Diazo compounds photolysis

No gas-phase photolyses have yet been reported with these diazirines. Pyrolysis of the cycloheptyldiazirine yields a mixture of hydrocarbons, and the relative yields of these products are very close to the values obtained by the pyrolysis of diazocycloheptane. Photolysis of these diazirines in the liquid phase yields a hydrocarbon mixture similar to that obtained by pyrolysis, but with small increases in some of the minor products. The photoisomerization reaction leading to the normal diazo compound is readily observed for these two diazirines since the diazo compounds formed are relatively stable. In both cases, during photolysis of the (colorless) diazirines, a deep orange solution develops, and ultraviolet and infrared absorption spectra indicate the formation of the diazo compounds. Photolysis of cycloheptyldiazirine in the presence of acetic acid yields 47% of cycloheptylacetate, which indicates that the photoisomerization reaction under these conditions is a major primary reaction. Under the same conditions cyclooctyldiazirine yields 28% of the corresponding acetate. 

Photolysis of dlazirines to nitrogen and carbenes is a general reaction and plays a greater role in carbene chemistry than photolysis of linear diazo compounds. Whereas the latter are often obtained only under the conditions of their thermal decomposition from suitable precursors, diazirines are obtainable in a pure state in most cases. Photolysis has the further advantage to permit nitrogen extrusion at atmospheric pressure, even with low-boiling materials. 

The photolysis of chlorodiazirine was investigated in several cases. From chloromethyl-diazirine (232) vinyl chloride was formed as the stable primary product of stabilization of chloromethylcarbene, with acetylene and hydrogen chloride as secondary products. Some 1,1-dichloroethane was assumed to have been formed through a linear diazo compound by reaction with HCl. Added HBr yielded 1-bromo-l-chloroethane (76MI5Q800). 

A final method of /3-lactam 3,4-bond formation which has found fairly wide application is based on carbenlc insertion (78T1731 p. 1739). The carbenic centre can be generated by photolysis of a diazo compound as in the case of (158) (72JA1629, 79CC846) or from organometalllc precursors, for example (159) (71ACS1927). 

We start the discussion of the group (2) of photo-de-diazoniations in imaging processes by noting that, after silver halide photography, systems based on the photolysis of diazo compounds form the second most important group of imaging processes. 

Despite the above, there is also considerable evidence to suggest that oxazole formation proceeds via an intermediate nitrile ylide, particularly in the catalysed reactions (see below). Nitrile ylides have been detected in laser flash photolysis studies of diazo compounds in the presence of nitriles, and stable nitrile ylides can be isolated in some cases.<94CRV1091>... 

Nevertheless, a more traditional approach to the stabilization of carbenes and the investigation of their spectral properties deals with the direct generation of carbenes in low-temperature matrices, e.g. by the photolysis of diazo-compounds or ketenes. The method allows stabilization of carbenes in their ground electronic state, prevents intramolecular isomerization and also facilitates direct spectroscopic monitoring of their chemical transformations in low-temperature matrices. 

A related issue is whether the carbene, when it is involved, is in equilibrium with a ring-closed isomer, an oxirene.233 This aspect of the reaction has been probed using isotopic labeling. If a symmetrical oxirene is formed, the label should be distributed to both the carbonyl and a-carbon. A concerted reaction or a carbene intermediate that did not equilibrate with the oxirene should have label only in the carbonyl carbon. The extent to which the oxirene is formed depends on the stmcture of the diazo compound. For diazoacetaldehyde, photolysis leads to only 8% migration of label, which would correspond to formation of 16% of the product through the oxirene.234... 

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... 

The photolysis of diazo compounds has been applied to some interesting synthetic problems, including the following<33 1) ... 

Thermolysis of 58a in butanol affords, together with 17% of 60a (R = C4H9) which evidences the intermediacy of the thiophosphene 59 a, a variety of partly atypical products which seriously impede the desired rearrangement38. Photolysis of 58b in methanol is also found to give only 18 % 1,2-P/C shift to form the heterocumulene 59b, from which the thiophosphinic rater 60b (R = CH3) results 39). As already mentioned in connection with the photolysis of diazo compounds of type 36 (see Sect. 2.2), Wolff rearrangement (9%) and O/H insertion (6%) once again compete with thiophosphinic ester formation. Moreover, solvolysis of the P(S)/C(N2) bond 391 prevents a greater contribution of carbene products to the overall yield. 

