Diazo compounds triplet carbene reactions

When monitoring the transient due to triplet carbenes is difficult because of the inherent weak nature of the bands and/or severe overlapping with the absorption bands of the parent diazo compounds, it is more convenient to follow the dynamics of the triplet carbene by measuring the rate of the products formed by reaction of triplet carbenes with quenchers such as radicals (Section 5.3) and carbonyl oxides (Section 6.5). In this case, note that the observed rate constant (feobs) of a triplet carbene reaction is the sum of the decay rate constants of the triplet. These may include decay via an associated but invisible singlet with which the triplet is in rapid equilibrium. Thus in general. 

There is much evidence that the mechanism" of the 1-pyrazoline reactions generally involves diradicals, though the mode of formation and detailed structure (e.g singlet vs. triplet) of these radicals may vary with the substrate and reaction conditions. The reactions of the 3 f-pyrazoles have been postulated to proceed through a diazo compound that loses N2 to give a vinylic carbene." ... 

Oxo-2,5-cyclohexadienylidene [83] was generated in solid argon at 9 K by irradiation of diazo compound [84] with visible light (A>495 nm) (Sander et al., 1988 Bucher and Sander, 1992 Bucher et al., 1992). The IR, UV, and esr spectra of [83] were in accord with a structure having a triplet state with one delocalized electron. In the IR spectrum of the carbene [83] the r (CO) mode was found at 1496 cm which indicates a bond order of the C—O bond considerably less than 2. The low-temperature reaction of carbene [83] with CO generated the keto-ketene [85]. Irradiation (A = 543 10 nm) of [83] led to its transformation into a very labile species, presumed to be [86], which rearranged back to [83] not only under UV or... 

The triplet state is usually the ground state for non-conjugated structures, but either species can be involved in reactions. The most common method for generating nitrene intermediates, analogous to formation of carbenes from diazo compounds, is by thermolysis or photolysis of azides.246... 

Direct irradiation or thermolysis of a diazo-compound (2) is believed to generate the carbene initially in its singlet spin state. Triplet sensitization (5) is presumed to give the triplet carbene directly without first forming its singlet state via the triplet diazo-compound. In some cases, careful comparison of the results of direct irradiation experiments with those from triplet sensitization can provide useful information to identify the spin state initiating a reaction. 

Triplet-Intermediates from Diazo-Compounds (Carbenes) Table 7. Reactions of triplet-carbenes in solution... 

In contrast to 2-alkylarylcarbenes, triplet carbonyl carbenes do not abstract H from 5- or e-CH bonds. Photolysis of diazo compounds (7) in methanol gave products due to Wolff rearrangement (8) and 0-H insertion (9). Sensitized photolysis led, in addition, to the H-abstraction product (10). Analysis of the results indicated that a large proportion of the insertion product (9) arises from the excited diazo compound and that spin inversion of the triplet carbene is faster than H-abstraction from the solvent. Intersystem crossing to the singlet state is a major reaction of all triplet carbonyl carbenes that are not rapidly scavenged intramolecularly. 

Photosensitized decomposition of 9b in substituted benzenes led to similar results [83DIS(B)(44)1113]. Therefore, either spin inversion from the triplet to the singlet form of 4-diazoimidazole is faster than the decomposition of the excited diazo compound, or intersystem crossing from the triplet to singlet carbene is easier and faster than the reaction of the triplet state with substrates. 

Take the reaction of carbenes generated by photolysis of diazo compounds (Scheme 9.1), for example. Direct irradiation of a diazo compound (12) is believed to generate the carbene initially in singlet state ( 3) via the singlet excited state of the precursor. Triplet sensization, on the other hand, is presumed to give the triplet... 

Non-stereospecific cyclopropanation reactions of the diazafluorenylidene (10), generated by photolysis of the diazo compound, indicated a triplet carbene.18 Competition experiments suggested a singlet-triplet equilibrium at room temperature and a Hammett study of additions to substituted styrenes indicated that the carbene reacts as an electrophile (p = —0.65). 

Tlie decomposition of diazo compounds can be run under very difierem conditions. Their photochemical decomposition has been largely investigated in the gas phase and in solution in the presence of olefinic and saturated hydrocarbons as substrates the fundamental concepts concerning the relative reactivity and selectivity of singlet-triplet states of carbenes are essentially the result of such studies and they have formed the basis for the determination of the correla tion between the spin state of the carbene and its selectivity and reactivity in imerliun and cycloaddition reactions. 

Triplet sensitization of two 1,2-diaryIdiazoethanes in methanol solution gave rise to stilbenes, by intramolecular 1,2-H shifts, and ethers, by reaction of the corresponding carbenes with methanol.This method of generating the intermediate carbenes bypasses the singlet excited diazo compounds, and thus eliminates the possibility that the products arise by direct reaction of these excited species. The results go some way towards establishing that 1,2-H and 1,2-C shifts can compete with alcohol trapping of a spin-equilibrated carbene. 

Some arylcarbenes, prepared by photodecomposition of the corresponding diazo compounds, undergo intersystem crossing so effectively that the alkenes present are mainly attacked by triplet-excited carbenes. This type of reactivity has, in particular, been associated with nitro-substituted arylcarbenes. The reaction between photochemically generated 4-nitrophenyl-carbene and (Z)- and ( )-but-2-ene, which has been studied in particular detail, has shown that nonstereospecific carbene addition may become just as important as the stereospecific reaction. ... 

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