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'N3 (R' = PhCH2, Pr, iBu, Ph) and with the nitrone PhHC=N(Me)=O are studied. The five‐membered ring [=(Me)B=N(tBu)‐N=N‐(PhCH2)N=] (2a) is formed from the four‐membered ring 1a and PhCH2N3 in the ratio 1:2. An exchange of NtBu for NR' with elimination of tBuN3 is observed, however, in the reaction of the ring compounds 1b –d with R'N3. Instead of four‐membered BN rings, the six‐membered BN ring (RBNR')3 (3a: R/R' = Et/PhCH2) and the eight‐membered BN rings (RBNR')4 (4a –h: R/R' = iPr/PhCH2, iBu/PhCH2, 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, PhCH2) 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.