Dienes in cycloaddition

The chalcogene heterocycles have been used as stable precursors for sulfur-said selenium-cantaining hetero-l,3-dienes in cycloaddition reactions 3//-l,2,4-Thiaselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-thia-3-aza-buta-1,3-dienes, and 3//-diselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-selena-3-azabuta-l,3-dienes as well as bis(tnfluoro-methyl)-substrtuted nitrile ylides [137]... 

Whereas tropones usually act as dienes in cycloaddition reactions (Section 5.4), tricarbonyl (tropone) iron 59 displays a reactivity that is almost identical to that of a normal enone. High pressure cycloadditions of 59 with 1-oxygen substituted dienes 60 gave the desired cycloadducts 61 in good to excellent yields (Equation 5.9). The subsequent decomplexation of the cycloadducts has been accomplished by treatment with CAN [20]. 

Other calculations were aimed at predicting the reactivity of some bicyclic 5-6 systems. For instance, the rare [l,4,2]diazaphospholo[4,5- ]pyridine 22 (Scheme 2) was examined for its reactivity (C=P bond) toward dienes in cycloaddition reactions <2005T10521>. The results of density functional theory (DFT) calculations were in good agreement with the experimentally obtained regioselectivity when using unsymmetrical dienes. 

Pyran-2-ones are effective dienes in cycloaddition reactions. 

There are many methods known for the synthesis of 1,2,4,5-tetrazines (39) but most afford the desired compounds only in low yield. There has been great interest in these compounds by physical organic chemists on account of their physical and spectroscopic properties, while preparative organic chemists have been interested in their high reactivity as dienes in cycloaddition reactions. 

Conjugation of a C=S moiety with a C=C or a C=0 bond brings the possibility of the compounds playing the role of dienes in cycloaddition (Tables 4 and 5). This is indeed observed with aromatic thioketones as demonstrated by the extensive work of Saito et al. at the Science University of Tokyo. Reaction of furyl phenyl thioketones or thiophenyl phenyl thioketones with... 

Five-membered heterocycles with two vicinal chalcogen atoms in the ring system can be used as stable precursors for sulfur as well as for selenium-containing hetero-1,3-dienes in cycloaddition reactions. Consequently, 3//-1,2,4-thiaselenazoles have been used for the in situ formation of 4,4-bis(trifluoromethyl)-l-thia-3-azabuta-1,3-dienes, which exist at room temperature only as 4,4-bis(trifluoromethyl)-2//-l,3-thiazetes. This strategy was applied to the synthesis of the first stable selenophosphorane from bis(trifluoromethyl)-substituted 3//-diselenazol and 2-methoxy-1,3,2-dioxaphospholan [78AG(E)774] (Scheme 83). 

Recently, an a-oxoquinodimethane has been used as a highly reactive diene in cycloadditions with simple aromatic aldehydes [Eq. (16)].47... 

Thiophene is generally considered the most aromatic of the five-membered heterocycles, hence its reluctance to function as a diene in cycloadditions. A detailed study of the reaction of thiophene with six common benzyne precursors showed that side reactions with intermediates en route to benzyne often dominate the reaction course. Best results were obtained with diphenyliodonium-2-carboxylate 294, which gave up to 30% yields of naphthalene, presumably mainly through intermediate 295. Other benzyne-derived products [benzo( )thiophene, 0.5% 2-phenylthiophene, trace diphenyl sulfide, trace 1- and 2-naphthyl phenyl sulfides, 9 and 6%, respectively o-(2-thienyl)biphenyl, 4%] bring the... 

Alkynyl Indoles and 2-Allenylindoles as Dienes in Cycloaddition Reactions 358... 

In frontier orbital terms the regioselectivity of Diels-Alder reactions is mainly controlled by the atomic orbital coefficients of the frontier orbitals of both reaction partners. The regioselectivily observed for substituted dienes with a, )-unsaturated carbonyl compounds is shown below. With substituents at C-1 of the diene the 3,4-disubstituted cyclohex-1-enes are preferably obtained. The formation of 1,4-disubstituted cyclohex-1-enes is favored in cycloaddition reactions of C-2-substituted dienes. In cycloaddition reactions of electron-poor alkenes with 1,3-disubstituted dienes generally a preference for the 1,3,4-trisubstituted cyclohex-1-enes is observed. 

Vinylmethylketenes 423 react as dienes in cycloaddition reactions with cyanoethylene (1979JCS(CC)10), eneamines (1982HCA2230) and 4-phenyl-4H-l,2,4-triazole-3,5-dione 424. From the reaction of the latter with 423 the [4 + 2] cycloadduct 425 is provided (Scheme 130) (1987JOC3289). 

Kitamura and coworkers reported that treatment of (phenyl)[2-(trimethylsilyl) phenyl]iodonium triflate with TBAF results in the formation of benzyne, which can be trapped with furan and other dienes in cycloaddition reactimis [35, 267, 268]. This feature was recently utilized with an iodonium salt containing an ort/io-(pyridyldiisopropyl)silyl group [269]. Diaryliodonium salts are also useful in covalent grafting of carbon surfaces useful in materials chemistry [270-273]. 

Of course, the carbonyl group is involved in the reactivity of unsaturated ketones or aldehydes toward dienes in cycloaddition pathways. Aqueous hetero Diels-Alder reactions of activated carbonyl compounds have also been reported, and are extensively described elsewhere in this book. 

The O -0X0 sulfines can also participate as dienes in cycloaddition reactions with electron-rich olefins, such as vinyl ethers. For example, the stable a-oxo sulfines 49 react with ethyl vinyl ether to give the cycloadducts 50 . 

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