Nichtklassische 1,2‐Diboretane und 1,2‐Diborolane

Abstract

Experiment und Theorie ergeben übereinstimmend, daß Vier‐ und Fünfringe mit zwei benachbarten Boratomen – wie entsprechende Dreiringe – die nichtklassischen Formen 1 bzw. 2 mit B‐H‐B‐Brücke bevorzugen, wenn der Elektronenmangel an den Boratomen nicht durch Donor‐Substituenten herabgesetzt ist.

1,2‐Diboracyclopentanes without Strong Donor Substituents: Synthesis, Reactions, and Computational Analysis

“…Reaction of 1a with n-butyllithium does not lead to deprotonation product 4a but to addition products 8a·Li(Et 2 O) 3 or 8b·Li(Et 2 O) depending on the workup conditions. B-B moiety -with subsequent rearrangement -is faster than its action as a base, in spite of the strong thermodynamic stabilization of anions of type 4.…”

“…Values included in Scheme 1 correspond to the MP2/6-31G* level. [3] Isomer 7u with two BHB bridges is the global minimum, and it is 18.3 kcal mol -1 lower in energy than 1u. In order to elucidate the mechanism leading to main products 5a and 6a of the thermolysis of 1a and to learn the reason why derivatives of 1u do not rearrange into derivatives of 3u in a similar way as derivatives of A and D into those of B and E, we looked for transition states leading from 1u to 2u and 3u as well as from 2u and 3u to 7u, respectively.…”

“…The structures were solved by direct methods and refined against all F 2 data by using full-matrix least-squares and difference Fourier techniques (SHELX programs [24,25] ). Most hydrogen atoms were kept riding on calculated positions with isotropic displacement factors taken as 1.2 times (1.5 times for CH 3 ) the U eq value of the corresponding C atom. The hydrogen atoms at the central rings were located and refined with individual isotropic displacement factors.…”

“…Crystallization at -30°C gave 7b 30°C (1a, 5a, 7a,b). Single crystals of 8a·Li(Et 2 O) 3 were grown from diethyl ether at -30°C. These crystals were investigated on imaging plate systems (IPDS Stoe) by using graphite monochromated Mo-K α radiation at 193 K. The space groups were determined from the systematic absences and intensity statistics; no absorption corrections were applied.…”

“…The latter have been computed to be preferred energetically in case of unsubstituted prototype 1u (Scheme 1): monohomoaromatic 2u and bishomoaromatic 3u are lower in energy than 1u by 2.5 and 18.9 kcal mol -1 , respectively, at the MP2/6-31G* level of theory. [3] Recent computations revealed 3u to be the strongest neutral homoaromatic ever discussed, 50.5 kcal mol -1 lower in energy than 3u*, which does not feature a threecenter-two-electron BCB bond. [4] Formation of derivatives of isoelectronic species B and E by isomerization of the corresponding derivatives of A and D, respectively, have been observed.…”