Chirale 1,3,2‐Oxazaborolidine aus chiralen 2,3‐Dihydro‐1H‐1,3,2‐diazaborolen und Diphenylketen

Chiral 1,3,2‐Oxazaborolidines from the Reaction of Chiral 2,3‐Dihydro‐1H‐1,3,2‐diazaboroles and Diphenylketene Reaction of equimolar amounts of diphenylketene with 1,3‐di‐tert‐butyl‐2‐isobutyl‐2,3‐dihydro‐1H‐1,3,2‐diazaborole tBuNCH=CHN( tBu)B- iBu (1) regioselectively afforded 1,3,2‐oxazaborolidine tBuN-CH(CH=N tBu)C(=CPh₂)OB- iBu (2). The employment of a series of chiral diazaboroles (S,S)-Me(Cy)CH-NCH=CH-N(Cy)Me (3a: X = nBu; b: iBu; c: CH2SiMe3; d: NHtBu) led to the formation of the diastereoisomeric oxazaborolidines Me(Cy)CHN-CH=NCH(Cy)Me(=CPh₂)OBX (4a–d) with diastereomeric excesses de, which increase with the steric demand of X from de = 55 % (X = nBu) to de ≥ 95 % (X = NHtBu). Under comparable conditions the treatment of the enantiomerically pure diazaborole (S)- tBuN CH=CHN( tBu)B-NHCH(Ph)Me (6) with the ketene yielded oxazaborolidine tBuN CH(CH=N tBu)C(=CPh₂)OBNHCH(Ph)Me (7) with a de‐value of only 52 %. The new compounds, with exception of 2 and 4d, are thermolabile solids, which were characterized mainly by spectroscopy (1H‐, 11B{1H}‐, 13C{1H}‐NMR, MS). The X‐ray structure analysis of 2 revealed a slightly puckered five‐membered heterocycle with a long B–O bond.