Photolysis of diazo compounds

Diazo compounds show a weak absorption at longer wavelengths in the visible region, and a somewhat stronger absorption at shorter wavelengths. An n — transition may be responsible for the weak absorption bands, but 

The question has been raised whether photoexcited states of diazo compounds, rather than or in addition to carbenes, participate in product formation. 

the quantum yield of the photolysis should depend on the nature and con- 

The quantum yields of the photolysis of diazocarbonyl compounds are solvent-dependent (Tables I and 2). These data do not necessarily imply solvent participation in the photodissociative step. The variation of solvent produces 

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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. 

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. 

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 term methylene is exclusively reserved for CH2. Spectroscopic investigations demonstrated that photolysis of diazo compounds in fact produces free carbenes. A flash photolysis of diazomethane gave methylene whose spectrum could be recorded 3i>. ESR-spectra were taken of a series of triplet carbenes which had been obtained by direct irradiation of diazo compounds in various matrices at low temperatures (see p. 97). 

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. 

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... 

Investigations similar to the ones just mentioned have focussed on (trimethylgermyl) (trimethylsilyl)carbene 3441. Photolysis of diazo compound 33 in MeOD yielded a mixture of methoxysilane 37 and methoxygermane 38 in a 79 21 ratio (equation 8). This result suggests that carbene 34 underwent a 1,2(Si C) and a l,2(Ge -C) methyl shift... 

The triplet excimer phosphorescence from liquid solutions of naphthalene and di-2-naphthylalkenes in isooctane previously reported has not been confirmed and the emission probably arose from impurities like biacetyl . Exciton trapping has been studied in doped 1,4-dibromonaphthalene . ENOOR applied to the study of triplet traps of phenanthrene-TCNB in naphthalene-TCNB , and triplet 1- and 2-naphthylphenylcarbenes produced by photolysis of diazo compounds . Conformers of triplet states of 2-naphthaldehyde at 300K , T and T2 states from dual... 

Tricyclic compound 371 was produced from diazoacetic ester 372 by the rhodium(II) acetate-catalyzed insertion into the C—H bond at the tertiary C atom and located in the e-position with respect to the diazo group (84JA5295). [2-(Alkoxymethyl)phenyl] carbenes generated by the photolysis of diazo compounds 373 undergo intramolecular C—H insertion with the formation of benzodihydropyran derivatives 374 in 32-96% yield (89JA1465 94TL1699). 

A series of four-membered Si- or P-containing heterocycles was produced in various yields by electrocyclization or [2-t-2]-dimerization of compounds with multiple carbon-silicon or carbon-phosphorus bonds. These unsaturated species are formed under photolysis of diazo compounds containing a P or Si atom in an a position with respect to the diazo group, following a [l,2]-sigmatropic shift in the intermediate carbene. 

The carbene generated by photolysis of diazo compound 499 rearranges via phenyl migration to a short-lived phosphene 500, which with aldehydes or ketones gives [2-l-2]-cycloadducts (A -l,2-oxaphosphetanes (501 (80CB3303 81T1039). 

The reaction of dichlorocarbene with ketones and diamines results in near quantitative formation of a mixture piperazinones 584 and 585 (80JOC754). As shown in Section III,C,2, piperazine 78 [R = H, R + R = (CH2)s], the minor product of the Rh2(OAc)4-catalyzed decomposition of diazo ester 73, is the result of the dimerization of the intermediate ylide 76 (84JOC113). Tetrahydropyrazines were synthesized through ring expansion of imidazolidines. Thermolysis or photolysis of diazo compounds... 

Photolysis of diazo compounds 613 yields 1,4-benzothiazines 614 (15-30%) by a [l,2]-N-shift of the carbene intermediates 615 along with methyl-enebenzothiazolines (23-42%) by [l,2]-H-shift (85BSB499). 

The two unshared electrons in singlet carbenes have opposite spins. Carbenes generated by a-elimination are singlets because the two unshared electrons are derived from the same MO. Triplet carbenes, in which the two unshared electrons have parallel spins, are usually generated by photolysis of diazo compounds. Triplet carbenes are discussed in Chapter 5. 

Regitz and co-workers have reported the matrix isolation of 1//-azirines generated by the photolysis of diazo compounds, as shown in Scheme 82 (81BSB615). 

Photolysis of diazo compounds (305) generates nitrogen and the carbene 289. Photolysis of diazomethane (311, R = H), for example, generates methylene (H2C ),242 Benzophenone is often added as a sensitizer for the photolysis of diazomethane. Under these conditions, triplet methylene is formed via intersystem crossing (5i Ti)243 (sec. II.IO.B). Energy transfer from triplet benzophenone to triplet diazomethane followed, and triplet diazomethane decomposed to triplet methylene.244 a similar thermal reaction gives the carbene and the usual carbene reactions.245... 

The photolysis of diazo compounds and the isomeric diazirines continues to attract a great deal of interest, ranging from fundamental theoretical studies to applications such as photoaffinity labelling. 

Carbenes can be obtained by pyrolysis or photolysis of diazo compound. 

The chemistry of high-spin polycarbenes started in 1967 when two groups led by K. Itoh [1] and E. Wasserman [2] reported independently their findings that OT-phenylene-dicarbene Ic generated by the photolysis of diazo compound Id has a ground quintet state. Prior to this report, A. M. Trozzolo and coworkers... 

Normally, the ideal photolabile reagent will possess an absorption maximum well clear of the ultraviolet or visible absorption of the receptor system. However, many biological preparations are reasonably stable to irradiation at 254 nm, at least for short times, and photolysis of diazo compounds and aryl azides is often complete well before even the most... 

Photolysis of diazo-compounds in the presence of the acetylene (112) has given rise to a general preparation of spiroheptatrienes (113). With... 

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