10 The Addition of Sulfur Atoms to Olefins O. P. STRAUSZ University of Alberta, Edmonton, Alberta, Canada
Absolute rate coefficients and Arrhenius parameters have been obtained for the cycloaddition reaction of S( P ) atoms with a representative series of olefins and acetylenes. The activation energies are small, and they exhibit a trend with molecular structure which is expected for an electrophilic reagent. The A-factors show a definite trend which can be attributed to steric repulsions and a generalized secondary α-isotope effect explained by activated complex theory. Secondary α-H/D kinetic isotope effects have been measured and their origin discussed. Hartree-Fock type MO calculations indicate that the primary product of the S( P) + olefin reaction is a ring-distorted, triplet state thiirane, with a considerable energy barrier with respect to rotation around the C-C bond. 3
2,1,0
3
Culfur atoms and the atoms of the group Via elements undergo facile ^ addition to olefins and acetylenes. These reactions represent the sim plest examples of a cycloaddition reaction. Among them, the sulfur atom reactions stand out with their experimental simplicity, cleanliness, and unique reactivity. Therefore the sulfur atom-olefin system is a model for studying some of the little explored aspects of cycloaddition reactions. Since the first report on sulfur atom addition reactions which appeared in 1962, a considerable amount of quantitative data and relevant informa tion have accumulated on the subject, which are summarized briefly here. In the ground electron configuration of the group VIA elements four electrons are distributed over three p-orbitals. This distribution gives rise to five spectroscopic states designated as P2,i,o> D , and ^o. The lowest lying state is the P state, though the spin-orbital splitting is small, only 1.6 kcal for sulfur. Radiative transitions among these states are for bidden by rigid selection rules; therefore the excited atoms have long lifetimes and are able to undergo bimolecular reactions. 3
3
2
137
1
2
138
SULFUR
RESEARCH
TRENDS
To date, studies have been confined largely to the reactions of ground state triplet and the lowest excited * D states of these atoms. Possible differences in reactivities among the triplet components have been ignored. For sulfur atoms in all kinetic studies reported, the source was the in situ photolysis of carbonyl sulfide or carbon disulfide, which proceed according to the equations: 2
COS + hv -+ CO + SOAO λ ^ 2800A
(1)
CSz + hv CS + S( P ,i,o) λ ^ 2100A
(2)
3
2
Thus, Reaction 1 produces sulfur atoms in their lowest excited singlet-D state with 26.4 kcal excitation energy while Reaction 2 results in their ground, triplet-P i states. Alternative sources of triplet sulfur atoms are the triplet mercury photosensitization of COS and the direct photolysis of COS in the presence of excess C 0 . Carbon dioxide is a quencher of singlet, excited sulfur atoms 2
2
> 0
2
S( D ) + C O ^ S W ) + C 0 , l
(3)
2
2
with a rate constant, k > 1 χ 10 liters mole" sec" (J). Both S ( D ) and S( P) atoms react readily with olefins and acety lenes, but they exhibit different reactivities. S( D ) atoms afford two principal products with olefins, thiirane, and mercaptan. Thiirane comes from the cycloaddition of the sulfur atom across the olefinic double bond. Mercaptans, which contain vinylic- and alkenyl-types, are characteristic insertion products formed upon a con certed, single step attack of the 8 ( Ό ) atom on the C H bond. The insertive ability of the S ( D ) atom has been demonstrated in separate studies. Vinylic type mercaptans are produced only from terminal olefins, and their formation may be related also to the isomerization of the chemically activated episulfides, C H 2 C H * -> C H = C H S H , con\ / S taining an excess energy of —85 kcal. The feasibility of this step is based on thermal studies of episulfides at elevated temperatures. The thiirane-forming cycloaddition follows a stereospecific path. This reaction has been shown for three pairs of olefins (2,3), cis-frarw-butene-2, cis-trans-l,2-dichloro- and difluoroethylenes. The reactivity of olefins with respect to S ( D ) atom increases as the number of alkyl substituents on the doubly bonded carbon atoms increase. The approximate values of rate constants for the reactions 10
3
1
J
2
1
1
3
2
2
1
2
2
1
2
2
10.
STRAUSZ
139
Addition to Olefins S Oft) + C2H
CH
4
CH /
2
\
S
-»S( P) + C H 3
Si !),) + C H 1
3
2
CH /
3
\
S
-*S( P) + C H 3
3
(4b)
4
CH CH
6
(4a)
2
(5b)
6
SOft) + i-C H -> (CH ) C 4
8
3
CH
2
\
(6a)
2
/
S
-+S( P) + i - C H 3
(5a)
2
4
(6b)
8
are (1,4): /c ~ 2 X 10 liters mole" sec" 10
4
/ c ~ 6 X 10 5
fc ~15X
1
10
10
e
1
10
The activation energy of the addition reactions is small, 1 kcal or less. The mercaptan-forming steps have about 0.5 kcal higher activation energies. Ground state, triplet sulfur atoms give only thiirane with olefins, and the yields with the simple, less reactive olefins are nearly ideal. The unique feature of the reaction is that it follows a stereospecific path and thereby provides the first example of a stereospecific cycloaddition of a divalent triplet state reagent. Quantitative studies of S( P) atom reactions have been carried out with about two dozen olefins (5, 6). The rate coefficients and Arrhenius parameters are summarized in Table I. The absolute rate coefficients were determined in flash photolysis experiments using kinetic absorption spectrometry (6, 7). Mixtures of 0.1 torr COS and 200 torr C 0 were flash photolyzed in the presence of an olefin, and S( P) atoms concentrations were monitored by measuring the optical densities of the 1807 ( P ) and 1820A ( Pi) atomic transitions. 3
2
3
3
2
3
The trend in activation energies (Table I) shows the electrophilic nature of attack by the sulfur atom; increasing number of alkyl substituents on the doubly bonded carbons decreases the value of E , while increasing number of halogen substituents on the doubly bonded carbons increases the value of E . These variations are correlated with molecular properties such as ionization potentials, excitation energies, and bond a
a
140
SULFUR
Table I.
RESEARCH
TRENDS
Arrhenius Parameters for the Addition of S ( P j ) Atoms to Olefins 3
E (C H ) - E (kcal/mole) &
t
4
a
(liter mole- sec ) X10* 1
e
k/k(C H ) t
k
a
C2H4
0.0
1.0
C D
0.0
1.14
0.0
1.07
cis-CHDCHD
0.0
1.04
^
1.14
1.0
6± 1
172
0.75
9± 1
2.09
0.53
2.01
0.65
12 ± 2
> =
2.36
0.97
45 ± 6
y=/
3.01
0.51
>=
