Cycloaddition to conjugated dienes

The third important reaction of singlet oxygen is 1,4-cycloaddition to conjugated dienes, resulting in the formation of endoperoxides.360-364,371 -373 Acyclic dienes give monocyclic endoperoxides that exist in rapidly equilibrating half-chair conformations 410,411... 

The [4 + 2] cycloaddition to conjugated dienes or anthracenes to yield endoperoxides... 

Another reaction unique to conjugated dienes is the Diels-Alder cycloaddition. Conjugated dienes react with electron-poor aikenes (dienophiles) in a single step through a cyclic transition slate to yield a cyclohexene product. The reaction is stereospecific, meaning that only a single product stereoisomer is formed, and can occur only if the diene is able to adopt an s-cis conformation. 

Trifluoroethene has been reported to undergo cycloadditions with conjugated dienes to give both [2 + 2] and [4 + 2] cycloadducts.25 Although mixtures of regio- and stereoisomeric cyclobutanes are obtained in these cases it still constitutes a synthetically useful reaction. 

In contrast to conjugated dienes, simple alkenes such as 2-butene do not react easily by photosensitized cycloaddition. But they will form [2 + 2] cycloadducts on direct irradiation. These additions occur by way of a singlet excited state and are stereospecific ... 

When tricarbonyl(diglyme)molybdenum(0) (53) (78) is heated with the hydrocarbons 50a-50d and 50t in n-hexane, the analogous molybdenum complexes of the chromium species 42 are readily formed (70) [Eq. (30)]. These results prove the second explanation to be the correct one as to why tricarbonyl-/ 6-l,3,5-cycloheptatrienemolybdenum shows no [4 + 6] cycloaddition with conjugated dienes. 

Let us consider, for example, the origin and evolution of the well-known Diels-Alder reaction. The net outcome of this process is the cycloaddition of conjugated dienes with alkenes, leading to the formation of cyclohexene derivatives (Scheme 2.7). Isolated examples of this cycloaddition, for example... 

Lewis acid coordination of caibonyl-conjugated dieno iles usually results in vastly improved regio-selectivities on [4 + 2] cycloadditions to nonsymmetrictd dienes (Section 4.1.4.1). Medium effects can also be usefully applied to achieve better regiochemical control (Section 4.1.4.2). 

Much attention has been directed to the chemistry of p-heteroatom-substituted allylic cations (5), especially oxyallyl cations (7), from both synthetic and mechanistic viewpoints. One of the characteristic reactions of oxyallyls (7) is their [4 -i- 3] 7 cycloadditions with conjugated dienes, which provide a... 

The second reaction unique to conjugated dienes is Diels-Alder cycloaddition. Conjugated dienes react with electron-poor alkenes (dienophilesi... 

Intramolecular cycloaddition involving conjugated diene and aUene units leads to bicyclic products that contain a 1,4-cycloheptadiene nucleus. [6 + 2]Cycloaddition of some cycloheptatrienes appended with a sidechain containing a triple bond occurs at room temperature, with PtCl2 to promote it. At higher temperature, the formal [6 + 1]-cycloadducts are formed. ... 

A number of reports of the 1,4-cycloaddition of phosphites to conjugated dienes and ajS-unsaturated ketones have appeared (see also Chapter 2). a-Allenyl ketones undergo the expected cycloaddition of phosphite or phosphinite to give the phosphoranes (45), and for the chloroketone (46) cycloaddition is preferred to the alternative Perkow reaction. The adduct (47) undergoes exothermic acidic hydrolysis to give a mixture of vinylphosphonates. 

Cycloaddition of allyl cations to conjugated dienes provides a route to seven-membered carbocycles. One of several methods can be used to generate the allyl cation, such as from an allyl halide and silver trifluoroacetate, or from an allyl alcohol by way of its trifluoroacetate or sulfonate. Cycloaddition of the allyl cation proceeds best with a cyclic diene, particularly for intermolecular reactions. Thus, cyclohexadiene and methylallyl cation gave the bicyclo[3.2.2]nonadiene 187 (3.125). Many intramolecular examples are known, such as the formation of the... 

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