A new hypervalent iodine precursor of a highly strained cyclic alkyne. Generation and trapping reactions of bicyclo[2.2.1]hept-2-en-5-yne

Full text of DOI:10.1021/jo9819483

680
J . Org. Chem. 1999, 64, 680-681
Communications
have found that (phenyl)[o-(trimethylsilyl)phenyl]iodonium
triflate (5) generates benzyne quantitatively under mild
conditions.8 This benzyne precursor 5 is stable and involves
the very mild reaction conditions such as neutral medium
and room temperature, giving high yields of the benzyne
adducts. Thus, we have applied the benzyne precursor 5 to
a cyclic alkyne system, namely bicyclo[2.2.1]hept-2-en-5-yne
(6) which can be generated by reaction of a hypervalent
iodine precursor, (phenyl)[3-(trimethylsilyl)bicyclo[2.2.1]-
hept-2,5-dien-2-yl]iodonium triflate (7).
The previous study8 on the benzyne precursor 5 suggests
that the most suitable substrate for the generation of cyclic
alkynes is the â-trimethylsilyl-substituted cyclic vinyliodo-
nium salt. The hypervalent iodine group has an extremely
high leaving ability,9 and the trimethylsilyl group is easily
cleavable by fluoride ion.10 Accordingly, this combination
constructs the alkyne precursor best.11 To prepare trimeth-
ylsilyl-substituted cyclic vinyliodonium triflates, we exam-
ined the Diels-Alder reaction of (phenyl)[(trimethylsilyl)-
ethynyl]iodonium triflate (8) with cyclic dienes, which
provides the very convenient and direct synthesis.
First, [(trimethylsilyl)ethynyl]iodonium triflate 812 was
prepared according to the modified method of Bachi and
Stang by using a hypervalent iodine reagent13 readily
prepared from PhI(OAc)2 and trifluoromethanesulfonic acid
(TfOH) or its anhydride (Tf2O). Treatment of bis(trimeth-
ylsilyl)acetylene with PhI(OAc)2 activated with TfOH or Tf2O
in dichloromethane gave alkynyliodonium triflate 8 in 72-
88% yields.
Next, we examined the Diels-Alder reaction with dienes
by the use of [(trimethylsilyl)ethynyl]iodonium triflate 8
according to the method of Stang.14,15 The reaction of 8 with
cyclopentadiene in acetonitrile proceeded efficiently to give
(phenyl)[3-(trimethylsilyl)bicyclo[2.2.1]hepta-2,5-dien-2-yl]-
A New Hyp er va len t Iod in e P r ecu r sor of a
High ly Str a in ed Cyclic Alk yn e. Gen er a tion
a n d Tr a p p in g Rea ction s of
Bicyclo[2.2.1]h ep t-2-en -5-yn e
Tsugio Kitamura,* Masashi Kotani,
Takanobu Yokoyama, and Yuzo Fujiwara
Department of Chemistry and Biochemistry, Graduate
School of Engineering, Kyushu University, Hakozaki,
Fukuoka 812-8581, J apan
Kenzi Hori
Department of Applied Chemistry and Chemical Engineering,
Yamaguchi University, Tokiwadai, Ube, 755-8611, J apan
Received September 28, 1998
Cyclic alkynes with small rings are of considerable inter-
est in chemistry because of their high strain and reactivity.1
Generally, alkynes bearing more than eight-membered ring
size are stable and isolable compounds, but cycloheptyne,
cyclohexyne, and cyclopentyne are unstable and exist only
as short-lived intermediates. Alkynes with three- and four-
membered rings have not been observed experimentally. As
a consequence, the smallest cyclic alkyne observed experi-
mentally is five-membered cyclopentyne (1), which can be
generated by reaction of 1,2-dibromopentene (2),2 by oxida-
tion of 1,2-bis(hydrazono)cyclopentane (3),2a,3 and by ring
expansion of cyclobutylidenecarbene (4).4 However, the
yields of adducts of cyclopentyne 1 with the trapping agents
are low.2-4 The unexpected side-reactions take place due to
the use of the unstable precursors and the severe reaction
conditions. In addition to the monocycloalkynes, strained
bicycloalkynes have been also reported.5
(7) For recent other reviews and books on hypervalent iodine compounds,
see: (a) Koser, G. F. In The Chemistry of Functional Groups, Supplement
D; Patai, S.; Rappoport, Z., Eds.; Wiley: Chichester, 1983; pp 721-811 and
pp 1265-1351. (b) Ochiai, M. Rev. Heteroatom Chem. 1989, 2, 92-111. (c)
Moriarty, R. M.; Vaid, R. K. Synthesis 1990, 431-447. (d) Varvoglis, A. The
Organic Chemistry of Polycoordinated Iodine; VCH Publishers: New York,
1992. (d) Koser, G. F. In Supplement D2: The Chemistry of Halides, Pseudo-
Halides and Azides; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, 1995;
pp 1173-1174. (f) Stang, P. J .; Zhdankin, V. V. Chem. Rev. 1996, 96, 1123-
1178. (g) Varvoglis, A. Hypervalent Iodine in Organic Synthesis; Academic
Press: San Diego, 1997.
