David J. Liptrot

Group 2 Promoted Hydrogen Release from NMe2H⋅BH3: Intermediates and Catalysis

Abstract Both homo‐ and heteroleptic alkyl and amide complexes of the Group 2 elements Mg and Ca are shown to be active for the catalytic dehydrocoupling of Me2NH⋅BH3. Reactions of either magnesium dialkyls or the β‐diketiminate complex [HC{(Me)CN(Dipp)}2MgnBu] with four or two equivalents of Me2NHBH3, respectively, produce compounds containing the [H3BNMe2BH2Me2N]− ion, which coordinates to the magnesium centers through Mg—N and Mg⋅⋅⋅HB interactions in both the solution and solid states. Thermolysis of these compounds at 60 °C produces the cyclic product [(H2BNMe2)2] and, it is proposed, magnesium hydrido species by an unprecedented δ‐hydride elimination process. Calcium‐based species, although less reactive than their magnesium‐based counterparts, are found to engage in similar dehydrocoupling reactivity and to produce a similar distribution of products under thermally promoted catalytic conditions. A mechanism for these observations is presented that involves initial production and insertion of H2B=NMe2 into polarized M—N bonds as the major B—N bond‐forming step. The efficacy of this insertion and subsequent β‐ or δ‐hydride elimination steps is proposed to be dependent upon the charge density and polarizing capability of the participating Group 2 center, providing a rationale for the observed differences in reactivity between magnesium and calcium.

Molybdenum-Mediated Carbonylation of Aryl Halides with Nucleophiles Using Microwave Irradiation

A new, efficient, and practical molybdenum-mediated carbonylation of aryl and heteroaryl halides with a variety of nucleophiles is described using microwave irradiation. A range of reactions illustrating the wide scope of this chemistry were carried out and proceeded in good to excellent yields.

Alkaline earths as main group reagents in molecular catalysis

The past decade has witnessed some remarkable advances in our appreciation of the structural and reaction chemistry of the heavier alkaline earth (Ae = Mg, Ca, Sr, Ba) elements. Derived from complexes of these metals in their immutable +2 oxidation state, a broad and widely applicable catalytic chemistry has also emerged, driven by considerations of cost and inherent low toxicity. The considerable adjustments incurred to ionic radius and resultant cation charge density also provide reactivity with significant mechanistic and kinetic variability as group 2 is descended. In an attempt to place these advances in the broader context of contemporary main group element chemistry, this review focusses on the developing state of the art in both multiple bond heterofunctionalisation and cross coupling catalysis. We review specific advances in alkene and alkyne hydroamination and hydrophosphination catalysis and related extensions of this reactivity that allow the synthesis of a wide variety of acyclic and heterocyclic small molecules. The use of heavier alkaline earth hydride derivatives as pre-catalysts and intermediates in multiple bond hydrogenation, hydrosilylation and hydroboration is also described along with the emergence of these and related reagents in a variety of dehydrocoupling processes that allow that facile catalytic construction of Si-C, Si-N and B-N bonds.

Selective reduction of CO2 to a methanol equivalent by B(C6F5)3-activated alkaline earth catalysis

Treatment of β-diketiminato Mg and Ca amidoborane compounds with B(C6F5)3 induces hydride elimination and formation of alkaline earth hydrido-tris(pentafluorophenyl)borate derivatives. Both species react with CO2 to provide formate complexes, one of which has been structurally characterised, and may be applied to the highly selective reductive hydroboration of CO2 with pinacolborane (HBpin) to provide the methanol equivalent, CH3OBpin.

