Organo Palladium(II) Complexes with PdC₄ Coordination: Phenylethinyl‐ and Methylphenylethinylcomplexes

Abstract

Tetrakis(p‐tolyl)oxalamidinato‐bis[acetylacetonatopalladium(II)] ([Pd2(acac)2(oxam)]) reacted with Li–C≡C–C6H5 in THF with formation of [Pd(C≡C–C6H5)4Li2(thf)4] (1a). Reaction of [Pd2(acac)2(oxam)] with a mixture of 6 equiv. Li–C≡C–C6H5 and 2 equiv. LiCH3 resulted in the formation of [Pd(CH3)(C≡C–C6H5)3Li2(thf)4] (2), and the dimeric complex [Pd2(CH3)4(C≡C–C6H5)4Li4(thf)6] (3) was isolated upon reaction of [Pd2(acac)2(oxam)] with a mixture of 4 equiv. Li–C≡C–C6H5 and 4 equiv. LiCH3. 1–3 are extremely reactive compounds, which were isolated as white needles in good yields (60–90%). They were fully characterized by IR, 1H‐, 13C‐, 7Li‐NMR spectroscopy, and by X‐ray crystallography of single crystals. In these compounds Li ions are bonded to the two carbon atoms of the alkinyl ligand. 1a reacted with Pd(PPh3)4 in the presence of oxygen to form the already known complexes trans‐[Pd(C≡C–C6H5)2(PPh3)2] and [Pd(η2‐O2)(PPh3)2]. In addition, 1a is an active catalyst for the Heck coupling reaction, but less active in the catalytic Sonogashira reaction.

Elucidating the reaction mechanism of a palladium-palladium dual catalytic process through kinetic studies of proposed elementary steps

“…Apart from our earlier reports 32,40 , palladium bisacetylides have not been described in the context of crosscoupling catalysis. There has been considerable interest in (transition)metal based acetylides 49,50 , while there are few reports on palladium counterparts [51][52][53][54][55][56][57][58] . We have previously described that under identical conditions the reaction of palladium oxidative addition complex A with palladium bisacetylide C into tolan product 3 occurs instantly, whereas induction period was present for the reaction of palladium oxidative addition complex A with terminal alkyne 2 in the presence of base, in which palladium acetylides are formed to initiate the reaction 32 .…”

Palladium End-Capped Polyynes via Oxidative Addition of 1-Haloalkynes to Pd(PPh3)4

“…The second method is a reaction between a palladium(II) complex and a metal alkynyl (method 2). In this regard, there are protocols which use trimethylstannyl alkynyl compounds and lithium, silver, or copper(I) acetylides . The third approach is an oxidative addition between a palladium(0) phosphine complex and 1-haloacetylene (method 3). , Similar approaches are used for other transition metal alkynyl and polyynyl compounds.…”