Structure, Conformation, and Stereodynamics of the Atropisomers of Highly Hindered Benzyl Ethers.

CAN 145:145171 AN 2006:476210 CAPLUS (COPYRIGHT (C) 2008 ACS ON SCIFINDER (R)) ABSTRACT LOW-TEMP. AND NOE NMR SPECTRA OF FOUR OF THE TITLE COMPDS. INDICATE THAT THEY ADOPT A SYNCLINAL (S.C.) CONFORMATION, IN AGREEMENT WITH THE PREDICTION OF AB INITIO COMPUTATIONS. IN THE CASE OF THE MOST-HINDERED DERIV. (COMPD. 4), THE CONFORMATION IS SYN-PERIPLANAR (SP), AS IS ALSO SHOWN BY X-RAY DIFFRACTION. SUCH STEREOLABILE SP- OR S.C.-ATROPISOMERS EXIST AS TWO CONFORMATIONAL ENANTIOMERS: THE CORRESPONDING ENANTIOMERIZATION BARRIERS, COVERING THE RANGE 6.6 TO 9.7 KCAL MOL-1, COULD BE MEASURED FOR ALL THE EXAMD. COMPDS. IN TWO CASES (COMPDS. 3 AND 5), THE MINOR ANTIPERIPLANAR (AP) ATROPISOMER HAS BEEN ALSO OBSD., AND THE S.C. TO AP INTERCONVERSION BARRIER MEASURED (11.7 AND 11.9 KCAL MOL-1, RESP.). IN ADDN., RESTRICTED ROTATION OF THE ISO-PR AND TERT-BU SUBSTITUENTS HAS BEEN DETECTED, AND THE CORRESPONDING BARRIERS HAVE BEEN DETD.