Substituent Effects on Vinyl Radical Cyclizations onto Aryl Rings

Article abstract of DOI:10.1021/jo971399l

Regioselective toluenesulfanyl radical addition to the C-1 carbon of propynyl benzyl ethers 2a-i leads to vinyl radicals 3a-j which mainly give methyl sulfides 9a-j and methyl ethers 8a-j through stereoselective 5-(π-endo)exo and 5-(π-exo)exo cyclization, respectively. Additionally, minor amounts of π-endo 6-membered cyclization products 5a-j and hydrogen abstraction products 4a-j are also formed. The role of steric and stereoelectronic factors in the 5-membered vs 6-membered and π-endo vs π-exo cyclization as well as the role of stabilization and polar factors has been studied. The substituent effect on the 5-(π-exo)exo cyclization has been estimated by the relative rate constants, κ_R, calculated for several substituents in the 4- and 3-position. Results show that stabilization and polar factors slightly affect the rate of the vinyl radical cyclization onto arene rings, which appears to be rather unselective with respect to the nature of the substituent. The nature of polar effects indicates that vinyl radicals are slightly electrophilic in character.

Full text of DOI:10.1021/jo971399l

J . Org. Chem. 1998, 63, 537-542
537
Su bstitu en t Effects on Vin yl Ra d ica l Cycliza tion s on to Ar yl Rin gs
Pier Carlo Montevecchi* and Maria Luisa Navacchia
Dipartimento di Chimica Organica “A. Mangini”, Universita` di Bologna, Viale Risorgimento 4.
I-40136 Bologna, Italy
Received J uly 28, 1997
Regioselective toluenesulfanyl radical addition to the C-1 carbon of propynyl benzyl ethers 2a -i
leads to vinyl radicals 3a -j which mainly give methyl sulfides 9a -j and methyl ethers 8a -j through
stereoselective 5-(π-endo)exo and 5-(π-exo)exo cyclization, respectively. Additionally, minor amounts
of π-endo 6-membered cyclization products 5a -j and hydrogen abstraction products 4a -j are also
formed. The role of steric and stereoelectronic factors in the 5-membered vs 6-membered and π-endo
vs π-exo cyclization as well as the role of stabilization and polar factors has been studied. The
substituent effect on the 5-(π-exo)exo cyclization has been estimated by the relative rate constants,
kR, calculated for several substituents in the 4- and 3-position. Results show that stabilization
and polar factors slightly affect the rate of the vinyl radical cyclization onto arene rings, which
appears to be rather unselective with respect to the nature of the substituent. The nature of polar
effects indicates that vinyl radicals are slightly electrophilic in character.
Carbon-centered radical additions to1 (and radical
Sch em e 1
cyclizations onto2 ) alkene and alkyne multiple bonds and
aromatic rings are governed by several factors3 (including
steric, stereoelectronic,4 polar, stabilization, and enthalpic
factors) which exert different roles depending on the
different nature of the radical and the radical acceptor.
Stabilization effects are determined by unpaired-
electron delocalization in the radical adduct intermediate,
substituted alkyl radicals indicate that the addition to
alkene double bonds is dominated by SOMO/HOMO
interaction and, therefore, is favored by alkene electron-
releasing substituents.7 In contrast, the addition to
alkyne triple bonds appears to be either SOMO/HOMO-
or SOMO/LUMO-controlled, depending on the nature of
the alkyne substituent.7d,8 Thus it is favored by both
strong electron-releasing and strong electron-withdraw-
ing (EWG) alkyne substituents.7d On the contrary, non-
EWG-substituted alkyl radicals, which are nucleophilic
in character, preferentially add to EWG-substituted
alkenes9 and alkynes.10
Both polar and stabilization factors play a little role
in homolytic aromatic substitutions at benzene rings.
Results reported in the 1960s by Hey et al.11 provided
evidence that the relative rates of para substitution by
phenyl radicals is generally slightly enhanced by both
while polar effects arise in the transition state by charge-
transfer from/to the radical to/from the radical acceptor
(Scheme 1). The direction of the charge transfer is
determined by the frontier molecular orbital approach
which can be controlled by either SOMO/HOMO or
SOMO/LUMO interaction.5,6
These effects do not always play a determining role,
especially when highly ergonic reactions are considered.
In these cases the enthalpic factor predominates. When
∆H , 0, the transition state is early and the reaction is
fast and unselective, while when ∆H . 0, the reaction is
thermodynamically controlled and the rate of the radical
addition is determined by the fate of the radical adduct.3,5
Polar and stabilization effects greatly depend on the
nature of substituents present on both the radical and
the radical acceptor.4,5 Results reported for electrophilic
(1) Carbon centered radical additions to C-C double and triple
bonds have been reviewed: Curran, D. P. Radical Addition Reactions.
In Comprehensive Organic Synthesis; Pergamon Press: Oxford, U. K.,
1991; Vol. 4, Chapter 4.1
(2) For leading references, see: (a) Curran, D. P. Radical Cycliza-
tions. In Comprehensive Organic Synthesis; Pergamon Press: Oxford,
U. K., 1991; Vol. 4, Chapter 4.2. (b) Curran, D. P.; Porter, N. A.; Giese,
B. In Stereochemistry of Radical Reactions; VCH Publishers: New
York, 1996; (c) Malacria, M. Chem. Rev. 1996, 96, 289.
(3) Tedder, J . M. Angew. Chem., Int. Ed. Engl. 1982, 21, 401.
(4) Stereoelectronic effects mainly govern cyclization reactions.
See: (a) Baldwin, J . E. J . Chem. Soc., Chem. Commun. 1976, 734. (b)
Beckwith, A. L. J .; Easton, C. J .; Serelis, A. K. J . Chem. Soc., Chem.
Commun. 1980, 482.
(5) Giese, B. Angew. Chem., Int. Ed. Engl. 1983, 22, 753.
(6) J asperse, C. P.; Curran, D. P.; Fevig, T. L. Chem. Rev. 1991, 91,
1237.
(7) (a) Reimschneider, R.; Drechsel,-Grau, E.; Boldt, P. Tetrahedron
Lett. 1979, 185. (b) Tedder, J . M.; Walton, J . C. Tetrahedron 1980, 36,
701. (c) Vertommen, L. L.; Tedder, J . M.; Walton, J . C. J . Chem. Res.
(S) 1977, 18. (d) Citterio, A.; Bergamini, F.; Nicolini, M.; Santi, R.;
Sebastiano, R. J . Org. Chem. 1992, 57, 4250.
(8) Kampmeier, J . A.; Kopchik, R. M. J . Am. Chem. Soc. 1968, 90,
6733; Kharash, M. S.; Sage, M. J . Org. Chem. 1949, 14, 537;
Haszeldine, R. N. . J . Chem. Soc. 1951, 388; Curran, D. P.; Kim, D.
