Origins of observed reactivity and specificity in the addition of B₂Cl₄and analogues to unsaturated compounds

Abstract

In 1954 Schlesinger and co-workers observed the direct reaction of diboron tetrachloride with simple organic compounds under mild conditions, the 1,2 addition product being formed with either ethylene or acetylene. In the following 25 years a series of addition reactions to simple alkenes, alkynes and dienes was demonstrated. B2F4 was shown to react in similar manner, albeit under more forcing conditions. Crucially, it was demonstrated that the addition to (E)- or (Z)-but-2-ene occurred with cis-stereospecificity. Only sporadic interest was shown in this field thereafter until catalysed addition reactions of diboron reagents were realized. Encouraged by this revival of interest through the discovery of transition-metal and nucleophilic catalysis of diboryl additions, DFT analysis of uncatalysed additions of B2X4 has been carried out and interpreted. This includes the relative reactivity of several B-B reagents with ethene, and that of B2Cl4vs. B2F4 additions, including benzene, naphthalene and C60 as reactants. This allows the analysis of relative reactivity vis-à-vis substitution on boron, and also direct comparison with hydroboration by HBCl2. [4 + 2] Addition of diboron reagents to dienes with B-B cleavage competes with direct [2 + 2] addition, favourably so for B2F4. The computational results demonstrate that the stereospecific addition to isomeric but-2-enes is a rare concerted [2σs + 2πs] process.

Mechanistic study on the reaction of pinB-BMes2with alkynes based on experimental investigation and DFT calculations: gradual change of mechanism depending on the substituent

“…It should be noted that the four-centered structures of the transition states in Fig. 2 are similar to those calculated for direct (non-catalyzed) haloboration 16 and diboration 5,12 of alkenes and alkynes. Confirmed by intrinsic reaction coordinate (IRC) calculations, the migration of the pinB group to an alkyne carbon proceeds in a concerted fashion, which is consistent with the first order kinetics as was observed in the experiments.…”

“…15 The calculated energy barriers are over 26.0 kcal/mol and the calculated reaction energies gained are more than 39.0 kcal/mol, showing that all the reactions are irreversible and the regioselectivity of the reaction is kinetically controlled. It should be noted that the four-centered structures of the transition states in Figure 2 are similar to those calculated for direct (non-catalyzed) haloboration 16 and diboration 5,12 of alkenes and alkynes.…”

“…The first example was reported for the reaction of acetylene with B 2 F 4 or B 2 Cl 4 to produce bis(dihaloboryl)ethylene in 1959 ( Scheme 1 ), 4 whose four-centered transition state was recently proposed by DFT calculations. 5 Two seminal reactions, alkoxide-catalyzed diboration of alkenes 6 and phosphine-catalyzed β-boration of electron-deficient alkenes, 7 induced the next evolution of the diboration of alkynes using a Lewis base as a catalyst. 8,9 These reactions involve the formation of sp 2 –sp 3 intermediates via the coordination of the Lewis base to the diborane(4) reagent.…”

Regioselective and Stereospecific Dehydrogenative Annulation Utilizing Silylium Ion-Activated Alkenes

“…Alternatively, the benzosilacyclobutenium ion 8 undergoes a [π2s + σ2a] reaction with the alkene to retain the stereochemistry (path b). 19 A partial positive charge appears at the βposition in the transition state of the [2 + 2] reaction, which causes the regioselectivity of cyclic products 4 and 6, similarly to path a. The four-membered ring intermediate 9 generated from deprotonation of 8 can be ruled out because the reaction of 9 with alkenes affords 1,2,3,4-tetrahydro-1-silanaphthalene derivatives via an o-silaquinone methide species in the case of the reaction using 1 (Scheme S1, Supporting Information).…”

Metallomimetic Chemistry of Boron

“…B2F4, B2Cl4 and B2Br4 were also found to spontaneously add to ethylene. [179][180][181][182] Since then, the diboration of many unsaturated bonds using diboranes has been established in the presence or absence of catalysts. 183,184 In recent years, departing from the chemistry of classical diborane(4) systems such as B2pin2, B2cat2, B2(OMe)4 and B2(NMe2)4, unusual complexes featuring reactive B-B single and multiple bonds have been the object of considerable interest.…”