Carbonyl oxides triplet carbenes

Some of the earliest studies of triplet carbenes in frozen media by epr spectroscopy revealed that these intermediates react rapidly with molecular oxygen (Trozzolo and Gibbons, 1967). This should not come as a surprise since the combination of a triplet carbene with triplet oxygen is a spin-allowed process. Indeed, recent measurements show that this reaction proceeds with a rate that is approximately at the diffusion limit. The product of this reaction (17) is the expected carbonyl oxide (Werstiuk et al., 1984 ... 

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. 

Three types of photoextrusion reactions have been identified in the irradiation of aryl-substituted 1,3,2-dioxathiolane 2-oxides [5 1-1-2 4-2] cycloelimination to produce a carbonyl compound, a carbene and sulfur dioxide extrusion of sulfur dioxide accompanied by a pinacol-like rearrangement to yield an aldehyde or ketone and extrusion of sulfur trioxide to give an alkene <72JOC2589>. Sensitization and quenching experiments indicate that a singlet state is responsible for the cycloelimination reaction, whereas the rearrangement and sulfur trioxide extrusion reactions arise from a triplet state <82JCR(S)175>. 

The other advantage of matrix IR spectroscopy is that one can follow the subsequent reaction (either thermally or photochemically) of triplet carbenes relatively precisely, again with the aid of computational methods. " This complementary approach will reinforce the assignment of triplet species. For example, triplet carbenes react with oxygen even at very low temperature to give oxidation products such as carbonyl oxides (see Section 6), which are easUy characterized by comparing the IR spectra with those calculated. 

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. 

Carbonyl oxides are extremely photolabile even under matrix conditions and irradiation with red light (600 nm) rapidly produces dioxrranes (82). The dioxi-ranes are stable under these conditions but at 400-nm irradiation are converted into esters (83) or lactones. Ketones have been observed as byproducts in the carbene-O2 reactions in frozen matrices. Since the reaction of triplet carbene with O2 is very... 

Carbonyl oxides are also easily detected by LFP with fast TRUV-vis spectroscopy. For example, LFP of a-diazophenylacetate in deaerated Freon-113 generates a transient absorption band at < 270 nm ascribable to triplet methoxycarbonylphenyl-carbene (53), which shows a pseudo-first-order decay with lifetime of 460 ns. When LFP is carried out in aerated solvents, a new transient band appears at 410 nm at the expense of the transient band due to triplet carbene. The decay rate of triplet car-bene increases as the concentration of oxygen increases. This correspondence indicates that the triplet carbene is trapped by oxygen to form the carbonyl oxides (53-O2). This result confirms that the transient absorption quenched by oxygen... 

Attempts to trap carbenes 3b and 3d with molecular oxygen, a reaction frequently used to characterize triplet carbenes, were successful at high (>5%) concentrations of O2 in the argon matrix22. In both cases, the corresponding silyl formate was detected, and it was assumed that a carbonyl oxide and a dioxirane are intermediates in this carbene trapping... 

Photolysis of matrix-isolated (trimethoxysilyl)diazomethane at X > 305 nm produced carbene 3e (equation 1), which was characterized by IR and UV-Vis spectroscopy23. Under these conditions, no other species besides the carbene could be detected spectroscopically. In an 02-doped (1%) argon matrix, the carbene rapidly reacted with oxygen to give carbonyl oxide 4 which was further photoisomerized to formylsilane 5. Again, the fast reaction of 3e with 302 points to a triplet ground state of the carbene. 

Thiols and sulphides quench triplet carbonyl compounds. Evidence (including that from CIDNP studies) indicates that these reactions occur by a radical rather than an electron-transfer pathway (Cohen et al., 1979 Ver-meesch et al., 1978). It is interesting to note that sulphides will deoxygenate ketones producing sulphoxides, sulphones and presumably carbenes (Fox et al., 1979). Phosphines quench triplet carbonyl compounds (Davidson and Lambeth, 1969). They also deoxygenate carbonyl compounds to produce phosphine oxides and carbenes, and in this case, the reaction was proposed as occurring by an electron-transfer process (Fox, 1979). 

Photolysis of 5-chloro-2-hydroxybenzonitrile (70) in aqueous solution gives the triplet carbene (71), which can be detected by transient absorption spectroscopy (X,max at 368 and 385 nm). The carbene was recognized by its reactions, e.g. with O2, to produce the corresponding carbonyl O-oxide (A ax at 470 nm), and with propan-2-ol to give 2-cyanophenoxyl radical. In deoxygenated solutions the main stable products are 2,5-dihydroxybenzonitriIe and two substituted biphenyls. 

When the triplet is an excited state, energy transfer occurs to form singlet oxygen. Ground state triplets react with oxygen by a spin-allowed process which, for carbenes in particular, produce carbonyl oxides [64], It seems that triplet nitrenes react with oxygen slowly. This will be discussed more fully later. Here we examine the products formed from reaction of photolysis of phenyl azide in the presence of oxygen. 

The thermal reaction of 2b with molecular oxygen produces, as expected for a triplet ground state carbene, the quinone (9-oxide 17. This carbonyl oxide is extremely photo-labile and irradiation with red light rapidly produces the spiro dioxirane 18, which on shorter wavelength irradiation yields the lactone 19. 

Thus, the reaction of triplet carbenes with 02 seems to be largely independent of the philicity of the carbene and always leads to a carbonyl oxide. 

Silylenes 1 are highly reactive homologues of the carbenes, and we have been interested to compare the reactivity and primary products of the oxidation of these divalent species. In principal one can expect two different primary adducts of a silylene and molecular oxygen the formal "end-on" adducts silanone 0-oxide 2 or "side-on" adducts dioxasilirane 3 (Scheme 1). It was shown by numerous matrix studies [8-12], experiments in solution using time resolved spectroscopy [13-16], and a preparative scale synthesis in solution [17] that triplet as well as singlet carbenes yield carbonyl 0-oxides as the primary oxidation products, while dioxiranes are products of secondary photolysis. Ando et al. reported on the synthesis of the silanone 0-oxide 2e by the reaction of dimesitylsilylene le and O2 in solid argon [1]. This is so far the only experimental evidence for a silanone O-oxide. 

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