S. Kushner for assistance in obtaining the nmr ... - ACS Publications

kcal/. AS*,. R no. “C. kR/kphb mole e U. CH3. 1. 240-260. P-CHaOCoHd. 2. 210. C5H6 ... GP-757 and GP-3375. ... In contrast to the racemization of 1-...
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S. Kushner for assistance in obtaining the nmr spectra and Dr. W. C. Baird, Jr., for a sample of syn-7-norbornenol. We are grateful to Professor S. Winstein for delaying the publication of his study (see adjoining communication) of the 7-norbornenyl cation so that our work could be published at the same time. (10) Alfred P. Sloan Foundation Research Fellow. (11) Supported (1964-1966) by a Fellowship (5-Fl-GM, 23-26-302) from the National Institute of General Medical Sciences. Herman G . Richey, Jr.,l0 Ronald K. Lustgartenl’ Department of Chemistry, The Pennsyluania State University University Park, Pennsylvania 16802 Receieed April 30, 1966

Mechanisms of Thermal Racemization of Sulfoxides1 Sir: It has long been known that sulfoxides suffer stereomutation on heating at elevated temperatures.2 We now wish to report preliminary results of a study aimed at correlating the rates of racemization of optically active sulfoxides with their structures. We restricted our investigation to sulfoxides which are incapable of 0 e l i m i n a t i ~ n . ~Salient results are collected in Table I. Table I.

Thermal Racemization of Sulfoxides, RSOC6H4CH3-pa

R

CH3 P-CHaOCoHd C5H6 p-CFaC& o-CHaC6Ha CY-C~OH~ 2,4,6-(CHa)CeHz CsHjCHz

CHe=CHCHz

AH*,

Compd

Temp range,

no.

“C

1 2 3 4

240-260 210 2oG-220 210 200-220

0.24c 0.60

2.2

37

-2

210

2.3 53. Od 2.8 X loae 5.6X106’

34 43 22

-3 +24

5 6

7 8

9

180-200 135-155 50-70

kR/kphb

kcal/ mole

AS*,

43

+3

1.o

eU

0

39

1.9

-9

In p-xylene or benzene. * First-order rate constant, k ~ of , RSOCsH4CH3-p,relative t o that of phenylp-tolyl sulfoxide, k ~ hat , l u 1e and s 7-9 were 210” (kphZ10= 3.19 X IO-5sec-1); k ~ ~ ~ ~ v afor obtained by extrapolation. = 6.7 X SeC-’. kR’” = 3.7 X sec-I. k~~~~ = 7.3 X sec-I corrected for de0 X lo-* sec-’. composition. f k ~ =~ 4.0 a

The rates of racemization of 1-7 are cleanly first order, and the racemized products are structurally identical with the starting materials. While the rates are relatively insensitive to electronic effects, a steric factor appears to be operative, for the attachment of (1) This work was supported by the National Science Foundation under Grants No. GP-757 and GP-3375. (2) I