Reactivity of the dimesityl-1,8-naphthalenediylborate anion: isolation of the borataalkene isomer and synthesis of 1,8-diborylnaphthalenes

The anionic boron peri-bridged naphthalene derivative, namely dimesityl-1,8-naphthalenediylborate ( 1), undergoes a hydrolysis reaction to afford dimesityl-1-naphthylborane (2) whose structure has been determined. Upon standing at room temperature in toluene for an extended period of time, 1 undergoes a ring expansion reaction to afford 8,10,11a-trimethyl-7-mesityl-11aH-7-boratabenzo[de] anthracene (3). As shown by its crystal structure, compound 3 constitutes a rare example of a borataalkene and features a carbon-boron double bond of 1.475(6) Angstrom incorporated in a conjugated hexa-1-boratatriene system. The reaction of 1 with 9-chloro-9-borafluorene and 5-bromo-10,11-dihydrodibenzo[ b, f] borepin results in the formation of diboranes 4 and 5 which bear two different boryl moieties at the peri-positions of naphthalene. These diboranes have been characterized by multinuclear NMR spectroscopy and X-ray single crystal analysis. The boron center of the borafluorenyl moiety is pi-coordinated to the ipso-carbon of a mesityl group with which it forms a contact of 2.730(3) Angstrom. The cyclic voltammogram of 2 in THF shows a quasi-reversible reduction wave at E-1/2 - 2.41 V (vs. Fc/Fc(+)) corresponding to the formation of the radical anion. In the case of diboranes 4, 5 and 1-(dimesitylboryl)-8-(diphenylboryl) naphthalene (6), two distinct waves are observed at E-1/2 - 2.14 and - 2.56 V for 4, E-1/2 - 2.26 and - 2.78 V for 5, and E-1/2 - 2.41 and - 2.84 V for 6. The first reduction wave most likely indicates the formation of a radical anion in which the unpaired electron is sigma-delocalized over the two boron centers.