Diazirine from photolysis

Methylene from diazirine has higher energy of vibration than the product from photolysis of ketene, but it is more discriminating in insertion reactions into primary and secondary C—H bonds. 

Carbene generation from photolysis of diazirine compounds leads to efficient insertion into C—H or N—H bonds and also causes addition reactions with points of unsaturation within target molecules. Diazirine-containing photoaffinity probes have... 

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). 

Preliminary investigations on the formation of carbenes from diazirines have already been made available. Frey and Stevens recently reported the photolysis of cyclic diazomethane. Cyclic diazomethane was irradiated in the gaseous phase with light of wavelength... 

The methylethylcarbene which is formed thermally from methyl-ethyldiazirine at 160°C gives the same products as that from butanone p-toluenesulfonylhydrazone and bases in aprotic solvents." However, photolysis of the same diazirine gives a different mixture of C4H8 hydrocarbons. Considerable amounts of 1-butene are formed, the trans-butene content is reduced by half, and the amount of methyl cyclopropane increased fivefold. ... 

Figure 4.1. TRIR difference spectra averaged over the timescales indicated following 355 nm laser photolysis of diazirine 8 (15.7mM) in C02-satnrated dichloromethane. Reprinted with permission from B. M. Showalter and J. P. Toscano, J. Phys. Org. Chem. 2004, 14, 743. Copyright 2004, John Wiley Sons Limited.

Now the 2-butenes comprise only 73% of the products, while 1-butene, a minor thermolytic product, increases to 23%. It is at least conceivable that some of the 1-butene here derives from an excited state of diazirine 8, rather than exclusively from carbene 6.3,15 Computational studies indicate that the 1,2-H shifts of 6 leading to the 2-butenes are preferred to the 1,2-H shift that yields 1-butene by a AAG of 2.6-3.3 kcal/mol. This again suggests that the substantial yield of 1-butene obtained from the photolysis of 8 is not simply derived from carbene 6.16... 

The Y-intercept ( 3 /k Eq. 13) of the reciprocal correlation for the photolysis of diazirine 21-C1 in TME was 2.18, which translated into a 68% incursion of diazirine excited state in the genesis of the rearrangement products, 22-C1 and 23-C1 carbene 17-C1 only accounted for 32% of these products.28 A similar conclusion followed from the ratio of rearr/addn (68 32) at a high concentration (6.7 M) of TME in pentane, where carbene 17-C1 was almost completely diverted to the cyclopropane, and 22 and 23 were exclusively derived from the excited diazirine.28... 

In the cyclobutylfluoro system, the excited diazirine, 21-F, is considerably less involved. A parallel analysis indicates that only about 12% of 22-F and 23-F arise from excited diazirine, while 88% of those products descend from carbene 17-F.28 The increased carbene involvement in the photolysis of 21-F presumably reflects the greater stability of fluorocarbene 17-F over its chloro analogue 17-C1 and, consequently, a more favorable partition (ki/k3) of excited diazirine 21-F to the carbene. 

In addition, 18-19% of isobutene and chloroacetylene formed via fragmentation. Photolysis of the diazirine in up to 9 M trimethylethylene in pentane led to a sharp decrease in 27 and 28 (to 32% and 8.5%), along with 40% of cyclopropanes formed via the capture of 19. However, the yield of isobutene and chloroacetylene was unchanged, indicating that these products did not stem from the carbene, but arose directly by fragmentation of its excited diazirine precursor.45... 

Cyclopropanated phenanthrenes revert to phenanthrene and carbenes on photolysis CH2,49 CC12,50 CBr2,51 CBrCl,51 and f-BuCH15d have been generated this way. Platz et al.25 used 37 (R=PhCH2) to generate benzylchlorocarbene (10a) for comparison with 10a generated from diazirine 9a cf. Scheme 2. 

It is suggested that the real carbene, generated by thermolysis of the diazo or diazirine precursors or photolysis of 40, gives mostly 1,3-insertion, whereas photolysis of either the diazoalkane or diazirine yields much 1,2-Me migration directly from precursor excited states.15 1 An analogous intervention of 1,2-Me migration via RIES was also observed in the photolytic decomposition of f-butylchlorodiazirine (24) to f-butylchlorocarbene (18) cf. Eq. 14.27... 

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... 

As in experiments with diazirine 1, the maximum yields of the ylides, and hence of the carbenes, increased with increasing strength of the alkyl group s a-CH bond.56 For example, the relative yields of carbene 45 from diazoalkane photolysis were 100% for R=cyc/o-C3H5, 96% for R=t-C4H9, and 72% for R=CH3 the relative carbene yields decreased to 14% with R=C2Hs and to only 7% with R=i-C3H7. 

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