Synergistic Effects of Hemilabile Coordination and Counterions in Homogeneous Catalysis:  New Tunable Monophosphine Ligands for Hydrovinylation Reactions

Abstract

Heterodimerization of olefins is a reaction of enormous synthetic potential, since it has been demonstrated that excellent yields and selectivities can be achieved under exceptionally mild conditions in many cases. 1,2 For example, we have recently shown that using catalysts derived from [(allyl)Ni(Br)] 2 and a tertiary phosphine in combination with a silver salt, a nearly quantitative hydrovinylation of vinylarenes can be achieved (eq 1). In our initial efforts to find a broadly applicable asymmetric version of this reaction, 2 we considered the requirement of a coordination site for ethylene on the putative cationic Ni intermediates, and chose (R)-2-diphenylphosphino-2′-alkoxy-1,1′-binaphthyl (MOP), 3 in which the 2′-methoxy group would play the role of a 'hemilabile′ ligand 4 (eq 2). With the fortuitous choice of this ligand, we also realized that counterions played a key role in the success of this reaction. For example, we found that AgOTf, which gave in many cases isolated yields >95% with Ph 3 P as a ligand, gave low yields in the hydrovinylation reactions using the MOP ligands. In sharp contrast, the use of Na + B[(3,5-(CF 3 ) 2 C 6 H 3 )] 4 -(NaBARF) in conjunction with MOP fully restored the activity of the catalyst (eq 2). 5 Since this original discovery, we have sought to clarify the role of hemilabile chelation and of the effect of various counterions on the efficiency and selectivity of the Ni-and Pd-catalysts for this reaction. With this goal, variation of the 2′ substituent of the MOP ligand was studied. In addition, we synthesized a new class of "tunable" hemilabile 1-aryl-2,5-dialkylphospholanes which were found to be exceptionally good ligands for the exacting hydrovinylation reaction, if and only if the appropriate counterions are used. The results of these studies are reported in this paper.

P-Stereogenic monophosphines in Pd-catalysed enantioselective hydrovinylation of styrene

“…The value of the 31 [55]. We easily prepared the selenides of some of the trialkylphosphines by direct treatment with elemental selenium.…”

“…Yield: 13.51 g (90%). 31 Compound 4 (13.50 g, 27.0 mmol) was dissolved in 200 ml of dry methanol and cooled to 0 C. Sulphuric acid (1.51 ml, 2.78 g, 27.0 mmol) was rapidly added and the mixture was left stirring overnight. Removal of the solvent furnished a pasty solid, which was suspended in 100 ml of ethyl acetate causing the precipitation of (À)-ephedrine sulphate, which was filtered out.…”

Catalytic Intermolecular Tail‐to‐Tail Hydroalkenylation of Styrenes with α Olefins: Regioselective Migratory Insertion Controlled by a Nickel/N‐Heterocyclic Carbene

“…[6e, 9] Nevertheless, all of the above methods are either characterized by head-to-head (h-h) or tail-to-head (t-h) C-C bond formation or use ethylene as one of the components; tail-totail (t-t) hetero-hydroalkenylation should provide branched terminal 1,1-disubstituted alkenes, but such products are observed only in small amounts, or in intramolecular cases wherein the geometric constraints of ring closure play a substantial role. Also, although phosphorus-based nickel(II) hydride complexes (Scheme 1 a) have been shown to achieve good selectivity and turnover numbers by exploiting various monodentate P ligands, hemilabile ligands, and counter ions, [10] successful cases of using long-chain a olefins as intermolecular hetero-hydroalkenylation substrates are not reported; as such substrates only isomerize into internal olefins. [3g, 11] N-heterocyclic carbene (NHC) nickel(II) hydride complexes ({(NHC)NiH}) have been rarely studied for this purpose, possibly because alkyl reductive elimination of NHCs can be a very effective process (Scheme 1 b), wherein the alkene hydrometalation is the first step, similar to that of a typical {PNiH}-catalyzed (P = P ligand) hydrovinylation.…”

Nickel‐Catalyzed Asymmetric Hydrovinylation Using Lewis Acid Activation

“…Among these activators, InI 3 appears to be the candidate of choice since it leads to the same chemoand enantioselectivity and even higher activity than the benchmark system based on NaBArF. In contrast to the activation with silver salts, [8] no filtration is needed before starting the reaction. Moreover, InI 3 is safe to handle, commercially available and relatively cheap, thus providing a simple and practical protocol for efficient hydrovinylation reaction.…”

“…This activation step can be achieved by replacing the halide ligand with a weakly coordinating anion (WCA) (Scheme 2a). Silver salts, [8] ionic liquids, [9] and in particular NaBArF [10] have been used successfully for this purpose. Since the pioneering work of Wilke [1] and Bogdanovič, [11] it is well established that the nature of the WCA plays a decisive role in controlling the activity, chemoselectivity and even enantioselectivity.…”