significant upfield shift of the guest signals (Fig. 2b and c). Then,
the resulting complex solution was irradiated with a high-
pressure mercury lamp at room temperature under anaerobic
and neutral conditions. After 6 h, the reaction mixture was
extracted with chloroform. Both NMR (Fig. 2d) and GC-MS
(Fig. S1, ESIw) revealed that alkyne 2 was completely consumed
and that only a single product was involved in the extract. By
comparison with an authentic sample, the product was deter-
mined to be benzyl butyl ketone (3), an anti-Markovnikov
hydration product. The yield estimated by GC analysis was 27%.
In this reaction, unfortunately, insoluble by-products were
formed as a precipitate, which, after dissolving in DMSO-d6,
showed very broad signals in the NMR spectrum (Fig. S2, ESIw).
It is, however, noteworthy that the Markovnikov adduct was
not detected both in the extract and in the precipitate by NMR
and GC-MS and that the photohydration proceeded in the
anti-Markovnikov fashion.
Benzyl ketone 3 is usually unstable towards UV light and
shows typical photochemical reactions of ketones, such as a-
cleavage at the carbonyl group and intramolecular g-hydrogen
abstraction (Fig. S3, ESIw). However, in the cavity of cage 1a,
further photoreactions of photo-hydrated product 3 were
prevented (Fig. S4, ESIw). Thus cage 1a serves as a nano-
metre-sized protection chamber for UV light.
The reaction rate of photo-driven alkyne hydration depends
on the cavity shape. Bowl-shaped cage 1b also encapsulated
arylalkyne 2 to form 1 : 1 inclusion complex 1bÁ2. When photo-
hydration was carried out for inclusion complex 1bÁ2, the
reaction was completed within 3 h to give 3 in 28% yield.10
We suppose that the open structure of 1b allowed the facile
attack of the guest by the surrounding water molecules.
Substituent effects on the benzene ring of arylalkyne 2 were
investigated for the photo-hydration in the cavity of cage 1b.
The product yields of arylalkynes 4a and 4b with an electron-
withdrawing group were almost the same as that in the
reaction with non-substituted 2. However, in the case of 4c
with a methoxy group, the reaction did not occur. Presumably,
the intermediary guest radical cation is too stable to react with
water before back electron transfer from the host occurs. For
4d and 4e, substrates were not encapsulated by cage 1b and no
reaction took place.
When the same reaction was carried out in D2O solution,
the benzylic protons (Hk) of photoproduct 3 were deuterated
(Fig. 2e). This result supports the hypothesis that one water
molecule from the bulk solvent added to alkyne 2 under UV
light irradiation. A hydroxyl radical is not involved as an active
species for the photo-hydration because treatment of Fenton’s
reagent did not afford 3. Neither (tmeda)Pd(ONO2)2 nor the
triazine ligand promoted the reaction. Internal arylalkynes
with different alkyl chains, Ph–CRC–R (R = n-Pr, Et, Me),
showed the same reactivity as alkyne 2.9
Based on the observations in the above reaction and the
previously reported alkane photo-oxidation,7 we could suggest
the following mechanism for the photo-hydration of arylalkynes
that proceeds via guest-to-host electron transfer (Scheme 1).
First, cage 1a is excited by photoirradiation to gain high
oxidative ability. Then, electron transfer from the alkyne to
the electron-deficient cage occurs to generate a highly active
phenyl alkyne radical cation. A water molecule (solvent)
attacks the radical cation intermediate. The regioselectivity is
controlled so that the resulting radical is stabilized at the
benzylic position, leading to anti-Markovnikov hydration.
Back electron transfer to the benzylic position, followed by
immediate protonation, affords the benzyl ketone derivatives
via keto–enol tautomerization.
In summary, we achieved anti-Markovnikov hydration
of internal arylalkynes within a coordination cage under UV
light irradiation and obtained photodegradable benzyl
ketones. In this reaction, the cages serve as photosensitizing
molecular flasks,11 offering the possibility of cage-mediated
guest-to-host electron transfer that subsequently brings out the
new reactivity and unusual reaction selectivity of encapsulated
guests.
We thank Mr Hongzhi Liu for experimental support. This
research was supported in part by the Global COE Program
(Chemistry Innovation through Cooperation of Science and
Engineering), MEXT, Japan.
Notes and references
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153, 228; (b) V. V. Markownikow, C. R. Hebd. Seances Acad. Sci.,
1875, 85, 668; (c) M. Beller, J. Seayad, A. Tillack and H. Jiao, Angew.
Chem., Int. Ed., 2004, 43, 3368; (d) L. Hintermann and A. Labonne,
Synthesis, 2007, 1121; (e) M. B. Smith and J. March, in March’s
Advanced Organic Chemistry: Reactions, Mechanisms, and Structure,
John Wiley & Sons., Hoboken, NJ, 6th edn, 2007, p. 1019.
2 (a) G. Zweifel and H. C. Brown, Org. React., 1963, 13, 1;
(b) H. C. Brown and S. K. Gupta, J. Am. Chem. Soc., 1972, 94, 4370.
3 T. Shimada and Y. Yamamoto, J. Am. Chem. Soc., 2002,
124, 12670.
4 (a) M. Tokunaga and Y. Wakatsuki, Angew. Chem., Int. Ed., 1998,
37, 2867; (b) T. Suzuki, M. Tokunaga and Y. Wakatsuki, Org.
Lett., 2001, 3, 735; (c) M. Tokunaga, T. Suzuki, N. Koga,
T. Fukushima, A. Horiuchi and Y. Wakatsuki, J. Am. Chem.
Soc., 2001, 123, 11917; (d) D. B. Grotjahn and D. A. Lev, J. Am.
Chem. Soc., 2004, 126, 12232.
Scheme 1 Plausible mechanism for the photo-hydration of arylalkynes
within a self-assembled coordination cage: (a) photoexcitation of the
coordination cage; (b) guest-to-host electron transfer; (c) hydration of
the alkyne (two possible pathways shown); (d) back electron transfer
then protonation; (e) keto–enol tautomerization.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 10960–10962 10961