July 20, 1955 ELECTROPHILIC DISPLACEMENT ON
+
ing negative signs. Values of 9extend from 100% for perfect correlation ( r = 0) to small or even negative values when there is serious scatter. Values of @ from 80 t o 100% are designated arbitrarily as “excellent,” 50-8070 as “good,” 20-50% as “fair,” and less than 20% as “poor,” which is generally in accord with subjective evaluation of the corresponding plots by independent observers. Figure 1 is a plot of one of the oldest linear freeenergy relationships, which is included simply to
[CONTRIBUTION FROM THE
Cis-2-METHOXYCYCLOHEXYLNEOPHYLMERCURY
3741
show how much a 9 = 77y0correlation scatters. The Bronsted equation implies that the freg energy of activation of a base- or acid-catalyzed reaction is a constant fraction of the free energy of ionization of the base or acid. The slope /3 is also a measure of the fraction of completion of the proton transfer a t the transition state. Table V gives data for typical fits, which vary from “poor” to “excellent.” CAMBRIDGE 39, MASSACHUSETTS
DEPARTMENT OF CHEMISTRY,
UNIVERSITY OF CALIFORNIA, L O S ANGELES]
Mechanisms of Reaction of Organomercurials.l I. Stereochemistry of Electrophilic Displacement on cis-2-Methoxycyclohexylneophylmercury by Radio-mercuric Chloride BY S. WINSTEIN,T. G. TRAY LOR^
C. S. GARNER
AND
RECEIVED JANUARY 25, 1955 Electrophilic substitution at a saturated carbon atom has been discussed sometimes as analogous to nucleophilic substitution. Possible contrast between the two varieties of substitution, based on electronic and stereoelectronic considerations, is discussed in the present paper. Further, the stereochemistry of electrophilic substitution a t a saturated carbon atom has been studied in the case of electrophilic substitution by mercuric chloride on cis-2-methoxycyclohexylneophylmercury. The use of radio-mercuric chloride has disclosed the proportions of cis-2-methoxycyclohexyl- and neophyl-mercury cleavage. The methoxycyclohexylmercuric chloride derived from the cleavage reaction has been shown, by a sensitive test for the trans isomer, to be very pure cis material. This result, coupled with the information on the extent of methoxycyclohexyl-mercury bond cleavage in the substitution, shows that substitution on cis-2-methoxycyclohexyl proceeds with retention predominating over inversion by a factor of a t least 100 to 1. Possible mechanisms for the substitution with retention of configuration are discussed.
Electrophilic substitution a t a saturated carbon atom, much less understood than the nucleophilic variety, has been discussed sometimes as analogous to nucleophilic substitution. Thus, Hughes and Ingold,s in their 1935 review of substitution, suggested an SEl-SE2 classification for electrophilic substitution analogous to SNl-SN2 for the nucleophilic case. They suggested a rate sequence, , to the one t-Bu > i-Pr > Et > Me, for S E ~opposite generally prevailing for S N ~ .Also, a t that time, they tentatively visualized, for the stereochemical outcome of electrophilic substitution, inversion of configuration in S E ~as, in S N ~and , ~retention of configuration in SEI, as in s ~ l Much . ~ more recently, Dewar6 has commented that ‘cationoid replacements undoubtedly conform to the same general principles as do their anionoid counterparts.” We have been interested in the analogy between electrophilic and nucleophilic substitution a t a saturated carbon atom. Just as for nucleophilic substitution, internal or cyclic mechanisms of electrophilic substitution, SEi, need to be considered. Also, for the spectrum of possible transition states in SE2 or SEi substitution, we must visualize various i
(1) Some of t h e material of this paper was presented a t t h e Organic Reaction Mechanisms Conference, Northwestern University, Evanston, Ill., Aug. 31, 1950. (2) U. S. R u b b e r Co. Fellow, 1951-1952. (3) E. D . Hughes and C. K. Ingold, J . Chem. Soc., 244 (1935). (4) Subsequent work on nucleophilic substitution proved inversion t h e rule in S N ~ .However. a number of possible outcomes of nucleophilic substitution by way of cationic intermediates is possible, deprnding on t h e stability a n d ion pair character of t h e intermediate, nucleophilic character of t h e solvent, anchimeric effects, etc. ( 5 ) M. J. S. Dewal, “ T h e Electronic Theory of Organic Chemistry,” Oxford University Press, Oxford, 1949, p 81.
