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...

The beta-diketiminate-stabilized calcium amide complex [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe(3))(2)}(THF)] (Ar = 2,6-diisopropylphenyl) and magnesium methyl complex [{ArNC(Me)CHC(Me)NAr}Mg(Me)(THF)] are reported as efficient precatalysts for hydroamination/cyclization of aminoalkenes. The reactions proceeded under mild conditions, allowing the synthesis of five-, six-, and seven-membered heterocyclic compounds. Qualitative assessment of these reactions revealed that the ease of catalytic turnover increases (i) for smaller ring sizes (5 > 6 > 7), (ii) substrates that benefit from favorable Thorpe-Ingold effects, and (iii) substrates that do not possess additional substitution on the alkene entity. Prochiral substrates may undergo diastereoselective hydroamination/cyclization depending upon the position of the existing stereocenter. Furthermore, a number of minor byproducts of these reactions, arising from competitive alkene isomerization reactions, were identified. A series of stoichiometric reactions between the precatalysts and primary amines provided an important model for catalyst initiation and suggested that these reactions are facile at room temperature, with the reaction of the calcium precatalyst with benzylamine proceeding with DeltaG(o)(298 K) = -2.7 kcal mol(-1). Both external amine/amide exchange and coordinated amine/amide exchange were observed in model complexes, and the data suggest that these processes occur via low-activation-energy pathways. As a result of the formation of potentially reactive byproducts such as hexamethyldisilazane, calcium-catalyst initiation is reversible, whereas for the magnesium precatalyst, this process is nonreversible. Further stoichiometric reactions of the two precatalysts with 1-amino-2,2-diphenyl-4-pentene demonstrated that the alkene insertion step proceeds via a highly reactive transient alkylmetal intermediate that readily reacts with N-H sigma bonds under catalytically relevant conditions. The results of deuterium-labeling studies are consistent with the formation of a single transient alkyl complex for both the magnesium and calcium precatalysts. Kinetic analysis of the nonreversible magnesium system revealed that the reaction rate depends directly upon catalyst concentration and inversely upon substrate concentration, suggesting that substrate-inhibited alkene insertion is rate-determining.

Despite the routine employment of Grignard reagents and Hauser bases as stoichiometric carbanion reagents in organic and inorganic synthesis, a defined reaction chemistry encompassing the heavier elements of Group II (M = Ca, Sr and Ba) has, until recently, remained unreported. This article provides details of the recent progress in heavier Group II catalysed small molecule transformations mediated by well-defined heteroleptic and homoleptic complexes of the form LMX or MX 2 ; where L is a mono-anionic ligand and X is a reactive s-bonded substituent. The intra-and intermolecular heterofunctionalization (hydroamination, hydrophosphination, hydrosilylation and hydrogenation) of alkenes, alkynes, dienes, carbodiimides, isocyanates and ketones is discussed.

The heavier group 2 complexes [M{N(SiMe(3))(2)}(2)](2)(1, M = Ca; 2, M = Sr) and [M{CH(SiMe(3))(2)}(2)(THF)(2)] (3, M = Ca; 4, M = Sr) are shown to be effective precatalysts for the intermolecular hydroamination of vinyl arenes and dienes under mild conditions. Initial studies revealed that the amide precatalysts, 1 and 2, while compromised in terms of absolute activity by a tendency toward transaminative behavior, offer greater stability toward polymerization/oligomerization side reactions. In every case the strontium species, 2 and 4, were found to outperform their calcium congeners. Reactions of piperidine with para-substituted styrenes are indicative of rate-determining alkene insertion in the catalytic cycle while the ease of addition of secondary cyclic amines was found to be dependent on ring size and reasoned to be a consequence of varying amine nucleophilicity. Hydroamination of conjugated dienes yielded isomeric products via η(3)-allyl intermediates and their relative distributions were explained through stereoelectronic considerations. The ability to carry out the hydroamination of internal alkynes was found to be dramatically dependent upon the identity of the alkyne substituents while reactions employing terminal alkynes resulted in the precipitation of insoluble and unreactive group 2 acetylides. The rate law for styrene hydroamination with piperidine catalyzed by [Sr{N(SiMe(3))(2)}(2)](2) was deduced to be first order in [amine] and [alkene] and second order in [catalyst], while large kinetic isotope effects and group 2 element-dependent ΔS(++) values implicated the formation of an amine-assisted rate-determining alkene insertion transition state in which there is a considerable entropic advantage associated with use of the larger strontium center.

Amides of the heavier group 2 elements Ca, Sr, and Ba are effective precatalysts for the atom-efficient addition of phosphine P−H bonds to carbodiimides. A number of intermediates within the catalytic cycle have been identified by in situ NMR methods and by stoichiometric synthesis.