Absorption diazo compounds

In contrast to the aliphatic diazo compounds, which are invariably colored, all the diazirines so far prepared are colorless. The UV absorption of diazirines corresponds approximately to that of the aliphatic azo compounds. Diazirine shows in methanol an absorption maximum at 321 mja. The IR spectrum of the diazirines shows a band at ca. 1580 cm". ... 

The arenediazocyanides have been known since 1879. They played an important role in the Hantzsch-Bamberger debate on the (Z)/( ,)-isomerism of diazo compounds (see Sec. 7.1). When an aqueous solution of a diazonium salt is added to a solution of sodium or potassium cyanide, both in relatively high concentration, at a temperature below 0°C, a yellow to red (Z)-arenediazocyanide starts to crystallize. Hantzsch and Schulze (1895 a) found that these compounds rearrange into the (ii)-isomers, which have a bathochromically shifted visible absorption (see Sec. 7.1). Under strongly alkaline conditions a 1 2 adduct is formed, to which Stephenson and Waters (1939) assigned the structure 6.36. It was never corroborated, however, by modern instrumental analysis. 

Figure 3. Time-dependent absorption spectra obtained after laser excitation (248 nm, 20 ns, 110 mJ/pulse) of 3,3-dimethyl-3H-indazole (182a) in oxygen-saturated HFIR90 Inset decay of the diazo compound 184a (at 410 nm) and of the cumyl cation 186a (at 320 nm).

The IR spectrum which can be measured in argon at 10 K after irradiation of diazo compound 18 with k = 313 nm is relatively complex. But the absorptions of 19 can be extracted by a subsequent irradiation with k > 570 nm. The signals of 19 decrease in intensity during this secondary irradiation. They fit much better with the bands calculated for T-19 than for S-19. The product formed under these conditions (X > 570 nm) is the ring-opened carbene 16, which in this case can directly be detected and shows an IR spectrum which is in agreement with that of S-16. Intermediate 16 can be transferred photochemically to 2-cyano-2/7-azirene (17) with X > 313 nm, which is the main product in the primary irradiation of diazocompound 18 with this wavelength. 

Irradiation of a benzene solution of DABA at room temperature with a nitrogen laser (Horn and Schuster, 1982) gives the transient absorption spectrum shown in Fig. 3. This spectrum was recorded 50 ns after irradiation of the diazo-compound and decays over a period of ca 250 ps by a path exhibiting complex kinetic behavior. This transient spectrum is essentially identical with the low temperature optical spectrum described above, and thus is similarly assigned to 3BA. 

Fig. 3 Transient absorption spectrum recorded 50 ns after irradiation of DABA in benzene solution. The region between 370 and 440 nm is obscured by the absorption of the diazo compound...

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... 

Triplet-Intermediates from Diazo-Compounds (Carbenes) Table 1. UV-absorptions and quantum yields of decomposition of some diazocompounds... 

Scheme 7) and the reaction was monitored by IR spectroscopy, no significant amount of 33 could be detected [106], Instead, at the initial stage of the irradiation, an intermediate diazo compound was observed, which was assigned the structure 33-D. Further irradiation gave rise to a compound, which was identified with the help of calculations as substituted cyclopropene Z-36 (Scheme 7). Although it is possible that 36 is formed directly from 33-D, it is more likely that 33 is an intermediate of the reaction, as the ESR data imply. ESR spectroscopy is generally more sensitive than IR, and the failure of the latter to detect 33 is likely due to its inherently weak-intensity vibrational absorptions (as indicated by calculations) and/or its high photoreactivity.

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. 

Photochemical reactions, indicated in reaction schemes by the symbol hv, are considered in Chapter 13. For the present, it is sufficient to note that absorption of light transforms a molecule to an excited state, which, in the case of diazo compounds, has sufficient energy for rupture of the C—N bond. Generation of carbenes from diazo compounds is reviewed by W. J. Baron, M. R. DeCamp, M. E. Hendrick, M. Jones, Jr., R. H. Levin, and M. B. Sohn, in Jones and Moss, Eds., Carbenes, Vol. I, p. 1. 

