Photoaddition of acetylene

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... 

Photoaddition of acetylenes to benzene provides a convenient synthetic route to cyclooctatet-raenes through or//io-photocycloaddition. Although the meta cycloaddition would be a symmetry-allowed concerted process from Sj or Tj excited acetylene and S, benzene, or from Sq acetylene and Si (Bjn) benzene, there are surprisingly few reported cases in which an acetylene underwent weM-photoaddition to a benzene ring to give a semibullvalene. 

Two modes for the photoaddition of acetylene to the cholestenone (74) have been reported. Thus [2 + 2]-cycloaddition yields the adducts (75) and (76) whereas an ene-type addition yields the ethylidene derivative (77). 

All the previously reported examples of photoaddition of acetylenes to benzene involve 1,2-cycloaddition and subsequent formation of cyclo-octatetraenes10 (but see ref. 43 for a discussion of the addition of perfluorobut-2-yne to benzene). The report of 1,3-cycloaddition of diphenylacetylene to the trimethyl ester of trimesic acid, is, therefore, of interest as this leads directly to the semibullvalene system.44 The reaction apparently requires 6 days, the yield of the product is low, and the adduct (28) is not isolable in the pure state. The triethyl ester and the acetylene also yield (29) via a stilbene-phenanthrene cyclization of the primary... 

Jorgenson, M. J., Photoaddition of acetylenic ketones to olefines. Tetrahedron Lett., 5811, 1966. 

Additions to Aromatic Hydrocarbons. A variety of photochemical additions to aromatic hydrocarbons have been reported. Benzene and its derivatives add to maleic anhydride74-76 as well as to simple olefins,77-80 isoprene,81 acetylene derivatives,79,82 and alcohols.83 The mechanism of the maleic anhydride-benzene reaction is discussed in Section IV. A.4. Naphthalene forms a photoadduct with dimethyl acetylenedicarboxylate62 and with acrylonitrile8211 while anthracene behaves similarly with maleic anhydride84 and with 1,2-benzanthracene.85 The photoaddition of several aromatic amines to anthracene has been reported to proceed via a charge transfer complex86,87 in fact, the majority of these addition reactions may proceed in this manner. 

The photoaddition of hydrogen bromide to olefines and acetylenes is used to synthesize alkyl and alkenyl bromides... 

Almost simultaneously, papers by Schmid et al.509 and by Gotthardt and Reiter510 appeared on the photoaddition of DMAD to sydnones the last authors also used propiolic and phenylpropiolic esters. Ohta et al.511 showed that similar reactions could be carried out with 3-alkyl-4-phenylsydnones, and other papers on the addition of acetylenic esters to sydnones have appeared.512-514... 

In the photoaddition of a saturated hydrocarbon to ethyl propiolate (equation 19) it is likely that the excited state of the acetylenic ester initiates reaction by abstracting a hydrogen atom from the hydrocarbon. The addition of cyclic ethers to an alkyne seems similar (equation 20), although a ketone sensitizer is required for addition of tetrahydropyran or dioxan . When reactions of this type involve a conjugated acetylenic ester, the first-formed a,p-unsaturated ester can normally undergo further photochemical reaction to produce the p,y isomer (see equations 19 and 20). 

In some systems an alcohol gives rise to photoreduction products of the alkyne rather than to products by photoaddition of the whole alcohol molecule. The acetylenic ester 13 gives the corresponding ethylenic ester on irradiation in methanol (equation 24). Under the same conditions the ketone (CH3)3C—C=C—COMe... 

Photodimerization of acetylene to give vinylacetylene (butenyne) and formation of polymers in the photolysis of alkynes generally are examples of photoaddition to alkynes. Photopolymerization of di- and poly-ynes has been studied, and for both conjugated diynes or triynes the polymerization process is a 1,4-addition reaction (equation 34). The products are highly unsaturated, and they tend to contain a high proportion of oxygen after exposure to the atmosphere. 

Under certain conditions ( — 78 °C, liquid phase) ethenethiol (CH2=CH—SH) is the major product of the photoaddition of hydrogen sulphide to acetylene . ... 