A new and excellent precursor for the generation of
strained cyclic alkynes is clearly required, especially one
which is stable, easy to handle, and efficient to generate a
strained cyclic alkyne under mild conditions. In continuation
of our studies on reactive hypervalent iodine reagents,6,7 we
(8) Kitamura, T.; Yamane, M. J . Chem. Soc., Chem. Commun. 1995, 983-
984.
(9) (a) Wiberg, K. B.; Pratt, W. E.; Matturro, M. G. J . Org. Chem. 1982,
47, 2720-2722. (b) Okuyama, T.; Takino, T.; Suede, T. Ochiai, M. J . Am.
Chem. Soc. 1995, 117, 3360-3367.
(10) For a fluoride ion-induced desilylation, see: Cunico, R. F.; Dexhe-
imer, E. M. J . Am. Chem. Soc. 1972, 94, 2868-2869. Cunico, R. F.;
Dexheimer, E. M. J . Organomet. Chem. 1973, 59, 153-160. Himeshima,
Y.; Sonoda, T.; Kobayashi, H. Chem. Lett. 1983, 1211-1214. Atanes, N. A.;
Escudero, S.; Pe´rez, D.; Guitia´, E.; Castedo, L. Tetrahedron Lett. 1998, 39,
3039-3040.
(11) The reaction of the corresponding 3-(trimethylsilyl)bicyclo[2.2.1]hept-
2,5-dien-2-yl tosylate with Bu4NF does not give alkyne 6 but takes place
only desilylation. See, Padwa, A.; Wannamaker, M. W. J . Chem. Soc. Chem.
Commun. 1987, 1742-1743.
(12) Bachi, M. D.; Bar-Ner. N.; Crittell, C. M.; Stang, P. J .; Williamson,
B. L. J . Org. Chem. 1991, 56, 3912-3915.
(1) (a) Hoffmann, R. W. Dehydrobenzene and Cycloalkynes; Academic
Press: New York, 1967. (b) Krebs, A. In Chemistry of Acetylenes; Viehe, H.
G., Ed.; Marcel Dekker: New York, 1969; pp 987-1062. (c) Nakagawa, M.
In The Chemistry of the Carbon-Carbon Triple Bond; Patai, S., Ed.;
Wiley: Chichester, 1978; pp 635-712. (d) Greenberg, A.; Liebman, J . F.
Strained Organic Molecules; Academic Press: New York, 1978; pp 133-
138. (e) Krebs, A.; Wilke, J . Top. Curr. Chem. 1983, 109, 189-233. (f) Gleiter,
R.; Merger, R. In Modern Acetylene Chemistry; Stang, P. J ., Diederich, F.,
Eds.; VCH: Weinheim, 1995; pp 285-319.
(2) (a) Wittig, G.; Krebs, A.; Pohlke, R. Angew. Chem. 1960, 72, 324. (b)
Wittig, G.; Pohlke, R. Chem. Ber. 1961, 94, 3276-3284. (c) Wittig, G.;
Weinlich, J .; Wilson, E. Chem. Ber. 1965, 98, 458-470.
(3) Wittig, G.; Krebs, A. Chem. Ber. 1961, 94, 3260-3275.