Stable diazo compounds may be characterized by the usual means including NMR, absorption spectroscopy, IR spectroscopy, microanalysis, mass spectroscopy. Much excellent information may be found in the papers from Westheimer s group quoted here. The UV and visible absorption characteristics of the reagents of interest has already been given (Table 3.1). Like azides, diazo compounds have a strong absorption band in the IR all the compounds discussed here absorb in the region 2,090 to 2,170 cm"1... 

In the course of cycloadditions of diazo compounds to cyanates the IR spectra of different substituted 4-aryloxy-l,2,3-triazoles (51) have been measured. The absorptions between 957 and 990 cm" have been attributed to the triazole nucleus while the y(CH) vibration of the triazole-H (R = H) is found in the range 1030-1045 cm" and what is presumably an N=N valence band occurs at 1720 cm" (66CB317). 

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. 

One possible candidate is again DM, which is also known to undergo a thermally induced Wolff rearrangement [252]. Two types of films were used, i.e., one prepared for irradiation in the UV (266 nm) and one prepared for irradiation at 1064 nm, to allow a comparison of the photochemical reaction with the thermal reaction. For the photochemically induced reaction, we used a mixture of PMMA and DMA. This diazo compound has two features which make it a perfect test molecule. It has the strong azo absorption band in the mid-IR (2176 cm-1) and has one known reaction product (keto-ke-tene), shown in Scheme 11, which also has a strong absorption band in the mid-IR (reported between 2000 and 2200 cm-1) [253]. For the thermal reaction of the diazo compound we used 1064-nm irradiation to excite a heater ... 

Solid-state polymerization of diazo compounds 18 and 20 was monitored by IR absorptions at 66 and 35 C, respectively. While an absorption at 3295 cm disappeared smoothly, more than 87<7o of an absorption at 2035 cm remained intact. Here again, the magnetization curves obtained did not obey the Brillouin function with any single S value but with a combination of S values in the range 2-2.5. The alignment of the spins is limited to a short range. 

Scaiano et al. (1989) characterized the parent benzoquinone oxide, various derivatives thereof being obtained from the corresponding cyclic diazo ketones as well as from 9-diazofluorene, and from diazodiphenylmethane by laser flash photolysis at room temperature. The parent benzoquinone oxide has an absorption maximum at 410 nm. The rate constants of 2-diazo-l,2-benzoquinone and of the carbene of this diazo compound with singlet oxygen were found to be 1.0 x 10 m s and... 

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 probe kinetic methodology developed by Scaiano [23] to study radicals and biradicals by laser flash photolysis (LFP) methodology is readily extended to carbenes. LFP (308, 337, 351, or 355 nm) of diazirine or diazo compounds in the presence of pyridine produces carbenes, which generally react rapidly lO M sec" ) to form ylides. [28,29] Ylides are much easier to monitor than the carbenes because they have intense UV-Vis absorption and microsecond lifetimes. Pyridine ylide methodology has enabled LFP studies of alkyl, dialkyl, alkylhalo, dihalo and carbonylcarbenes. It is now a standard tool and will be used as long as LFP studies of simple carbenes are performed. 

The reaction of stable N-heterocyclic carbenes (NHCs) with carbon monoxide and the carbonylation of diazo compounds also provide routes to ketene formation. Ketene formation by carbon monoxide addition to a stable carbene (with 55% conversion) has been confirmed by the characteristic IR absorption, and the ketene has been isolated as a purple solid at low temperature with the structure determined by X-ray crystallography (Scheme 7.42). 

Infra-red spectra are most useful for characterising diazocyclo-pentadienes. There is strong absorption at about 2100 cm"l [30], characteristic of aliphatic diazo-compounds. However an analysis of the spectrum of diazocyclopentadiene itself and its perdeuterio derivative is said to give evidence for delocalisation of the TT-electrons in the ring [67]. 

Photolysis and Thermolysis. The absorption maxima of diazo compounds BTf and BP in methanol were at 396 nm and 392 nm as shown in Table 1. 

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