Reaction intermediates, photochemical generation

Photochemical irradiation of dimethyl and diethyl sulphoxides yields the corresponding sulphone in the presence of air and a photosensitizer such as methylene blue in yields up to 99% . Sulphoxides are also oxidized when they act as traps for persulphoxides, the intermediate formed on reaction of a sulphide with photochemically generated singlet oxygen - , equation (9). Isotope studies have shown that such reactions proceed through a linear sulphurane intermediate . Persulphones also react with sulphoxides in a similar manner , equation (10). 

In this article we have summarized the use of both photochemical and more classical thermal kinetics techniques to deduce the nature of intermediates in the ambient temperature, fluid solution chemistry of several triruthenium clusters. In some cases the photochemically generated intermediates appear to be the same as those proposed to be formed along thermal reaction coordinates, while in other cases unique pathways are the results of electronic excitation. The use of pulse photolysis methodology allows direct observation, and the measurement of the reaction dynamics of such transients and provides quantitative evaluation of the absolute reactivities of these species. In some cases, detailed complementary information regarding... 

Clearly, mechanistic investigations can provide circumstantial evidence for the participation of particular intermediates in a reaction but, here, we are concerned with the definitive observation of these species. If the intermediates are relatively stable then direct spectroscopic observation of the species during a room-temperature reaction may be possible As a rather extreme example of this, the zero-valent manganese radicals, Mn(CO>3L2 (L phosphine) can be photochemically generated from Mh2(CO)gL2, and, in the absence of O2 or other radical scavengers, are stable in hydrocarbon solution for several weeks (2, 3) However, we are usually more anxious to probe reactions in which unstable intermediates are postulated. There are, broadly speaking, three approaches - continuous generation, instantaneous methods and matrix isolation. 

Since differences were often reported in product yields from photochemical and thermal reactions, it was not clear that the same intermediate was generated in both cases. This issue was complicated by the fact that the temperatures under whieh the two experiments were run were usually quite different. The acid-base chemistry of nitrenium ions was largely unexplored so it was not known under what conditions these species could be protonated or deprotonated. It had also not been demonstrated that nitrenium ions played any role in the biological activity of mutagenic and carcinogenic esters of N-arylhydroxylamines or hydroxamic acids, particularly in their reactions with the DNA bases. Over the next decade these issues would be resolved but many questions about nitrenium ion chemistry would remain unanswered. 

Figure 9.9 Bergman cycloaromatization reactions for ltex-3-ett-l,5-diyne and its perfluorinated congener, as well as a photochemical reaction scheme for generating the perfluorinated diradical from an iodinated precursor. What spectral features would be expected to be most diagnostic of the different intermediates What levels of theory would be appropriate for predicting these spectral signatures (Note that equilibrium arrows of unequal length indicate which species predominates at equilibrium.)...

Finally, it should be pointed out that methods used to study short-lived chemical intermediates in fast thermal reactions may be applicable also to photochemical studies. Radical intermediates, however generated, can be studied by CIDNP ichemically induced dynamic nuclear spin polarization), in which the n.m.r. spectrum of the reaction mixture is recorded during the reaction period. II a substrate is continuously irradiated with ultraviolet/visible light in the cavity of an n.m.r. spectrometer, the resulting n.m.r. spectrum of the substrate/product mixture exhibits intensity variations as compared with the normal spectrum—intensity enhancement, reduction or even reversal (i.e. emissionl. Note that the spectrum involved is not... 

The photochemistry of aryl azides is quite complex, suggesting that the nitrene 14 may not be the only reactive intermediate and that insertion reactions may not be the only route to form photoconjugates.Although aryl nitrenes are much less susceptible to rearrangements than acyl nitrenes, they may still occur and lead to the formation of reactive intermediates such as azepines, which may go on to react with nucleophiles.[911 141 Addition of nitrenes to double bonds will generate azirines, while dimerization will produce azobenzenesJ11 Aryl azides are stable to most of the procedures used in the course of peptide synthesis except for reduction reactions. Non-photochemical reduction of aryl azides to the primary amines by thiols has been reported by Staros et al.[15]... 

Photochemically generated carbene 231a reacts with (tcrf-butylmethylidyne)phosphine to form the stable l(X5),2(X3)-diphosphete 234137. It has been proposed that 2//-phosphi-rene 235 is a transient intermediate in this reaction (equation 79) it would be formed by [1+2] cycloaddition of 231a to the P,C triple bond in complete analogy to azirene 233 and would undergo spontaneous ring-expansion to 234. 

Oxasiletene, the unsaturated analogue of oxasiletane, was first postulated by Seyferth and coworkers as a reactive intermediate in the reaction of l,l-dimethyl-2,3-bis(trimethylsilyl)-l-silirene with dimethyl sulfoxide25. The photochemical generation of siladienone intermediate 28 from (pentamethyldisilanyl)diazomethyl 1-adamantyl ketone... 

One laser pulse is used for the generation of a transient species (excited state or reaction intermediate) and a second laser pulse conveniently delayed with respect to the first one is used for the photolysis of the transient24-30 Because most of these photochemical processes occur in the nanosecond-second time domain, the probe of preference has been the pulsed Xe lamp described in Section 6.5.2. A short-lived laser pulse, delayed with respect to the second laser flash, can also be used as a probe of the photolyzed transient species. 

Functionalized chromium carbene complexes can be prepared by the reaction of polyfunctional diorganozincs with photochemically generated Cr(CO)5.THF. The resulting intermediate ate complexes furnish under 1 atm of carbon monoxide an acyl complex which can be treated with Meerwein salt (Me30+ BF4" in dry CH2C12) at -30°C to give a chromium carbene complex (Scheme 9.3). 

A major supposition of this computational mechanistic study is the separation of the photochemical and thermal reaction events. It has been assumed, Scheme 1, that a photochemical reaction takes place to generate a coordinatively unsaturated intermediate that subsequently reacts thermally with dihydrogen. In other words, we are assuming that the reaction is photochemically initiated but that light plays no role in later steps of the reaction (for at least one cycle). 

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