The hydroboration of benzo-, tolu-, p-methoxybenzo-, or pi-rather stable. Compound 2a is converted to benzylbis( 1,5-cyvalonitriles with bis(9-borabicyclo[3.3.ljnonane) (9H-9-BBN)* clooctanediylbory1)amine (3a) only after 48 h at reflux in meaffords the corresponding 9-aldimino-9-borabicyclo[3.3.1]-sitylene. Compounds 1 a -d are stable towards strong N-bases.nonanes (la-d) in about 80% yields. Compounds la and Id Compound Id reacts with EtzO . BF3 to form 9-fluoro-9-borareadily react with an excess of (9H-9-BBN)2 to form the 1: 1 bicyclo[3.3.l]nonane (4) and the 1: 1 cyclic adduct (5d) of (2,2-adducts 2 a or 2 d, respectively, containing a NBzH four-mem-dimethylpropy1ideneamino)difluoroborane (6 d) and 1 d. bered ring (X-ray structure of 2a). The adducts 2 are thermallyThe direct hydroboration of nitriles with hydrodiorganoboranes was studied some twenty yearsThe aldimino-diorganoboranes I1 formed are, in contrast to the ketimino-diorganoboranes I which are often monomeric[31, generally reported to be unstable and to dimerize to compounds of the type I1 with tetracoordinated boron atoms[41. Presumably this dimeric structure prevents the further hydroboration by an excess of >B -H borane reagent. Similar dimers are formed when nitriles are heated with an excess of triethylborane at 200 0C[51. In this reaction the diethylhydroborane generated as an intermediate is thought to be the reactive species. We have in the past frequently observed that the 9-borabicyclo[3.3.1]non-9-yl group (9-BBN) displays special steric properties which alter the stabilities of certain labile intermediates and thus the course of the reactions[61. Therefore, we have reinvestigated the hydroboration of nitriles by using (9H-9-BBN)2 in the hope of isolating monomeric aldiminoboranes which are stable enough to allow a chemistry other than simple dimerization. In this paper we report on our preliminary results.
R' R'\ /
I1
Results and DiscussionTo avoid the complicating effects of the expected iminoenamino equilibria[',*] only some derivatives of benzonitrile and pivalonitrile which have no a-hydrogen atoms, have been chosen for this study. The reaction of an excess of these nitriles with (9H-9-BBN)2 at 60-110°C furnishes the corresponding products in high yields, as revealed by their "B-NMR spectra exhibiting signals at about 6 = 37. The colorless monomeric aldiminoboranes 1 a -d are generated from benzo-, tolu-, p-methoxybenzo-, and pivalonitrile, respectively. Compounds 1 a -d have proved to be extremely air-sensitive. Even the slow passage of dry and oxygen-free argon gas through a vessel containing them causes partial decomposition, as indicated by rapid yellow coloration[91. The mass spectra of the products show molecular ions corresponding to the monomeric species. The strong absorption bands at 1800 for the aryl-and 1840 cm-' for the tert-butylsubstituted products indicate considerable lone-pair overlap from the nitrogen to the boron atom resulting in a heterocumulene limiting structure. Another indication for the existence of such a s...
