U”.Rev. 1999.93, 487-505
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Techniques and Applications of Far-UV Photochemistry in Solution. The Photochemistry of the C3H4and C4HsHydrocarbons William J. Leigh oapamnanr Of chanlray.m s l w l h m , nammon. ontarlo, ceneda LBS 4M1 Recslved
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17. 1 9 9 2 ( R e M Manuscript Recehgd Novembw 25. 1992)
Contents I. Introduction 11. Far-UV Photochemistry In Solution: Techniques A. Light Sources and Flkers E. Apparatus, Solvents. and Procedures C. Actinometers 111. The Photochemistry of Cyclopropenes. Allenes. and Alkynes: Interconversions of the C3H1 Hydrocarbons A. Background E. Theory C. The Photochemistry of Akylcyclopropenes and Allenes D. The Photochemlsby of Alkynes E. Summary IV. orbital Symmetry and the Photochemistry of Alkylcyclobutenes
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A. Theory E. me Photochemistry of Cyclobutene: Histwical C. The Photochemistry of Alkylcyclobutenes 1. Identification of the Excked State@) Responsible for Ring Opening and Cycloreversbn 2. Scope and Mechanism of the Cycloreversion Reaction 3. Scope and Mechanism of the Ring-Openlng Reaction D. Recent Theoretical Resuns The Photochemisby of Elcyclo[l.l.O]b~~tanes A. Thermal Rearrangements E. Eicyclo[l.l.O]butanes from Photolysis of 1.3-Dlenes c. The Photochemistry of Elcyclo[l.l.O]butanes D. Theory The Photochemistry of Methylenecyclopropanes A. Thermal Rearrangement E. Photochemical Rearrangement c. Theory Summary and Prospects References
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VII. VIII.
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I . Introduction Far-UV light sources have been employed in studies of the photochemistry of small molecules in the gas 0009-2665/93/0793-0487$12.00/0
William J. L a m was raised in Chalham. Ontario, and anended tJM Unhrersilyof Western Ontario, where he received an Honours B.Sc. In 1976 and the W.D. degree in 1981 under D. R. Arnold’s supervision. He underwent postdoctoral training wlth R. Srinivasan at the IBM Research Center in Yorktown Heights. NY. and later wntr J. C. Scaiano at the National Research Council of Canada in Ottawa. He accepted an appointment with McMastar University In 1983 as Assistant Professor and NSERC Universlty Research Fellow and was promoted to the rank of Professor in 1992. I n addlion to his ongoing interest in the far-UV photochemistry of s m a l l ~ u i e s i n s o l u i i o nLegh’scunentresearchinterestsindude . the study of reactive intermediatesin organosilicon chemistry using nanosecond laser flash photolysis techniques and lhe effects of wdered madla such as liquid crystals and cyclodextrins on thermal and photochemical reactions.
phase for many years, but their use in solution-phase studies has become common only relatively recently. There are a number of special considerations which come into play in studies of this type, but in spite of this, solution-phase far-UV photochemistry is now a well-established field and the special techniques involved are becoming routine. Far-UVtechniquesmake it possible to study the photochemistry of molecules which contain simple chromophores, unencumbered with conjugatingsubstituentsthatdelocalize the lowest excited state and lower its energy. In short, they make it possible to photolyze systems which absorb only a t wavelengths less than ca. 200 nm-the practical cutoff wavelength for ‘conventional” photochemical studies with medium-pressure mercury lamps and ordinary quartz reaction vessels. The use of far-UV techniques for the study of photochemical processes in condensed phases was pioneered independently by N. C. Yang and C. von Sonntag in the late 19608, in studies of the photochemistry of aliphatic alcohols and ethers in the neat liquid phase.’ The extension to studies of small molecule photochemistry in dilute solution (principally alkenes and cyclopropanes) was brought about later by Y. Inoue2and R. Srinivasan? who demonstrated (among 0 1993 Amalcan Chemical Society
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Chemical Reviews, 1993, Vol. 93,No. 1
