Conformational study by CD of chirally tethered naphthalene moieties: toward an understanding of the asymmetric intramolecular coupling reaction?
The carbonate (R,R)-1 and the diester (R,R)-2, precursors of optically active 1,1'-binaphthyl derivatives through asymmetric intramolecular coupling, were prepared and fully characterized. The absorption and CD spectra, together with the cholesteric induction measurement of (R,R)-1, indicate that it assumes a conformation in which the naphthalene rings are M-skewed. Since the intramolecular coupling of (R,R)-1 affords a P-twisted (S) binaphthyl derivative, a change of twist from M to P must occur during the reaction. Moreover, this conformation allows the coupling to occur along only one pathway, thus explaining the high stereoselectivity. The CD analysis of (R,R)-2 indicates that it assumes a rigid conformation as well, with the naphthalene rings fixed relative to each other. However, in this compound the naphthalene moieties can give rise to a coupling reaction following two different pathways, leading to oppositely configured binaphthyls with almost the same probability so that low stereoselectivity results. Copyright 2000 Wiley-Liss, Inc.
Rosini
C
Dipartimento di Chimica, Universita della Basilicata, Potenza, Italy.
Superchi
S
Bianco
G
Mecca
T