Addition photochemical

Photochemical additions to give four-membered rings are known. Thus the reactions of imidazoles across the 4,5-bond with benzophenone and acrylonitrile are illustrated by (278) (279) and (280) (281), respectively (80AHC(27)24l). Oxazolin-2-one undergoes... 

The photochemical addition of azirines to the carbonyl group of aldehydes, ketones, and esters is also completely regiospecific (77H(6)143). Besides the formation of the isomeric oxazolines (50) from (39) and ethyl cyanoformate, there is also formed the imidazole (51) from addition to C=N in the expected regioselective manner. Thioesters lead to thiazolines (52), while isocyanates and ketenes produce heterocycles (53). 

Other isocyanates undergo [2 + 2] cycloaddition, but only with very electron rich alkenes. Thus phenyl isocyanate gives /3-lactams with ketene acetals and tetramethoxyethylene. With enamines, unstable /3-lactams are formed if the enamine has a /3-H atom, ring opened amides are produced 2 1 adducts are also found. Photochemical addition of cis- and traH5-stilbene to phenyl isocyanate has also been reported (72CC362). 

Azirine, trans-2-methyl-3-phenyl-racemization, 7, 33, 34 1-Azirine, 2-phenyl-reactions, 7, 69 with carbon disulfide, S, 153 1-Azirine, 3-vinyl-rearrangements, 7, 67 Azirines, 7, 47-93 cycloaddition reactions, 7, 26 fused ring derivatives, 7, 47-93 imidazole synthesis from, 5, 487-488 photochemical addition reactions to carbonyl compounds, 7, 56 photolysis, 5, 780, 7, 28 protonated... 

Imidazole, l-methyl-2,4,5-triphenyl-photochemical addition reactions, 4, 421 Imidazole, nitro-applications, 5, 498 IR spectra, 5, 358 mass spectra, 5, 359 quatemization, 5, 386 reactions, 5, 441 reduction, 5, 441 UV spectra, 5, 356 Imidazole, 1-nitro-reactions, 5, 454 Imidazole, 2-nitro-, S, 415 applications, 5, 498 reactions, 5, 96 reduction, 5, 441 synthesis, 5, 378, 395 Imidazole, 4-nitro-deuteration, 5, 417 methylation, 5, 383, 388, 389... 

The initial discussion in this chapter will focus on addition reactions. The discussion is restricted to reactions that involve polar or ionic mechanisms. There are other important classes of addition reactions which are discussed elsewhere these include concerted addition reactions proceeding through nonpolar transition states (Chapter 11), radical additions (Chapter 12), photochemical additions (Chapter 13), and nucleophilic addition to electrophilic alkenes (Part B, Chi iter 1, Section 1.10). 

The photochemical addition of trifiuoroiodomethane to unsaturated systems has been thoroughly investigated by Haszeldine. Little use has been made of this reaction in the steroid field. Irradiation of the enol ether (64) in trifiuoroiodomethane containing pyridine in a quartz vessel furnishes in 60 %... 

The photochemical addition of simple olefins to a,j -unsaturated ketones is a reaction of great current interest. The steroidal A -20-ketone system is especially prone to cycloaddition under mild conditions. Sunder-Plassmann et irradiated 3j5-acetoxypregna-5,16-dien-20-one (67) in the presence... 

Photochemical addition of hexafiuoroacetone to (67) affords an adduct, which is formulated as the oxetane (71). 

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... 

Addition of fluorine and carbon to nucleophilic olefins is little studied, a rare example being the photochemical addition of benzotrifluonde to electron-rich cyclic olefins [202] (equation 40)... 