Tetrahedron, 1991, 32, 6171; Montevecchi, P. C.; Navacchia, M. L.;
Spagnolo, P. Tetrahedron 1997, 53, 7929.
(9) Egger, K. W.; Cocks, A. T. Helv. Chim. Acta 1973, 56, 1537. Zabel,
F.; Benson, S. W.; Golden, D. M. Int. J . Chem. Kinet. 1978, 10, 295.
Ruchardt, C.; Beckhaus, H. D. Angew. Chem., Int. Ed. engl. 1980, 19,
429. Caronna, T.; Citterio, A.; Ghirardini, M.; Minisci, F. Tetrahedron
1977, 33, 793. Baban, J . A.; Roberts, B. P. J . Chem. Soc., Perkin Trns.
2 1991, 161. Shiraishi, H.; Ramby, B. Chem. Sci. 1977, 118. Korus,
R.; O’Driscol, K. F. In Polymer Handbook; Brandrup J ., Immergut, E.
H., Eds.; Wiley: New York 1975, 11. Giese, B.; Kretzschmar, G.;
Meixner, J . Chem. Ber. 1980, 113, 2787. Cekovic, Z.; Saicic, R.
Tetrahedron Lett. 1986, 27, 5893. Clive, D. L. J .; Anogh, A. G. J . Chem.
Soc., Chem. Commun., 1985, 980. Curran, D. P.; Chen, M. H. J . Am.
Chem. Soc., 1987, 109, 6558.
(10) Curran, D. P.; Kim, D.; Ziegler, C. Tetrahedron 1991, 32, 6189.
Ichinose, Y.; Matsunaga, S.; Fugami, K.; Oshima, K.; Utimoto, K.
Tetrahedron Lett. 1989, 30, 31555.
(11) Hey, D. H. Adv. Free Radical Chem. 1967, 2, 47.
S0022-3263(97)01399-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/22/1998

538 J . Org. Chem., Vol. 63, No. 3, 1998
Montevecchi and Navacchia
Sch em e 2
EWG and electron-releasing substituents, while for the
more electrophilic 4-cyano-, 4-chloro-, and 4-nitro-sub-
stituted phenyl radicals, a rate increase has been ob-
served for electron-releasing substituents and a rate
decrease for EWG substituents present on the benzene
radical acceptor. This behavior can be explained by
considering that EWG substituents are capable of ac-
celerating the substitution rate by unpaired electron
delocalization, but meanwhile, they exert unfavorable
polar effects in these SOMO/HOMO controlled reactions.
These unfavorable effects prevail for highly electrophilic
aryl radicals, while stabilization effects prevail for weakly
electrophilic phenyl radicals. On the other hand, kmeta
values have been generally found to be higher than kH
for electron-releasing substituents and lower for EWG
substituents, because the attack in the meta position is
only governed by polar factors.
In contrast to the above homolytic aromatic substitu-
tions, polar effects are dramatically important in the
Minisci reaction, which provides a useful synthetic route
to alkylated heterocycles through nucleophilic radical
substitution at protonated heterocycles under oxidative
conditions.12
At present, no data are available concerning the role
of polar and stabilization factors for inter- or intramo-
lecular vinyl radical homolytic aromatic substitutions.
Therefore, in our interest for vinyl radical rearrange-
ments and cyclizations,13 we have undertaken a study
to investigate the main factors affecting the rate of vinyl
radical cyclizations onto aryl rings, with particular
attention to the effect of the substituent present on the
aromatic ring.
Resu lts a n d Discu ssion
For our purpose we considered the fate of vinyl radicals
3a -i generated by regioselective 4-cyanotoluenesulfanyl
radical addition to a number of propynyl benzyl ethers
2a -i. Sulfanyl radicals were produced from thiol 1, R′
) CN, under thermal conditions in boiling benzene in
the presence of 0.2 mol equiv of AIBN as initiator.
Chromatography of the reaction mixtures generally
gave variable amounts of the 1,4-aryl migration products
9a -i and 8a -i, which were derived from vinyl radicals
3a -i through 5-(π-endo)exo and 5-(π-exo)exo cyclization,
respectively14 (Scheme 2). The stereoselective π-endo
cyclization led to spiro radicals 7a -i from which (E)-
methyl sulfides (E)-9 arose through cleavage of the spiro
ring. Similarly, the π-exo cyclization led to spiro radicals
6a -i and then to 1,4-aryl migration products 8a -i.
The π-exo cyclization onto aryl rings occurred in a
highly stereoselective fashion, leading to configuration-
ally pure ethers (Z)-8a -i, which were derived from
radicals (Z)-3a -i. The actual configuration of (Z)-8a -i
was established by NOE measurements. Irradiation on
the vinyl proton signal caused an enhancement of the
S-methylene group, but no enhancement of the O-
methylene group was observed. Analogous stereoselec-
tivity has been reported for π-exo vinyl radical cycliza-
tions onto C-C double bonds. As previously suggested,13a
the π-exo cyclization of (E)-vinyl radicals could be dis-
couraged by steric repulsion occurring in the transition
state between the radical acceptor moiety (alkene double
bond or aromatic ring) and the â-substituent.
In addition to products 9a -g and 8a -g, column
chromatography generally separated minor amounts (ca.
10%) of (E)- and (Z)-adducts 4b-g (40:60 E/ Z ratio)
resulting from radicals 3b-g by hydrogen abstraction.
Formation of small amounts of possible adducts 4a ,h ,i
1
was detected by H NMR spectroscopy of the correspond-
ing reaction mixtures, but subsequent column chroma-
tography did not allow for the separation of these
compounds.
The substituent effect on the vinyl radical cyclization
onto aryl rings was estimated by comparison of the
(12) Minisci, F.; Vismara, E.; Fontana, F. Heterocycles 1989, 28, 489.
Minisci, F.; Citterio, A.; Giordano, C. Acc. Chem. Res. 1983, 16, 27.
Minisci, F. In Substituents Effects in Radical Chemistry; Viehe H. G.,
Ed., Reidel: Dordrecht, 1986; p 391. Minisci, F. Top. Curr. Chem. 1976,
62, 1.
(13) For our reports on 2-sulfanyl vinyl radical cyclization and
rearrangements, see: (a) Montevecchi, P. C.; Navacchia, M. L. J . Org.
Chem. 1997, 62, 5600. (b) Montevecchi, P. C.; Navacchia, M. L.
Tetrahedron Lett. 1996, 37, 6583. (c) Capella, L.; Montevecchi, P. C.;
Navacchia, M. L. J . Org. Chem. 1996, 61, 6783. (d) Benati, L.; Capella,
L.; Montevecchi, P. C.; Spagnolo, P. J . Org. Chem. 1995, 60, 7941. (e)
Benati, L.; Capella, L.; Montevecchi, P. C.; Spagnolo, P. J . Org. Chem.
1994, 59, 2818.
