Cyclopropene photochem

Photochemical Transformation of the Obtained 3H-Pyrazoles into Cyclopropene Derivatives... 

The photochemical study of 3H-pyrazoles was carried out in the search for a route to cyclopropenyl tertiary alcohols. Irradiation of 63a in dry dichloromethane at 300 nm and at room temperature for 0.5 h led to the exclusive formation of the gem-dimethylcyclopropene 65 (Scheme 17). The formation of cyclopropene 65 arises from the loss of N2 and cycUzation of the vinylcarbene intermediate (III). 

The analogous photochemical reaction (300 nm) of 3H-pyrazole 64 in dichloromethane at room temperature led to cyclopropene derivative 66, possessing a a-hydroxy group, isolated in 70% yield (Scheme 18). 

This study demonstrates that the addition of the 2-diazopropane with the triple bond of propargyl alcohols is regioselective, and affords new antibacterial 3H-pyrazoles. The photochemical reaction of these 3H-pyrazoles selectively leads to a- and 6-hydroxy cyclopropenes. The overall transformation constitutes a simple straightforward route to substituted cyclopropenyl alcohols without initial protection of the propargyl alcohol hydroxyl group. 

H-Pyr azoles readily undergo photochemically induced elimination of nitrogen to yield the corresponding cyclopropenes, often by way of detectable vinyl diazo intermediates. l-Aryl-3-methyl-2-phenyl-l-diazobut-2-enes have, in fact, been prepared in this way from the appropriate pyrazoles.345 The vinyl diazo compounds 416, obtained by irradiation of the 3//-pyrazoles 417, were further converted to cyclopropenes 418 via vinylmethylene intermediates 419 by irradiation at 10°C.346 1-Acylcyclopropenes have been... 

The photochemical behavior of methylene cyclopropenes is a subject of current investigation170 Previous results with some 4,4-diacyl and 4,4-dicyano triafulvenes indicate that mainly dimerization, but sometimes additional solvent incorporation and hydrogen abstraction occurs. In the case of the photodimer of 1,2-diphenyl-4,4-diacetyl triafulvene (180) the structure 460 can be assigned from spectral evidence ... 

Compared to the parent system 3a, the barrier for formation of 3d is the highest in this series whereas the formation of 3b should be the most facile according to our computations. Although the reactions of carbenes la-c are initiated photochemically, the observed reactivity seems to be in line with the computed ground state properties. Thus, while methyl substitution in 3-and 5-position inhibits the vinylcarbene-cyclopropene rearrangement, methyl substitution in 2- and 6-position has the opposite effect. 

Fig. 5). A photochemical activation (irradiation into the visible absorption of carbene 18) is required to induce the ring-closure to the spiro-cyclopropene 19. Both 18 and 19 could be identified by comparison of the experimental with DFT-calculated IR spectra. 

Carbene lv is photolabile, and 400 nm irradiation produces a mixture of products.108 By comparison with calculated IR spectra the major product was identified as cyclopropene 3v. The formation of 3v is irreversible, and it cannot be thermally (by annealing the matrix) nor photochemically converted back to carbene lv. The lv -> 3v rearrangement is calculated (B3LYP/6-31G(d) + ZPE) to be endothermic by only 5.4 kcal/mol with an activation barrier of 18.2 kcal/mol. Due to the two Si-C bonds in the five-membered ring of 3v this cyclopropene is less strained than 3s, which is reflected by the smaller destabilization relative to carbene lv. The thermal energy available at temperatures below 40 K is much too low to overcome the calculated barrier of 12.8 kcal/mol for the rearrangement of 3v back to lv, and consequently 3v is stable under the conditions of matrix isolation. 

Similar to alkynes, cyclopropenes also readily form transition metal 7i-complexes which can isomerize to carbene complexes thermally, photochemically, or che-... 

The formation of the cyclopropene derivative Z-36 is very similar to the isomerization of p-phenylenebisnitrene (13) to 14 (Scheme 2) and both processes are likely to be photochemical, rather than thermal. Further irradiation of Z-36 causes its isomerization to another compound, which according to DFT simulations, is compatible with the structure of its E-isomer (E-36, Scheme 7). 