(4) (a) Erickson, K. L.; Wolinsky, J . Am. Chem. Soc. 1965, 87, 1142-
1143. (b) Fitjer, L.; Modaressi, S. Tetrahedron Lett. 1983, 24, 5495-5498.
(c) Gilbert, J . C.; Baze, M. E. J . Am. Chem. Soc. 1983, 105, 664-665. (e)
Gilbert, J . C.; Baze, M. E. J . Am. Chem. Soc. 1984, 106, 1885-1886.
(5) Gassman, P. G.; Valcho, J . J . J . Am. Chem. Soc. 1975, 97, 4768-
4770. Shahlai, K.; Hart, H. J . Am. Chem. Soc. 1988, 110, 7136-7140.
Komatsu, K.; Aonuma, S.; J inbu, Y.; Tsuji, R.; Hirosawa, C.; Takeuchi, K.
J . Org. Chem. 1991, 56, 195-203.
(13) (a) Kitamura, T.; Matsuyuki, J .; Taniguchi, H. Synthesis 1994, 147-
148. (b) Kitamura, T.; Kotani, M.; Fujiwara, Y. Synthesis 1998, 1416-1418.
(14) (a) Stang, P. J .; Zhdankin, V. V. J . Am. Chem. Soc. 1991, 113, 4571-
4576. (b) Ryan, J . H.; Stang, P. J . J . Org. Chem. 1996, 61, 6162-6165.
(15) Williamson, B. L.; Stang, P. J .; Arif, A. M. J . Am. Chem. Soc. 1993,
115, 2590-2597.
(16) Willey, F. G. Angew. Chem. 1964, 76, 144. Very recently, 2-(trim-
ethylsilyl)cyclohexenyl triflate was reported as a new source of cyclohexyne,
see: Atanes, N.; Escudero, S.; Pe´rez, D.; Guitia´n, E.; Castedo, L. Tetrahedron
Lett. 1998, 39, 3039-3040.
(6) (a) Kitamura, T. J . Synth. Org. Chem. J pn. 1995, 53, 893-905. (b)
Kitamura, T.; Fujiwara, Y. Org. Prep. Proced. Int. 1997, 29, 409-458.
10.1021/jo9819483 CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/20/1999

Communications
J . Org. Chem., Vol. 64, No. 3, 1999 681
Sch em e 1
zofuran in dichloromethane, the carbonyl-containing adduct
(11) was obtained in 43% yield after isolation by column
chromatography on silica gel. Surprisingly, we obtained the
product different from that we have expected. According to
the Wittig’s reports,2b,3 the cycloadducts derived from the
reaction of 1,3-diphenylisobenzofuran with cycloheptyne and
cyclooctyne are isomerized to the corresponding carbonyl
compounds on heating. On the basis of these results, this
carbonyl compound 11 should be formed by the similar
isomerization reaction of the primary cycloadduct (12)
derived from the reaction of bicyclo[2.2.1]hept-2-en-5-yne 6
with diphenylisobenzofuran (Scheme 1).
Since it is seemed that the use of 7 is useful for generation
of cyclic alkynes, we compared the reaction of bicyclo[2.2.1]-
hept-2-en-5-yne precursor 7 with those of the cyclopentyne
precursors. There are three major methods generating
cyclopentyne 1 from 1,2-dibromopentene 2,2 1,2-bis(hydra-
zono)cyclopentane 3,2a,3 and bromomethylenecyclobutane.4
In the reactions with 1,3-diphenylisobenzofuran, these
methods give the cycloadduct in 0.5-12% yields. On the
other hand, the reaction of the cyclic iodonium triflate 7
using Bu4NF gives the cycloadduct, although it is isomerized
one, in 43% yield. Apparently this type of cyclic iodonium
triflate gives better results than the reported cyclic alkyne
precursors.