(14) We have previously shown that toluenesufanyl radical addition
to terminal alkynes having radical acceptor moieties in the radical
chain (terminal and styrene double bonds or aryl rings) leads to vinyl
radicals which undergo π-exo cyclization onto the radical acceptor in
competition with the 5-(π-endo) cyclization onto the aryl ring. See ref
13a.

Vinyl Radical Cyclizations
J . Org. Chem., Vol. 63, No. 3, 1998 539
Ta ble 1. Rela tive Yield s (%) of P r od u cts 9 a n d 8 fr om
5-(π-En d o)exo a n d 5-(π-Exo)exo Cycliza tion s of Ra d ica ls 3
a n d Rela tive Ra te Va lu es (kR) for th e π-Exo Cycliza tion s
Sch em e 3
(relative yields, %)
entry
R′
R
sulfide 9
ether 8
kR
1
2
3
4
5
6
7
8
9
CN
CN
CN
CN
CN
CN
CN
CN
CN
H
H
9a (87)
9b (81)
9c (86)
9d (82)
9e (83)
9f (77)
9g (83)
9h (57)
9i (87)
9j (57)
8a (13)
8b (19)
8c (14)
8d (18)
8e (17)
8f (23)
8g (17)
8h (43)
8i (13)
8j (43)
1.0
1.57
1.09
1.47
1.37
2.0
1.37
5.05
1.0
4-OMe
3-OMe
4-Me
3-Me
4-Cl
3-Cl
4-CN
3-CN
H
10
(1.0)
relative rate constants, kR, obtained for the 5-(π-exo)exo
cyclizations (Table 1). These values are referred to kH
range 1-5. On these bases it can be inferred that the
enthalpic factor predominates, as expected from a highly
exothermic reaction occurring through an early transition
state.21
)
1 and were calculated through the equation kR ) ([9]H-
[8]R)/([8]H[9]R). The [9]/[8] ratios were obtained from
direct 1H NMR analyses of the corresponding reaction
mixtures by measuring the integrals of the S-methyl
singlet at 2.3 and the O-methyl singlet at 3.3. For this
purpose the reaction mixtures were preliminarily worked
up to eliminate unreacted thiol 1 and alkyne 2 (see
Experimental Section).
In all cases, 1H NMR analyses of the reaction mixtures
showed the presence of small amounts of thiopyrans 5a -i
(see Scheme 2), which were characterized by methylene
signals at 3.8 (CH2S), 4.4 (CdCCH2O), and 4.5 (ArCH2O)
and by the vinyl proton singlet at 6.6-6.7 in the 1H NMR
spectrum. The ratio between the methyl sulfides 9 and
the corresponding thiopyrans 5, irrespective of the nature
of the R substituent, was found to be ca. 12-13:1.
Unfortunately, complete characterization was only per-
formed for the chloro derivative 5f. In all the other cases
it was not possible to obtain a suitable sample owing to
the small amounts of these products.
Thiopyrans 5a -i might in principle arise through two
reaction pathways: a direct 6-membered π-endo cycliza-
tion (Scheme 3, path a), which would occur in competition
with the 5-(π-endo)exo cyclization, and/or ring-expansion
of the spiro radical 7 through three-membered cyclization
followed by intraanular bond scission22 (Scheme 3, path
b). To clarify the actual mechanism, we carried out the
reaction of benzenethiol 1, R′ ) H, with propynyl benzyl
ether 2a . From this reaction the expected products 9j
and 5j, derived from the radical 3j through formal 5- and
6-membered π-endo cyclizations, respectively, were ob-
tained in a 2.5:1 ratio23 in addition to the methyl ether
8j and the adduct 4j. The observed decrease of the [9]/[5]
ratio from 12-13:1 to 2.5:1 on passing from R′ ) 4-CN
to R′ ) H is in very good agreement with a mechanism
involving competing 5- and 6-membered cyclizations.24
In contrast, no change in the [9]/[5] ratio would be
From data reported in Table 1 it can be observed that
both electron-releasing and EWG substituents in the
4-position activate the aromatic ring, although to a small
extent (kpara > 1). Moreover, in all cases kpara values are
slightly higher than the corresponding kmeta values.
These findings indicate that stabilization effects prevail
over polar effects. It is noteworthy that the cyano group
presents the higher kpara value, as a consequence of its
greater capability of delocalizing the unpaired electron,
and the lower kmeta value, as a consequence of its strong
electron-withdrawing effect. This trend suggests that
vinyl radical cyclizations onto aryl rings are SOMO/
HOMO-controlled, as expected from an electrophilic
radical addition. It has been previously suggested15 that
vinyl radicals are electrophilic in character, even though
no definite evidence has been reported yet.16 The ob-
served kCl and kCN values (Table 1, entries 6-9) are quite
similar to the partial rate factors reported in the litera-
ture for the phenylation of chlorobenzene17 and benzoni-
trile18 with benzoyl peroxide. This finding would suggest
that vinyl and phenyl radicals have a comparable phi-
licity, although the reported electronegativity of vinyl
radicals (ø ) (IP + EA)/2 ) 4.63 eV)19 is rather lower
than that of phenyl radicals (ø ) 5.59 eV).20
Nevertheless, it appears that both polar and stabiliza-
tion effects do not play a great role. In fact, vinyl radical
cyclizations onto aromatic rings are rather unselective,
as evidenced by the finding that all kR values lie in the
(21) This claim was been supported by semiempirical calculations
(performed with a AM1 Spartan program) which gave ∆H ) ca. -22
kcal/mol.
(22) Ring expansion of 5-exo to 6-endo radicals are well documented
in cyclizations of â-multiply bonded alkyl or vinyl radicals. See:
Beckwith A. L. J .; O’Shea, D. M. Tetrahedron Lett. 1986, 27, 4525.
Stork, G.; Mook, R., J r. Tetrahedron Lett. 1986, 27, 4529. J asperse, C.
P.; Curran, D. P.; Fevig, D. L. Chem. Rev. 1991, 91 1237. Beckwith A.
L. J .; O’Shea, D. M.; Westwood, S. W. J . Am. Chem. Soc. 1988, 110,
2565. Nanni, D.; Pareschi, P.; Tundo, A. Tetrahedron Lett. 1996, 37,
9337.
(23) A k5/k6 ratio ) 6-7:1 has been reported for related π-endo vinyl
radical cyclizations carried out at room temperature. See: Citterio,
A.; Sebastiano, R.; Maronati A.; Santi, R.; Bergamini, F. J . Chem. Soc.,
Chem. Commun. 1994, 1517.
(24) The cyano group do not affect the rate of vinyl radical cyclization
in 3-position (see Table, entry 9) and thus the 6-membered ortho-
cyclization reactions leading to 5h and 5j, respectively, would occur
at the same rate. In contrast, the formation of sulfide 9j should be
5.05 times slower than 9h (see Table, entry 8). The calculated [9h ]/
[5h ] ) 5.05[9j]/[5j] ) 12.75 ratio value is actually strictly closed to
the observed [9h ]/[5h ] ) 12-13.
