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From: Bob Gawley (rgawley$##$
Date: Wed May 27 1998 - 09:17:40 EDT

>HI all,
>One more theoretical question of mine: Is SBINAP pS-Binap or aS-Binap; for
>what type of chirality does S stand??
>Thank you


They are all the same, sort of...

Cahn, Ingold, and Prelog (CIP) codified the R,S system in 1966 (Angew
Chemie Int Ed Engl vol 5, p. 385; Angew Chemie vol 78, p. 413). Prelog and
Helmchen (PH) modified it slightly and proposed some changes relevant to
your question in 1982 (Angew Chemie Int Ed Engl v. 21, p. 567; Angew Chemie
v. 94, p. 614).

CIP defined R/S to describe (specify) absolute configuration, most usually
about a chirality CENTER. The prefixes "a" and "p" refer to chirality AXES
and PLANES. The rules are in the 1966 paper, but the prefixes are often
omitted in the literature nowadays. It doesn't really matter if you use the
prefixes, since aS = pS = S.

I should also point out that PH recommended in the 1982 paper that the
prefixes P (for Plus) and M (for Minus) could (should) be used in place of
R/S for chirality axes and planes, and my own experience suggests that it
is infinitely easier to derive a P/M descriptor for an axis or a plane. Let
me see if I can produce a diagram that illustrates how easy it is:

For 1 having higher priority than 2, a chirality axis might look like this,
viewed along the chirality axis:


(I intend for the horizontal line to be toward the viewer).

Now, to proceed from the higher priority ligand in front to the higher
priority ligand in back, you go clockwise, therefore the configuration is P.

If you want to convert from P/M to R/S, arrange them alphabetically, then
equate them vertically:

R / S
M / P

(R=M, P=S).

For definitions and further details (and better pictures), see pp. 30-31 of
"Principles of Asymmetric Synthesis" R. E. Gawley and J. Aube, Elsevier:
Oxford, 1996; or for considerable detail, see Ch. 14 of "Stereochemistry
of Organic Compounds", E. L. Eliel, S. H. Wilen, L. N. Mander, Wiley: New
York, 1994.


Bob Gawley

Robert E. Gawley, Professor
Department of Chemistry
University of Miami
P. O. Box 249118
Coral Gables, Florida 33124-0431
Phone: 305-284-3279
FAX: 305-668-3313


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