Nonclassical 1,2‐Diboretanes and 1,2‐Diborolanes

Homoaromaticity

“…Berndt et al demonstrated that 1,2-diboretanes with electron-deficient boron atoms prefer the nonclassical 125a to the classical 125b structure. 154 The nonclassical structure is characterized by two 3c/2e interactions, one for the hydrogen-bridged B-H-B and one for the homoaromatic B-C-B "threemembered ring" (isoelectronic with the homocyclopropenium cation). Berndt et al prepared three derivatives ( 126 X-ray structure on 126a and compared this and the experimental chemical shifts of 126a-c with the calculated structural and NMR parameters obtained for 125a and 125b.…”

Syntheses and Reactions of a Stable 1,2‐Dichloro‐1,2‐diborolane and Aromatic Tetraboranes

“…This result explains the failure of earlier efforts to prepare 1a lacking any substituents. [2] So far, reaction of metals with 1,2-dichloro-1,2-diboranes(4) having appropriate substituents, like sterically demanding amino or tert-butyl substituents, have led to the elimination of both chlorine atoms and generation of folded or tetrahedral tetraboranes (4). [28,29] The reductive dimerization of 1b to cis-11 is the first example where only one chlorine atom of each 1,2-dichloro-1,2-diborane(4) is eliminated.…”

“…[4] Attempts to prepare 1,2-dichloro-1,2-diborolane (1a) by reaction of the 1,2-diamino-1,2-diborolane 2a with BCl 3 were unsuccessful, [2] as were efforts to obtain 2b by reaction of 1,2-dichloro-1,2-bis(dimethylamino)diborane(4) (3) with 1,3-dilithio-2-methylenepropane (Scheme 1). [3] In this paper we describe the synthesis of the stable 1,2-dichloro-1,2-diborolane 1b (Scheme 3) via the new 1,2-diamino-1,2-diborolane 2c (Scheme 2).…”

Ab Initio Study of Rearrangements on the (CH)2(BR)2, RH, and NH2 Potential Energy Surfaces

“…1,2 Several of these structures have been characterized experimentally by using appropriate substituents in place of hydrogen. [3][4][5][6][7][8][9] The nonclassical bonding in these systems has attracted considerable experimental [10][11][12][13][14][15][16][17][18][19] and theoretical attention. [20][21][22][23][24][25] While the formal electron count of (CH) 2 (BH) 2 places it in the "closo" category (n + 1 bonding electron pairs, where n ) number of vertexes) and thus a member of the familiar (CH) 2 (BH) n † Dedicated to Professor Paul von Rague ´Schleyer on the occasion of his 70th birthday.…”

“…The (CH) 2 (BH) 2 potential energy surface is very rich in the number of structural types. On one hand, there are classical structures (Scheme ) such as 1,2-diboretene ( 2 ) and diborylacetylene ( 9 ), while on the other hand, there are nonclassical structures such as 1,2-diboretane-3-ylidene ( 3 ). , Several of these structures have been characterized experimentally by using appropriate substituents in place of hydrogen. − The nonclassical bonding in these systems has attracted considerable experimental − and theoretical attention. − While the formal electron count of (CH) 2 (BH) 2 places it in the “closo” category ( n + 1 bonding electron pairs, where n = number of vertexes) and thus a member of the familiar (CH) 2 (BH) n carborane family, the potential energy surface bears no similarity to other carboranes. Instead, the nonclassical structures are characterized by σ- and π-three-center-two-electron (π-3c-2e) bonds. , The π-3c-2e bond is located on a central Hückel-aromatic three-membered ring.…”