degrees of importance of bond formation to the carbon atom undergoing substitution. Considering stereochemical outcome of s ~ 2 substitution, the most stable transition state I is attained by trigonal (spz) hybridization of orbitals on the central carbon atom, a p orbital serving for the partial bonds to the leaving and entering nucleophiles. This arrangement, leading to inverted product, apparently maximizes bonding6 and minimizes repulsion’ between electron pairs, of which there are five, in separate bonds. I t does not follow that this type of orbital hybridization will be favored also for the transition state in SE2, involving one less pair of electrons in the five full
I y---c..--x
,X
>p:\y
A I
I1
or partial bonds to the central carbon atom. S o t only is repulsion between separate electron pairs in a transition state of the type I1 less serious in electrophilic than in nucleophilic substitution, but, in some cases extra stabilization may be associated with this variety of transition state. In electrophilic substitution on carbon by an electrophilic reagent, E, the transition state iiiay be regarded as electron-deficient, and, in some cases a t least, extra stabilization may be derived from bonding between the leaving group X and the incoming group E. This is symbolized with the contribution of structure IIIc to the hybrid transition 111, or by the summary symbol IV. A4tany rate, we ap( 6 ) Reference 5, page 64. (7) E. D. Hughes a n d C (1937).
K
Ingold, et ai., J . Chein. S o c , 1252
proached electrophilic substitution with thc expectation that retention of configuration would be
IIIa
IIIh
I\'
IIIC ~
an entirely plausible and possibly general stereochemical outcome of concerted substitution.8 Cases of concerted electrophilic substitution a t a saturated carbon atom are a t lcast very rare, except with organometallic substances. Many electrophilic substitutions at a saturated carbon atom, such as halogenation or deuterium exchange of ketones or certain decarboxylations, are to be classed as SEI. &o, many attacks of electrophilic reagents on saturated carbon compounds occur on an atom other than carbon and thus lead to nucleophilic substitution on carbon. For example, the powerfully electrophilic carbonium ions tend to attack hydrocarbons or alkyl halides on hydrogen or halogen, respectively. The carboxylic acid silver salt-bromine reaction has been suggestedlO to proceed by an s E 2 mechanism with Walden inversion, but the mechanism of this reaction is clearly radical.'' A simple way to provide for concerted electrophilic substitution on saturated carbon is to employ an organometallic substance as the substrate. Organomercurials represent some of the most convenient materials for experimental study. Also, with organomercurials there already exists a broad qualitative background and some semiquantitative impression of relative reactivities. I f This serics of papers reports some mechanistic studies with organomercurials, not only in electrophilic substitution but in other reactions. In the present paper we report the results of a study of the stereochemistry of substitution by mercuric chloride on an unsymmetrical dialkylmercury,13 R-Hg-R', as in equation 1. The reaction of mercuric chloride with dialkylmercuries ( 8 ) In m a n y rearrangements of t h e Wagner-hleerffein, Beckmann and Lossen t y p e , electrophilic displacement on t h e migrating group R by t h e electrophilic center, in what can b e classified a s a n SEi substitution, occurs with retention of configuration. However, these rewits m a y n o t be extrapolated t o S E substitution. ~ 4ince even an ssi
R
. .-
v-
/C-E
R .-I;-
CH3-X-CH-COCsIIj
I@
CHa SEi
Ssi
analog of t h e above Sxi cases, where substitution on 12 is b y a nucleophilic center proceeds with retention [A. Campbell, A. H. J. Houston a n d J. Kenyon, J . Ckem. S O L ,93, ( 1 9 1 7 ) ; C. K . Ingold, "Structure a n d Mechanism in Organic Chemistry,'' Cornell Univ. Press, Ithaca. Pi. Y . , 1953, p. 527; J. H. Brrwster and M. W. Kline, Abstracts of 122nd Meeting of American Chemical Society, Atlantic City, N. J., Sept. 14-19, 1952, p. 29hil. (9) P. D. B a r t i e t t , F. E . Condon and A. Schneider, Tnis J O U R N A L , 6 6 , 1531 (1944). (10) J , Kenyon, C. L. Arcus a n d A. Campbell, J . Chern. Soc., 1510 (1949). (11) E . g . , C. Berr, Dissertation, U. C . I>. A , , 1952, and many references quoted therein. (12) F. C. Whitmore. "Organic Compounds of Mercury," Chemical Catalog Co. (Reinhold Publ. Corp), A'ew I'ork, K.Y., 1921. (13) h l . S . Kharasch and R. Marker, THIS J O U R N A L48, , 3130 (1926).
