Photochemical addition, of alkenes

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>. 

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. ... 

The photochemical addition of ethene at 0°C in methylene chloride to the enedione (77) affords a high yield of the adduct (78). This was converted to the monochloro derivative (79) which also undergoes photoaddition of ethene to yield the Z> adduct (80). This on elimination of HCl yielded the quinol (81) which can be oxidised to the quinone (82). Cycloaddition of alkenes (cyclopentene, cyclohexene, and cycloheptene) has been carried out to the same enedione (77) to yield the adducts (83). lyoda et al. have also described a convenient synthesis of the bicyclo-octanediones(84) by a photochemical addition of alkenes to the enedione (77). The adducts (84) can be reduced by zinc in acetic acid to the desired products. Cycloaddition of ethyne to the same enedione followed by reduction affords the bicyclooctanes (85). The photoaddition of alkenes to the dibromo-enedione (86) is also effective and yields, after reduction, the adducts (87). 

Photochemical addition of alkenes to C=N systems has been studied and interpreted by means of perturbation MO theory.(2- -2)-Cycloadducts (180) and (181) are formed on irradiation of the isoxazoline (182) in the presence of indene. ... 

Photochemical additions of alkenes to aromatic compounds are now well established and similar additions to pentafluoropyridine (86) occur,335 whereas a novel insertion into the carbon-chlorine bond occurs with 105, in a process that does not appear to involve tetrafluoropyridyl radicals.336 It has also been demonstrated recently that Diels-Alder additions occur with... 

Ortho addition is the favoured mode of photochemical addition of alkenes to hexafluorobenzene and this has now been extended to the addition of the dienes 1,4-octadiene and 1,3-hexadiene. Ortho adducts are also formed in the irradiation of pentafluoroalkoxy-benzenes with cyclic alkenes the reaction is regioselective and... 

The photochemical addition of alkenes to the pyrrole ring of indole to give cyclobutane products was first reported by Julian and Foster in 1973 [43], They found that both electron-rich and electron-poor alkenes add to indole, but only if an acyl group is present on the indole nitrogen atom. With monosubstituted alkenes the reaction was regioselective and, with the exception of vinyl acetate, the exo stereoisomer was produced as either the major or the only product (Scheme 15). A mechanism involving the indole triplet excited state was suggested, based on the observation that the reaction could be sensitized by acetophenone and quenched by naphthalene. 

The synthesis of 2,3,4,5-tetrahydro-lH-2-benzazepine-l,5-diones by the photochemical addition of alkenes to phthalimides has been known for a number of years. More recently this method has been extended to the reaction of N-alkylphalimides prepared from phthalic anhydride and amino acids, with a further example being published during the period under review <94T3627>. 2,3-Dihydro-l/f-2-benzazepine-l,3-diones have been prepared by the reaction of... 

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). 

Ordinary aldehydes and ketones can add to alkenes, under the influence of UV light, to give oxetanes. Quinones also react to give spirocyclic oxetanes. This reaction, called the Patemo-BUchi reaction,is similar to the photochemical dimerization of alkenes discussed at 15-61.In general, the mechanism consists of the addition of an excited state of the carbonyl compound to the ground state of the alkene. Both singlet (5i) and n,n triplet states have been shown to add to... 

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

This is a general situation for thermal, concerted additions those involving Ane + 2ne systems proceed readily, e.g. the Diels-Alder reaction, whereas those involving 2ne + 2ne systems, e.g. the cyclo-dimerisation of alkenes, do not. We might, however, expect that photochemical cyclodimerisation of alkenes would be symmetry... 

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. 

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]. 

Among these reactions, the photochemical cycloadditions of C=C bom which can create up to four asymmetric carbons during the photochemical sti are particularly interesting, and numerous synthetic applications of this react have been reported. Advances in the understanding of the origin of asymmefa induction, during addition of alkenes with carbonyl derivatives, cyclic enom and aromatic compounds, will be discussed in detail. 

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

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