Alkene photochemical additions

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). 

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). 

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. 

Carbohydrates remain an attractive source of chirality in preparation of ligands for asymmetric catalysis. Functionalized phospholanes, 192 [167], and chiral bisphosphinites 193 [168] with an attached crown ether unit were obtained recently from D-mannitol and from phenyl 2,3-di-0-allyl-4,6-0-benzylidene-p-D-glucopyranoside, respectively (Figure 18). Compounds 194 and 195 were obtained in the photochemical addition of H2P(CH2)3PPH2 onto the crresponding alkenes - Pd-complexes of these new bisphosphines were successfully applied as catalysts in the copolymerization of CO and... 

Dioxazolidines are produced by cyclization of A,A-dialkoxy- or A -alkoxy-A -chloroamines (Scheme 11) <93CRV725>. The synthetic potential of this method far surpasses that of the photochemical addition of nitrobenzene to alkenes <84CHEC-1(6)-914). 

The photochemical addition of pofyhalomethanes to alkenes has been a known reaction for a long time. Efficient cleavage occurs preferentially at the weakest carbon-halogen bond, and the formation of the adduct f5.64) may well be a chain process. 

In 1966, two groups simultaneously discovered photochemical addition reactions of simple alkenes to excited benzenes. Wilzbach and Kaplan [4] reported that upon irradiation of 10% solutions of r/.v-but-2-ene, cyclopentene, and 2,3-dimethylbut-2-ene in benzene with light of 254 nm, meta photocycloadducts are formed. Bryce-Smith, et al. [5] irradiated an equimolar mixture of benzene and cA-cyclooctene with 254-nm light which yielded a mixture of 1 1 adducts. The main component ( 85%) was a 1 1 adduct similar to that found by Wilzbach and Kaplan, a meta photocycloadduct. The minor (10-15%) 1 1 adduct was unstable and underwent thermal isomerization. It was suggested with some reservations that the minor adduct was tetracydo[6.6.0.02,7.03,6]tetradec-4-ene (I, Fig. 1). In 1968, this structure was rejected [37], and in 1973, the product was shown to be the ortho adduct, tricyclo[6.6.0.02,7]tetradeca-3,5-diene [38] (II, Fig. 1). 

Synthesis of 1,3,2-dithiazoles has been the most extensively studied. Various structural types of these compounds have been synthesized in 1990s from raV-dithiols, bis(sulfenylchlorides), and alkynes <1996CHEC-II(4)433>. Much attention has been paid to the preparation of stable 1,3,2-dithiazolyl radicals and, especially, cations. The synthetic potential of 1,3,2-oxathiazoles and 1,3,2-dioxazoles is restricted by several uncommon procedures including nitrosation of thiolcarboxylic acids and photochemical addition of nitrobenzene to alkenes <1996CHEC-II(4)433>. 

In fact the workers who wanted 7 for photochemical addition to alkenes (chapter 32) chose1 to use propargyl bromide 14 and an enamine 13 of the aldehyde 12. Mercury-catalysed hydration of 15 gave 8 which cyclised to 7 in base. 

Glycoconjugate polymers were prepared by the copolymerization of different monomers, and immobilized on cellulose membrane for affinity separation. The synthesis of the mannose-based monomer involved the photochemical addition of 2-aminoethanethiol hydrochloride onto an alkenyl a-D-mannoside derivative 36. Of note, 37 was formed in a quantitative yield upon irradiation in MeOH at room temperature, with no competitive addition of the amino group onto the alkene (Scheme 3.24) [59]. 

Other methods such as disulfide contraction [43] or photochemical addition of alkenes [44] or alkynes [45] have also been used for the preparation of thioether-linked... 

Photochemical addition of alkenes to conjugated a-diketones leads to hiradicals of type 15 that form allyl carbene intermediates of type 16 which readily isomerize to furnish tetrasuhstituted furans in good yields (Scheme 17) <1996JOC3388>. 

Irradiation of an alkene in the presence of molecular oxygen and an a-diketone furnishes the core-sponding oxirane in high yields. The reaction proceeds in the complete absence of nucleophiles, and thus can avoid formation of by-products arising from the reaction of nuclec hiles with sensitive oxiranes. The photoepoxidation proceeds via addition of an acylperoxy radical to the alkene. Photochemical epoxidation of cholesteryl acetate (176) has been carried out (equation 64a) the major epoxidation product is the sp,6 -epoxide (177a). In MCPBA epoxidation of (176) the major product is (177b). 

