Snapshots of magnesium-centred diborane heterolysis by an outer sphere S_N2 process

Abstract

Reactions of a magnesium diboranate as a source of [Bpin]− anions are initiated by ‘outer sphere’ attack of CN bonded substrates.

Reactivity of a magnesium diboranate with organic nitriles

“…[53][54][55] Although species such as VI are readily envisaged as sources of the {Bpin}  anion, compound V has, in its own right, been shown to act as a nucleophilic boron surrogate and as a conveniently generated reagent for nucleophilic B-C and B-B bond formation when reacted with suitable C-or B-centred electrophiles. 53,[56][57][58][59] Reactions of V with triphenylphosphine oxide and a variety of organic imines have implied that the mode of B-B heterolysis can encompass significant mechanistic diversity invoking either an 'inner'…”

“…or 'outer sphere' approach of the Lewis basic reagent. 55,56 Addition of triphenylphosphine oxide to compound V provided a Ph 3 P=O-coordinated magnesium boryl species, VII, which is otherwise analogous to compound VI (Scheme 2(a)). Although computational (DFT) assessment of this reaction implied that the B-B' cleavage was induced by inner sphere coordination of the P=O bond to magnesium, 55 reactions of V with more sterically encumbered imine reagents provided the N-B bonded organomagnesium compounds as the initially formed kinetic products (Scheme 2(b)).…”

Diborane heterolysis and P(v) reduction by Ph3PO coordination to magnesium

“…This deduction, therefore, contrasts with the outer sphere attack at boron implicated in reactions of 9 with more sterically encumbered imine reagents (Scheme 1). 50 In both cases, however, B-B bond cleavage is facile, indicating that the mode of [pinBB(n-Bu)pin] À heterolysis is subtly dependent on the degree of kinetic discrimination provided by the co-reagent. We are, thus, continuing to examine the implications of these observations for a wider range of substrates and element centres.…”

“…[47][48][49] Although this is consistent with the generation of an intermediate boryl species, we have also observed that reaction of 9 with imines provides the N-B bonded organomagnesium compounds rather than the product of nucleophilic C-borylation (Scheme 1). 50 In this case, B-B cleavage does not result in a terminal magnesium boryl species but occurs through attack of the imine nitrogen at the three-coordinate boron of the diboranate anion. 50 In contrast, treatment of 9 with 4-dimethylaminopyridine does provide the Mg-B bonded species, [(BDI)Mg(Bpin)DMAP] (10), 45,46 raising questions about the generality of such an 'outer sphere' process.…”

“…50 In this case, B-B cleavage does not result in a terminal magnesium boryl species but occurs through attack of the imine nitrogen at the three-coordinate boron of the diboranate anion. 50 In contrast, treatment of 9 with 4-dimethylaminopyridine does provide the Mg-B bonded species, [(BDI)Mg(Bpin)DMAP] (10), 45,46 raising questions about the generality of such an 'outer sphere' process. In this contribution, we extend our study to the reaction of compound 9 with triphenylphosphine oxide and address the mechanism of the resultant B-B heterolysis.…”