O,N,B-Containing eight-membered heterocycles by ring expansion of boroles with nitrones

Abstract

By a ring expansion reaction of boroles with nitrones a straightforward route to eight-membered O,N,B-containing ring systems was developed, which present a new structural motif in heterocyclic chemistry. Formally, these compounds can be considered as BN-analogues of eight-membered ring ethers containing three double bonds. The X-ray crystallographic data of a series of these oxazaborocine derivatives show that they adopt a boat conformation in the solid state.

Expedient Synthesis of 1,2‐Thiaborines by Means of Sulfur Insertion into Boroles

“…Developing the chemistry of 1,2‐thiaborines could provide access to molecules that exhibit properties similar to thiophene, yet exhibit a profile akin to that of benzene. In contemplating the design of a new route to 1,2‐thiaborines, we were attracted to boroles as they have been shown to be effective reagents for the generation of unsaturated boron ring systems of six to eight atoms . The synthesis of 1,2‐thiaborines by means of this route could provide access to a monocyclic species lacking an amino group on boron.…”

“…In contemplating the design of an ew route to 1,2-thiaborines, we were attracted to boroles as they have been shownt obeeffective reagents for the generation of unsaturated boron ring systems of six to eight atoms. [10,18,19,[25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] The synthesis of 1,2-thiaborines by means of this route couldp rovide access to am onocyclic species lacking an amino group on boron.…”

Electronic Push–Pull Modulation by Peripheral Substituents in Pentaaryl Boroles

“…Being an isolable antiaromatic compound class, boroles have received growing interest in recent years . Due to their peculiar electronic situation, they feature intriguing reactivity including E−H bond splitting, cycloadditions and ring expansions, as well as Lewis acidity enabling adduct formation even with weak Lewis bases such as CO …”

“…[9] Being an isolable antiaromatic compound class, boroles have received growing interesti nr ecent years. [6,9] Due to their peculiar electronic situation, they feature intriguing reactivity includingE ÀHb ond splitting, [4,7,[10][11][12] cycloadditions and ring expansions, [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] as well as Lewis acidity enabling adduct formation even with weakL ewis bases such as CO. [30] The first report on free borole by Eisch et al in 1969 applied five phenyl substituents on the C 4 B-ring, which were sufficiently bulky to suppress Diels-Alder dimerization and other rear-rangements. [2,5,31,32] Such behavior is documented for less bulky derivatives such as (MeC) 4 BPh.…”

Ring expansion reactions of anti-aromatic boroles: a promising synthetic avenue to unsaturated boracycles

“…The photolysis of [PPh] 5 to generate phosphinidene synthons was exploited for the 1,1--insertion of a PPh fragment into the borole rings of 1 and 28 to furnish 1,2--phosphaborines 29 and 30 (Fig 11). 59 The mechanism to form these species was rationalized via oxygen coordination (Int20), followed by the formation of bicyclic intermediate Int21 via intramolecular attack of the butadiene to the carbon of the nitrone and a subsequent B--C bond cleavage. Collectively, all of the metrical parameters indicate significant delocalization within the conjugated π--system and the 1,2-phosphaborines represent a new type of heteroarene.…”