UV-light initiation is used for the perfluoroalkyl iodide addition to more fragile alkenes. The synthesis of 3-trifluoromethyl-substituted steroids is an example of such a photochemical addition. In the addition of trifluoroiodomcthanc to 5a-pregn-3-en-20-one. only one product 4 was isolated from the reaction mixture although eight isomeric adducts might be formed from this reaction. ... 

Photochemical Addition of A -Halo Amides and A -Halo Imides to Alkenes General Procedure62> 3 ... 

The addition of radicals follows a different pattern which is determined by access to the alkene. The addition of hydrogen bromide under photochemical conditions, or in the presence of peroxides, involves the initial attack of a bromine radical on the alkene. The consequences are illustrated by the reaction of hydrogen bromide with allyl bromide (Scheme 3.4). 

One limitation encountered in synthetic studies of this reaction is the lack of chemoselectivity in the addition of aldehydes to unsymmetrically substituted furans. For example, the photochemical addition of benzaldehyde to 2-methylfuran provides a 1 3 1 mixture of oxetanes resulting from the exo addition of aldehyde to the more and less substituted alkenes, respectively. Ketone photocycloadditions are more selective. For instance, irradiation of 2-butanone and furan forms a 7 1 mixture of regioisomeric oxetanes (167) and (168) (1 1 mixture of epimers). 

Photochemical addition of electron deficient alkenes (72) to the pyridine thione (73a) affords the substituted pyridines (74). This reaction arises by the formation of an unstable spiro-thietane (75) which fragments to produce the isolated products. In the case of the Al-methylpyridine thione (73b)... 

Photocycloaddition of 1,1-dichloroethene to the quinolone (110) affords the adduct (111). The triplet excited state of the enones (112) are photoreactive and undergo addition to alkenes to afford reasonable yields of the azetidines (113). - Both electron rich and electron deficient alkenes photochemically add to the enone (114) to afford the cyclobutane adducts (115). Normally the C=N Is unreactive to (2+2)-cycloadditions but the authors believe that in this case the C=N system is activated by the trifluoromethyl group. The azetidine-2-ones (116) can be readily prepared by irradiation of the enones (114) in the presence of ketene. ... 

A study of the photochemical addition of 2,3-dimethylbut-2-ene to the furocoumarin derivatives (117-119) has sought to establish an order of reactivity for the cycloaddition of alkenes to the enone double bond. The (2+2) adduct (120) exhibits temperature dependent fluorescence. ... 

Photoaddition of alkenes to the azathymine (227) affords the adducts (228) the structures of which were determined by -ray crystallography. Acetone-sensitized irradiation of the azauracil derivative (229) yields the cyclobutane (230). Irradiation of the uracil (231) in isotropic solvents is known to yield all four (2-1 2)-dimers in low yield especially at low concentrations. A study has shown that dimerization in smectic media affords a high yield (94X) of the trans-anti dimer (232). In frozen solutions the specificity is reversed and the CIS-anti dimer (233) is formed. The influence of a variety of media on this process was studied in detail. A theoretical treatment of the photochemical addition of alkenes to psoralens such as (234) has been published. ... 

Paterno-Buchi reaction The photochemical addition of a ketone to an alkene to give an oxetane. 

Chung and Ho have reported the details of the photochemical addition of electron-deficient alkenes to the substituted adamantanones (73). All of the ketones undergo reaction from the singlet state and, for example, the addition to acrylonitrile affords the two adducts (74) and (75) in an anti syn ratio of 60 40 which is the approximate value for the products of all systems studied. The irradiation at 300 nm of the alkene (76) in neat spectral-grade acetone affords (77), referred to as a photo-Conia product, in 52% yield. A trace of the oxetane (78) was also found. In hexadeuterioacetone the ratio of the products was found to be 2 1... 

Quinones - Quinone methides are produced on irradiation of /7-benzoqui-nones in the presence of the eneyne (267) in methylene chloride as solvent. 1,4-Naphthoquinones also afford quinone methides but unstable spirooxetenes were also detected. The photochemical addition of phenanthraquinone and ace-naphthenequinone to eneynes has also been studied. 1,4-Benzoquinones undergo addition to electron donating alkenes to yield spirooxetanes. The resultant cyclohexadienones are themselves photochemically reactive and irradiation converts these into dihydrobenzofurans via a dienone-phenol rearrangement. " ... 

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