The cycloaddition of the allenes (63) to cyclopentenone results in the formation of the two adducts (64) and (65) in a ratio of 4 1. The addition reaction occurs primarily across the less substituted double bond of the aliens. Cycloaddition of acetylene and but-l-yne to the enones (66) can be brought about using left-circularly polarised light. The resultant cyclobutane derivatives are readily converted to the optically active enones (67). The photoaddition of hex-1-yne to the lactone (68) affords the two isomeric adducts... 

The u.v.-induced addition of propan-2-ol to acetylene has been studied with and without acetone as sensitizer.168 The acetylene (241) undergoes photoaddition of isopropylamine and isopropyl ether to yield (242) and (243a) respectively.16 The same acetylene also reacts with ethyl 2-methylpropionate giving /rans-(243b) and the cyclic product (244). 

When acetylenes are irradiated in aqueous solution [27], in acetic acid [28], or in alcohols [29,30], photoaddition reactions take place to give a ketone, an enol acetate, or an enol ether, respectively. In the photohydration reaction, a hydrated proton attacks the singlet excited state of the acetylene directly [31], On the other hand, alcohols give addition products by attack on the excited states of acetylenes in a radical-like mechanism. Radical photoaddition to acetylenes occurs also with saturated hydrocarbons such as cyclohexane [29], and with cyclic ethers such as tetrahydrofiiran [32], Simple acetylenes are photoreduced on irradiation in hydrocarbon solvents for example photolysis of dec-l-yne or dec-5-yne in pentane gives the corresponding alk-ene (dec-l-ene or trans- and ds-dec-5-enes) [32]. 

Neutral carbon of organic compounds is inherently a very weak base due to the lack of nonbonding electrons. However, the 7r-system of alkenes, alkynes, and allenes are potential basic sites due to the availability of 7t-electrons. Wooldridge and Roberts [61] first reported the facile photohydration of arylacetylenes to the corresponding ketones (in an overall Markownikoff sense) in dilute aqueous acid solutions. The facile protonation of the excited state was attributed to the enhanced basicity of the alkyne moiety in the excited state, more specifically as a result of extensive charge migration from the aryl group to the acetylene moiety. The photoaddition of methanol [62] and acetic acid to 7c-electron systems such as alkenes, alkynes, and allenes may be rationalized in the same way. 

Photoaddition of dimethyl acetylenedicarboxylate to cis- and trans-but-2-enes gives four [(76), (77), (78), and (79) E = C02Me] of the seven possible bicyclic products. The structures of the adducts were established by 220 MHz n.m.r. spectroscopy and by consideration of the symmetry properties of the molecules. The acetylenic ester is the excited partner in the addition. Under the conditions described, no mono-adducts could be detected in the products. 

Koltzenburg et have shown that sensitized photoaddition of MA to acetylene gives, in addition to the cyclobutene derivatives 46, a 2 1 MA acetylene derivative 47. They further showed that the cyclobutene adduct... 

Photoaddition of MA to acetylenes has been also extended to trimethyl-... 

Fig. 6-5. General scheme for photoaddition of MA to simple olefins and acetylenes. From Reference 43.

Acylbutyrolactones (55) have been prepared in high yields by a rearrangement reaction between j3y-butenolide (54) and an aldehyde in the presence of boron trifluoride etherate. Japanese workers have also reported a study of the scope of photoadditions of olefins and acetylenes to unsaturated lactones to give products of type (56). Conditions have been recorded for the reduction of a-hydroxyamino-butenolides to the corresponding a-aminobutyrolactones. 

Also noted are further details of the conversion of barbaralone into (249) via dibromides of triasteranone (253X photosensitized addition of oxygen to (254) to give (255) after reductive work-up, the photoaddition of benzo-phenone to barbaralone to give (256), and the thermal addition of substituted acetylenes to tropone to give, for example, (257). Products of bromination of (258) are described. ... 

Photoaddition of bis(methylthio)acetylene to a thione gives an a-(thiomethoxy-thiocarbonyl) vinyl sulphide R R C=C(CSSMe)SMe. Vinyl sulphides of particular interest, prepared by straightforward methods, include optically active alkyl vinyl sulphides EtCHMe(CH8) SCH=CHj amides R NHCMe= C(SR )CONHPh and 2-pyridyl vinyl sulphides. The novel sulphur-linked squaric acid analogue (23), prepared from the corresponding thiolate anion by... 

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