(15) Crich, D.; Sun, S.; Brunckova, J . J . Org. Chem. 1996, 61, 605.
Mawson, S. D.; Routledge, A.; Weavers, R. T. Tetrahedron 1995, 51,
4665.
(16) Nevertheless, it should be stated that the “philicity” is a relative
concept. In principle, a radical can behave as an electrophile or
nucleophile, depending on the nature of the radical-acceptor counter-
part. For example, reactions of vinyl radicals with olefins are usually
dominated by SOMO-LUMO interaction; in this cases the vinyl radical
behaves as a nucleophile and the olefin as an electrophile (see ref 6).
(17) Hey, D. H.; Orman, S.; Williams, G. H. J . Chem. Soc. 1961,
565.
(18) Danley, R. L.; Gregg, E. C. J . Am. Chem. Soc. 1954, 76, 2997.
(19) Nicolaides, A.; Borden, W. J . Am. Chem. Soc. 1991, 113, 6750.
(20) Pierini, A. B.; Borosky, G. L.; Baungartner, M. T. Int. J .
Quantum Chem. 1992, 44, 759; Hacaloglu, I.; Gaines, A.; Suzer, S. Org.
Mass Spectrosc. 1993, 28, 285.

540 J . Org. Chem., Vol. 63, No. 3, 1998
Montevecchi and Navacchia
Sch em e 5
Sch em e 4
expected if the mechanism leading to both 5 and 9
occurred through intermediacy of the spiro radical 7.
It is interesting to note that the same [9]/[8] ratio (57:
43) was found in the cases of R ) R′ ) 4-CN and R ) R′
) H (see Table, entries 8 and 10), as expected from
cyclizations occurring from rapidly interconverting (E)-
and (Z)-vinyl radicals 3. This finding rules out the
possibility that the sulfanyl radical addition to alkynes
2 preferentially leads to radicals (Z)-3 from which π-endo
cyclization might in principle occur in competition with
the interconversion to the more stable radicals (E)-3.25
In this case a higher [9]/[8] ratio should be expected for
R ) R′ ) 4-CN, owing to the effect of the 4-cyano group.
The observed slight preference for the 5-(π-endo) cycliza-
â-alkyl substituent in these sp2-hybridized vinyl radicals.
However, a careful chromatographic analysis of the
reaction mixture obtained from thiol 1, R′ ) CN, and
alkyne 2f allowed us to separate two products which were
assigned the structure of the regioisomeric adduct 15f
and the methyl sulfide 16f. 1H NMR analysis showed a
ca. 1:1 15f/16f ratio and a ca. 30:1 9f/16f ratio. Com-
pounds 16f and 15f were clearly derived from the
regioisomeric vinyl radical 14f through hydrogen ab-
straction and 5-(π-exo)exo cyclization, respectively (Scheme
5). To our knowledge, the finding of products deriving
from sulfanyl radical addition to the alkyl-substituted C-2
carbon atom is unprecedented.
tion mode over the 5-(π-exo) one ([9h ,j]/[8h ,j] ) Kekendo
/
Con clu sion s
kexo ) 1.3) might result from the fact that the more stable
(E)-radicals could be preferred at the equilibrium (Ke >
1) (see Scheme 2), even though the role played by possible
different stereoelectronic conditions cannot be valued.
Despite our efforts, no traces of any products derived
from radicals 3a -j through π-exo 6-membered ortho-
cyclization were found, possibly as a consequence of
unfavorable stereoelectronic conditions for this cyclization
mode. This behavior parallels that exhibited by the
related vinyl radicals 10, X ) S, Se, which have been
previously found to undergo cyclization onto the phenyl
ring only in a 5-(π-exo) mode.26 The formal 6-(π-exo)
cyclization product 12 has been suggested to arise from
intermediate spiro radical 11, X ) S, through a ring-
expansion process which competes with the C-S bond
cleavage, eventually leading to 13, X ) S. A similar ring
expansion was not provided by 11, X ) Se, owing to the
lower C-Se bond energy (Scheme 4). In the case of spiro
radicals 6a -j, the ring expansion might be prevented by
the more feasibile spiro-ring scission leading to stable
R-oxymethyl radicals.
In this work we have shown that toluenesulfanyl
radical addition to the C-1 carbon of propynyl benzyl
ethers 2 leads to vinyl radicals 3 which can undergo three
kinds of cyclization, stereoselective 5-(π-endo)exo and
5-(π-exo)exo cyclization, eventually leading to the 1,4-aryl
migration products 9 and 8, respectively, and π-endo
6-membered cyclization, leading to thiopyrans 5.
The rate of the vinyl radical cyclization onto aryl rings
is determined by (a) steric factors which inhibit (E)-vinyl
radicals from undergoing π-exo cyclization, (b) stereo-
electronic factors which favor the 5-membered over the
6-membered cyclization [k5(π-endo)/k6(π-endo) ) 2.5, k5(π-exo)
/
k6(π-exo) . 1], and (c) stabilization and polar factors
determined by the presence of aryl ring substituents. The
nature of the substituent effect would indicate that vinyl
radicals are slightly electrophilic in character. However,
vinyl radical cyclizations appear to be rather unselective
with respect to the nature of the aryl substituent. This
finding suggests that the enthalpic factor might play a
major role.
As mentioned above, radicals 3 arose from regioselec-
tive sulfanyl radical addition to the alkyne triple bond
of 2. Radical addition of thiols to terminal alkylacet-
ylenes has been extensively studied and in all cases only
formation of products derived from sulfanyl radical
addition to the terminal C-1 carbon atom has been
reported.27 The regioselectivity is governed by steric
effects; in fact, no stabilization can be provided by the
Exp er im en ta l Section
Sta r tin g Ma ter ia ls. Toluenethiol 1, R′ ) H, is com-
mercially available. 4-Cyanotoluenethiol 1, R′ ) CN, was
prepared as described in the literature.13d Propynyl benzyl
ethers 2a -g,i were obtained following a previously reported
method.13a According to this procedure, a THF solution (50
mL) of sodium propargylate (50 mmol) (prepared from equimo-
lar amounts of propargyl alcohol and sodium hydride in
anhydrous THF) and the appropriate commercially available
benzyl chloride (50 mmol) was heated in a sealed tube at 80
°C for 8 h. Subsequent work up and column chromatography
gave ethers 2a ,28 2b,29 2c [1H NMR 2.45 (1 H, t, J ) 2.5 Hz),
(25) We have previously suggested that samarium diiodide-promoted
vinyl radicals can undergo 5-exo cyclization onto C-C triple bonds in
competition with the E/ Z interconversion. (Capella, L.; Montevecchi,
P. C.; Navacchia, M. L. J . Org. Chem. 1995, 60, 7424.)
(26) Capella, L.; Montevecchi, P. C.; Nanni, D. J . Org. Chem. 1994,
59, 3368.
