Photocycloaddition reactions

A number of photochemically induced 4 + 2 cycloadditions have been observed. This reaction can be either a concerted (,4, + a2 ) or a (b4o + 2,) addition. The stereochemical consequences of the allowed reactions are as follows  

One of the first examples of the formation of the bicyclo[3.1.0]hex-2-ene structure from an acyclic conjugated triene was the formation of suprasterol I (24) and suprasterol II (25) from vitamin 

It is known from x-ray studies that the most stable conformation of vitamin D2 is that shown in Eq. (9.77), s trans, s-c and not the s-cis, s-cis structure (26). 

Dauben et al. have shown from the study of simple model trienes that this conformation favors the formation of the ( 4 2) product (see Table 

Padwa and Clough examined the photochemistry of (31) and (32) and found that they did not give the expected products from either a simple b4, + 2a or a 4 + 2, cycloaddition  

A cycloaddition reaction produces a ring of atoms by forming two new G-bonds, for example the formation of a cyclobutane dimer from two alkene molecules. The direct photoreaction involves the concerted reaction of the singlet Jtpt ) excited state of one alkene with the ground state of the other. Stereospecific reactions in which the dimers preserve the ground-state geometry occur when liquid cis- or trans-but-2-ene are irradiated at low temperature  

For rigid alkenes, triplet sensitisation brings about photocycloaddition via the 3(Jt,7t ) state. These reactions are neither concerted nor stereospecific. Cyclopentene produces a tricyclic dimer  

Cycloadditions represent an important class of photochemical reactions. We discussed thermal cycloadditions extensively in Chapter 15, with the prototype being the [4+2] cycloaddition of the Diels-Alder reaction. Orbital symmetry reasoning would lead us to expect that photochemical cycloadditions should be typified by a [2+2] reaction. Indeed, formal [2+2] photocycloadditions are common. However, most photochemical cycloadditions involve triplet states and biradical intermediates. Concerted photochemical cycloadditions are rare. As such, orbital symmetry arguments are not directly relevant, and instead we must focus on potential biradical intermediates and possible funnels and other surface crossing points. Some photochemical cycloadditions do proceed via singlet states, and usually these involve the formation of exciplexes. 

Funnel forms here where the ground state and excited state surfaces approach 

State correlation diagram for a [2+2] cycloaddition. There is a substantial barrier on the ground state energy surface, but the first excited state surface approaches the ground state surface, and a funnel forms that allows the excited state to exit to the ground state, facilitating the reaction. 

Formal [2+2] cycloadditions can be conveniently divided into three classes addition of two simple olefins to form a cyclobutane addition of an olefin to a carbonyl to form an oxe-tane and reaction of an a,p-unsaturated carbonyl with an olefin to form a cyclobutane. We will briefly discuss each reaction here. 

In contrast to acyclic olefins, sensitized photodimerization of cyclic olefins is often fairly efficient (Eq. 16.28). A major reason for this is that cis-trans isomerization is no longer an efficient side reaction. As shown with the example of cyclohexene, this is an excellent way to make cyclobutanes, but control of stereochemistry is problematical. 

The reaction mechanism of photochemical cycloadditions is not always well known. A concerted pericyclic process is likely when the reaction is highly stereospecific, while a two-step mechanism involving a diradical or diion intermediate is more probable when a mixture of stereoisomers is obtained. This is not a rule and a recent example is the [2 - - 2] photodimerization reaction of diethyl l,2-benzoxaphosphorine-6-bromo-3-carboxylate(an analog to coumarin) performed in solvents of different polarities (H2O, MeOH, PhH, CH3C02H). In water, the reaction is highly stereoselective in favor of a centrosymmetric anti-head-to-tail stereoisomer. Theoretical data, however, indicate that the process is not pericyclic, but that the reaction proceeds through a diradical or dipolar intermediate. 

Photocycloadditions are currently carried out in organic solvent the study in pure water is stiU at an infancy stage even if there is great interest fi-om an environmental point of view. Most of the investigations carried out in aqueous medium concern [2 - - 2] processes and have been performed in the presence of micelles, CD, or in inclusion complexes, with the scope of favoring the molecular aggregation and therefore the regio- and stereoselectivity of the reaction.  

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In recent years the application of photocycloaddition reactions to organic synthesis has been growing in importance. - The procedure described is illustrative of a general method based on a photocycloaddition reaction for the introduction of an activated alkyl group specifically to the a-carhon atom of an a,/3-unsaturated cyclohexenone. Especially significant is the fact that the method is also applicable to... 

Stereoselective intermolecular [2- -2]-photocycloaddition reactions of unsaturated heterocycles with formation of fused systems 98S683. 

In this synthesis, we have witnessed the dramatic productivity of the intramolecular enone-olefin [2+2] photocycloaddition reaction. This single reaction creates three contiguous and fully substituted stereocenters and a strained four-membered ring that eventually provides the driving force for a skeletal rearrangement to give isocomene. 

Dihydropyran-4-ones are a source of phenols via an intramolecular [2+2] photocycloaddition reaction and a Lewis-acid catalysed cleavage of the cyclobutane moiety <96TL1663>. 

Photochemical ring contraction of 2-ethoxypyrrolin-5-ones, 59, 132 Photocycloaddition reactions, 57, 116 Photolysis, apparatus for, 55, 17 59, 132, 195... 

Photocycloaddition Reactions ofEnones. Cyclic a,(3-unsaturated ketones are another class of molecules that undergo photochemical cycloadditions.188 The reactive... 

Photocycloaddition Reactions of Carbonyl Compounds and Alkenes. Photocycloaddition of ketones and aldehydes with alkenes can result in formation of four-membered cyclic ethers (oxetanes), a process often referred to as the Paterno-Buchi reaction.196... 

Scheme 6.11. Photocycloaddition Reactions of Carbonyl Compounds and Alkenes...

The subject of the first section of this chapter is the photodimerization and photocycloaddition reactions of olefins. In this category we include only those compounds in which the photoreactive olefin or polyene is not part of an aromatic system although it may bear an aromatic substituent, such as in styrene. 

Many examples of photocycloaddition reactions of olefins and polyenes yielding cyclobutane derivatives have been reported. Irradiation of mixtures of butadiene and 1,1-dichloroethylene in the presence of a... 

PHOTODIMERIZATION AND PHOTOCYCLOADDITION REACTIONS OF ,j8-UNSATURATED CARBONYLS AND ACID DERIVATIVES... 

Table 2. Reaction time, products, and yields of photocycloaddition reactions of inclusion complexes involving 11,17, and 21 in the solid state...

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