Allenes photochemical

Photoaddition of acetylene, 349 Photoaddition of allene, 349 Photoaddition of dichloroethylene, 349 Photoaddition of ethylene, 348 Photoaddition of hexafluoroacetone, 345 Photoaddition of maleic anhydride, 348 Photochemical addition of ethylenes and acetylenes to steroidal enones and dienones, 343... 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

Finally, thermally induced isomerizations which generate carbon-centered biradical organic molecules have been shown to serve as alternative for conventional chemical and photochemical methods [71]. A straightforward procedure to accomplish such biradicals was described by Myers using a thermal conversion of yne-allenes [72]. According to this scheme, Wang and coworkers [73] heated 3-178 in 1,4-cyclohexadiene to 75 °C and obtained 3-181 in 22% yield via the biradicals 3-179 and 3-180 (Scheme 3.48). 

The photochemical addition of diphenyl(trimethylstannyl)phosphine to either alkynes or allenes has been investigated23. The -isomer is usually predominant (reaction 9) except... 

Me3SnPPh2 will add to allenes under photochemical conditions, giving two regioiso-meric products32. The predominant species is that in which the phosphine residue attaches to the central carbon atom (reaction 23). The overall yield, and relative proportions of 16 and 17 produced, depends on the nature of the substituent R. For R = H, yield = 78%, ratio 16 17 = 89 11 for R = Me, yield = 67%, ratio = 73 27 for R = Bu, yield = 58%, ratio = 88 12. 

As has already been discussed (Section m.B.3) we were able to demonstrate that the three C3H2 isomers cyclopropenylidene (2), propargylene (36), and vinylidenecarbene (37), interconvert photochemically in low-temperature matrices. Unlike 36 vinylidenecarbene (37) was predicted to be a singlet.108-110 To aid the spectroscopic identification of S-37 we calculated (MP2/6-31G ) IR frequencies and intensities of this species.26 Comparison with the experimental IR spectrum (most intense band at 1952 cm-1) confirmed the allenic structure S-37. For T-37 a completely different IR spectrum was expected. An additional structural proof for S-37 was its reversible transformation into the other two C3H2 isomers S-2 and T-36. 

When the terminal alkynes 96 are treated with the trimethylsilylalkyne 97 in the presence of HfCl4 as a Lewis acid, the silylated vinylallenes 98 are produced in acceptable yields. In an intramolecular variant of this process, 100 was obtained from the diyne 99 [32]. Vinylallenes, incorporated into a cyclic framework and hence of restricted conformational mobility, are of interest for photochemical studies [33] and are among the photoproducts in ring-enlargement reactions of polycyclic allenes [34]. 

Am. Chem. Soc. 1993, 115, 459—466 for some leading references on modern photochemical reactions of allenes, see... 

The photochemical reaction of diiron //-alkcnylidc complexes with diazo compounds leads to -allene complexes, which can be cleaved by several methods to yield allenes, for example allenic esters [32]. 

In 1996, Malacria et al. [139] reported on cobalt-mediated reactions of the related allenynes. Heating the allenyne 222 in the presence of cpCo(CO)2 accompanied by a photochemical activation of this organometallic compound delivered the cross-conjugated trienes 223 (Scheme 15.71). The second triple bond present in the substrate did not participate in the reaction, underlining the higer reactivity of the allene unit. 

The thermal isomerization of higher terminal alkynes also delivered some allene, from 1-hexyne and 1-heptyne, for example, some 1,2-diene was formed [30]. With an ,/l-unsaturated unit in the alkyne 9, a photochemical isomerization to 10 was successful but delivered only a low yield and 11 as a significant side-product [31]. These reactions tolerate different functional groups alcohols, ethers or, as in 12, tertiary amines and nitriles have been used (Scheme 1.5) [32, 33],... 

Observation of the list of borazine derivatives produced photochemically (Table 3) reveals that all of the reagents yielding photochemical products have an electronegative element (either 0,N,F,C1, or Br) which substitutes at the boron site. No B-C bonds have been formed photochemically, even when, as in the CH3Br and the HFA reactions, methyl or perfluoromethyl radicals are present Also, no B-C bonded compounds were formed when borazine was photolyzed with benzene, pyridine, cyclopentadiene, allene, ketene, acetylene, or acetone. 

Enamine carbaldehyde 76 (Scheme 20) by sequential photochemical cycloaddition and iminium ion-propargylsilane cyclimtion furnishes allenes 77a-c in good yield and with high diastereoselectivity (92X2081). (Gas chromatographic fR values and thin-layer chromatographic Rp values have been reported.) Radical cyclization of ca-iodoalkyl isoquinolone 78a under... 

Efforts have been made to find stereoselective routes which provide disubstituted azetidines. Palladium catalysed cyclization of an enantiomer of allene-substituted amines and amino acids gives the azetidine ester 2 and a tetrahydropyridine in variable yield and ratio, depending on the substituents and conditions <990L717>. The (TRIS)- and (253I )-isomeis of the substituted azetidine-2-carboxylic acids 3 (R = COjH) are obtained in several steps from the corresponding 3 (R = CHjOSiMejBu ) which, in turn, is produced in high yield by photochemical intramolecular cyclization <98HCA1803>. 

Anastasio, C., B. C. Faust, and J. M. Allen, Aqueous Phase Photochemical Formation of Hydrogen Peroxide in Authentic Cloud Waters, J. Geophys. Res., 99, 8231-8248 (1994). 

Rearrangements of disilanes to a-silylsilenes are well established and are involved in the exchange of substituents between a silylene center and the adjacent silicon.Pulsed flash pyrolysis of acetylenic disilane (41) gave rise to the acetylenic silene (42), which subsequently rearranged to the cyclic silylene, 1-silacyclopropenylidene (43). Irradiation of the cyclic silylene resulted in the isomerization to the isomeric 42, which itself could be photochemically converted into the allenic silylene (44). Both 42 and 43 also were reported to isomerize on photolysis to the unusual (45), which was characterized spectroscopically (Scheme 14.24). 

Thermal and photochemical cycloaddition reactions of 27r-electron species represent an important synthetic approach to four-membered rings. The reactions summarized in this section include 2 + 2 cycloaddition reactions of thioketones, thioketenes, isothiocyanates, sulfenes and iminosulfenes with alkenes, allenes, ketenes, ketenimines and alkynes. 

Intramolecular photoaddition of (terminal) allenes or ketenes to a cyclohex-2-enone C —C double bond have been investigated. The former reaction occurs regiospecifically in high yield,2 the latter, wherein the ketene moiety is photochemically generated, proceeds in moderate yield.3... 

Photochemical reactions of quinones with allenes have also been studied and in some cases cyclobutane formation occurs, although in competition with products derived from attack of the allene on the carbonyl oxygen. Thus, photocycloaddition of tetramethyl-l,4-benzoquinone with 1,1-dimethylallene affords the four-membered carbocycle 6 in good yield.12... 

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