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

Chiral 1,3,2‐Oxazaborolidines from the Reaction of Chiral 2,3‐Dihydro‐1 H ‐1,3,2‐diazaboroles and Diphenylketene Reaction of equimolar amounts of diphenylketene with 1,3‐di‐ tert ‐butyl‐2‐isobutyl‐2,3‐dihydro‐1 H ‐1,3,2‐diazaborole ${\rm{\it t}Bu\overline{NCH=CHN({\it t}Bu)B}-{\it i}Bu}$ ( 1 ) regioselectively afforded 1,3,2‐oxazaborolidine ${\rm{\it t}Bu\overline{N-CH(CH=N{\it t}Bu)C(=CPh_{2})OB}-{\it i}Bu}$ ( 2 ). The employment of a series of chiral diazaboroles ${\rm{\it (S,S)}-Me(Cy)CH-\overline{NCH=CH-N\{CH(Cy)Me\}B}X}$ ( 3a : X = n Bu; b: i Bu; c: CH 2 SiMe 3 ; d: NH t Bu) led to the formation of the diastereoisomeric oxazaborolidines ${\rm Me(Cy)CH\overline{N-CH\{CH=NCH(Cy)Me\}C(=CPh_{2})OB}X}$ ( 4a – d ) with diastereomeric excesses de , which increase with the steric demand of X from de = 55 % (X = n Bu) to de ≥ 95 % (X = NH t Bu). Under comparable conditions the treatment of the enantiomerically pure diazaborole ${\rm{\it (S)}-{\it t}Bu\overline{N CH=CHN({\it t}Bu)B}-NHCH(Ph)Me}$ ( 6 ) with the ketene yielded oxazaborolidine ${\rm{\it t}Bu\overline{N CH(CH=N{\it t}Bu)C(=CPh_{2})OB}NHCH(Ph)Me}$ ( 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 ( 1 H‐, 11 B{ 1 H}‐, 13 C{ 1 H}‐NMR, MS). The X‐ray structure analysis of 2 revealed a slightly puckered five‐membered heterocycle with a long B–O bond.