Direct photolysis, triplet carbenes

The yield of trans product (18) is decreased by the presence of a radical scavenger such as 1,1-diphenylethylene and increased by dilution of the reactants with methylene chloride or butane, indicating this product to result from the triplet carbene. A heavy-atom effect on the carbene intermediate was observed by photolysis of a-methylmercuridiazoacetonitrile. With c/s-2-butene as the trapping agent either direct photolysis or triplet benzophenone-sensitized decomposition results in formation of cyclopropanes (19) and (20) in a 1 1 ratio ... 

The term methylene is exclusively reserved for CH2. Spectroscopic investigations demonstrated that photolysis of diazo compounds in fact produces free carbenes. A flash photolysis of diazomethane gave methylene whose spectrum could be recorded 3i>. ESR-spectra were taken of a series of triplet carbenes which had been obtained by direct irradiation of diazo compounds in various matrices at low temperatures (see p. 97). 

Photolysis of diazomethane in carbon-tetrachloride in the presence of benzophenone yields 1,1,1,2-tetrachloroethane showing an enhanced absorption due to the triplet carbene. The direct photolysis of diazomethane proceeds via singlet methylene CIDNP-studies of the photolysis of methyl-diazoacetate, for which a radical pair mechanism was suggested, were recently challenged 2). 

The transition state of singlet carbene cycloaddition to alkenes involves an electrophilic approach of the vacant p orbital to the n bond of alkenes. By contrast, the first step of the triplet addition process may involve the in-plane a orbital of the carbene. As in the case of C—H insertion (see Section 5.1), the difference in the transition structure between the singlet and triplet cycloaddition becomes important in the intramolecular process, especially when approach to a double bond is restricted by ring strain. Direct photolysis of ( )-2-(2-butenyl)phenyldiazomethane (99) in the presence of methanol gives l-ethenyl-l,la,6,6fl-tetrahydrocycloprop [fljindene [100, 29%, (E/Z)= 10 1] and l-(2-butenyl)-2-(methoxymethyl)benzene (101, 67%). Triplet-sensitized photolysis results in a marked increase in the indene (52%, EjZ) = 1.3.T) at the expense of the ether formation (4%) (Scheme 9.30). On the other hand, direct photolysis of phenyldiazomethane in an equimolar mixture of... 

More direct evidence for the intervention of excited states of triplet carbenes in reactions in solution is obtained by spectroscopic studies. Thus, picosecond lasers make it possible to study the quenching of carbene fluorescence by various substrates in solution at room temperature. Diphenylcarbene is generated upon laser photolysis of 30 and a second UV laser pulse is time delayed by 8 ns and is used to excite the carbene, thereby producing the excited triplet DPC (Scheme 9.32). The fluorescence of DPC is then monitored with a streak camera. The fluorescence... 

The photolysis of diazoalkanes both in the gas phase and in solution is a carbenoid reaction. Moreover, the results of EPR-spectroscopic investigations (Section IIB) demonstrate that triplet carbenes can be generated by irradiation of diazoalkanes. That the reactive intermediates in carbenoid reactions are free carbenes is usually taken as self-evident. While such an assumption is probably wholly justified in most cases, it is worth remembering that both in the gas phase and in solvents such as n-hexane, the electronic absorption spectra of simple diazoalkanes show definite fine structure (Bradley etal., 1964a). This implies that the photo-excited state is bonding (Hoffmann, 1966) and consequently may have a life-time long enough to enable it to react directly with another molecule... 

Photolysis of the 3if-pyrazole 60 with a Cu/Ni-salt filter (A 330—410 nm) afforded the vinyldiazoalkane 61. This could be interconverted to the cyclopropene 63 via the vinylcarbene 62. Direct photolysis with light A > 350 nm gives 63 directly. ESR spectroscopy after irradiation at 5 K indicated clearly a triplet carbene 62 76>. 

The direct photolysis leads after ring opening to vinyldiazocompound 71 which decomposes via the carbene 72 to 73. Mechanistic studies demonstrated that the rather selective carbene 72 involved adds nonstereospecifically to cis- or trans-4-methyl-pent-2-ene and therefore from a triplet state 81>. The difference of the reaction path for the photolysis of 37f-pyrazoles 74 leading to 68d, e (R =H, m—Cl) or [1.2]-spirene 75 (R =p-Cl) may be attributed to intersystem crossing induced... 

On the other hand, cycloaddition of cyclobutene by bis(methoxycarbonyl)carbene yielded dimethyl bicyclo[2.1.0]pentane-5,5-dicarboxylate (10) as the sole product (55% yield) in the direct photolysis, whilst the triplet carbene reaction in addition to 10 produced two vinylcy-clopropanes 11 and 12 which probably arise from the same intermediate 1,3-diradical. 

At that time, it was not possible to measure any of the rate constants of Scheme 1 directly but in some cases it was possible to measure ratios of rate constants or to determine if spin equilibration was much faster or slower than intermolecular reactions. Organic chemists could then only dream of determining the absolute rate constants of Scheme 1. This would become possible around 1980 with the invention of laser flash photolysis with nanosecond (ns) time resolution. But successful application of this tool would require knowledge of the electronic spectra of singlet and triplet carbenes. Low temperature spectroscopy was enormously helpful in this regard. 

ESR parameters for triplet carbenes and nitrenes have been summarized, and it has been shown that phenylnitrene is produced predominantly (87-88%) in the singlet state by direct photolysis of phenyl azide in low-temperature matrices. The first spectroscopic observation of a singlet nitrene has been reported nanosecond-laser photolysis of 1-azidopyrene gives the So nitrene (A ax 450 nm) which has a lifetime of 22 nsec at room temperature (in benzene) and 34 nsec at 77 K in rigid solution. At room temperature it decays to the triplet ground state (Tj, A ax 415 nm) with a rate constant of about 4.4 x 10 sec T, is formed directly by biacetyl sensitized photolysis of the azide. The lifetime of the excited triplet (Tj) was about 7 nsec. Tj dimerizes to azopyrene. ... 

All the thermal protocols discussed above initially form singlet carbenes, as do the photolysis of diazo and diazirine compounds. For cases where the triplet is the ground state, relaxation to the triplet state can occur in competition with additions and insertions. In addition, it is possible to use a photochemical sensitizer (see Chapter 16) to produce a triplet state directly. Here, the sensitizer is a photo-excited triplet state of an additive that transfers its energy to the carbene precursor, creating the triplet carbene directly. 

---