Alkenes photochemical

Other cations (Cu2+, Pd2+, Ru3+, Ni2+, Rh3+) incorporated into Nafion-H have been found to promote hydration.36 Other metals that catalyze hydration of alkynes include gold(III),37 ruthenium(in),38 and platinum(II) (Zeise s salt39 40 and halides40), p-Methoxybenzenetellurinic acid is very effective in the hydration of terminal alkynes 41 Similar to the hydration of alkenes, photochemical acid-catalyzed hydration of alkynes is possible ... 

The photolysis of N(3- or 4-alkenyl)mono- or di-thioglutarimides seems to proceed in several steps involving initial intramolecular thietane formation between thiocarbonyl and N-substituted alkene. Photochemical cleavage of both C-S and C-C bonds of a thietane resulted in the formation of the corresponding indolizines or quinolizines (Equation 3) <2000H(53)2781>. 

Cubane-type compounds have been synthesized by a variety of routes. Most take advantage of the tethered 2+2 photocycloaddition. The first synthesis was reported by Eaton [33], although his synthetic route employed an enone+alkene photocycloaddition. An alkene+alkene cycloaddition was used to synthesize the propellacubane shown in Sch. 21 [34]. A more recent report decribed the synthesis of permethylated cubane using the alkene+ alkene photochemical approach [35]. That photocycloaddition is shown in Sch. 22. Although this particular reaction is inefficient, the product was made in sufficient quantity to allow for its complete characterization. 

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). 

Photochemically induced [2 + 2] cycloaddition is of extraordinary importance in organic synthesis,as this is a method ideally suited for the preparation of sterically congested compounds. The reaction may occur by a concerted mechanism allowed by rules of orbital symmetry, or, more often, via a biradical pathway. For preparative purposes, the most widely exploited is the enone-alkene photochemical [2 + 2] cycloaddition. This reaction proceeds with high regioselectivity, although its stereoselectivity might be low. The first example of the utilization of this reaction for the synthesis of a natural compound, a-cariophyllene 385, was described by Corey (Scheme 2.129). Adduct 386, formed as a mixture of stereoisomers in high yield from simple precursors, was further transformed via the tricyclic intermediate 387 into the... 

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. ... 

II alkene photochemical fragmentation. The singlet, by contrast, undergoes chemistry similar to that of a benzyl cation. Examples of hydrogen abstraction by singlet alkene excited states are also well documented 12,13). 

Cycloadditions. Methylenec alkenes photochemically and in the pre leads to methylenecyclopentane product tituent is at an allylic position. 

Cycloadditions. Methylenecyclopropane reacts with electron-deficient alkenes photochemically and in the presence of BU2S2. This free radical reaction leads to methylenecyclopentane products, in which the electron-withdrawing substituent is at an allylic position. 

We will show here the classification procedure with a specific dataset [28]. A reaction center, the addition of a C-H bond to a C=C double bond, was chosen that comprised a variety of different reaction types such as Michael additions, Friedel-Crafts alkylation of aromatic compounds by alkenes, or photochemical reactions. We wanted to see whether these different reaction types can be discerned by this... 

Compound A (C4H10) gives two different monochlondes on photochemical chlorination Treatment of either of these monochlondes with potassium tert butoxide in dimethyl sulfoxide gives the same alkene B (CaHg) as the only product What are the structures of compound A the two monochlondes and alkene B2... 

Compound A (CgHi4) gives three different monochlondes on photochemical chlonnation One of these monochlondes is inert to E2 elimination The other two monochlondes yield the same alkene B (CgHi2) on being heated with potassium tert butoxide in tert butyl alcohol Men tify compound A the three monochlondes and alkene B... 

The regioselectivity of addition of HBr to alkenes under normal (electrophilic addi tion) conditions is controlled by the tendency of a proton to add to the double bond so as to produce the more stable carbocatwn Under free radical conditions the regioselec tivity IS governed by addition of a bromine atom to give the more stable alkyl radical Free radical addition of hydrogen bromide to the double bond can also be initiated photochemically either with or without added peroxides... 

Among the hydrogen halides only hydrogen bromide reacts with alkenes by both electrophilic and free radical addition mechanisms Hydrogen iodide and hydrogen chlo ride always add to alkenes by electrophilic addition and follow Markovmkov s rule Hydrogen bromide normally reacts by electrophilic addition but if peroxides are pres ent or if the reaction is initiated photochemically the free radical mechanism is followed... 

Hydrogen bromide is unique among the hydrogen halides m that it can add to alkenes either by electrophilic or free radical addition Under photochemical conditions or m the presence of peroxides free radical addition is observed and HBr adds to the double bond with a regio selectivity opposite to that of Markovmkov s rule... 

The reaction is illustrated by the intramolecular cycloaddition of the nitrilimine (374) with the alkenic double bond separated from the dipole by three methylene units. The nitrilimine (374) was generated photochemically from the corresponding tetrazole (373) and the pyrrolidino[l,2-6]pyrazoline (375) was obtained in high yield 82JOC4256). Applications of a variety of these reactions will be found in Chapter 4.36. Other aspects of intramolecular 1,3-dipolar cycloadditions leading to complex, fused systems, especially when the 1,3-dipole and the dipolarophile are substituted into a benzene ring in the ortho positions, have been described (76AG(E)123). 

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). 

Photochemical [2 + 2] cycloaddition of benzonitrile and of 1- and 2-naphthonitriles to electron-rich alkenes such as 2,3-dimethyIbut-2-ene gives the corresponding 2-aryl-l-azetines in poor yield 72JA5929,76CC729, 77JOC4238). This does not appear to be a versatile route to 1-azetines. 

The important hydrocarbon classes are alkanes, alkenes, aromatics, and oxygenates. The first three classes are generally released to the atmosphere, whereas the fourth class, the oxygenates, is generally formed in the atmosphere. Propene will be used to illustrate the types of reactions that take place with alkenes. Propene reactions are initiated by a chemical reaction of OH or O3 with the carbon-carbon double bond. The chemical steps that follow result in the formation of free radicals of several different types which can undergo reaction with O2, NO, SO2, and NO2 to promote the formation of photochemical smog products. 

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 irradiation of 1,3-dioxolane in the presence of alkenes and a photochemically activated initiator at 30°C leads to 2-alkyldioxolanes ... 

The photochemical reactions of organic compounds attracted great interest in the 1960s. As a result, many useful and fascinating reactions were uncovered, and photochemistry is now an important synthetic tool in organic chemistry. A firm basis for mechanistic description of many photochemical reactions has been developed. Some of the more general types of photochemical reactions will be discussed in this chapter. In Section 13.2, the relationship of photochemical reactions to the principles of orbital symmetry will be considered. In later sections, characteristic photochemical reactions of alkenes, dienes, carbonyl compounds, and aromatic rings will be introduced. 

The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the same reaction types discussed in Chapter 11 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2ti + 2ti] cycloaddition of two alkenes can serve as an example. This reaction was classified as a forbidden thermal reaction (Section 11.3) The correlation diagram for cycloaddition of two ethylene molecules (Fig. 13.2) shows that the ground-state molecules would lead to an excited state of cyclobutane and that the cycloaddition would therefore involve a prohibitive thermal activation energy. 

Consideration of the HOMO-LUMO interactions also indicates that the [2n + 2ti] additions would be allowed photochemically. The HOMO in this case is the excited alkene 71 orbital. The LUMO is the ti of the ground-state alkene, and a bonding interaction is present between the carbons where new bonds must be formed ... 

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

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