3302
IRVIXROTHBERG A N D EDWARD R . THORNTOS [CONTRIBUTION FROM
THE
Vol. 8G
DEPARTMENT OF CHEMISTRY, UNIVERSITY OF PENSSYLVANIA, PHILADELPHIA 4, PA ]
Mechanisms of a-Elimination. BY
p-Nitrobenzylsulfonium Ions Reacting with Aqueous Hydroxide'-3
IRVINROTHBERG AND EDWARD R.
THORNTON
RECEIVED MARCH12, 1964 The four sulfonium salts ~ - O ~ S C ~ H ~ C HCH3)RXPS"( (where R , X - = C6Hs, c104-; p-cIc6H1, CIOI-; p-CH&H4, CIO-; and (CH3)?CH, B r r ) were synthesized, and a study of their decomposition in aqueous sodium hydroxide solution a t 60' was made. Under nitrogen, the per cent yields of p-nitrobenzyl alcohol and p,p'-dinitrostilbene were, respectively: 19, 19; 10, 16; 12, 21; ylvania. 1963
(-1) ( 6 0 Hinsberg, Chem Bpi, , 69, 1 9 2 l l 9 3 6 ) , N. J . L e o n a r d , T. W. Slilligan, and T I. AI-own, J A m Chem Soc , 32, 4075 (1960) T W M i l l i gan a n d B. C . SIinoi- J O I P C h r m , 23, 235 (1963).
+ HO-
.ArCH2-S+-R
fast
~
CHt
CH3
Xr-C-H -%r--?!--H
+ H20
.ArCH-S--R fast
+ .ArCHS-(CH3)R
In all cases mixtures of products formed and were isolated by filtration and chromatographic separation, both thin layer and column, on alumina. In addition, the aqueous filtrates were extracted with ether to isolate p-nitrobenzyl alcohol if formed. In each case a number of products was formed, only part of which could be identified. Some of the experiments were perfortned with and some without a protective cover of nitrogen. The fact that a quantitative yield of p.p'-dinitrostilbene was not formed from any of these salts indicates that the stilbene-forming reaction is extraordinarily sensitive to the nature of the leaving group.
o r fui-thei- details
The yields of the three products which could be identified, p-nitrobenzyl alcohol, p,p'-dinitrostilbene, and p,p'-dinitrostilbene oxide, are given in Table I. p,p'-Dinitrostilbene was almost quantitatively recovered (97%) when passed through an alumina column ; therefore the relatively low yields are not due to decomposition on work-up. The yield of p , p ' dinitrostilbene was much smaller (maximum 487,) with these four salts than with p-nitrobenzyldimethylsulfonium p-toluenesulfonate (100%'). Variation in the yield of p,p'-dinitrostilbene is relatively small when the substituents in the phenyl ring of the leaving group are varied, indicating that the electrical effect of the substituents is quite small. Therefore it seems probable that a steric factor is responsible for the differences among R = CGHS,(CHa),C H , and CHs. An attractive hypothesis based on a proposed'>jmechanism involving p-nitrophenylcarbene is t h a t the carbene is produced and will attack the species present in solution by the pathway which requires the least energy. \X7hen R = CH3 the attack on the reversibly-formed ylid requires the least energy and is the predominant reaction
Jslow + CH3SR
fast
+ ArCH-S--R
--+-ArCH-CHhr
CHI
R-S
+-CH3
J fast + CH3SR
ArCH=CHXr
(1)
or alternatively' carbene insertion in a C H bond of the sulfonium ion occurs instead of reaction 1 hr-C-H
+ ArCH2-S+-R CH3
fast d
,
HO -
XrCH2-CHXr
R-S XrCH=CHAr
-+ fast
+-CH3 CHISR
+
+ H20
However, when the ylid (or the sulfonium ion) is sufficiently sterically hindered, the carbene could attack other species present (e.g., insertion reactions or attacks on the double bonds of the leaving group, reaction with oxygen to form 9-nitrobenzaldehyde, ( 5 ) C. G Swain a n d E. R . T h o r n t o n , J . A m . Chem SOL.,83,4033 (1961).
Aug. 20, 1964
~-NITROBEXZYLSULFOSIUM IONSWITH HYDROXIDE TABLE I
PRODUCTS OF REACTION OF P - O Z S C ~ H ~ C H ZCH,)RXS+( WITH AQUEOUS SODIUM HYDROXIDE AT 60” yo----
----Yield,
R
X-
Conditions
+Nitrobenzyl alcohol
p,p’-Di-
p,p‘-Dinitro-
nitrostilbene
stilbene oxide
21 7 19 -0 10 13 16 -0 21 -0 48 -0 0.1 M sulfonium
Air 8 Under N2 19 p-CICsHaa c104- Air 10 Under N2 p-CH3C6H4’ C104- Under I ~ z 12 (CH,)#2Hb BrUnder iX2
