Photocycloaddition alkyne

The photochemical cycloadditions of alkenes and alkynes with aromatic compounds have received by far the most attention. Yields of [2+2] cydoadducts can be good, but reaction times are often long and secondary rearrangement products are common [139, 140, 141,142, 143,144, 145,146] (equations 63-65). The pioneering mechanistic and synthetic work on aromatic photocycloadditions has been reviewed [147],... 

Scheme 6.10. Photocycloadditions of Enones with Alkenes and Alkynes... 

Both saturated or unsaturated thioimides and dithioimides behave like thioketones in their photocycloaddition reaction with alkenes (equation 129) and alkynes (equation 130)185. 

When enamines are employed as sulfene traps, thiete 1,1-dioxides (e.g. 142) can be prepared by subsequent Hofmann elimination of amine. An alternative route to thiete 1,1-dioxides involves trapping sulfenes with ynamines, as illustrated by preparation of (144) (73S534). Certain thietes such as (145) (80LA873, 78TL3617, 78TL4839) are available directly from thioketones by 2 + 2 photocycloaddition with alkynes. 

Oxetanes can be formed by intramolecular reaction between a carbonyl group and an alkene, and this has been used (4.74) in making analogues of thromboxane A, (one of the compounds responsible for the control of blood clotting), albeit usually as the minor product. A special case of intramolecular reaction is seen for a,p-unsaturated carboxylic acids 14.75), where the product is an oxete that is tautomeric with a p-lactone. Oxetes may also be formed by photocycloaddition of ketones or aldehydes with alkynes the oxete normally ring-opens at room temperature to give an a,p-unsaturated carbonyl compound (4.76), but at lower temperatures its spectral... 

Sensitized photocycloaddition of thiophene and 2,5-dimethylthiophene to acetylenedicar-boxylic ester leads to the phthalic esters (303) (73CB674). In the mechanism suggested, the triplet state of the thiophene adds to the alkyne to form a biradical, which cyclizes to the... 

Photocycloaddition of oxygen heterocycles to alkynes has been of some use in the preparation of cyclobutene derivatives.279 Irradiation of tetrakis(trifluoromethyl)furan (336) and dimethylacetylene, for example, affords the adduct (337), which on further irradiation, is converted to the... 

Ortho Photocycloaddition of Alkenes and Alkynes to the Benzene Ring... 

In this chapter, we will be concerned with the ortho photocycloaddition of arenes to the benzene ring. This implies that photocycloadditions to larger aromatic systems and to heterocyclic aromatic molecules will not be discussed. The photoadditions of alkynes to benzene and derivatives of benzene, however, are included in this review. The material is organized in sections, according to the pathway that is followed from the ground state of the addends to the ortho photocycloadducts. 

Most of the ortho photocycloadditions of alkenes and alkynes to the benzene ring that have been reported during the past 40 years have been tabulated in the 4 major sections of this chapter. Many ortho photocycloadducts are unstable under the conditions of irradiation. In some cases, they undergo spontaneous photochemical or thermal rearrangements in other cases, the investigators add a reagent that turns the unstable ortho adduct into a more stable product that can be isolated and identified. Tables 1-7, for the sake of comparison, list the primary ortho adducts, even if they have never been isolated or detected. In Section VI, the many types of secondary reactions that ortho photocycloadducts can undergo are discussed. 

III. ORTHO PHOTOCYCLOADDITION VIA EXCITATION OF THE ALKENE OR ALKYNE FOLLOWED BY REACTION WITH GROUND-STATE ARENE... 

Ortho photocycloaddition to benzene of derivatives of acetylene and maleimide proceeds via excitation of the alkyne or the maleimide. A few other alkenes follow the same route to ortho photocycloadducts among those are dichlorovinylene carbonate and some alkenes in which the double bond is conjugated with a cyano, carbonyl, or phenyl group, which makes it possible to excite them in the presence of the arene. 

Intermolecular and intramolecular ortho photocycloadditions of derivatives of benzene to alkynes are collected in Table 2. Nitriles that give ortho cycloadducts are listed at the end of this table, although mechanistically, as explained, they fall in another category. Table 3 contains the ortho photocycloaddi-... 

Table 2 Ortho Photocycloaddition of Arenes to Alkynes and Nitriles...

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