Diverse Reactivity of Dienes with Pentaphenylborole and 1‐Phenyl‐2,3,4,5‐Tetramethylborole Dimer

Abstract

The reactions of a monomeric borole and a dimeric borole with 2,3‐dimethyl‐1,3‐butadiene and 1,3‐cyclohexadiene were investigated. The monomeric borole reacted at ambient temperature whereas heat was required to crack the dimer to form the monomer and induce reactivity. 2,3‐Dimethyl‐1,3‐butadiene reacts to give diverse products resulting from a cycloaddition process with the B−C moiety of the boroles acting as a dienophile, followed by rearrangements to furnish bicyclic species. For 1,3‐cyclohexadiene, a [4+2] process is observed in which 1,3‐cyclohexadiene serves as the dienophile and the boroles as the diene partner. The experimental results are corroborated with mechanistic theoretical calculations that indicate boroles can serve as either a diene or dienophile in cycloaddition reactions with dienes.

Boron and Silicon-Substituted 1,3-Dienes and Dienophiles and Their Use in Diels-Alder Reactions

“…Martin and co-workers continued their work and reported Diels-Alder reactions of pentaphenylborole ( 42 ) and the 1-phenyl-2,3,4,5-tetramethylborole dimer with 2,3-dimethyl-1,3-butadiene ( 52 ) and 1,3-cyclohexadiene ( 55 ) ( Scheme 16 ) [ 18 ]. Pentaphenylborole ( 42 ) served as the dienophile in reactions with 2,3-dimethylbutadiene ( 52 ) whereas it functioned as the diene in [4+2] cycloaddition with cyclohexadiene ( 55 ).…”

2,5-Bis-trimethylsilyl substituted boroles

“…Isoelectronic to the elusive cyclopentadienyl cation, with four π-electrons in cyclic conjugation via the empty p-orbital of boron, these systems exhibit a weakly anti-aromatic character. [1][2][3][4][5][6] Free boroles are very reactive species that add dihydrogen or silane Si-H bonds across the π-system, 7-10 react as potent Lewis-acids also towards weak bases, 2,[11][12][13] engage in various Diels-Alder and ring-expansion reactions [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] or readily accept two electrons to form dianionic 6π-electron systems. [32][33][34][35] Borolediides are isoelectronic to cyclopentadienyl anions and have thus been used and studied as ligands in transition metal coordination chemistry.…”

“…These dimers can provide a source of monomeric borole synthons upon thermal treatment. [49][50][51] With free boroles being so reactive, to date, their synthesis and successful isolation is limited to relatively few substituents around the central C 4 B cycle. 52 Most reports on free borole chemistry discuss pentaaryl boroles 34,53,54 and particularly (PhC) 4 BAr for which reliable synthetic protocols exist.…”

Dimeric boroles: effective sources of monomeric boroles for heterocycle synthesis

“…In contrast to cyclopentadiene, the monomers are not isolable, however, reactivity studies suggest that the retro Diels-Alder process can be thermally induced. 3,13 Despite these dimers being known since 1985, the only studies have been heating the dimers in the presence of metal precursors to access h 5 -metal complexes 14 or examining their Diels-Alder reactivity. 3,13 If dimeric boroles could be utilized as reagents for ring expansion reactions, it would circumvent the requirement of bulky substituted monomeric boroles enabling access to a diverse library of products.…”