Interactions of Isonitriles with Metal-Boron Bonds: Insertions, Coupling, Ring Formation, and Liberation of Monovalent Boron.

Boryl, borylene, and base-stabilized borylene complexes of manganese and iron undergo a range of different reactions when treated with isonitriles including single, double, and partial isonitrile insertions into metal-boron bonds, ring formation, isonitrile coupling, and the liberation of new monovalent boron species. Two of the resulting cyclic species have also been found to react selectively with anhydrous HCl to form ring-opened products. The diverse isonitrile-promoted reactivity of transition-metal-boron compounds has been explored computationally.

Activation of boryl-, borylene and metalloborylene complexes by isonitriles.

The reactions of isonitriles with a variety of metalloboryl and metalloborylene species are shown to yield a range of products, none of which display the typical Lewis acid-base reactivity previously observed between such metal complexes and other Lewis bases. Insertions of one, or several, isonitriles into the metal-boron bond give cyclic and spiro compounds with bonding controlled by the electron count at the metal and in the ring.

Metathesis reactions of a manganese borylene complex with polar heteroatom-carbon double bonds: a pathway to previously inaccessible carbene complexes.

A comprehensive study has been carried out to investigate the metathesis reactivity of the terminal alkylborylene complex [(η(5)-C5H5)(OC)2Mn═B(tBu)] (1). Its reactions with 3,3',5,5'-tetrakis(trifluoromethyl)benzophenone, 4,4'-dimethylbenzophenone, 2-adamantanone, 4,4'-bis(diethylamino)benzophenone, and 1,2-diphenylcyclopropen-3-one afforded the metathesis products [(η(5)-C5H5)(OC)2Mn═CR2] (R = C6H3-3,5-(CF3)23a, C6H4-4-Me 3b, C6H4-4-NEt23d; CR2 = adamantylidene 3c, cyclo-C3Ph23e). The cycloaddition intermediates were detected by NMR spectroscopy from reactions involving ketones with more electron-withdrawing substituents. The reaction of 1 with dicyclohexylcarbodiimide (DCC) only proceeds to form the cycloaddition product [(η(5)-C5H5)(OC)2Mn{κ(2)-C,B-C(═NCy)N(Cy)B(tBu)}] (4), which upon warming, rearranges to afford complex [(η(5)-C5H5)(OC)2Mn{CN(Cy)B(tBu)CN(Cy)}] (5). The reaction of 1 with triphenylphosphine sulfide SPPh3 also yields the metathesis product [(η(5)-C5H5)(OC)2Mn(PPh3)] via an intermediate which is likely to be a η(2)-thioboryl complex [(η(5)-C5H5)(OC)2Mn{(η(2)-SB(tBu)}] (6). Similar reactions have been studied using an iron borylene complex [(Me3P)(OC)3Fe═B(Dur)] (Dur = 2,3,5,6-tetramethylphenyl, 9). Extensive computational studies have been also carried out to gain mechanistic insights in these reactions, which provided reaction pathways that fit well with the experimental data.

Quaternizing diboranes(4): highly divergent outcomes and an inorganic Wagner-Meerwein rearrangement.

Apart from a few compounds under heavy use in organic chemistry, diboranes are relatively exotic and poorly understood. Recently, interest in these molecules has intensified with the advent of so-called "sp(2)-sp(3)" diboranes which exhibit useful reactivity toward organic substrates. In our hands, addition of Lewis bases to dihalodiorganyl diboranes(4) has previously shown some very surprising reactivity, including a substituent exchange between the boron atoms, and diboranes in which halide atoms bridge the B-B bond. Herein we have expanded the range of diborane(4) and Lewis base reaction partners, in the process uncovering three new reactivity patterns as well as some cases where mixtures are obtained. Trends are established for the preferential formation of certain products which rationalize the results based on electronic and steric effects. The substituent exchange, clearly based on an inorganic version of the well-known Wagner-Meerwein rearrangement, was also found to be an equilibrium reaction with the halide-bridged Lewis adducts on the other side.