Synthesis of N-heterocyclic carbene palladium(ii) bis-phosphine complexes and their decomposition in the presence of aryl halides

Abstract

Methylpalladium(II) carbene complexes of the type [Pd(NHC)Me(P-P)]BF(4) (NHC = N-heterocyclic carbene, P-P = chelating phosphine) have been synthesised, the complex [Pd(tmiy)Me(dcype)]BF(4) (tmiy = 1,3,4,5-tetramethylimidazol-2-ylidene, dcype = 1,2-bis(dicyclohexylphosphino)ethane) being characterised crystallographically. Complexes bearing the tmiy ligands were shown to decompose in an analogous manner to complexes bearing monodentate phosphine ligands, with the rate of decomposition being nominally linked to the size of the chelate ring. The decomposition of these complexes in the presence of aryl halides-expected to yield Pd(Ar)X(P-P)-was studied and shown instead to yield PdX(2)(P-P) and [Pd(tmiy)X(P-P)]BF(4). Additionally, Pd(Me)X(P-P) and Pd(Ar)X(P-P) were observed in some cases. Intermolecular cross-over reactions between the starting complex and Pd(Ar)X(P-P) were found to be the source of these unexpected products.

Switching the nature of catalytic centers in Pd/NHC systems by solvent effect driven non‐classical R‐NHC Coupling

“…Dissociation of the [R-NHC]X species was confirmed by the experiment and the species were detected. [14,[29][30][31][32][35][36][37][38][39][40][41][42][43][44] Thus, elimination of R-NHC coupling products is a wellestablished fact and in the present work we have modeled only the corresponding overall stages.…”

“…[35] The process was studied experimentally with NMR spectroscopy, electron microscopy, and mass-spectrometry. [29][30][31] A number of experimental studies have observed facile R-NHC coupling in stoichiometric reactions, [14,[36][37][38][39][40][41][42][43][44] under catalytic conditions [31,[45][46][47] and were modeled with gas-phase calculations. [33,34,[48][49][50][51][52] Although gas phase calculations may serve as initial model, this model cannot be compared with experimental reactions, which are exclusively carried out in the presence of solvent.…”

Mechanisms in the Reaction of Palladium(II)–π‐Allyl Complexes with Aryl Halides: Evidence for NHC Exchange Between Two Palladium Complexes

“…An interesting feature of this reaction is that it provides an alternative explanation to mechanisms involving Pd IV species. [28][29][30][31][32] While some claims of Pd IV intermediates are backed with considerable experimental evidence, [33,34] Pd IV mechanisms are sometimes invoked simply because no other explanation can be found. [35] Considering the general reaction between a Pd II complex noted [Pd(A)(B)] and an organic substrate CD (Scheme 1), two possible mechanisms can be envisaged: in the first (Scheme 1 a), a Pd IV intermediate is generated by oxidative addition, and then collapses to give final products [Pd(C)(B)] and AD; in the second, s-bond metathesis (Scheme 1 b), a four-membered cyclic transition state is formed in which Pd is in the + II oxidation state.…”

“…carbene ligands · density functional calculations · palladium · transmetalation Another alternative, often overlooked, mechanism is ligand transfer between two Pd complexes. [28][29][30][31][32] We have recently reported an intriguing reaction between Pd II p-allyl complex 1 a and phenyl iodide (Scheme 3). [45] Compound 1 a was designed as a Pd 0 precatalyst: [46][47][48][49][50][51][52] upon thermal activation, 2-propenylimidazolium 2 a salt is eliminated, thus yielding highly reactive, monoligated phosphane complex 3 a.…”

“…[33,57,58] Thus, the unprecedented nature and high selectivity [59] of this reaction prompted us to conduct a detailed experimental and DFT study of its mechanism. Notably, in light of previous studies on reactions seemingly involving Pd IV intermediates but which actually proceed by ligand transfer between two Pd centres, [28][29][30][31][32] we investigated two other possible mechanisms, that is, s-bond metathesis and ligand transfer. [36] In this contribution, we demonstrate that the latter is operating, and that the key transmetalation step most likely involves the transfer of an NHC between two Pd atoms.…”