Since bicyclo[2.2.1]hept-2-en-5-yne 6 is a bicyclic alkyne
constructed by five- and six-membered rings, it is expected
to have more strain than cyclopentyne 1. To estimate the
strain of the bicyclic alkyne 6, we examined the π bond strain
of 6 derived by comparison to the reference molecules
according to the calculation by J ohnson and Daoust.18 We
considered the similar isodesmic reaction shown in eq 2 for
comparison. By the calculation at MP2/6-31G* level, we got
72.7 kcal/mol of the in-plane π bond strain. This value is
between those obtained from cyclobutyne and cyclopentyne.18
If we add the strain energy of norbornadiene, 24.0 kcal/mol,19
as an approximation for other strain components, the total
strain energy of 96.7 kcal/mol is obtained. This strain energy
is much higher than that of cyclopentyne (68.1 kcal/mol) but
lower than that of cyclobutyne (101.8 kcal/mol).18 Conse-
quently, bicyclo[2.2.1]hept-2-en-5-yne 6 is a highly strained
cyclic alkyne.
iodonium triflate 7 in 96% yield (eq 1). The similar reaction
with 1,3-diphenylisobenzofuran in dichloromethane also
gave the corresponding cycloadduct in 66% yield. Unfortu-
nately, the reactions with other dienes such as butadiene,
2,3-dimethylbutadiene, and furan did not give the cycload-
ducts. Although [(trimethylsilyl)ethynyl]iodonium triflate 8
is not so active, the Diels-Alder reaction of 8 with cyclo-
pentadiene or 1,3-diphenylisobenzofuran gives the satisfac-
tory results.
The generation of cyclic alkyne 6 was examined by the
reaction of cyclic vinyliodonium triflate 7 with Bu4NF in the
presence of tetraphenylcyclopentadienone. Treatment of
cyclic vinyliodonium triflate 7 with a THF solution of Bu4-
NF in the presence of tetraphenylcyclopentadienone in
dichloromethane gave a crystalline cycloadduct, 1,2,3,4-
tetraphenylbenzonorbornadiene (9), in 69% isolated yield.
The formation of the cycloadduct 9 strongly supports the
generation of bicyclo[2.2.1]hept-2-en-5-yne 6, which under-
goes the [4 + 2] cycloaddition with tetraphenylcyclopenta-
dienone to give the intermediate cycloadduct (10) followed
by release of CO to form the product 9, as shown in Scheme
1. The similar reactions giving benzocycloalkenes have been
observed in the cases of cyclic alkynes such as cyclohexyne,16
cycloheptyne,16,17 and cyclooctyne.2a,16 Therefore, the forma-
tion of benzoadduct 9 confirms the intervention of cyclic
alkyne 6.
In conclusion, we have succeeded in the generation of a
highly strained cyclic alkyne, bicyclo[2.2.1]hept-2-en-5-yne
6, by using the hypervalent iodine precursor 7 and found
that the cyclic alkyne 6 is efficiently trapped with tetraphen-
ylcyclopentadiene or 1,3-diphenylisobenzofuran. We believe
that bicyclo[2.2.1]hept-2-en-5-yne 6 is one of the highly
strained cyclic alkynes observed experimentally by the
trapping reactions.
Then, we conducted the reaction in the presence of 1,3-
diphenylisobenzofuran which is also well-known as a trap-
ping agent. When cyclic iodonium triflate 7 was treated with
a THF solution of Bu4NF in the presence of diphenylisoben-
(17) Wittig, G.; Meske-Schu¨ller, J . J ustus Liebigs Ann. Chem. 1968, 711,
65-75.
Ack n ow led gm en t. This work was partly supported by
Grants-in-Aid for Scientific Research from the Ministry of
Education, Science, Sports and Culture, J apan.
(18) J ohnson, R. P.; Daoust, K. J . J . Am. Chem. Soc. 1995, 117, 362-
367.
(19) Wiberg, K. B. Angew. Chem., Int. Ed. Engl. 1986, 25, 312-322. This
value is obtained by adding 4.8 kcal/mol of the olefinic strain to norbornene
and smaller than those reported earlier (34.720 and 31.6 kcal/mol21).
(20) Schleyer, P. von R.; Williams, J r., J . E.; Blanchard, K. R. J . Am.
Chem. Soc. 1970, 92, 2377-2386.
Su p p or tin g In for m a tion Ava ila ble: Characterization data
for compounds 7, 9, and 11. This material is available free of
charge via the Internet at http://pubs.acs.org.
(21) Allinger, N. L.; Sprague, J . T. J . Am. Chem. Soc. 1972, 94, 5734-
5747.
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