(27) Benati, L.; Montevecchi, P. C.; Spagnolo, P. J . Chem Soc., Perkin
Trans 1 1991, 2103 and references therein.
(28) Kwart, H.; Sarner, S. F.; Slutsky, J . J . Am. Chem. Soc. 1973,
95, 5234.

Vinyl Radical Cyclizations
J . Org. Chem., Vol. 63, No. 3, 1998 541
3.80 (3 h, s), 4.17 (2 H, d, J ) 2.5 Hz), 4.6 (2 H, s), 6.8-7.3 (4
H, m); MS m/z (rel intensity) 176 (M+, 30), 135 (30), 122 (100).
Anal. Calcd for C11H12O2: C, 74.98; H, 6.86; O, 18.16. Found:
C, 75.20; H, 6.90.], 2d ,30 2e,31 2f [1H NMR 2.45 (1 H, t, J ) 2.5
Hz), 4.15 (2 H, d, J ) 2.5 Hz), 4.55 (2 H, s), 7.2-7.4 (4 H, m);
MS m/z (rel intensity) 182, 180 (M+, 20), 127, 125 (80), 77 (100).
Anal. Calcd for C10H9ClO: C, 66.49; H, 5.02; Cl, 19.63; O, 8.86.
Found: C, 66.70; H, 5.0.], 2g [1H NMR 2.50 (1 H, t, J ) 2.5
Hz), 4.20 (2 H, d, J ) 2.5 Hz), 4.60 (2 H, s), 7.2-7.3 (4 H, m);
MS m/z (rel intensity) 182, 178 (M+, 4), 181, 179 (20), 141,
139 (80), 127, 125 (80), 77 (100). Anal. Calcd for C10H9ClO:
C, 66.49; H, 5.02; Cl, 19.63; O, 8.86. Found: C, 66.28; H, 5.05.],
2h ,12a and 2i [1H NMR 2.45 (1 H, t, J ) 2.5 Hz), 4.20 (2 H, d,
J ) 2.5 Hz), 4.60 (2 H, s), 7.2-7.7 (4 H, m); MS m/z (rel
intensity) 170 (M+-1, 10), 132 (80), 130 (90), 117 (90), 116 (100).
Anal. Calcd for C11H9NO: C, 77.17; H, 5.30; N, 8.18; O, 9.35.
Found: C, 77.4; H, 5.27; N, 8.2.] in ca. 75-80% yield.
(3H, s), 4.32 (2H, br s; irradiation at 6.62 caused a 6%
enhancement), 4.55 (2H, s), 6.62 (1H, br s), 7.2-7.7 (aromatic
protons); GC-MS m/z 329 (M+).], and 1:1 adduct 15f [1H NMR
(300 MHz) 3.95 (2H, s), 4.05 (2H, br s), 4.45 (2H, s), 5.0 (1H,
t, J ) 1.0 Hz, collapsing to singlet upon irradiation at 4.05),
5.35 (1H, t, J ) 1 Hz, collapsing to singlet upon irradiation at
4.05), 7.2-7.7 (aromatic protons); GC-MS m/z 329 (M+).]. A
repeated column chromatography led to the separation of
almost pure thiopyran 5f (6 mg) [1H NMR (300 MHz) 3.80 (2H,
AB system, J AB ) 10 Hz, inner line separation 2.5 Hz), 4.40
(2H, s; irradiation at 6.72 caused a 5% enhancement), 4.47
(2H, s), 6.72 (1H, s), 7.2-7.7 (7H, m); GC-MS m/z (rel intensity)
329, 327 (M+, 20), 186 (100)].
The following new methyl vinyl sulfides (E)-9 were obtained
as oily products. 9a : 1H NMR (200 MHz) 2.32 (3H, s), 4.28
(2H, s), 4.52 (2H, s), 6.46 (1H, s), 7.2-7.4 (5H, m), 7.6 (AB
system, J AB ) 8.5 Hz); MS m/z (rel intensity) 295 (M+, 15), 91
(100). Anal. Calcd for C18H17NOS: C, 73.19; H, 5.80; N, 4.74;
O, 5.42; S, 10.85. Found: C, 73.40; H, 5.82; N, 4.71; S, 10.88.
9b: 1H NMR (200 MHz) 2.35 (3 H, S), 3.80 (3H, s), 4.25 (2H,
br s), 4.45 (2H, s), 6.45 (1H, br s), 6.90 (2H, d, J ) 8.5 Hz),
7.20 (2H, d, J ) 8.5 Hz), 7,6 (4H, AB system, J AB ) 8.5 Hz);
MS m/z (rel intensity) 325 (M+, 15), 121 (100). Anal. Calcd
for C19H19NO2S: C, 70.13; H, 5.88; N, 4.30; O, 9.83; S, 9.85.
Found: C, 70.35; H, 5.90; N, 4.32; S, 9.82. 9c: 1H NMR (300
MHz) 2.32 (3 H, S), 3.78 (3H, s), 4.28 (2H, br s), 4.50 (2H, s),
6.45 (1H, br s), 6.8-6.90 (2H, m), 7.2-7.3 (2H, m), 7.6 (4H,
AB system, J AB ) 8.5 Hz); MS m/z (rel intensity) 325 (M+, 15),
122 (100), 121 (70). Anal. Calcd for C19H19NO2S: C, 70.13; H,
5.88; N, 4.30; O, 9.83; S, 9.85. Found: C, 70.40; H, 5.9; N,
4.28; S, 9.88. 9d : 1H NMR (300 MHz) 2.35 (6H, s), 4.25 (2H,
s), 4.46 (2H, s), 6.45 (1H, s), 7.1-7.7 (8H, m); MS m/z (rel
intensity) 309 (M+, 5), 105 (100). Anal. Calcd for C19H19NOS:
C, 73.75; H, 6.19; N, 4.53; O, 5.17; S, 10.36. Found: C, 74.0;
H, 6.15; N, 4.5; S, 10.3. 9e: 1H NMR (200 MHz) 2.32 (6H, s),
4.28 (2H, s), 4.48 (2H, s), 6.46 (1H, s), 7.0-7.3 (4H, m), 7.6
(4H, AB system, J ) 8.5 Hz); MS m/z (rel intensity) 309 (M+,
10), 105 (100). Anal. Calcd for C19H19NOS: C, 73.75; H, 6.19;
N, 4.53; O, 5.17; S, 10.36. Found: C, 73.95; H, 6.22; N, 4.5; S,
10.32. 9f: 1H NMR (300 MHz) 2.35 (3 H, S), 4.28 (2H, s), 4.48
(2H, s), 6.45 (1H, s), 7.2-7.7 (8H, m); MS m/z (rel intensity)
331, 329 (M+, 40), 127, 125 (70), 116 (100). Anal. Calcd for
Rea ction P r od u cts. Methyl sulfide 9h and methyl ether
8h have been previously reported.13a Structural assignments
for the unknown sulfides 9a -g,i,j, ethers 8a -g,i,j, adducts
1
4b-g,j, and thiopyran 5j came from H NMR and MS spectral
data in addition to elemental analysis. Thiopyran 5f, methyl
sulfide 16f, and adduct 15f were characterized by GC-MS and
1H NMR spectral analysis. Elemental analysis was not
performed owing to the impossibility of obtaining a pure
sample. Thiopyrans 5a -d ,e-i were not separated and thus
not characterized. Their identification arose from 1H NMR
spectral analyses of the reaction mixtures by assignment of
signals at ca. 3.8 (singlet, CH2S), 4.5 (br singlet, HCdCCH2O),
4.6 (singlet, OCH2Ar), and 6.6 (br singlet, vinylic proton).