Examples of perfluoroalkyl iodide addition to the triple bond include free radical addition of perfluoropropyl iodide to 1 -heptyne [28] (equation 21), thermal and free radical-initiated addition of lodoperfluoroalkanesulfonyl fluorides to acetylene [29] (equation 22), thermal addition of perfluoropropyl iodide to hexa-fluoro 2 butyne [30] (equation 23), and palladium-catalyzed addition of per-fluorobutyl iodide to phenylacetylene [31] (equation 24) The E isomers predominate in these reactions Photochemical addition of tnfluoromethyl iodide to vinylacetylene gives predominantly the 1 4 adduct by addition to the double bond [32] Platinum catalyzed addition of perfluorooctyl iodide to l-hexyne in the presence of potassium carbonate, carbon monoxide, and ethanol gives ethyl () per fluorooctyl-a-butylpropenoate [JJ] (equation 25)... 

Irradiation at 20 C of perfluoro ethers containing a carbonyl group causes almost quantitative decarbonylation [179. Azindine undergoes photochemical addition with methyl trifluoroacetate [180 (equation 47). 

Azabicyclo[4.2.0]octatriene systems (e.g. 8), formed as nonisolable intermediates during the photochemical addition of benzonitrile or 1-naphthonitriles to phenols,16 -19 isomerize to 2-hydroxyazocines which exist predominantly as the lactam tautomers. 

Polymers are often side products. Photochemical addition of aldehyde to conjugated C=C units can be efficient when a triplet sensitizer (p. 316) such as benzophenone is... 

As discussed in Section 10.4 of Part A, concerted suprafacial [2tt + 2tt] cycloadditions are forbidden by orbital symmetry rules. Two types of [2 + 2] cycloadditions are of synthetic value addition reactions of ketenes and photochemical additions. The latter group includes reactions of alkenes, dienes, enones, and carbonyl compounds, and these additions are discussed in the sections that follow. 

Alcohols add to olefins upon irradiation in two modes, to produce higher alcohols and to produce ethers. In most photochemical additions of alcohols the first mode occurs ... 

The products consistently exhibit retention of the original olefin stereochemistry and are probably formed in a concerted manner. An exciplex formed from singlet excited benzene and the ground state olefin (allowing relaxation of the orbital symmetry requirements for concerted 1,3- and 1,4-cycloaddition) has been proposed to account for these products/126 Srinivasan and Hill reported an unusual photochemical addition to benzene to form cycloadduct (52)<74) ... 

In 1949 Sinsheimer and Hastings<64) reported the almost complete loss of the 260-nm absorption of the pyrimidines upon irradiation in aqueous solution. This effect is now known to be caused by the photochemical addition of water across the 5,6 double bond, as shown below for cytosine ... 

These remarks represent only the barest outline of at least two aspects of PVC degradation which have been the focus of attention for several years and remain incompletely understood namely the mechanism involved and the related problem of the involvement of HC1. Several excellent reviews give more comprehensive summaries of the earlier work (10, 11, 12). More recent work has made it clear that under appropriate conditions the presence of HC1 can affect the initiation, propagation and termination steps as well as influencing the distribution of polyene sequence lengths. In addition it can undergo photochemical addition reactions with the polyenes, i.e. the reverse of the dehydrochlorination process, as well as forming colored polyene/HCl complexes. These various possibilities will be considered in turn. 

The photochemical addition of HC1 to polyenes in degraded PVC has been shown (27) to proceed smoothly in the solid state. 

Four years later, Nelson and Stoddart designed di- and tetra-valent lactosylated dendrons according to a convergent pathway and under mild and chemoselective conditions (Scheme 46).320 The strategy was based on the photochemical addition of hepta-0-acetyl-l-thio-/j-lactose (448) onto bisallyl trisaccharide 449 to form divalent adduct 450. Two nearly quantitative deprotection steps consisting in standard... 

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

Indeed, access to matrix-isolated thioxoethenone C2OS (117) was gained by us in 1997.148 As proposed the photochemical addition of CS, generated by a microwave discharge in CS2, to CO was achieved by selective excitation (A. = 254 nm) of CS. This reaction could be reversed completely by changing the excitation wavelength to k = 313 nm. 

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