Calcium-Catalyzed Intermolecular Hydrophosphination

Abstract

The -diketiminato complex [{HC(C(Me) 2 N-2,6-iPr 2 C 6 H 3 ) 2 }Ca{N(SiMe 3 ) 2 }(THF)] effects intermolecular hydrophosphination of a range of alkenes and alkynes. In behaVior reminiscent of lanthanocene(III) catalysis, a more electrophilic alkene is polymerized to phosphine-terminated macromolecules.Organophosphines, R 3 P, are an important class of compound widely employed in transition metal catalysis and organic synthesis. Hydrophosphination, the addition of the P-H bond of a primary or secondary phosphine to an unsaturated C-C bond, is a potentially powerful and, importantly, atom-efficient route to such compounds. 1 The transformation can be achieved under radical conditions or, alternatively, may be promoted by group 1, 2 late transition metal, 3 or lanthanide based catalysts. 4 On the basis of a proposed analogy between catalytic lanthanide and heavier group 2 metal centers, we have previously reported the -diketiminato-stabilized calcium amide 1 as an effective catalyst for the intramolecular hydroamination of aminoalkenes and aminoalkynes. 5 The reaction was postulated to occur by the generalized catalytic cycle outlined in Scheme 1, via (i) initiation of the precatalyst by a σ-bond metathesis (or protonolysis) of 1 with a primary amine to form a calcium primary amide, (ii) an intramolecular insertion of the alkene into the Ca-N bond, and (iii) the σ-bond metathesis of the resultant calcium alkyl with a further equivalent of amine to liberate the product and regenerate the active catalyst. On the basis of the numerous applications of lanthanide-based catalysts to the heterofunctionalization of unsaturated carboncarbon bonds, we speculated that the observed reactivity was not confined to the intramolecular hydroamination of alkenes and alkynes. Indeed, Harder has very recently shown that homoleptic benzyl alkaline earth complexes may act as precatalysts for the hydrosilylation of alkenes. 6 Lanthanocene catalysts of the form Cp* 2 LnX (X ) H, CH-(SiMe 3 ) 2 , Ln ) La, Sm, Y, Lu) have been applied to the intramolecular hydrophosphination/cyclization of a variety of phosphinoalkenes. 7 In this case, the reaction mechanism has been studied in depth and occurs via a pathway analogous to that depicted in Scheme 1. Both experimental and theoretical studies suggest that the σ-bond metathesis of the Ln-C bond of the intermediate is the rate-determining step (cf. Scheme 1, step iii). Furthermore, the intermolecular hydrophosphination of alkenes with such catalysts has not been achieved; rather, a lanthanocene phosphide mediated polymerization of ethylene has been reported. 8 Although divalent ytterbium catalysts have been applied to the intermolecular variant of this reaction, 4 the reaction mechanism in these cases is potentially complicated by reductive initiation.We now present a preliminary account of the application of 1 to the intermolecular hydrophosphination of unsaturated C-C bonds. In this regard it is noteworthy that limited evidence exists for both σ-bond metathesis and insertion steps requisi...

Hydrophosphination of Styrene and Polymerization of Vinylpyridine: A Computational Investigation of Calcium-Catalyzed Reactions and the Role of Fluxional Noncovalent Interactions

“…The presence of a sterically demanding spectator ligand helps to stabilise the Ae metal centre against lability and brings to the fore the importance of Ae···π, Ae···H-X and CH···π interactions. 16,38,39,[40][41][42] Such interactions have previously been overlooked due to difficulties in synthetic procedures as a consequence of high reactivity. 43 As a corollary, these interactions have also been largely ignored computationally.…”

“…Nevertheless, the local energy barriers (∆GL) associated with alkene insertion (76 kJ/mol for styrene and 68 kJ/mol for vinylpyridine) correspond to the ratelimiting step for both pathways, concurrent with experimental observation. 16,52 Figure 2. Representative diagrams of the three different coordination modes, X is the structure identifier.…”

“…14,15 Experimentally, there are a number of Ca-based complexes that have been shown to catalyse the hydrophosphination reaction. [16][17][18][19] Unlike transition metal catalysts, Ca (and heavier alkaline earth (Ae)) cationic (2+) complexes consist of an electron deficient (d 0 ) metal centre. Unwanted oxidation/reduction pathways typically associated with transition metals can be avoided as the metal oxidation state remains unperturbed during the reaction.…”

“…This includes, the stabilising effect of the Ae-bound ancillary ligands, the impact of the ancillary ligands on reactivity and the effect of activated vinylarenes. [16][17][18][19]23 All of which are addressed within this article.…”

“…16,17,22,23 When 2-vinylpyridine (referred to as vinylpyridine in the following) was used in place of styrene, the reaction did not undergo hydrophosphination.…”