1H NMR spectra were recorded at 200 (or 300 MHz) with
Me4Si as internal standard. Mass spectra were recorded with
the electronic impact method.
Rea ction s of Th iols 1, R′ ) H, CN w ith P r op yn yl
Ben zyl Eth er s 2a -i. A benzene solution (50 mL) of the
appropriate thiol 1, R′ ) H, CN (4 mmol), the appropriate ether
2a -i (8 mmol), and AIBN (0.8 mmol) was refluxed for 3 h.
The reaction mixture was washed twice with NaOH 10% and
once with water, the organic layer was dried over Na2SO4, and
the solvent evaporated off. The unreacted thiol 1, R′ ) H, CN,
was generally recovered by acidification of the aqueous layer
and extraction with diethyl ether in 45-50% yield. The
organic residue was chromatographed on a Merck silica gel
column (0.040-0.063 particle size). Elution with petroleum
ether (bp 40-70 °C) (for the reaction of 1, R′ ) H, with ether
1a ) or gradual elution with light petroleum-diethyl ether (in
all the other cases) separated the unreacted ether 2a -i and a
mixture of the appropriate sulfide 9a -j, ether 8a -j, thiopyran
5a -j, and 1:1 adduct 4a -g,j contaminated by small amounts
of unidentified products (ca. 560-600 mg; ca. 80-85% overall
yield based on reacted thiol 1). These mixtures were analyzed
C
18H16ClNOS: C, 65.54; H, 4.89; Cl, 10.75; N, 4.25; O, 4.85;
S, 9.72. Found: C, 65.75; H, 4.86; N, 4.23; S, 9.7. 9g: 1H
NMR (300 MHz) 2.35 (3 H, S), 4.28 (2H, s), 4.48 (2H, s), 6.45
(1H, s), 7.2-7.7 (8H, m); MS m/z (rel intensity) 331, 329 (M+,
40), 127, 125 (100), 116 (100). Anal. Calcd for C18H16ClNOS:
C, 65.54; H, 4.89; Cl, 10.75; N, 4.25; O, 4.85; S, 9.72. Found:
C, 65.85; H, 4.92; N, 4.22; S, 9.66. 9i: 1H NMR (200 MHz)
2.35 (3 H, S), 4.30 (2H, s), 4.52 (2H, s), 6.48 (1H, s), 7.2-7.7
(8H, m); MS m/z (rel intensity) 320 (M+, 4), 132 (60), 116 (100).
Anal. Calcd for C19H16N2OS: C, 71.22; H, 5.03; N, 8.74; O, 4.99;
S, 10.01. Found: C, 71.0; H, 5.0; N, 8.7; S, 10.05. 9j: 1H NMR
(300 MHz) 2.30 (3 H, S), 4.30 (2H, d, J ) 1.1 Hz), 4.52 (2H, s),
6.30 (1H, t, J ) 1.1 Hz), 7.2-7.4 (10 H, m); MS m/z (rel
intensity) 270 (M+, 20), 91 (100). Anal. Calcd for C17H18OS:
C, 75.51; H, 6.71; O, 5.92; S, 11.86. Found: C, 75.25; H, 6.73;
S, 11.83.
1
by H NMR to determine the relative ratios of products 5a -j,
8a -j, and 9a -j. The relative yields of sulfides 9a -j and
ethers 8a -j are reported in Table 1. The 9a -i/5a -i ratios
were found to be ca. 12-13:1. The 9j/5j ratio was found to be
2.5:1. Subsequent repeated column chromatography of the
above mixtures led to the separation of a head fraction (ca. 50
mg) containing unidentified products and fractions containing
analytical samples of sulfides 9a -j, ethers 8a -j (60-65%
overall yield), adducts 4b-g, j (ca. 10%), and thiopyran 5j
(15%) [1H NMR (300 MHz) 3.80 (2H, AB system, J AB ) 10 Hz,
inner line separation ) 2.5 Hz), 4.47 (2H, d, J ) 1.0 Hz), 4.58
(2H, s), 6.55 (1H, t, J ) 1.0 Hz), 7.1-7.5 (9H, m); MS m/z (rel
intensity) 268 (M+, 50), 161 (100), 128 (80), 115 (80), 91 (100).
Anal. Calcd for C17H16OS: C, 76.08; H, 6.01; O, 5.96; S, 11.95.
Found: C, 76.35; H, 5.98; S, 12.0.]. The head fraction from
the reaction of 1, R ) CN, with 2f was subjected to a further
chromatographic separation to give a ca. 2:1:1 mixture (30 mg)
of thiopyran 5f, methyl sulfide 16f [1H NMR (300 MHz) 2.40
The following new methyl allyl ethers (Z)-8 were obtained
as oily products. 8a : 1H NMR (200 MHz) 3.30 (3H, s), 4.0
(2H, s), 4.30 (2H, s), 6.40 (1H, s), 7.2-7.4 (5H, m), 7.6 (AB
system, J AB ) 8.5 Hz); MS m/z (rel intensity) 295 (M+, 15), 91
(100). Anal. Calcd for C18H17NOS: C, 73.19; H, 5.80; N, 4.74;
O, 5.42; S, 10.85. Found: C, 73.4; H, 5.78; N, 4.76; S, 10.9.
8b: 1H NMR (200 MHz) 3.25 (3 H, S), 3.80 (3H, s), 3.95 (2H,
s), 4.35 (2H, s), 6.32 (1H, s), 6.90 (2H, d, J ) 8.5 Hz), 7.20
(2H, d, J ) 8.5 Hz), 7,6 (4H, AB system, J AB ) 8.5 Hz); MS
m/z (rel intensity) 325 (M+, 15), 121 (100). Anal. Calcd for
C
19H19NO2S: C, 70.13; H, 5.88; N, 4.30; O, 9.83; S, 9.85.
