Cyclopropane from photolysis

Table 14. Cyclopropanes from Photolysis of Various Diazobenzocycloheptene Derivatives in the Presence of Alkenes... 

This investigation of successive bimolecular reactions is analogous to that carried out on the successive unimolecular processes represented by equations (1.64a) and (1.64b). There an estimate was made of the internal excitation of cyclopropane from photolysis of 1-pyrazoline from their experiments, Endo et al concluded that the average recoil energy from (1.74) amounted to 84 1% of that reaction s exoergicity. 

The preparation of cyclopropanes by photolysis of monoaryldiazomethanes in an organic solvent in the presence of an alkene is usually carried out at or below room temperature using broad-band irradiation from a high-pressure or medium-pressure mercury lamp. In the absence... 

More recent approaches addressing dynamical effects on the reaction reveal a greater propensity for double inversion via mostly conrotatory motions over single inversion in the 1,2-dideuterio case by factors of 2.3-3.5 and 4.7, which are significantly less than that determined by Berson and Baldwin (refs. 21 and 22, respectively). Another significant milestone in the trimethylene story is the observation of the singlet species derived from photolysis of cyclobutanone and a determination of its lifetime before formation of cyclopropane at 122 fs. For the 2,2-dideuterio species the lifetime is 129 fs and that for the 1,1,3,3-tetradeuterio species is 183 fs. These effects were reasonably reproduced by DZP-TCSCF wave functions, which indicate that double inversion occurs four times more often by conrotatory over disrotatory paths. 

Stabilization of the cyclopropane molecule. For example, with cyclohutanone photolysis (4.5 Torr) at 265.4 nm, the addition of 33.5 Torr of hexane decreased the ratio of propene to cyclopropane from 1.04 to 0.56 (McGee, 1968). 

Cyclopropanations by carbenes from chlorodiazirines were observed in several cases, e.g. with the r-butyl compound. Cyclopropanation and stabilization by ring enlargement and by elimination compete in chlorocyclobutyldiazirine photolysis. 

The photolysis of a-diazosulfones dissolved in alkenes provides sulfonyl-substituted cyclopropanes in high yields. This is exemplified by the preparation of l-(p-methoxyphenylsulfonyl)-2,2,3,3-tetra-methylcyclopropane in 75% yield from -methoxybenzenesulfonyl-diazomethane and 2,3-dimethyl-2-butene. A similar addition to [Pg.101]

There has been new information on the products of photolysis of derivatives of compound 1. Low temperature irradiation of the ester 254 gives a ketene (93JACS8621) the isolation of an isomeric ketene from a 3-pyridyldiazo ester suggests the involvement of the open chain form 255. Photolysis of the 3-phenyl derivative 256 in the presence of cyclopentadiene gives exo and endo cyclopropanes and a dipyridylstilhene, suggesting the intermediacy of the carhene 257 (99JOC6635). 

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. ... 

The yield of trans product (18) is decreased by the presence of a radical scavenger such as 1,1-diphenylethylene and increased by dilution of the reactants with methylene chloride or butane, indicating this product to result from the triplet carbene. A heavy-atom effect on the carbene intermediate was observed by photolysis of a-methylmercuridiazoacetonitrile. With c/s-2-butene as the trapping agent either direct photolysis or triplet benzophenone-sensitized decomposition results in formation of cyclopropanes (19) and (20) in a 1 1 ratio ... 

The yields of cyclopropanes in this case are low in relation to the amount of acetophenone formed. However, similar cyclopropane product ratios are obtained when photolysis is carried out in the presence of Michler s ketone as sensitizer. Thus the carbene intermediate produced in the direct irradiation is thought to be a triplet, as suggested by the nonstereospecificity of its addition. Whether this intermediate arose from singlet diazoacetophenone (via singlet decomposition and intersystem crossing of the singlet carbene) or by decomposition of the triplet molecule was not determined. 

An interesting gas-phase study of the photolysis of ftmy-l-phenyl-2-butene has been published by Comtet.(39,40) He has found that (a) it was not possible to quench the formation of the cyclopropane product under conditions that reduced the fluorescence quantum yield, (b) sensitization by acetophenone only gave cis-trans isomerization, and (c) the quantum yield of cyclopropane formation in the direct photolysis decreases as n-butane is added to the reaction mixture. Comtet suggests that the data are consistent with a reaction from the second triplet state. 

