BN‐Achtringe (RBNR') 4 aus BN‐Vierringen (RBNtBu) 2 durch NtBu/NR'‐Austausch

Eight‐Membered BN Rings (RBNR') 4 from Four‐Membered BN Rings (RBNtBu) 2 by an Exchange NtBu/NR' Reactions of the four‐membered BN ring compounds (RBNtBu) 2 ( 1a – e : R = Me, Et, iPr, iBu, Pr) with azides R'N 3 (R' = PhCH 2 , Pr, iBu, Ph) and with the nitrone PhHC=N(Me)=O are studied. The five‐membered ring [=(Me)B=N(tBu)‐N=N‐(PhCH 2 )N=] ( 2a ) is formed from the four‐membered ring 1a and PhCH 2 N 3 in the ratio 1:2. An exchange of NtBu for NR' with elimination of tBuN 3 is observed, however, in the reaction of the ring compounds 1b – d with R'N 3 . Instead of four‐membered BN rings, the six‐membered BN ring (RBNR') 3 ( 3a : R/R' = Et/PhCH 2 ) and the eight‐membered BN rings (RBNR') 4 ( 4a – h : R/R' = iPr/PhCH 2 , iBu/PhCH 2 , iPr/Pr, iBu/Pr, iPr/iBu, iBu/iBu, Et/Ph, iBu/Ph) are formed, respectively. The NMR spectra reveal diastereotopic methylene protons (Et, Pr, iBu, PhCH 2 ) and methyl groups (iPr, iBu) and thus elucidate the tub‐like structure of the eight‐membered rings. 3a and 4a crystallize in the space group P2,/c. The reaction of the four‐membered rings 1c – e with the nitrone gives five‐membered rings [=(R)B=N(tBu)=B(R)=N(Me) – O=] ( 5c – e ). Both of the reactions of the four‐membered rings with azides and with the nitrone can be rationalized in terms of a similar sequence of reaction steps.