Found: C, 70.35; H, 5.85; N, 4.28; S, 9.8. 8c: 1H NMR (200
MHz) 3.30 (3 H, S), 3.78 (3H, s), 4.0 (2H, s), 4.35 (2H, s), 6.41
(1H, s), 6.8-6.9 (2H, m), 7.2-7.3 (2H, m), 7.6 (4H, AB system,
J AB ) 8.5 Hz); MS m/z (rel intensity) 325 (M+, 15), 122 (100),
121 (70). Anal. Calcd for C19H19NO2S: C, 70.13; H, 5.88; N,
4.30; O, 9.83; S, 9.85. Found: C, 69.95; H, 5.85; N, 4.32; S,
(29) Marshall, J . A.; Robinson, E. D.; Zapata, A. J . Org. Chem. 1989,
54, 5854.
(30) Alonso, F.; Barba, I.; Yus, M. Tetrahedron 1990, 46, 2069.
(31) Viola, A.; Collins. J . J .; Filipp, N. Prep., Div. Pet. Chem., Am.
Chem. Soc. 1979, 24, 206.

542 J . Org. Chem., Vol. 63, No. 3, 1998
Montevecchi and Navacchia
irradiation at 5.70), 4.4 (2H, s), 5.7 (1H, dt, J d ) 15 Hz, J t
9.88. 8d : 1H NMR (300 MHz) 2.33 (3H, s), 3.30 (3H, s), 3.96
(2H, s), 4.40 (2H, s), 6.37 (1H, s), 7.1-7.7 (8H, m); MS m/z (rel
intensity) 309 (M+, 5), 105 (100). Anal. Calcd for C19H19NOS:
C, 73.75; H, 6.19; N, 4.53; O, 5.17; S, 10.36. Found: C, 74.05;
H, 6.22; N, 4.5; S, 10.3. 8e: 1H NMR (300 MHz) 2.33 (3H, s),
3.30 (3H, s), 4.0 (2H, s), 4.35 (2H, s), 6.40 (1H, s), 7.0-7.3 (4H,
m), 7.6 (4H, AB system, J AB ) 8.5 Hz); MS m/z (rel intensity)
309 (M+, 5), 105 (100). Anal. Calcd for C19H19NOS: C, 73.75;
H, 6.19; N, 4.53; O, 5.17; S, 10.36. Found: C, 73.95; H, 6.16;
N, 4.5; S, 10.4. 8f: 1H NMR (200 MHz) 3.30 (3H, s), 4.0 (2H,
s), 4.35 (2H, s), 6.40 (1H, s), 7.2-7.7 (8H, m); MS m/z (rel
intensity) 331, 329 (M+, 40), 127, 125 (100), 116 (100). Anal.
Calcd for C18H16ClNOS: C, 65.54; H, 4.89; Cl, 10.75; N, 4.25;
O, 4.85; S, 9.72. Found: C, 65.85; H, 4.87; N, 4.22; S, 9.75.
8g: 1H NMR (200 MHz) 3.30 (3H, s), 4.0 (2H, s), 4.35 (2H, s),
6.45 (1H, s), 7.2-7.7 (8H, m); MS m/z (rel intensity) 331, 329
)
6.5 Hz), 6.20 (1H, dt, J d ) 15 Hz, J t ) 1.5 Hz), 7.1-7.7 (8H,
m); MS m/z (rel intensity) 309 (M+, 5), 105 (100). Anal. Calcd
for C19H19NOS: C, 73.75; H, 6.19; N, 4.53; O, 5.17; S, 10.36.
Found: C, 73.5; H, 6.16; N, 4.5; S, 10.3. 4e: 1H NMRZ-isomer
(200 MHz) 2.3 (3H, s), 3.88 (2H, s), 4.10 (2H, dd, J 1 ) 6.5 Hz,
J 2 ) 1.5 Hz; collapsing to d, J ) 1.5 Hz upon irradiation at
5.80), 4.38 (2H, s), 5.80 (1H, dt, J d ) 10 Hz, J t ) 6.5 Hz), 6.05
(1H, dt, J d ) 10 Hz, J t ) 1.5 Hz), 7.1-7.7 (8H, m); 1H
NMRE-isomer (200 MHz) 2.3 (3H, s), 3.92 (2H, s), 3.95 (2H, dd,
J 1 ) 6.5 Hz, J 2 ) 1.5 Hz; collapsing to d, J ) 1.5 Hz upon
irradiation at 5.70), 4.38 (2H, s), 5.7 (1H, dt, J d ) 15 Hz, J t
)
6.5 Hz), 6.20 (1H, dt, J d ) 15 Hz, J t ) 1.5 Hz), 7.1-7.7 (8H,
m); MS m/z (rel intensity) 309 (M+, 5), 105 (100). Anal. Calcd
for C19H19NOS: C, 73.75; H, 6.19; N, 4.53; O, 5.17; S, 10.36.
Found: C, 73.55; H, 6.23; N, 4.5; S, 10.4. 4f: 1H NMRZ-isomer
(200 MHz) 3.88 (2H, s), 4.08 (2H, dd, J 1 ) 6.5 Hz, J 2 ) 1 Hz;
collapsing to d, J ) 1 Hz upon irradiation at 5.77), 4.40 (2H,
s), 5.77 (1H, dt, J d ) 10 Hz, J t ) 6.5 Hz), 6.08 (1H, dt, J d ) 10
Hz, J t ) 1 Hz), 7.2-7.7 (8H, m); 1H NMRE-isomer (200 MHz)
3.92 (2H, s), 3.97 (2H, dd, J 1 ) 6.5 Hz, J 2 ) 1 Hz; collapsing
to d, J ) 1 Hz upon irradiation at 5.68), 4.40 (2H, s), 5.68 (1H,
dt, J d ) 15 Hz, J t ) 6.5 Hz), 6.20 (1H, dt, J d ) 15 Hz, J t ) 1
Hz), 7.2-7.7 (8H, m); MS m/z (rel intensity) 331, 329 (M+, 40),
127, 125 (100), 116 (100). Anal. Calcd for C18H16ClNOS: C,
65.54; H, 4.89; Cl, 10.75; N, 4.25; O, 4.85; S, 9.72. Found: C,
65.8; H, 4.92; N, 4.2; S, 9.75. 4g: 1H NMRZ-isomer (200 MHz)
3.90 (2H, s), 4.09 (2H, dd, J 1 ) 6.5 Hz, J 2 ) 1 Hz; collapsing
to d, J ) 1 Hz upon irradiation at 5.78), 4.42 (2H, s), 5.78 (1H,
dt, J d ) 10 Hz, J t ) 6.5 Hz), 6.10 (1H, dt, J d ) 10 Hz, J t ) 1
Hz), 7.2-7.7 (8H, m); 1H NMRE-isomer (200 MHz) 3.93 (2H, s),
3.98 (2H, dd, J 1 ) 6.5 Hz, J 2 ) 1 Hz; collapsing to d, J ) 1 Hz
upon irradiation at 5.70), 4.42 (2H, s), 5.70 (1H, dt, J d ) 15
Hz, J t ) 6.5 Hz), 6.20 (1H, dt, J d ) 15 Hz, J t ) 1 Hz), 7.2-7.7
(8H, m); MS m/z (rel intensity) 331, 329 (M+, 40), 127, 125
(70), 116 (100). Anal. Calcd for C18H16ClNOS: C, 65.54; H,
4.89; Cl, 10.75; N, 4.25; O, 4.85; S, 9.72. Found: C, 65.4; H,
4.87; N, 4.22; S, 9.75. 4j: 1H NMRZ-isomer (300 MHz) 3.87 (2H,
s), 4.10 (2H, dd, J 1 ) 6.5 Hz, J 2 ) 1.1 Hz; collapsing to d, J )
1.1 Hz upon irradiation at 5.74), 4.45 (2H, s), 5.74 (1H, dt, J d
) 10 Hz, J t ) 6.5 Hz), 6.15 (1H, dt, J d ) 10 Hz, J t ) 1.1 Hz),
7.2-7.4 (10H, m); 1H NMRE-isomer (200 MHz) 3.90 (2H, s), 3.98
(2H, dd, J 1 ) 6.5 Hz, J 2 ) 1 Hz; collapsing to d, J ) 1.1 Hz
upon irradiation at 5.70), 4.45 (2H, s), 5.70 (1H, dt, J d ) 15
Hz, J t ) 6.5 Hz), 6.24 (1H, dt, J d ) 15 Hz, J t ) 1.1 Hz), 7.2-
7.4 (8H, m); MS m/z (rel intensity) 270 (M+, 20), 91 (100). Anal.
