4486
J. Am. Chem. Soc. 1996, 118, 4486-4487
Scheme 1
Alkynylation of C-H Bonds via Reaction with
Acetylenic Triflones1
Jianchun Gong and P. L. Fuchs*
Department of Chemistry, Purdue UniVersity
West Lafayette, Indiana 47907
ReceiVed October 20, 1995
In connection with our program investigating the chemistry
of the sulfone functional group, we had occasion to prepare
acetylenic triflones 1a,b (Scheme 1) via the direct reaction of
acetylenic anions with triflic anhydride.2 An exceptional
observation attended our attempts to use THF as a solvent for
the chemistry of acetylenic triflone 1a. Simply adding 1a to
THF resulted in an exothermic reaction which produced
R-alkynylated tetrahydrofuran in essentially quantitative yield
(Table 1, entry 1). Similar reactions with five- and six-
membered ethers and sulfides results in the formation of
R-functionalized ethers and sulfides in good to excellent yields
(Table 1, entries 3-5, 10-13). In contrast, anisole and phenyl
isopropyl ether are recovered unchanged from the reaction.3 For
those reactions run in solution, the best compromise solvent
appears to be 1,2-dichloroethane.4 While even this compound
undergoes alkynylation, the reaction is quite slow and most
substrates are efficiently alkynylated with minimal competitive
production of the propargyl chloride (Table 1, entry 14).5
In an effort to probe the regiospecificity of the reaction, the
substrates of Figure 1 were similarly investigated. Lower
regiospecificities were observed upon initiation with AIBN at
reflux (procedure B) as compared to reaction at 25 °C without
added initiator (procedure A).
Table 1. Reaction with Acetylenic Triflones 1a,b
Even more astounding is the observation that unactiVated
hydrocarbons are alkynylated when heated in the presence of
1a,b and AIBN or via photochemical activation. Cyclopentane,
cyclohexane, cycloheptane,6 cis-1,4-dimethylcyclohexane (Fig-
ure 1),7 2,3-dimethylbutane, adamantane, and norbornane8 afford
the products shown in Table 1 (entries 6, 7, 9, 15, 17, 19) while
cubane, which has an olefin-like hybridization of ∼31% s
character,9 does not undergo successful alkynylation.
(1) Syntheses via Vinyl Sulfones. 61. Triflone Chemistry 5.
(2) Acetylenic triflones have traditionally been prepared in 50-70% yield
by addition of triflic anhydride to a solution of acetylenic anion (Hanack,
M.; Wilhelm, B.; Subramanian, L. R. Synthesis 1988, 88, 592-595);
however, we have found that inVerse addition substantially improves the
yield (J. Xiang, A. Mahadevan, P. L. Fuchs, submitted for publication).
(3) Acetylenic triflones have been previously reported to react with
alcohols, DMF, and DMSO: (a) Massa, F.; Hanack, M.; Subramanian, L.
R. J. Fluorine Chem. 1982, 19, 601. (b) Hanack, M.; Willhelm, B. Angew.
Chem., Int. Ed. Engl. 1989, 28, 1057.
(4) While Freon 112 and perfluorohexane are unreactive with 1a,b, they
do not accommodate clean alkynylation reactions.
(5) The 1,2-dichloroethane adduct (Table 1, entry 14) is easily detected
as a distinctive pink-staining UV-active spot when visualized on silica TLC
using acidic ethanolic anisaldehyde reagent. Note Added in Proof: CH3CN
is also a good reaction solvent. Adamantane gives 81% product with
acetylenic triflone 1a (compare Table 1, entry 17).
a Procedure A: neat substrate, probable trace peroxides, 25 °C.
Procedure B: neat substrate, ∼20% AIBN, reflux. Procedure C: 1.1-
1.2 equiv of substrate, AIBN, ClCH2CH2Cl, reflux. Procedure D: neat
substrate, Rayonet reactor, hν 254 nm, 25 °C. *Based upon 50%
recovered 1a.
(6) When this reaction was conducted under the photolytic conditions
(condition D), a 1:1 mixture of cycloheptylphenylacetylene and compound
22 was produced in 80% yield. Examination of the crude product from the
thermal reaction (condition B) reveals only traces of compound 22.
Apparently the cycloheptyl radical undergoes partial oxidation to cyclo-
heptene (20), which then produces 22 as shown in Scheme 2. No evidence
for similar products was seen in the reactions of cyclopentane or cyclo-
hexane.
(7) Both products 8 and 9 are one major (>3:1) diastereomer. The
stereochemical assignment has yet to be undertaken, but on the basis of
mechanistic arguments (axial attack from the most stable radical intermedi-
ate), it is expected that 8 bears the trans-dimethyl configuration, while 9
has the trans-methyl/alkynyl geometry.
The reaction has the characteristics of a radical process.
Highly purified THF does not react with alkynyl triflone 1a
when heated in an argon atmosphere, but warming in the
presence of AIBN, peroxides, or small amounts of oxygen serves
to initiate the process. Conversely, addition of BHT or
m-dinitrobenzene prevents the reaction from occurring.
While mechanistic experiments are underway, the tentative
hypothesis shown in Scheme 1 appears to be consistent with
the information currently available. The reaction is postulated
to be initiated (Z• ) init) and propagated (Z ) R′•) by addition
of a radical to the â-carbon of acetylenic triflones 1a,b thereby
(8) Radical reactions of norbornane are known to occur at the secondary
center because of the greater strain associated with the formation of the
bridgehead radical via abstraction of the tertiary hydrogen (see ref 18). The
13C NMR of the product uniquely defines the regiochemistry, while
decoupling of the proton spectra is indicative of the exostereochemistry.
(9) Eaton, P. E.; Galoppini, E.; Gilardi, R. J. Am. Chem. Soc. 1994, 116,
7588.
S0002-7863(95)03518-9 CCC: $12.00 © 1996 American Chemical Society
Gong