Photolysis of diazocyclopentadiene in trans- and cw-methyl-2-pentene results in products (12) and (13), respectively, in an essentially stereospecific manner (1-2% of the t/ww-cyclopropane was observed to result from addition to the cis olefin)<26> ... 

The compound 251 decarbonylates on photolysis to bis(4-hydroxyaryl) acetylene 253, which is easily oxidized to the quinonoid cumulene 254. This is also obtained by thermal decarbonylation of the product of oxidation of cyclopropenone 251, the diquinocyclopropanone 252. Likewise, the blue derivative of 3-radialene 256 (a phenylogue of triketo cyclopropane) is formed from tris-(4-hydroxyaryl) cyclopropenium cation 255 by oxidation34. ... 

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 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. 

Repetition25 of the 9a photolysis in the presence of TME in either isooctane or CH2C12 confirmed that correlations of addn/rearr vs. [TME] were indeed curved, as previously observed.17,19,33 However, generation of PhCH2CCl from 37 by photolysis at 308 nm gave linear correlations of addn/rearr vs. [TME].25 Thus, curvature in the addn/rearr correlations is precursor dependent, and is not due to the intervention of a CAC. If such a complex does form, it must continue on to cyclopropane faster than it reverts or rearranges to fl-chlorostyrene by... 

Certain transition metal complexes catalyze the decomposition of diazo compounds. The metal-bonded carbene intermediates behave differently from the free species generated via photolysis or thermolysis of the corresponding carbene precursor. The first catalytic asymmetric cyclopropanation reaction was reported in 1966 when Nozaki et al.93 showed that the cyclopropane compound trans- 182 was obtained as the major product from the cyclopropanation of styrene with diazoacetate with an ee value of 6% (Scheme 5-56). This reaction was effected by a copper(II) complex 181 that bears a salicyladimine ligand. 

Photolysis of diazoaJkanes in liquid phase yields carbenes in a vibrationally relaxed state, since deactivation in solution immediately removes all excess vibrational energy. The addition of carbenes to the olefins, which results in nonstereospecific formation of cyclopropanes, must therefore result from the different multiplicity of carbenes — singlet or triplet. Since most of these multiplicity... 

Ab initio and RRKM calculations indicate that the reactions of C, CH, and (H2C ) with acetylene occur with no barrier." Laser flash photolysis of the cyclopropanes (69) and (70) was used to generate the corresponding dihalocarbenes. The absolute rate constant for the formation of a pyridine ylide from Br2C was (4-11) x 10 lmoP s. The rates of additions of these carbenes to alkenes were measured by competition with pyridine ylide formation and the reactivity of BrClC was found to resemble that of Br2C rather than CI2C . 

The double )5-scission pathway becomes dominant in bicyclic systems (Equations (7)-(9) and Scheme 13). Thus, cyclopentene ozonide (69) gives cyclopropane (Equation (7)) <68TL329l>. Photolysis of the ozonide derived from 1,4-benzodioxins (70) provides a method for the preparation of labile o-benzoquinones (71) (Scheme 13) <87JOC56l6>. Photolysis can also provide a route to unstable compounds and transient species such as the aziridine-2,3-dione (72) (Equation (8)), identified at 77 K using infrared spectroscopy <80JA6902>. Relatively unstable azacarbapenems (73) have been prepared by photolysis of tricyclic compounds containing a cyclobutene ozonide (Equation (9)). On silica, the 1,2,4-trithiolane (74) underwent photo-equilibration (Equation (10)) with the 1,3-dithetane (75) and sulfur. 

The photochemistry of thietanes involves entirely different pathways from those encountered in azetidines the low bond dissociation energy of the C—S bond seems to be mainly responsible. The direct photolysis of thietane vapor with 213.9-228,8- and 253.7-nm light leads to ethylene and propylene, cyclopropane, and thiocyclopropene. A white polymer appeared as a constant by-product. ... 

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