Only one heterocyclic cycloproparene having the heteroatom in the ring adjacent to the cyclopropene has been isolated, namely the l,l-dimethylcyclopropa[c]pyri-dine derivative 170 which is available in moderate yield upon photochemical extrusion of Nj from the 3Ff-indazole 169. In contrast, irradiation of 171 produces no cycloproparene 173, but other products derived from the intermediate biradical 172. Apparently, the radical centers in 172 are too far apart to allow ring-closure. ... 

Oxidation of cyclopropenes by peroxy acids (67HCA1669, 69JA6534, 70JOC450) or photochemically 80JOC2334, 81H(15)1643> gives enones. The products and kinetics are... 

Although cyclopropene formation has been observed to be reversible,150 and its conversion to indene has been achieved thermally,43 the two products are not interconvertible under the photochemical reaction conditions, showing that they are formed by different pathways.45,153 An elegant proof of this is the isolation of different indenes from 116 and 118 (Scheme 59).71... 

Photochemical scrambling of ring atoms can involve a ring-walk or a cyclopropene mechanism. 

The question as to whether the thermal azirine formation proceeds through a vinyl nitrene intermediate or by a concerted mechanism is not as yet resolved. A nitrene intermediate seems most probable on the basis of its similarity with cyclopropene formation from alkenylcarbenesM>> and because either thermal or photochemical decomposition of vinyl azides yields die azirines (Table XI). 

The Cu(I)-catalyzed decomposition of (alkynyloxysilyl)diazoacetates 119 furnishes the silaheterocycles 120 and/or 121 (equation 30) in modest yield63. In these cases, the photochemical extrusion of nitrogen from 119 does not lead to defined products and the thermal reaction is dominated by the 1,3-dipolar cycloaddition ability of these diazo compounds. In mechanistic terms, carbene 122 or more likely a derived copper carbene complex, is transformed into cyclopropene 123 by an intramolecular [1 + 2] cycloaddition to the triple bond. The strained cyclopropene rearranges to a vinylcarbene either with an exo-cyclic (124) or an endocyclic (125) carbene center, and typical carbene reactions then lead to the observed products. Analogous carbene-to-carbene rearrangements are involved in carbenoid transformations of other alkynylcarbenes64. 

Cycloaddition of the carbene derived from 205 to bis(trimethylsilyl)acetylene yields the expected cyclopropene in low yield both photochemically (20%) and under catalysis by copper triflate at 80 °C (10-13%)119. The latter version of the reaction is accompanied by [3 + 2] cycloaddition of the diazo compound to the alkyne, and the photochemical route yields a by-product which obviously comes from carbenic C,H insertion at a SiMe3 group of the alkyne. 

Silyl-substituted cyclopropenes have been observed to rearrange photochemically giving high yields of allenes. A typical example is shown in equation 2 where the silylcyclo-propene 3 apparently ring-opened to the carbene 4 which, following 1,2-silyl migration, led to the allene 513. 

C6o reacts with diazomethane to yield fulleroids [97,99,100,234], Carbene generated from the thermolysis of precursors such as diazirines, sodium trichloroac-etate, cyclopropene, oxadiazole, and tosylhydrazone [60,235] adds onto C6o leading to methanofullerenes [12,15,236], Recently, Akasaka et al. described the photochemical reaction of diazirine with C6o [237], Irradiation of a benzene solution of 2-adamantane-2,3 -[3H]-diazirine 58 and C6o with a high pressure mercury lamp (cutoff <300 nm) at 15°C in a Pyrex tube resulted in the formation of mixture of the isomers 59a and 59b in a ratio of 51/49 (Scheme 24). 

Obata, N., and J. Moritani Photochemical transformation of acyl-cyclopropene derivatives. Tetrahedron Letters 1966, 1503. 

Cyclopropenes may also be made to undergo [2+ 2]-addition to other alkenes thus 3,3-dialkylcyclopropenes add to norbornadiene in the presence of Ph3P CuCl at low temperature to produce bicyclo[2.1.0]pentanes at higher temperature the product is apparently derived by addition of a vinylcarbene R 2C = CH—CH to the diene 106). Photochemically induced intramolecular [2+ 2]-addition is also successful 107a,c). 

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