Calcd for C17H18OS: C, 75.51; H, 6.71; O, 5.92; S, 11.86.
Found: C, 75.25; H, 6.7; S, 11.9.
(M+, 40), 127, 125 (100), 116 (100). Anal. Calcd for C18H16
-
ClNOS: C, 65.54; H, 4.89; Cl, 10.75; N, 4.25; O, 4.85; S, 9.72.
Found: C, 65.8; H, 4.92; N, 4.27; S, 9.76. 8i: 1H NMR (200
MHz) 3.30 (3H, s), 4.05 (2H, s), 4.35 (2H, s), 6.52 (1H, s), 7.3-
7.7 (8H, m); MS m/z (rel intensity) 320 (M+, 4), 132 (70), 116
(100). Anal. Calcd for C19H16N2OS: C, 71.22; H, 5.03; N, 8.74;
O, 4.99; S, 10.01. Found: C, 71.0; H, 5.0; N, 8.7; S, 9.97. 8j:
1H NMR (300 MHz) 3.30 (3 H, S), 4.0 (2H, s), 4.40 (2H, s),
6.53 (1H, s), 7.2-7.4 (10H, m); MS m/z (rel intensity) 270 (M+,
20), 91 (100). Anal. Calcd for C17H18OS: C, 75.51; H, 6.71; O,
5.92; S, 11.86. Found: C, 75.2; H, 6.73; S, 11.9.
The following new 1:1 adducts 4 were obtained as 60:40 Z/E
mixtures. 4b: 1H NMRZ-isomer (200 MHz) 3.80 (3H, s), 3.85
(2H, s), 4.02 (2H, br d, J ) 6.5 Hz; collapsing to br s upon
irradiation at 5.75), 4.37 (2H, s), 5.75 (1H, dt, J d ) 10 Hz, J t
) 6.5 Hz), 6.05 (1H, br d, J ) 10 Hz), 6.8-7.4 (8H, m); 1H
NMRE-isomer (200 MHz) 3.80 (3H, s), 3.90 (2H, s), 3.92 (2H, br
d, J ) 6.5 Hz; collapsing to br s upon irradiation at 5.7), 4.37
(2H, s), 5.7 (1H, dt, J d ) 15 Hz, J t ) 6.5 Hz), 6.20 (1H, br d,
J ) 15 Hz), 6.8-7.4 (8H, m); MS m/z (rel intensity) 325 (M+,
15), 121 (100). Anal. Calcd for C19H19NO2S: C, 70.13; H, 5.88;
N, 4.30; O, 9.83; S, 9.85. Found: C, 70.4; H, 5.85; N, 4.32; S,
9.88. 4c: 1H NMRZ-isomer (300 MHz) 3.78 (3H, s), 3.88 (2H,
s), 4.09 (2H, dd, J 1 ) 6.5 Hz, J 2 ) 1 Hz; collapsing to d, J )
1.0 Hz upon irradiation at 5.70), 4.38 (2H, s), 5.7 (1H, dt, J d
10 Hz, J t ) 6.5 Hz), 6.06 (1H, dd, J 1 ) 10 Hz, J 2 ) 1.0 Hz),
6.8-6.9 (2H, m), 7.2-7.3 (2H, m), 7.,6 (4H, AB system, J AB
)
)
1
8.5 Hz); H NMRE-isomer (200 MHz) 3.78 (3H, s), 3.92 (2H, s),
3.98 (2H, dd, J 1 ) 6.5 Hz, J 2 ) 1 Hz; collapsing to d, J ) 1.0
Hz upon irradiation at 5.80), 4.38 (2H, s), 5.8 (1H, dt, J d ) 15
Hz, J t ) 6.5 Hz), 6.20 (1H, dd, J 1 ) 15 Hz, J 2 ) 1.0 Hz), 6.8-
6.9 (2H, m), 7.2-7.3 (2H, m), 7.6 (4H, AB system, J AB ) 8.5
Hz); MS m/z (rel intensity) 325 (M+, 15), 121 (100). Anal. Calcd
for C19H19NO2S: C, 70.13; H, 5.88; N, 4.30; O, 9.83; S, 9.85.
Found: C, 70.45; H, 5.9; N, 4.33; S, 9.9. 4d : 1H NMRZ-isomer
(200 MHz) 2.34 (3H, s), 3.88 (2H, s), 4.07 (2H, dd, J 1 ) 6.5 Hz,
J 2 ) 1.5 Hz; collapsing to d, J ) 1.5 Hz upon irradiation at
5.78), 4.4 (2H, s), 5.78 (1H, dt, J d ) 10 Hz, J t ) 6.5 Hz), 6.05
(1H, dt, J d ) 10 Hz, J t ) 1.5 Hz), 7.1-7.7 (8H, m); 1H
NMRE-isomer (200 MHz) 2.34 (3H, s), 3.92 (2H, s), 3.96 (2H, dd,
J 1 ) 6.5 Hz, J 2 ) 1.5 Hz; collapsing to d, J ) 1.5 Hz upon
Ack n ow led gm en t. This work has been carried out
under the “Progetto di Finanziamento Triennale Ateneo
di Bologna”. We also acknowledge financial support from
the Ministero dell′Universita` e della Ricerca Scientifica
e Tecnologica (MURST) and CNR (Rome).
J O971399L