The quantum yield of diazo compound formation from diazirines 43 and 44 was only 0.10-0.13, however, so that RIES was not considered an important product source during the diazirine photolysis. In the case of 44, for example, the substantial RIES contribution to chloroisobutene formation stems from excited 44, not from the excited state of its diazo isomer.55 The situation is analogous to the RIES formation of a-chlorostyrene during the photolysis of benzylchlorodiazirine.25... 

The methylated carbenes lb-d are easily generated in argon matrices by photolysis of the corresponding diazo compounds.24 The IR and UV/vis spectra are very similar to that of la. In addition, the carbenes were chemically characterized by their reaction with molecular oxygen to give the corresponding quinone O-oxides 7b-d. 

The anthronylidenes lp-lr are easily matrix-isolated by photolysis of the corresponding diazo compounds 2 (Scheme 17).99 The reaction of the carbenes with molecular oxygen results in the formation of anthraquinone 0-oxides 7,... 

Carbenes are commonly generated by irradiation or pyrolysis of an appropriate diazo-compound (2). Apart from differences readily traced to the change in temperature, the chemical properties of the carbenes formed from photolysis and from thermolysis are usually quite similar. These observations... 

The synthesis of 9-diazofluorene 19 is easily accomplished from commercially available 9-fluorenonehydrazone (HgO, Et20 followed by KOH in EtOH yield = 98%) [72], Diazo compounds on photolytic decomposition tend to generate carbenes [73,74], Fluorenylidene, generated by the photolysis of 9-diazofluorene adds to olefins with negligible amounts of hydrogen abstraction [75,76]. Copper and its salts, however, have been shown to lead to... 

Certain transition metal complexes catalyze the decomposition of diazo compounds. The metal-bonded carbene intermediates behave differently from the free species generated via photolysis or thermolysis of the corresponding carbene precursor. The first catalytic asymmetric cyclopropanation reaction was reported in 1966 when Nozaki et al.93 showed that the cyclopropane compound trans- 182 was obtained as the major product from the cyclopropanation of styrene with diazoacetate with an ee value of 6% (Scheme 5-56). This reaction was effected by a copper(II) complex 181 that bears a salicyladimine ligand. 

The isomeric stilbene dicarbenes [20] were generated in 2-MTHF matrices in an epr cavity at 16 K (Murata et ai, 1987 Iwamura, 1988). The spectra obtained by the photolysis of the diazo-compounds [20a], precursors to m,p -[20] and w,m -[20], at 16 K exhibited conspicuous signals at ca. 250 mT, characteristic of quintet species. The signals of /w,w -[20] showed a dramatic temperature dependence (Fig. 16). First, their intensity increased as the temperature was raised, reaching a maximum at SO K, and then decreased somewhat and eventually irreversibly at above 65 K. In contrast, the intensity of the strong signal at ca. 250 mT and some weak signals due to m,p -[20] decreased linearly with the reciprocal of the temperature as dictated... 

The photolysis or thermolysis of diazo compounds results in the formation of carbenes. These reactions were identified as early as 1901 by Hantzsch and Lehmann D and Staudinger and Kupfer who decomposed diazomethane photochemically and thermally, respectively. The subsequent work of Hine 3) and Doering ) started the era of carbene chemistry. Excellent reviews of the chemistry of carbenes are available 5-27),... 

Photolysis In photochemical generation of a caxbene, excitation of the diazo compound must occur. The absorption of diazo compounds between 400 — 500 nm, with a low extinction coefficient e10), has been assigned to a forbidden n - tt transition 29), structure of this band indicates that the photoexcited... 

The rate of disappearance of the diazo compound, e.g. the quantum yield of the photolysis, should be independent of the nature and concentration of the reactants. This is the kinetic criterion for the intermediacy of a free carbene. The quantum yield data of diazo compound photolyses are shown in Table 1. Carbenes are named in the usual way by using... 

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