is well known,lJ but the only previous cases where R-Hg-R'
+ HgC12
--f
RHgCl
+ ClHgR'
(1)
the stereochemical result is clear involve divinylmercury compounds. Bis-2-chlorovinylmercury reacts with mercuric chloride to yield 2-chlorovinylmercuric chloride Retention ~ of with retention of c o n f i g ~ r a t i o n . ~ configuration a t a trigonal carbon atom also is observed in other substitution reactions of organomercury or antimony compounds.16 I n fact, retention of configuration a t a trigonal carbon atom is common in other c o n v e r s i o n ~of ~ ~vinyl . ~ ~ compounds, >C=,-the fraction of the mercurial lrhi-h was counted. c trans-2-Methosycyclohexylmercuric 35yc of the 2-methoxycyclohexylmercuric chloride chloride. (VI) which was isolated was derived from 2-methoxyTEsrs
FOR
EXCHANGE OF MERCCRY BETTVEEN MERCURIC neophylmercuric
c).
T A ~ LI1E R A D I O ACTIVITY O F P R O D U C T S AND S T A R T I S G M A T E R I A L S IS THE R E A C T I O N O F C i s - ' - S I E , r H O X Y C ~ C L O ~ ~ ~ ~ Y L ~ E O P H Y L M ~ R C ~ R \ ' WITH
Comiiotind
I)atc
R;*tlio -HgCI. 1 bt crop of T.1 2iid crop of 1.1
l i t crop of I S 2nd crop of I S Same soln. used i n linc 1 1.0 g . HgCI? Same as ahovc
Set count, c./rn.
11)
.iOROh
:j4iO
12
5OUO
1 lU.i
1)(i
51,i;' 3090 .ilj7
399 1007
j090
952
21
+
Cotmter tube nt-rnher
L'li "ii
101ii
MERCURIC CHLORIDE Count per 30 ml. soh, c./m.
\vt prod in 30 ml. soln., P
JIM) 1135 49v'
0,i74;j
1807 1%~' 111"
7,317 :$18,j ,j(ll2
-,.),,4
Ilmoles" prod. in 30 ml. soln.
Count per C o u n t per m . y - a t o m mg.-atrim Hg in Hg decay ]prod., cqr..
2115 21Oil il 911 1357 1.i~ii1
c.;ing.-
at.
in.
I!I~O fiX1
237 WO
S'X
c 'm"
nt
m
1R7O w 4 (i70 102!4 10:i3
.4c
based I I R N A I .6 , 9,
G97 (1947).
(32) I 1 Gilman, rf R I . , { D i d . , 4 6 , I,>(; (19%;) (:i3) 4iiuIy.is I i y 111 A . liirl, 1 , Augrlrs, ~ Ctlli(
