Diazoalkanes photolysis

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. 

The yields of carbene (and pyridinium ylide) are thus controlled by competition between simple nitrogen loss to the carbene or 1,2-H (or 1,2-C) shift linked to nitrogen loss in the excited state of the diazoalkane precursor 56 When R=i-C3H7, the a-CH bond is weak, there is much RIES upon diazoalkane photolysis, and carbene production is inefficient. When R=/-C4H9 (no a-CH bond) or R=cyclo-CjU5 (strong a-CH bond), diazoalkane photolysis gives little RIES and much carbene.56... 

The rearranged carbenes can be trapped by alkenes see also Blaustein and Berson. Incidentally, the reaction to give (3) described by Munro and Sharp provides an unusual example of diazoalkane photolysis that does not involve a carbene intermediate. 

REACTIONS INVOLVING Diazoalkanes Photolysis, thermolysis, or metal-ion R2C +N2 a... 

Diazirine, fluoromethoxy-nitrogen extrusion, 7, 224 Diazirine, methylvinyl-rearrangement, 7, 221 Diazirines addition reactions to Grignard compounds, 7, 2 0 as carbene precursors, 7, 236 IR spectra, 7, 203 microwave spectrum, 7, 199 molecular spectra, 7, 202-204 nitrogen extrusion, 7, 223 NMR, 7, 202 photoconversion to diazoalkanes, 7, 234 photoisomerization, 7, 221 photolysis, 7, 225-227 quantum chemical investigations, 7, 197 reactions... 

Salts of sulfonylhydrazones [R2C = NNS02Ar]- Photolysis or thermolysis diazoalkanes are intermediates R2C + N2 + ArS02 b... 

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... 

RIES from diazoalkanes is also sensitive to the dihedral angle between the migrating a-H and the C-N bond of the diazo moiety.57 For example, the A values for the pyridine capture of the photolytically generated carbenes from 46 and 47 are in the ratio of 1.7 1. Similarly, the carbene from 46 is more efficiently generated and trapped in methanol, whereas the photolysis of 47 in methanol affords twice as much olefin (by 1,2-H RIES) compared to the photolysis of 46. These phenomena are attributed to conformational factors that favor RIES during the photolysis of 47, with the proximal excited state represented as a pyramidalized 1,3-C-N=N diradical.57... 

Generation of 78 by thermolysis or photolysis of a diazoalkane or diazirine precursor, however, affords the singlet carbene, whose 1,2-H shift to ethene is opposed by a barrier of only 0.678 to 1.298 kcal/mol. Consequently, even in cryogenic matrices, singlet 78 rearranges more rapidly than it intersystem crosses to the triplet, which has therefore not been detected by UV or ESR in either an Ar matrix at 8 K or a Xe matrix at 15 K." The lifetime of singlet 78 at ambient temperature has been estimated at <0.5 ns.89,98b (Note the enormous spectator substituent effect of Cl the lifetime of MeCCl is 740 ns,60 at least 1500 times longer than that of MeCH.)... 

This technique is most useful in studying triplet carbenes since it responds to triplet states only. The carbenes are mostly generated at low temperatures (4 or 77 °K) by photolysis of diazoalkanes in a) solid solutions in single crystals or b) in randomly oriented glasses. 

The triplet carbenes fluorenylidene 20 and anthronylidene 24, which can be generated from the diazoalkanes by photolysis, show a similar behaviour. Fluorenylidene in cyclohexane yields 9-cyclohexenyl-fluorene 27, and 9,9 -difluo-renyl 22 which are clearly formed by an abstraction-recombination process Another example is anthronylidene 24 in cyclohexane or toluene, which yields the products 25,26,27 resulting from an abstraction-recombination process Benzene, on the contrary, failed to give the radical pair product 20... 

The experimental ratio of ds- to trans-cyclopropane 43 46, i.e. the stereo-specifity of the reaction cannot be considered as a simple indication of singlet or triplet percentage of RaC , since the stereochemistry of the cyclo-addition depends on many factors. Photolysis produces the exdted 5i-state of the diazoalkane 41. This compound can lose nitrogen and form the singlet carbene 42 (So-state). 42 can add directly in a stereospecific manner if ki is large. If, however, intersystem crossing 42 45 (Aisc is large) competes favorably with... 

One of the most fascinating transformations of free carbenes, generated for instance by photolysis of diazoalkanes or by a-elimination, is their insertion into aliphatic C-H bonds. This ability of carbenes is not only of theoretical interest, but also a unique tool for the synthesis of highly strained compounds such as, e.g., bicyclo[l. 1.0]butanes. 

Carbenes and transition metal carbene complexes are among the few reagents available for the direct derivatization of simple, unactivated alkanes. Free carbenes, generated, e.g., by photolysis of diazoalkanes, are poorly selective in inter- or intramolecular C-H insertion reactions. Unlike free carbenes, acceptor-substituted carbene complexes often undergo highly regio- and stereoselective intramolecular C-H insertions into aliphatic and aromatic C-H bonds [995,1072-1074,1076,1085,1086],... 

An interesting preparation of aliphatic diazoalkanes (R R C = N2 R, R = alkyl) involves the photolysis of 2-alkoxy-2,5-dihydro-1,3.4-oxadiazoles (see Scheme 8.49). When the photolysis is carried out in the presence of an appropriate dipolarophUe, the diazo compounds can be intercepted (prior to their further photolysis) by a [3 + 2] cycloaddition reaction (54). As an example, 2-diazopropane was intercepted with A-phenylmaleimide (54) and norbornenes (55) to give the corresponding A -pyrazolines. 

Walling297 suggested that the electronic structure of the carbenes, the photolysis products from diazoalkanes, is somewhat analogous to carbon monoxide, and most of their subsequent reactions, e.g., addition to aromatic58 59 or rearrangements98 are those of electron-deficient electrophilic entities rather than radicals.51 However, radical photoaddition of diazomethane to carbon tetrachloride289 290 should not be overlooked. 

If compound 8 is irradiated (X > 300 nm) in the presence of diazoalkanes, the product pattern shifts dramatically for the diazoacetates and -malonates which have oxygen-containing substituents capable of coordination. At temperatures of about -40 to -20°C (tetrahydrofuran), photolysis predominantly yields mononuclear metallacycles 12 which are not yet accessible by any other means (135, 136). An X-ray diffraction study revealed the strict planarity of the cyclic system (136). The bidentate ligands that wind around the metal arise from metal-mediated carbonyla-tion of the respective carbenes (Scheme 6). 

The formation of cyclopropane derivatives by photolysis of diazoalkanes in the presence of alkenes is believed to occur by photolytic decomposition of the diazoalkane to yield the carbene, followed by addition of this carbene to the alkene. Cycloaddition of this type has been reported in furan, dihydrofuran, and thiophene.198 Thus, photolysis of ethyl diazoacetate in thiophene yields the bicyclic sulfur heterocycle (215). Alternatively, photolysis of 3-diazo-l-methyl-oxindole (216) in cyclohexene leads to the formation of two isomers which are thought to have the spirocyclopropyl structure (217) photolysis in ethanol yields 3-ethoxy-1-methyloxindole.194... 

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