Diazirines from carbenes

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... 

Photoelimination of nitrogen from diazirines, for example, proceeds via carbene intermediates. 3-tert-Butyldiazirine (395) is converted into the cyclopropane 396 and the alkene 397 with the formation of the carbene insertion product being favored from the singlet state.328 3-Cyclopropyl-3-chlorodiazirine (398) has similarly been converted to the carbene 399 which undergoes both rearrangement to l-chlorocyclobutene(400)329 and addition... 

The fluorescence is not quenched by pyridine (i.e., the excited diazirines do not react with pyridine), although the carbenes do react with pyridine to form ylides. Importandy, the fluorescence from diazirine 35-dt is 50% stronger than that of 35, and the pyridine-trappable yield of carbene 36-dt (from 35-df) is 50% greater than that of 36 from diazirine 35. These results parallel deuterium kinetic isotope effects on RIES.22... 

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. 

Accordingly, a re-examination of the benzylchlorocarbene system was performed, with close attention paid to the products formed at low temperature.71 Carbene 10a was photolytically generated from diazirine 9a in isooctane, methylcyclohexane, and tetrachloroethane at temperatures ranging from 30 to —75°C. At —70°C in isooctane, the products included 47% of P-chlorostyrenes 11a and 12a, 2.4% of a-chlorostyrene (49), 2% of dichloride 50, 5.5% of a C-H insertion product of 10a and isooctane, 4% of the dimers of 10a, and 30% of azine 48.71 The sum of the intermolecular products at —70°C was thus 41.5%, of which azine was the principal component. 

In general, 1,2-C shifts do not compete effectively with the 1,2-H shifts of acyclic alkyl and alkylhalocarbenes. However, r-butylchlorocarbene (18) lacks the a-H needed for a 1,2-H shift, and so affords 1,3-CH insertion and 1,2-Me migration Eq. 14. Note that only for the thermally generated 18 is the 1,2-Me shift product (26) derived from the carbene. Photolytic generation of 18 from diazirine 24 gives only 1,3-CH insertion to dimethylchlorocyclopropane 25 in this case, the 1,2-Me shift product is formed by RIES of the diazirine.27 Based on the rate constant for the 1,3-CH insertion of t-BuCCl at 25°C (9.3 x 105 s 1), we can estimate A 105 s 1 for the 1,2-Me shift at 78°C. 

Data are at 25 °C except for CCI2 (80 °C). AU carbenes were generated from diazirines except CCI2 which was produced by thermolysis of CgHsHgCClBr. 

Unsubstituted alkyl diazirines are ruled out because carbenes from them rearrange to olefins (Fig. 3.14). The constraints of the caged ring system do not allow this possibility with the useful adamantane diazirine, although here a fraction of the carbene is lost by intramolecular insertion (Fig. 3.15). 

Control of the irradiating wavelength following the matrix-decomposition of 2-benzo-thienyl trifluoromethyl carbene from the diazirine enables selective generation of the thio-quinomethide and the strained benzothiopyran allene 57 <07OL3177>. 

Diazirines (isomeric with diazoaUcanes) give carbenes/ but aryhnethyl radicals have also been generated from diazirines. In a different study, thermolysis of diaryloxydiazirines gave the anticipated carbene products, but photolysis gave both carbenes and aryloxy radicals by a-scission. ... 

The dramatic increase in 2,4-didehydroadamantane (tetracyclo[3.3.1.13,7.02,4]de-cane, 22) production when diazirine 16 is photolyzed in CyDs and FAU zeolites (Table 1) is demonstrative of the powerful influence these hosts can have on reaction outcomes. Since there are no solvent molecules with which supramolecular carbene 21 can react and because a 1,2-H shift by carbene 21 is prohibited, decay via 1,3-CH insertion is greatly enhanced. In contrast, cyclopropane 22 is only formed in trace amounts from diazirine 16 in organic solvents.101,103 When a physical mixture... 

A study of aryl(acyloxy)carbenes, generated photolytically from diazirine... 

Laser flash-photolysis investigations have been carried out for benzylchloro-carbene, phenylchlorocarbene, methoxyphenylcarbene and fluorenylidene, and some other carbenes, generated from diazirine, diazo and other precursors, in the presence of oxiranes and thiiranes. These carbenes abstract oxygen or sulfur atoms with bimolecular rate constants in the range 10 to 10 s ... 

Addition of chloro(phenyl)carbene, generated from diazirine, to alkenes is stereospecific, yet diethyl (Z)-but-2-enedioate isomerized partially to give a mixture of stereoisomeric cyclopropanes(for a more detailed discussion see Houben-Weyl, Vol. E19b, p995). 

Table 37. l-Alkoxy(or -Aryloxy)-l-halocyclopropanes from Carbenes, Generated from Diazirines, and Alkenes... 

Further cycloadditions include the 1,3-dipolar cycloadditions as well as the respective [2-i-l]-reactions. The latter have already been mentioned in the reaction of carbenes photochemically generated from diazirines (Section 6.5.2.4). Experimental examples for [3-i-2]-cycloadditions have not yet been reported. Still theoretical considerations gave rise to the assumption that such reactions with, for example, azides, diazomethane or other classical 1,3-dipoles should readily take place on the surface dimers. The calculations further revealed that the cleavage of nitrogen under formation of the respective azacyclopropane as known for normal alkenes should also take place for the [3-i-2]-adducts bound to diamond. For the reaction with ozone (Section 6.5.2.3) it has not yet been clarified whether or not an ozonide is initially formed by [3-i-2]-cycloaddition, only then to be transformed into the carbonyl compound. 

In the 1950s and 1960s chemists learned how to generate carbenes from haloform, diazirine and diazo compound precursors as transient intermediates in solution. [1,5,6] The chemists of this era identified the products of carbene reactions and learned how to make these reactions synthetically useful. They postulated a framework for carbenes (Scheme 1) with ground or low lying triplet states which is still the starting point in all mechanistic discussions. 

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