Carbene triplet sensitizers

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. 

Cyclohexane and cyclohexane-d12 have been used as the probe for crossover and, hence, the reactive multiplicity of the subject carbenes. By combining direct and triplet-sensitized generation of the carbene with kinetic analysis from laser spectroscopy and the results of the crossover experiments, a rather complete picture of the reaction of aromatic carbenes with hydrocarbons emerges. 

The significance of a value of AG 5.2 kcal mol -1 is that most of the bimolecular reactions of the triplet carbene will be faster than the formation of the singlet from the triplet. Thus intersystem crossing from BA to 3BA is irreversible, and if 3BA is formed first (triplet sensitization), then there will be no reaction at all of BA. However, in the direct irradiation of DABA, some... 

In contrast to the behavior of BA, triplet sensitized formation of XA in the presence of alcohol gives exactly the same result as does the direct irradiation ether is formed in very high yield. Thus, even when 3XA is formed first, the chemical behavior observed is that of XA. This finding is consistent with a carbene having a singlet ground state a view at least supported by the absence of an epr spectrum for the triplet carbene as noted above. 

The species identified as XA reacts with styrene to give the expected cyclopropane. The rate constant for this reaction is ca 200 times less than the corresponding rate constant for 3BA (Table 6). Also, use of the deuterium-labeled a-methylstyrene reveals that the cyclopropanation occurs with essentially total retention of stereochemistry. Moreover, precisely the same result is obtained when this carbene is formed by triplet sensitization rather than by direct irradiation. These findings also point to a reaction originating from a singlet carbene. 

Irradiation of DAAN in benzene gives 3AN. This carbene reacts with oxygen very rapidly to give an intermediate believed to be the carbonyl oxide. The triplet carbene reacts with labeled a-methylstyrene to give the cyclopropane with total loss of stereochemistry (Table 6). Direct irradiation in neat isopropyl alcohol gives the ether in low yield (relative to the yields from XA, DMFL, FL, and BFL). The other products are those expected to result from hydrogen-atom abstraction. Triplet-sensitized irradiation of DAAN in the alcohol does not give a detectable amount of the ether. 

The chemical behavior of PM is characteristic of a carbene for which equilibration of spin-states is faster than irreversible reaction. In particular, direct and triplet-sensitized irradiations of 2-n-butylphenyldiazomethane give exactly the same products (Baer and Gutsche, 1971). A similar conclusion was reached in the study of ANM (Hadel et al., 1983). 

As has already been mentioned (vide supra, p. 118) the population of the triplet state of a carbene depends on the effectiveness of the intersystem crossing steps kisc and k lse- These rates can be altered by dilution. Another technique which exclusively populates the triplet state of a carbene is sensitization, or energy transfer. A triplet sensitizer is required for this purpose — usually an aromatic ketone. In these ketones the intersystem crossing efficiency is almost 100%. Energy is then transferred from the sensitizer triplet to the diazoalkane, thus populating the triplet state of the latter. 

Carbenes with a singlet ground state far below the triplet react in the singlet state even if generated by triplet-sensitized photolysis. 

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... 

Carbenes, as synthetic species, are fascinating chemicals, and the synthesis of cyclopropanes by a [2 +1] cycloaddition of alkenes with carbenes represents an extremely fruitful approach. Recently, highly effective intramolecular [2 + 1] cycloadditions, novel triplet sensitizers, metal-catalyzed cyclopropanations, and novel precursors of carbenes have been developed. 

The 4,5-corane (84) is obtained in SOX yield on photo-decarbonylation of the pentacyclic ketone (85). Photochemical decomposition of the carbonate (86), by the loss of carbon dioxide, affords a mixture of products containing oxirane. styrene oxide, bibenzyl and phenylacetaldehyde. Triplet sensitized irradiation yields products solely from benzyl radicals. - An earlier study of the irradiation (at 254 nm) of the carbonate (87) reported that benzaldehyde, phenyl carbene, and carbon dioxide were produced. A reinvestigation of the irradiation of this compound (at 254 nm in acetonitrile) has provided evidence that the cis- and trans-stilbene oxides (88) and (89) are formed as well as deoxybenzoin and smaller amounts of diphenylacetaldehyde and bibenzyl. When methanol is used as the solvent the same products are produced accompanied by benzylmethyl ether, 1,2-diphenylethanol, and 2,2-diphenylethanol. These authors suggest that the oxiranes (88) and (89) are formed by way of... 

Previous work by Jeger and his coworkers has studied the photoreactivity of y.5-epoxy enones. More recent work by Ishii et aJ has examined the photochemical reactivity of the related nitriles (145) and (146). The direct irradiation of (145) brings about isomerization by ring opening of the cyclopropane to afford (147) as the major product. This is accompanied by the cyclopropenyl nitrile (148) presumably formed via a carbene intermediate. Triplet sensitized irradiation of (145) only brings about trans-cis isomerization. The epoxide (149) is also photoreactive and direct irradiation affords the products (150) - (156) either by the intermediacy of an ylide (157) or a carbene (156). ... 

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. 

It is particularly informative to compare the results of direct and triplet-sensitized irradiation of DCDAF in acetonitrile containing both methyl alcohol and a-methylstyrene. As the reactions shown in Scheme 5 clearly demonstrate, both ether and cyclopropane products are to be expected. The relative yields of these products, however, could depend on whether the reaction is direct or sensitized, and on the rate of reaction with methyl alcohol and a-methylstyrene in comparison with intersystem crossing (/ sT and Ts)- ratio of ether to cyclopropane for the triplet-sensitized reaction of DCDAF should vary according to (29). This prediction is verified by experiments where the alcohol concentration is held constant and the a-methylstyrene concentration is varied. Combination of this result with the rate constant for the reaction of the triplet carbene with a-methylstyrene and assuming that DCFL reacts with methyl alcohol at approximately the diffusion-limited rate (cf. XA and DMFL) gives X, = KMX) which corresponds to ACs = 4 kcal mol". ... 

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