Synthesis and Ring Strain of a Benzoborirene‐
<i>N</i>
‐Heterocyclic Carbene Adduct
作者:Jennifer Hahn、Constanze Keck、Cäcilia Maichle‐Mössmer、Esther von Grotthuss、Paul Niklas Ruth、Alexander Paesch、Dietmar Stalke、Holger F. Bettinger
DOI:10.1002/chem.201804629
日期:2018.12.12
The reduction of an N‐heterocyclic carbene (1,3‐diisopropyl‐4,5‐dimethylimidazolin‐2‐ylidene, IiPr ) adduct of dichloro(ortho‐bromophenyl)borane by tert‐butyl lithium at low temperature yields the IiPr adduct A of parent benzoborirene, a highly strained boron‐containing bicyclic compound. A is unstable at room temperature and dimerizes at low temperature to the bis‐IiPr adduct of 9,10‐dihydro‐9,10‐diboraanthracene
9,10-Dihydro-9,10-diboraanthracene ([1](n)) and its SMe2 adduct 1(SMe2)(2) are readily reduced with lithium in THF to the dianionic 9,10-diborataanthracene Li-2[1]. An X-ray crystal structure analysis of (Li(thf)(2))(2)[1] revealed monomeric inverse sandwich complexes, each of them containing two Li(thf)(2) moieties coordinated to both sides of the central B2C4 ring. Compared to 9,10-dimethyl-9,10-dihydro-9,10-diboraanthracene, the four B-C-Ar bonds of (Li(thf)(2))(2)[1] are shorter by 0.046(4) angstrom, thereby indicating an increased degree of B=C-Ar double-bond character. Consequently,(Li(thf)2)2[1] reacts with 4,4' -dimethylbenzophenone as a B=C-Ar-C-Ar=B diene and undergoes a[4+2] cycloaddition reaction with formation of a bicyclic product. In contrast, tert-butylacetylene reacts with (Li(thf)(2))(2)[1]under formal 1,4-addition of its methinic C H group instead of its C C triple bond to the two boron atoms.