Species Containing an Allene-Type NBC Sequence

1885 cm reaction with CH3OH, reaction with H2O to give R(HO)B-NHR  

Compounds containing a B=C double bond have been prepared only recently [24]. In all of these compounds the electron deficiency at the boron atom has to be reduced by intramolecular K bonding. This can be done in a nonclassical arrangement through a three-membered borirane ring [24 to 27] in most cases, however, a nitrogen atom serves as the n donor. For (CH3)2N-B=CH2, see p. 225. 

The bond lengths in the linear allene-type central NBC moiety are r(BN) =136.3 and r(BC) = 139.1 pm [28]. Benzaldehyde reacts by [2 -1- 2] cycloaddition to the B=C double bond to give the 

83 was prepared by dehydrohalogenation of fluoren-9-yl(2,2,6,6-tetramethylpiperidin-1-yl)haloboranes (halogen = F, Cl, Br) with (CH3)3Si-NM(C4Hg-f) (M = Li, Na) in toluene and was obtained as a 1 1 solvate solvent-free crystals were formed upon recrystallization from CH2Cl2/pentane [30, 31]. 83 was also postulated as an intermediate in the dehydrofluorination of fluoren-9-yl(fluoro)-(2,2,6,6-tetramethylpiperidin-1-yl)borane [32]. 83, melting point 164 to 166°C, has been characterized by mass spectrometry and NMR ( H, B, C) data 6 B = 59.2 ppm with hi/2 = 700 Hz. In the IR spectrum, Vas(NBC) is observed at 1717 cm with high frequency satellites up to 1780 cm T The chemical shift of the C(9) atom of the fluorene moiety is 6 C = 83.2 ppm in accordance with other cumulene structures [30]. 

The amino(methylene)borane 84 has been postulated as an intermediate in the liquid-phase pyrolysis of [diphenyl(trimethylsilanyl)methyl]fluoro-(2,2,6,6-tetramethylpiperidin-1-yl)-borane in boiling hexadecane at 282°C. If this pyrolysis is performed in the presence of benzophenone, tetraphenylethene is obtained, which could originate from a Wittig-type reaction of 84 and (CeH5)2CO [35]. 

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