Photochemical initiation alkenes

Addition of iodine to alkenes can be accomplished by a photochemically initiated reaction. Elimination of iodine is catalyzed by excess iodine, but the diiodo compounds can be obtained if unreacted iodine is removed.52... 

Radical chloroaminations are known, using radical, transition metal ion or photochemical initiation. They also occur without overt initiation, thus anti-Markovnikov additions to terminal alkenes occur with N,N-dichlorourethane in benzene at 5-40 C (yields <= 60%)P Similar reactions occur with N,N-dichlo-roarenesulfonamides in CH2CI2 at or below room temperature (yields mostly 53-91% 10% with isobu-tylene). ° The remaining N—Cl bond is reaiUly reduced if desired with sodium sulflte. N-Halosulfoximines also add to alkenes thermally or photolytically. ... 

Both the thermally and the photochemically initiated extrusion of nitrogen from bicyclic diazene (1) leads to the formation of a 1,3-diradical (diyl) which is structurally related to trimethylenemethane. As shown in the cascade mechanism illustrated, two forms of the diyl can be intercepted by alkenes bearing electron-withdrawing groups (diylophiles), viz. the open form of the singlet diyl and the triplet diyl which is formed via intersystem crossing from the singlet (Scheme 1). ... 

The synthesis of cyclopropanes via cyclopropene ring opening with subsequent [1 +2] cycloaddition is restricted to thermolytically or photochemically initiated, and transition metal catalyzed reactions, and furthermore is only applicable to suitably substituted alkenes. 

If the [3-h 2]-cycloaddition reaction of a methylenecyclopropane is performed with a remote alkene or alkyne moiety within the same molecule, products 2 of the bicyclo[3.n.0]-type (n < 3) or the bicyclo-[n.3.0]-type (n > 3) can, in principle, be obtained. The former of these product types can be considered as unhkely, as an alkene or alkyne separated by only one or two atoms from the methylenecyclopropane would give rise to a strained annulated ring, i.e. a cyclopropane or a cyclobutane. It is known from investigations of photochemically initiated intramolecular [2 3-2] cycloadditions of alkenes that the minimum size required for a spacer between 7t-systems is three atoms in a reaction of a non-crossed type. There are actually a few examples of photoinduced intramolecular cycloadditions of the crossed type involving substrates with short spacers (n =... 

Trialkyltin hydrides add in a similar way to give alkenylstannanes 4. In both cases, mixtures of E- and Z-alkenes are formed, from which the sulfur or trialkyltin group can be removed quite easily. Thus, photochemically initiated reactions of trimethyltin hydride and tributyltin hydride with alkyl derivatives of vinylcyclopropane 3, either neat or in hexane solution, pro-... 

Carbohydrate-derived enones have been employed as chiral, electrophilic alkenes in addition reactions to alcohols, ethers, or aldehydes90. Through benzophenone-sensilized photochemical initiation, hydrogen abstraction occurs from the a-carbon atoms in alcohols and ethers or from the carbonyl atom in aldehydes. Addition then occurs exclusively on the /(-face of the enone,... 

Addition polymerization of alkenes can be performed in several ways. Most commonly polymerization is initiated using reactive free radicals provided by an added reagent. Initiation is usually achieved thermally, but photochemical initiation is also relevant here. Initiation procedures using anionic or cationic reagents are attractive alternatives since they offer better control of molecular mass some cationic procedures are mentioned below (Section 7.5.2.1). 

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... 

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 ... 

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. 

As pericyclic reactions are largely unaffected by polar reagents, solvent changes, radical initiators, etc., the only means of influencing them is thermally or photochemically. It is a significant feature of pericyclic reactions that these two influences often effect markedly different results, either in terms of whether a reaction can be induced to proceed readily (or at all), or in terms of the stereochemical course that it then follows. Thus the Diels-Alder reaction (cf. above), an example of a cycloaddition process, can normally be induced thermally but not photochemically, while the cycloaddition of two molecules of alkene, e.g. (4) to form a cyclobutane (5),... 

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