Theoretical Study of the BF₃-Promoted Rearrangement of Oxiranyl N-Methyliminodiacetic Acid Boronates

Abstract

The mechanism of the rearrangement of oxiranyl N-methyliminodiacetyl (MIDA) boronates in dicholoromethane has been extensively investigated with density functional theory. Several reaction pathways were examined. Our results revealed that the most-favorable mechanisms for the BF-promoted rearrangement of 2-phenyl oxiranyl MIDA boronate (1) and 1-phenyl oxiranyl MIDA boronate (24) comprise two steps: ring opening of the epoxide to a carbocation intermediate followed by migration of a MIDA-boryl group (for the reaction of 1) and hydrogen (for the reaction of 24), to give the same BF-coordinated α-boryl aldehyde in both cases. The first step of the ring opening of the epoxide is the rate-determining step of these reactions. In the rearrangement step for the reaction of 1, the MIDA-boryl group migrates easily, probably because of its electron-rich sp-hybridized boron center. For 24, the most-favorable pathway involves a rare boryl-substituted carbocation. The course of these reactions is mainly controlled by electronic effects, although steric effects are also significant. The higher energy barrier calculated for the unsubstituted oxiranyl MIDA boronate (31) explains the lack of reactivity in the studied BF-promoted rearrangement.

Electrophilic Fluorination of Alkenes via Bora‐Wagner–Meerwein Rearrangement. Access to β‐Difluoroalkyl Boronates

“…From Int4 , the bora‐Wagner–Meerwein rearrangement occurs via TS2 , with a barrier of 12.4 kcal mol −1 . Similarly to the studies of the groups of Yudin [9] and Pellegrinet [10] the migration occurs via a bora‐cyclopropane type structure ( TS2 ). Formation of a three‐membered ring‐shaped TS including electron deficient carbon centers represents a clear analogy to the Wagner–Meerwein rearrangement involving aryl/alkyl/H groups [19] .…”

“…Mechanistic studies by Yudin and co‐workers [9] revealed that the high migration aptitude of MIDA boronate in this [1,2] process is due to the hemilabile bonding of nitrogen to boron in the Bmida group. Interestingly, this B‐N hemilability lends a migration aptitude to Bmida group, which is in the same magnitude as the ability of H/alkyl/aryl groups to undergo [1,2] migration to electron deficient carbon centers [10] . Electrophilic fluorination reactions are also known to proceed via carbocations or electron‐deficient carbon centers.…”

Electrophilic Fluorination of Alkenes via Bora‐Wagner–Meerwein Rearrangement. Access to β‐Difluoroalkyl Boronates

“…From Int4, the bora-Wagner-Meerwein rearrangement occurs via TS2, with a barrier of 12.4 kcal mol À1 . Similarly to the studies of the groups of Yudin [9] and Pellegrinet [10] the migration occurs via a bora-cyclopropane type structure (TS2). Formation of a three-membered ring-shaped TS including electron deficient carbon centers represents a clear analogy to the Wagner-Meerwein rearrangement involving aryl/alkyl/H groups.…”

Substituent Effects of Tetracoordinate Boron in Organic Synthesis

“…On the other hand, stabilization of the positive charge by 2‐alkyl or 2‐aryl groups seems to be crucial for the rearrangement because the absence of a β‐substituent induces no rearrangement. A theoretical study suggested the formation of a carbocation by full ring‐opening from 2 in the case of 2‐aryl substituted substrates [8] . After that report, Li and Burke reported a stereoselective rearrangement of oxiranyl boronate derivatives having a chiral pinene‐derived iminodiacetic acid ligand [9] …”

“…A theoretical study suggested the formation of a carbocation by full ring-opening from 2 in the case of 2-aryl substituted substrates. [8] After that report, Li and Burke reported a stereoselective rearrangement of oxiranyl boronate derivatives having a chiral pinene-derived iminodiacetic acid ligand. [9] In 2015, Yudin and co-workers reported rhodium-catalyzed intramolecular CÀ H amination of MIDA boryl sulfamate esters 4 (Scheme 2).…”