Photochemical carbene addition

Azirines are also made by carbene addition to nitriles (89 — 90) and by thermal or photochemical (68JA2869) elimination of N2 from vinyl azides (e.g. 91 — 92). Vinyl azides are prepared by the Hassner reaction (68JOC2686, 71ACR9), where iodine azide is first added to an alkene and the resultant (3-iodoazide is dehydrohalogenated with base (Scheme 37) (86RTC456). 

The reverse of the carbene addition (Equation 12.6) occurs photochemically upon ultraviolet irradiation of cyclopropanes.7 Were the extrusion of the carbene... 

The most generally employed approach for the formation of cyclopropanes is the addition of a carbene or carbenoid to an alkene. In many cases, a free carbene is not involved as an actual intermediate, but instead the net, overall transformation of an alkene to a cyclopropane corresponds, in at least a formal sense, to carbene addition. In turn, the most traditional method for effecting these reactions is to employ diazo compounds, R R2 —N2, as precursors. Thermal, photochemical and metal-catalyzed reactions of these diazo compounds have been studied thoroughly and are treated separately in the discussion below. These reactions have been subjects of several comprehensive reviews,8 to which the reader is referred for further details and literature citations. Emphasis in the present chapter is placed on recent examples. 

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. ... 

Particular interest has been shown in the reactions of photochemically generated arylcarbenes. A study of the photodecomposition of phenyldiazomethane in 2-chloropropane has revealed that carbon-chlorine bond insertion of singlet phenylcarbene predominates at low temperature in solution, whereas carbon-hydrogen bond insertion is preferred in a rigid matrix. The principal products of irradiation of diazoalkane (63) are phenylacetylene (64) and the cyclobutene (65), even in the presence of alkenes. At lower temperatures, however, carbene addition predominates as shown, for example, with isobutene as shown in Scheme 7. 

A novel synthesis of the lavandulyl skeleton depends on the hydrolysis of the spiro-compound (71), obtainable by a carbene addition on the allene (72). The resulting alcohol is converted into the bromide (73) from which isolavandulyl acetate (74) can be obtained. Allyl rearrangement of the bromide (73) during acetolysis and subsequent formation of the hydrocarbon (75), also mentioned in this paper, has been previously observed (c/ ref. 114). Photochemical sensitized oxygenation of lavandulyl acetate (76) is described it yields the expected products (77) and (78). ... 

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

Additional evidence for a photochemically produced noncarbene precursor to ketene 29 is provided by product analysis. Photolysis of 25 in neat methanol leads to both carbene-derived (i.e., 30 in 75% absolute yield) and ketene-derived adducts (i.e., 31 in 18% absolute yield), but thermolysis of 25 in neat methanol (sealed tube at 170°C) provides only carbene-derived adduct 30 (91% absolute yield) The ratio... 

Biscarbene 34 was characterized by IR and UV/vis spectroscopy [49], The analysis of the experimental data showed that these are compatible with the presence of two phenylchlorocarbene (6) subunits in 34. This interpretation was further supported by the reactivity behavior of 34, which, like 6, is unreactive toward oxygen under conditions where triplet carbenes react fast. In contrast to its para isomer (22), 34 appears to undergo photochemical ring expansion analogous to that of 6[105]. In addition, the computed [RHF/6-31G(d)] IR spectrum of 34, which is in good agreement with the observed one, is based on the wave function for the singlet (cr /cr ) biscarbene (54 of Fig. 9). 

Carbocations have also been obtained by protonation of photochemically generated carbenes (see Eq. 17), by the fragmentation of photochemically generated cation radicals (see Eq. 18), and by the addition of one photochemically generated cation to an arene (or aUcene) to generate a second cation. As illustrated in Eq. 19, the last method has been employed to convert invisible carbocations into visible ones. Short-hved aryl cations and secondary alkyl cations are quenched by electron-rich aromatics such as mesitylene and 1,3,5-trimethoxybenzene in HEIP to give benzenium ions that can be observed by LEP in this solvent. 

Luminescence is seldom observed from free radicals and radical ions because of the low energy of the lowest excited states of open-shell species, the benzophenone ketyl radical being however a noteworthy exception. There are few reports of actual photochemical reactions of free radicals, but the situation is different with biradicals such as carbenes. These have two unpaired electrons and can exist in singlet or triplet states and they take part in addition and insertion reactions (Figure 4.90). 

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