Stoichiometric reactivity of dialkylamine boranes with alkaline earth silylamides

Reactions of β-diketiminato group 2 silylamides, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)M(THF)(n){N(SiMe(3))(2)}] (M = Mg, n = 0; M = Ca, Sr, n = 1), and an equimolar quantity of pyrrolidine borane, (CH(2))(4)NH·BH(3), were found to produce amidoborane derivatives of the form [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)MN(CH(2))(4)·BH(3)]. In reactivity reminiscent of analogous reactions performed with dimethylamine borane, addition of a second equivalent of (CH(2))(4)NH·BH(3) to the Mg derivative induced the formation of a species, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)Mg{N(CH(2))(4) BH(2)NMe(2)BH(3)}], containing an anion in which two molecules of the amine borane substrate have been coupled together through the elimination of one molecule of H(2). Both this species and a calcium amidoborane derivative have been characterised by X-ray diffraction techniques and the coupled species is proposed as a key intermediate in catalytic amine borane dehydrocoupling, in reactivity dictated by the charge density of the group 2 centre involved. On the basis of further stoichiometric reactions of the homoleptic group 2 silylamides, [M{N(SiMe(3))(2)}(2)] (M = Mg, Ca, Sr, Ba), with (CH(3))(2)NH·BH(3) and (i)Pr(2)NH·BH(3) reactivity consistent with successive amidoborane β-hydride elimination and [R(2)N[double bond, length as m-dash]BH(2)] insertion is described as a means to induce the B-N dehydrocoupling between amine borane substrates.

Hetero-dehydrocoupling of silanes and amines by heavier alkaline earth catalysis

The homoleptic alkaline earth hexamethyldisilazides, [M{N(SiMe3)2}2]2 (1: M = Mg; 2: M = Ca; 3: M = Sr), have been demonstrated as active pre-catalysts for the cross-dehydrocoupling of Si–H and N–H bonds under mild (25–60 °C) conditions. The reactions are applicable to the coupling of a wide variety of amine and silane substrates and are proposed to occur via a sequence of discrete Si–H/M–N and N–H/M–H metathesis steps. Whereas reactions of dialkyl group 2 species with 2,6-di-iso-propylaniline and phenylsilane delivered a series of well-defined compounds consistent with this rationale, kinetic analysis of the cross-coupling of diethylamine with diphenylsilane provided evidence for a more complex and subtly variable mechanistic landscape. Although reactions performed with all three pre-catalysts presented a number of common features, in every case the calcium species, 2, was found to provide notably superior catalytic activity, an order of magnitude higher than both 1 and 3 and in excess of many previously described benchmark transition metal- or f-element-mediated processes. Variations in overall reaction order, mode of pre-catalyst activation and the nature of the rate determining process are postulated to arise as a consequence of the marked change in M2+ radius and resultant charge density as group 2 is descended.

Stereoselective double Friedel-Crafts alkylation of indoles with divinyl ketones

A tandem double Friedel-Crafts reaction of indoles and nonsymmetrical divinyl ketones has been achieved. The tandem reaction forms complex [6-5-7]-tricyclic indoles in excellent yields. The reaction is completely regioselective and offers high levels of syn diastereoselectivity. The reaction is also seen to be sensitive to substrate structure and catalyst.

Alkylstrontium diamidoboranes: β-hydride elimination and Sr–C insertion

Alkylstrontium secondary amidoboranes are shown to undergo β-hydride elimination and Sr-C insertion reactions; observations which provide support for similar processes in d(0)-catalysed dialkylamine borane dehydrocoupling.

Novel Aryl and Heteroaryl Acyl Sulfamide Synthesis via Microwave-Assisted Palladium-Catalyzed Carbonylation

A novel, simple synthesis of aryl and heteroaryl acyl sulfamides has been developed via palladium-catalyzed carbonylation of aryl or heteroaryl halides in the presence of sulfamide nucleophiles. A range of reactions illustrating the wide scope of this reaction was carried out under microwave irradiation in a vessel equipped with a gas inlet adapter and proceeded in good to excellent yields.

Alkaline-earth-catalyzed dehydrocoupling of amines and boranes

Dehydrocoupling reactions between the boranes HBpin and 9-borabicyclo[3.3.1]nonane and a range of amines and anilines ensue under very mild reaction conditions in the presence of a simple β-diketiminato magnesium n-butyl precatalyst. The facility of the reactions is suggested to be a function of the Lewis acidity of the borane substrate, and is dictated by resultant pre-equilibria between, and the relative stability of, magnesium hydride and borohydride intermediates during the course of the catalysis.