11986
J. Am. Chem. Soc. 1996, 118, 11986-11987
Scheme 1
Alkenylation of C-H Bonds via Reaction with
Vinyl and Dienyl Triflones. Stereospecific Synthesis
of Trisubstituted Vinyl Triflones via Organocopper
Addition to Acetylenic Triflones1
Jason Xiang and P. L. Fuchs*
Department of Chemistry, Purdue UniVersity
West Lafayette, Indiana 47907
ReceiVed August 9, 1996
We recently reported that reaction of ethers, sulfides, and
hydrocarbons with acetylenic triflones such as 2a,b provides
facile access to substituted alkynes 3 (Scheme 1).2 The reaction
proceeds via radical C-H abstraction3 by the very electrophilic
trifluoromethyl radical 44 in a process involving addition of alkyl
radical 5 to the R-carbon of the acetylenic triflone 2 followed
by elimination of the vinyl radical 6 to alkyne 3 and trifluo-
romethylsulfonyl radical 7. Fragmentation5 of 7 to sulfur
dioxide and the trifluoromethyl radical 4 propagates the chain.6
Consistent with this mechanism is the finding by Russell that
photogenerated cyclohexyl radicals undergo reaction with
â-phenylethynyl phenyl sulfone (8)7 to generate phenyl cyclo-
hexyl acetylene (10) and phenyl sulfonyl radical (which does
not fragment to sulfur dioxide and phenyl radical,8 thereby
unable to propagate a similar C-H activation event). In the
same study, Russell made the important observation that radical
addition to the vinyl sulfone 11 provides alkene 13 via a similar
addition-elimination process (Scheme 2).7
Scheme 2
Scheme 3
The second example suggested that our C-H functionaliza-
tion protocol shown in Scheme 1 might be extended to the
domain of olefins. In order to assess that possibility, we
examined the reaction of THF (15a) and cyclohexane (15b) with
the vinyl triflone E-14 and the dienyl triflone E,E-17, which
we had previously prepared via a Peterson olefination protocol.9
As can be seen in Scheme 3, these reactions provide direct
access to C-H functionalized olefins E-16a,b and dienes E,E-
18a,b.
The stereochemistry of the radical addition-elimination
reaction was investigated using isomeric vinyl triflone Z-14.10
Not surprisingly, reaction with THF (15a) again provides adduct
E-16a to the total exclusion of adduct Z-16a, indicating that
intermediate 19 undergoes bond rotation prior to elimination
of the trifluoromethyl sulfonyl radical 7 or that 7 can re-add to
the Z-olefin Z-16a (Scheme 4).
Scheme 4
We next explored the C-H alkenylation reaction with a set
of trisubstituted vinyl triflones Z,E-20, Z-21, and Z,E-22. As
with previous examples, radical addition-elimination reaction
with THF (15a) was high yielding, but the products 24-26 were
(1) Syntheses Via Vinyl Sulfones 68. Triflone Chemistry 7. For triflone
paper 6, see ref 6.
(2) Gong, J.; Fuchs, P. L. J. Am. Chem. Soc. 1996, 118, 4486.
(3) Medebielle, M.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1991,
113, 6872.
(4) Bond dissociation energy HCF3 ) 107 kcal/mol: Handbook of
Chemistry and Physics, 74th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton,
FL, 1993-1994; pp 9-137.
Z/E mixtures of the trisubstituted olefin, consistent with
expectations from equilibration of the â-trifluoromethylsulfonyl
radical intermediate 23 (Scheme 5).
(5) (a) Langlois, B. R.; Laurent, E.; Roidot, N. Tetrahedron Lett. 1992,
33, 1291. (b) Hu, C.-M.; Qing, F.-L.; Huang, W.-Y. J. Org. Chem. 1991,
56, 2801. (c) Huang, W.-Y.; Hu, L.-Q. J. Fluorine Chem. 1989, 44, 25. (d)
Langlois, B. R.; Laurent, E.; Roidot, N. Tetrahedron Lett. 1991, 32, 7525.
(6) Xiang, J.; Fuchs, P. L. Tetrahedron Lett. 1996, 37, 5269.
(7) Russell, G. A.; Ngoviwatchai, P. J. Org. Chem. 1989, 54, 1836 and
references cited therein. This finding has recently been extended to the
additions of R-alkoxy radicals to styryl sulfoximines (Clark, A. J.; Rooke,
S.; Sparey, T. J.; Taylor, P. C. Tetrahedron Lett. 1996, 37, 909). For general
references to olefin formation via â-sulfonyl radical chemistry, see: Ono
N.; Kamimura, A.; Kaji, A. J. Org. Chem. 1987, 52, 5111 and references
cited therein.
Examination of Table 1 reveals that intermediate 23 retains
a substantial memory of the stereochemistry of its vinyl triflone
precursor, with fragmentation occurring predominantly via a
least motion retention process. The decrease in reaction rate
and accompanying increase in stereospecificity as one proceeds
down the table is consistent with progressively smaller stability
of the â-trifluoromethylsulfonyl radical intermediate 23.
More interesting is the observation that the reactions of diaryl-
substituted vinyl triflones Z- and E-20a initially exhibit high
stereospecificity, which is substantially degraded as the reaction
proceeds (Table 1, entries 2a-c, 4a-c). Control reactions
(8) (a) Bertrand, M. P. Org. Prep. Proc. Int. 1994, 26, 257. (b)
Chatgilialoglu, C. Sulfonyl Radicals. In Chemistry of Sulphones and
Sulphoxides; Patai, S., Ed.; John Wiley: New York, 1988; pp 1089-1113.
(9) Mahadevan, A.; Fuchs, P. L. Tetrahedron Lett. 1994, 35, 6025.
(10) This material was prepared by photochemical isomerization of E-14.
See ref 12.
S0002-7863(96)02790-4 CCC: $12.00 © 1996 American Chemical Society
Xiang, Jason
