C O M M U N I C A T I O N S
Scheme 3
respective pathways “a” and “b” as supported by laser irradiation
at 355 nm. Under continuous irradiation with broadband UV, any
pyrrole 6 regenerated from 8 is recycled to the photometathesis
product. Laser irradiation of pure 7 (R ) CONHEt, R′ ) Me, R′′
) H) at 355 nm showed no reaction, indicating that this final step
is irreversible.
In summary a new photochemically mediated intramolecular
metathesis sequence of pyrroles has been described. The reaction
has been shown to proceed via two sequential wavelength-dependent
reactions involving [2+2] cycloaddition at 220-280 nm followed
by a retro-[2+2] of the resulting cyclobutane at wavelengths longer
than this. Reversibility in cyclobutane formation is well documented,
and in the context of metathesis there have been reports of
photochemically mediated [2+2] followed by thermally mediated
retro-[2+2].9 However, to the best of our knowledge the present
study represents the first example of a metathesis sequence where
both steps are photochemically controlled. Notably these two
discrete steps can be decoupled and studied by wavelength selection
from tunable UV lasers.
Scheme 4. Proposed Mechanism for the Wavelength-Dependent
Photometathesis of Pyrroles (R ) CONHEt, R′ ) Me, R′′ ) H)
Acknowledgment. We thank EPSRC (GR/S25593) and GSK
for generous funding of this study and Keith N. Rosser for help
with the lasers. A.J.O.E. thanks the Leverhulme Trust for the award
of a Senior Research Fellowship.
As quantities of the thermally stable cyclobutane intermediate 8
(R ) CONHEt, R′ ) Me, R′′ ) H) were available from irradiations
using low-pressure lamps we undertook a study to gain some insight
into the likely mechanism of the photometathesis sequence.
Significantly, it was found that the overall sequence proceeds via
two discrete wavelength-dependent reactions which could be
decoupled and studied by appropriate choice of monochromatic
source. The fact that irradiation of 6 (R ) CONHEt, R′ ) Me, R′′
) H) yields predominantly 8 (R ) CONHEt, R′ ) Me, R′′ ) H)
at 222 nm (laser) and at 254 nm (low-pressure lamp) suggests that
the strong UV absorption of ∼250 nm exhibited for this pyrrole is
the key region for excitation for the initial [2+2] cycloaddition.
The cyclobutane 8 (R ) CONHEt, R′ ) Me, R′′ ) H) exhibits a
strong absorption feature at 300-350 nm in the UV spectrum. It
was found that irradiation of pure 8 (R ) CONHEt, R′ ) Me, R′′
) H) with monochromatic 355 nm UV from a Nd:YAG laser gave
equimolar amounts of the starting pyrrole 6 (R ) CONHEt, R′ )
Me, R′′ ) H) and photometathesis product 7 (R ) CONHEt, R′ )
Me, R′′ ) H).
A more elaborate example involving photometathesis of a fused
pyrrole was realized by submitting cyclohexane-1,4-dione to a Knorr
pyrrole synthesis followed by N-alkylation. Irradiation of the
resulting pyrrole resulted in clean photometathesis to the complex
enone-enamine 10 (Scheme 3).
On the basis of these above findings a general mechanism for
the complete photometathesis sequence using broadband UV light
is proposed in Scheme 4. Absorption of shortwave UV (220-280
nm) first results in excitation of the pyrrole nucleus in 6 engaging
the initial [2+2] cycloaddition to 8. Continued irradiation (>280
nm) likely results in the triplet diradical 9 (initially via n f π*
excitation of the enone CdO). This diradical then undergoes
fragmentation to product 7 and back to starting pyrrole 6 via the
Supporting Information Available: Experimental procedures and
characterization data for pyrroles and photoproducts and X-ray crystal-
lographic data. This material is available free of charge via the Internet
References
(1) For a recent highlight of arene phototchemistry see: Mattay, J. Angew.
Chem., Int. Ed. 2007, 46, 663.
(2) (a) Pe´rez-Ruiz, R.; Miranda, M. A.; Alle, R.; Meerholz, K.; Griesbeck,
A. G. Photochem. Photobiol. Sci. 2006, 5, 51. (b) D’Auria, M.; Emanuele,
L.; Racioppi, R. J. Photochem. Photobiol., A 2004, 163, 103.
(3) (a) Bellas, M.; Bryce-Smith, D; Gilbert, A. Chem. Commun. 1967, 263.
(b) Basaric, N.; Marinic, Z.; Sindler-Kulyk, M. J. Org. Chem. 2006, 71,
9382.
(4) (a) Hiraoka, A. Chem. Commun. 1970, 1306. (b) Barltrop, J.; Day, A. C.;
Moxon, P. D.; Ward, R. R. J. Chem Soc., Chem. Commun. 1975, 786.
(5) Cantrell, T. S. J. Chem. Soc., Chem. Commun. 1972, 155.
(6) Booker-Milburn, K. I.; Anson, C. E. A.; Clissold, C.; Costin, N. J.; Dainty,
R. F.; Murray, M.; Patel, D.; Sharpe, A. Eur. J. Org. Chem. 2001, 1473.
(7) (a) Mazzocchi, P. H.; Bowen, M. J.; Narain, N. K. J. Am. Chem. Soc.
1977, 99, 7063. (b) Mazzocchi, P. H.; Minamikawa, S.; Bowen, M. J. J.
Org. Chem. 1978, 43, 3079. (c) Mazzocchi, P. H.; Minamikawa, S.;
Wilson, P. J. Org. Chem. 1979, 44, 1186. (d) Mazzocchi, P. H.; Wilson,
P.; Khachik, F.; Klingler, L.; Minamikawa, S. J. Org. Chem. 1983, 48,
2981.
(8) Substituted pyrroles used were known compounds synthesized by selective
4-acylation of 2-trichloroacetyl pyrrole. N-akylations were carried out on
the pyrrole anions and 5-bromo-1-pentene or with 4-pentene-1-ol by
Mitsunobu reaction.
(9) (a) Inaki, I. CRC Handbook of Organic Photochemistry and Photobiology;
Horspool, W., Lenci, F., Eds.; CRC Press: Boca Raton, FL, 2004; p 104.
(b) Kammermeier, S.; Herges, R. Angew. Chem., Int. Ed. 1996, 35, 417.
(c) Wiberg, K. B.; Matturro, M.; Adams, R. J. Am. Chem. Soc. 1981,
103, 1600. (d) Camps, P.; Font-Bardia, M.; Perez, F.; Solans, X.; Vazquez,
S. Angew. Chem., Int. Ed. 1995, 34, 912. (e) Fessner, W. D.; Murty, B.
A. R. C.; Prinzbach, H. Angew. Chem., Int. Ed. 1987, 26, 451.
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