Pericyclic Reactions between Aminoboranes R₂BNR₂‘ and Alkenes:  [4+2] vs [2+2] Transition States

Abstract

Computational comparison of the [2+2] and [4+2] reactions between aminoboranes R2BNR‘2 and alkenes CH2CH(R‘ ‘) shows that the latter are strongly preferred. The preference arises from the [4+2] reactions exhibiting lower reaction barriers than do the [2+2] reactions; the products formed are of nearly equal stability. This behavior mimics that of reactions between alkenes and supports viewing aminoboranes, particularly (F3C)2BNMe2, as acting like polar alkenes.

Hydrogen and Amine Activation by a Frustrated Lewis Pair of a Bulky N‐Heterocyclic Carbene and B(C6F5)3

“…For example, the unsymmetrical aminoborane 7 has six separate 19 F signals at 25 °C. This is consistent with a significant overlap of the orbital occupied by the nitrogen lone pair and the empty boron p Z orbital 32. 33…”

“…This is consistent with a significant overlap of the orbital occupied by the nitrogen lone pair and the empty boron p Z orbital. [32,33] The presence and necessity of the carbene in the formation of 6-8 suggests that these reactions are catalytic in carbene, and indeed formation is readily accomplished with the use of 2-3 mol % of ItBu. In these cases, the quantitative formation of the aminoboranes [33,34] is complete within 2 h at 25 8C.…”

Conformational and Substituent Effects on The Rehybridization of Boron in β‐Substituted Ethylboranes

“…In order to check for the role of the substituent (amino) effect on the reactivity of an organoboron compound, which resembles the model compounds studied herein, we used the reaction example depicted in Figure 5. In the xanthate addition to alkenes, the stability of the intermediate adduct radical plays a pivotal role in the effectiveness of the reaction; whereas a stable radical is formed from the reaction of vinyl pinacolato boronate (Bpin) with an initiator, resulting in low‐yielding due to the side‐formation of oligomers, the reaction with vinyl boronic acid MIDA ester [B(MIDA)] is more successful, since the intermediate radical is not stabilized by delocalization into the empty p B orbital (Figure 5a) [14] . This delocalization is inhibited by a transannular interaction n N → p B in the B(MIDA) radical, which can be confirmed by NBO analysis and comparing the s ‐character of boron in the Bpin and B(MIDA) analog radicals (Figure 5b).…”

“…[13] Boron rehybridization is crucial in determining the stability of radical intermediates and, consequently, in the yield of a specific reaction to produce terminal or internal boronates. [14] This example demonstrates that increased reaction yields and avoidance of oligomer production involve the modulation of boron hybridization by vicinal groups; specifically, for this reaction, hybridization was tuned using pinacolato boronates (with a free p-orbital) or diethanolaminoboronates (with the boron p-orbital partially fulfilled), as instances. [14] The conformational analysis of β-substituted ethylboranes (Figure 1) can be used to search for the vicinal substituent effects on the ability of boron to rehybridize, since throughspace orbital interactions with boron and the substituent can shift the conformational equilibrium and change the s character at the boron atom.…”

“…Aminoboranes behave like polar alkenes in pericyclic reactions and the result is an sp 2 – sp 3 rehybridization of boron [13] . Boron rehybridization is crucial in determining the stability of radical intermediates and, consequently, in the yield of a specific reaction to produce terminal or internal boronates [14] . This example demonstrates that increased reaction yields and avoidance of oligomer production involve the modulation of boron hybridization by vicinal groups; specifically, for this reaction, hybridization was tuned using pinacolato boronates (with a free p ‐orbital) or diethanolaminoboronates (with the boron p ‐orbital partially fulfilled), as instances [14] …”