Chiral 1,3,2-oxazaborolidines from the reaction of chiral 2,3-dihydro-1H-1,3,2-diazaboroles and diphenylketene

Article abstract of DOI:10.1002/zaac.200500054

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=NtBu)C(=CPh ₂)OB-iBu (2). The employment of a series of chiral diazaboroles (S,S)-Me(Cy)CH-NCH=CH-N{CH(Cy)Me}BX (3a: X = nBu; b: iBu; c: CH ₂SiMe₃; d: NHtBu) led to the formation of the diastereoisomeric oxazaborolidines Me(Cy)CHN-CH{CH=NCH(Cy)Me}C(=CPh ₂)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)-tBuNCH=CHN(tBu)B-NHCH(Ph)Me (6) with the ketene yielded oxazaborolidine tBuNCH(CH= NtBu)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.