Cycloaddition of 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... 

The formal 4 + 1-cycloaddition of conjugated dienes and nittene precursors, PhI=NR, in the presence of the active catalyst, [Cu(hfacac)2] (hfacac = 1,1,1,5,5,5-hexafluoroacetylacetonate), yielded 3-pyrrolines with a good to excellent stereo-selectivity. The cycloaddition proceeds through diene aziridination followed by ring expansion. The Ni(I)-catalysed 4 + 1-cycloaddition of methylene... 

Cycloadditions of conjugated dienes to alkenes (dienophiles) provide rapid access to substituted cyclohexenes. In view of the widespread occurrence of carbocyclic compounds in nature, and the recent demands for the synthesis of enantiomerically pure compounds, it is not surprising that carbohydrates were examined as substrates for Diels-Alder reactions. Both carbohydrate-derived dienophiles and dienes have been described in the literature. The products of reactions involving these types of compounds, referred to as annulated sugars, are most often formed stereoselectively. The hi levels of both diastereofacial selectivity and endo/exo selectivity sometimes result in the formation of a single cycloadduct from among several possibilities. 

Binaphthol-derived titanium complexes [64], prepared from chiral ligands 65 (Figure 3.13), also performed very well in the cycloadditions of conjugated aldehydes with cyclic and acyclic dienes. Judging from the absolute configurations of endo and exo adducts, this catalyst should cover the re-face of carbonyl on its u tz-coordination to s-trans a,/l-unsaturated aldehydes, and hence dienes should approach selectively from the si-face. 

Beyond dimerization and oligomerization, [2 + 2]- and [4+ 2]-cycloadditions with conjugated dienes and styrenes and the addition of nucleophiles are typical reactions of strained cyclic allenes. These transformations have been studied most thoroughly with 1,2-cyclohexadiene (6) and its derivatives [1, 2]. Concerning the cycloadditions, a theoretical study had the surprising result that even the [4+ 2]-cycloadditions should proceed in two steps via a diradical intermediate [9]. In the case of nucleophiles, the sites of attack at several 1,2-cyclohexadiene derivatives having an... 

The formation of heterocycles by cycloaddition reactions of conjugated dienes is the subject of this chapter. Almost the entire account is devoted to the Diels-Alder reaction of dienes with heterodienophiles to yield six-membered ring compounds (equation 1). Many such reactions have been reported and there is a plethora of reviews. Somela p are general others are cited at appropriate places in the text. This account is highly selective, concentrating on recent work with particular regard to the stereochemistry of these processes. 

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. 

Certain allyl cations react quite efficiently with nonactivated alkenes to give cyclobutanes. Such cations can be generated by Lewis acid catalyzed dehalogenation of allyl halides, protonation of conjugated dienes and Lewis acid eomplexation of conjugated carbonyl derivatives. For example, 2-chloro-2,4-dimethylpent-3-ene ( ) reacts with alkenes in the presence of zinc(II) chloride to give the corresponding cyclobulanes.1 Alkyl substitution of the allyl cation at the 2-position results in [3 + 2]-cycloaddition products. 

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... 

Ultraviolet irradiation of [Cr(CO)3(ty6-l,3,5-cycloheptatriene)] (41) in the presence of conjugated dienes causes no CO evolution. A photochemical [6 + 4] cycloaddition takes place with the dienes la-Id, li, and 2,3-dimethyl-1,3-butadiene (It) (69-71) [Eq. (21)]. Within the coordination sphere of chromium, two C—C bonds are formed between C-l and C-6... 

As already mentioned, no [6 + 4] cycloaddition is observed when tri-carbonyl-i 6-l,3,5-cycloheptatrienemolybdenum(0) is irradiated in presence of conjugated dienes. So an explanation of these experimental results has to be provided. One reason might be an unfavorable fit of the CuH14 n(CH3 ) and C, 4H18 - (CH3) hydrocarbons into the coordination sphere of molybdenum, arising from the increased Mo—C bond lengths. The other reason concerns the possible different stabilities of the intermediates of the photochemical [6 + 4] cycloaddition with molybdenum as the central metal. 

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. 

The photochemical reactivity of alkenes is also of great interest [1,2]. Studies in this area have led to an expansion of the synthetic utility of these substances. Typical photochemical reactions include cis-trans isomeriza-tions, inter- and intramolecular cycloadditions, photooxidations, and electrocyclic ring opening and closing of conjugated dienes and polyenes. Many of these photoreactions have thermal counterparts. In contrast,... 

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 cation radical cycloaddition reactions of conjugated dienes which can adopt either the s-cis or s-trans conformation with a dienophile can proceed to yield either CB or DA adducts or an admixture of both, as noted above. In the case where the dienophile is an electron rich alkene, the tendency toward CB adduct formation appears to be even more pronounced than in the case of a dienophile of the diene or... 

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. ... 

---