other things) that UV light of energy high enough to break any bond in an organic molecule does in fact usually result in fairly clean and selective photochemistry. In addition to laying out the basic experimental requirements and establishing a reliable actinometer for use in experiments of this type: they reported comprehensive investigations of the photochemistry of simple aliphatic alkenes and cyclopropanes5~6-studies which have provided invaluable comparisons with the photochemistry of more highly substituted analogs. This work was quickly extended to encompassmore complex, bichromophore and mixed chromophore systems such as nonconjugated dienes, allyl- and vinylcyclopropanes, bis-cyclopropyl systems, and enol ethers. All of this work has been comprehensively reviewed, and the reader is directed elsewhere for descriptions of the photochemistry of these systemsm6P6 Out of these studies emerged one of the most significant recent developments in the use of pulsed UV lasers-the ablative photodecomposition (APD) of polymeric materials, which was originally discovered using the ArF excimer line a t 193 nm.' This technique is now used widely in technological and medical applications8and employs lasers of wavelengths in both the far- and mid-UV ranges (193, 248, and 308 nm). The focus of the present review is on the use of solution-phase far-UV techniques to study the photochemistry of organic molecules which are distinguished by their particular theoretical importance. Indeed, the ability to study directly-under conditions of complete thermalization of the excited state-the photochemistry of molecules small enough to lend themselves to independent ab initio theoretical study at the highest levels of sophistication is arguably the most significant aspect of the contribution that solution-phase far-UV techniques can make to the field of organic photochemistry. In recent years, the excited-state behavior of several such systems has been studied in detail experimentally and theoretically. These are the C3H4 hydrocarbons [cyclopropene (l),allene (2), and propyne (3); Scheme I3 and the cyclic members of the C4H6 hydrocarbon family [cyclobutene (4), bicyclobutane (5), and methylenecyclopropane (S)] (eqs 1-3). Scheme I. The C3H4 Hydrocarbons H
x
HA
H
or A
hv or {hv
H
1
hv or\v
A
H2C'C=CH2
H
hv or A
or A
- = CH3
2
El
hv -,
3
\w/
'
+
= +
=
[l]
4
a>-% 5
'\
+
Ll
121
I I. Far-UV Photochemistry In Solutlon: Technlques A. Light Sources and Filters
Early studies in the area employed unfiltered lowpressure mercury lamps, whose main emission lines arise at 184.7 and 253.8 nm in an intensity ratio of about 1:7. These lamps are still the most economical sources of far-UV radiation for solution-phase studies and are particularly convenient for applications where the substrate and its photoproducts are transparent at 254 nm. For quantitative and semipreparative (100-300 mg) work, small U-shaped lamps (10or 20 W) are ideal: although larger lamps (up to ca. 1m in length and power 40 W) are available for larger scale preparative photolyses.lo The smaller lamps can be used in conjunction with an interference filter (at the sacrifice of most of the lamp's radiant energy) to reduce the intensity of the 254-nm component (to 1-2% of its normal value) in cases where it is absorbed by photoactive products.ll Commercially available interference filters have lifetimes of about 200 h with a 10-W lamp. As an alternative, we have employed 3-mm-thick lithium fluoride windows (vacuum-UV grade) which have been irradiated with y-rays from a 'Wo s ~ u r c e . ~The ~J~ irradiated windows exhibit strong absorption centered at 249 nm (OD > 5 for 3-mm windows) with very little absorption at 185 nm.13 The color centers giving rise to the 249-nm absorption degrade upon UV irradiation and must be monitored closely, but they can be regenerated repeatedly by resubjecting the window to 'Wo radiation. Unfortunately, the "stability" of these filters (toward UV bleaching of the 249-nm absorption) varies substantially from crystal to crystal. Good quality crystals can provide filters stable for dozens of hours of UV irradiation, but acquisition of "good quality" crystals is evidently a matter of providence. Filters made using seemingly identical windows obtained from the same supplier have showed very poor UV stability, and for this reason we have abandoned this approach to obtaining monochromatic light in the far-UV range. Zinc and cadmium resonance lamps (16 W) provide monochromatic radiation at 214 and 228 nm,14respectively, with insignificant output at other wavelengths below ca. 275 nm.15 These lamps are marketed as spectroscopic sources and are about the same size as the 10-W low-pressure mercury lamps. They can thus be used with the same apparatus (vide infra). A somewhat more expensive source of monochromatic far-UV radiation is the ArF excimer laser, which should be operated at low power ( < l e 2 0 mJ) and with the beam unfocused in order to avoid potential problems from two-photon processes. Quantitative work should be carried out at low repetition rates (
