Norbornadienes cycloaddition with

Only a few reactions of benzodithiadiazines have been investigated. In common with dithiatriazines 12.8, the anti-aromatic system 12.12 (R = H) undergoes a reversible 5,5 -cycloaddition with norbornadiene. The reaction of 12.12 (R = F) with triphenylphosphine results in a ring contraction to give the imino 2 -phosphane 12.13. ... 

A cobalt-based catalyst, prepared by reducing Co(acac)3 with diethylalumi-num chloride in the presence of the bidentate ligand l,2-bis(triphenylphosphi-no)ethane, accelerates [87] the cycloadditions of norbornadiene (88) with a variety of acetylenes (Equation 3.30). 

There are several known examples of silenes participating in [2 + 2 + 2] cycloadditions, including reactions with norbornadiene and with quadricyclane. Examples of these reactions with norbornadiene are shown in Eq. (30), where the silene bridges the 2,6-positions.38 5, 53 ,7U85 ,88 This reaction has only been investigated using some of the Auner-type silenes. In many cases mixtures of stereoisomers were obtained, and in some cases products of an ene reaction were also observed. 

The intramolecular 2 - - 2 - - 1-cycloadditions of allene, alkyne (106), and carbon monoxide yield a -methylene-(107) or 4-alkylidene-cyclopentenones (108) depending on the allene structure or the reaction conditions (Scheme 4i).i59.i6o The cobalt-catalysed 4 - - 2 - - 2-cycloaddition of norbornadienes (109) with buta-1,3-dienes readily produces cycloadducts (110) when a bimetal system is used (Scheme A2) ... 

Benzo[ ]furan-based azide was also reported to undergo a 1,3-dipolar cycloaddition with norbornadiene as dipolar-ophile to give a triazole after extrusion of cyclopentadiene (Equation 180) <2001T7729>. 

If the silole bears hydrogen atoms in 2,5-positions (e.g., 48), it can undergo cycloaddition with the very weak dienophile 47 to give 7-sila-norbornadiene 49 or with itself during its formation to give 7-sila-norbornenes such as SO <2002ZNB741>. 

The homo-Diels-Alder adduct of two molecules of norbornadiene to methyl propynoate 6 was isolated in very low yield (3%) in the above reaction it is however possible, by the use of high-pressure conditions, to effect this second cycloaddition with a yield of 60%. ... 

As shown in Section 2.2.2.3.1., bicyclopropylidene (1) is capable of undergoing [2-1-2] cycloadditions with electron-deficient alkenes such as diethyl fumarate under nickel(O) catalysis. The [3 -I- 2] cyclodimer and a cyclotrimer are obtained only as minor products from this reaction. In contrast, exclusive [3 -I- 2] cycloaddition can be achieved with many other substrates when palladium(O) catalysts are employed. These cycloaddition products are also produced with phosphite-modified nickel(O) catalysts, but both yields and selectivities are markedly lowered. The reactions of 1 with norbornadiene and norbornene serve as examples for the reaction with strained hydrocarbons, providing the cyclodimers 2 and 3 in 61% and 66% yield, respectively. ... 

Subsequently, Sammes showed that while simple olefins were unreactive, strained olefins such as norbornadiene, and electron-rich olefins such as vinyl ethers also undergo cycloaddition with 41 [54]. In the latter case, the endo cycloadduct tends to predominate. Sammes also successfully induced the formation of 41 from 40 in the presence of catalytic base at ambient temperatures, thus allowing the cycloaddition to proceed under mild conditions, Eq. 27. 

However, carbenes react with dienes 6.457 to give vinylcyclopropanes 6.458, avoiding the symmetry-allowed [2 + 4] cycloaddition with a linear approach giving cyclopentenes 6.456. Almost the only exceptions to this pattern are the reaction of difluorocarbene with norbornadiene, where a [2 + 2 + 2] reaction is in competition with the [2 + 2],934 and the [2 + 4] pathway taking place in the opposite direction in the easy loss of carbon monoxide from strained cyclopentenones as in the decarbonylation 6.408 6.409. 

Dimerization of norbornadiene, a formal [2 + 2 + 2 + 2] cycloaddition with stereospecific cis-endo addition, is possible with molybdenum carbonyl complexes20 and other transition metal compounds21 23. With unsymmetrically substituted norbornadienes (e.g., 1-tert-butoxynorbornadiene), the corresponding substituted cage molecules of heptacy-clo[6.6.0.02,6.03 13.04 11.05,9.0lo,14]tetradccane (cydooetaquinaue) are formed20,24,25. 

Cycloadditions with norbornadiene, cyclopentene, cyclopentadiene, tra 5-cyclooctene, and sty-... 

Norbornadiene may undergo cycloaddition either once or twice the latter process is not usually a major complication 30-60 l o yields with simple acetylenes are typical and are further improved by adding a phosphine oxide [3, 99]. Under thermal conditions yields of cyclopentenones are limited by side-reactions of both diene and acetylene with the cobalt reagent. Cycloaddition with 4-pentyn-l-ol is such a case [Eq. (40)]. Much better yields, at the expense of some stereoselectivity, are obtained using an amine oxide to facilitate CO loss and alkene coordination [100]. Prior attachment of the alkynol to a Merrifield polymer is another method to suppress interfering processes and improve yields (Eq. (41)] (101). 

The nickel-catalyzed homo-Diels-Alder cycloaddition with norbornadienes and electron-deficient alkenes is an effective method for generating strained polycyclic compounds. At the time of writing, this method is the only strategy for carrying out a nickel-catalyzed [2+2+2] cycloaddition with three alkene Jt-systems such that six contiguous stereocenters may be generated. Both acyclic and cyclic enones participate in the process (Scheme 80). 

Norbornadiene reacts with bis-(cis-l,2-perfluoromethylethylene-l,2-di-thiolato)nickel (74) to give the 1,8-cycloaddition adduct (75). This oxidative cycloaddition reaction obeys second-order kinetics. In cyclohexane as solvent... 

As mentioned in the previous section, substituted norbornadienes can proceed [2+2] cycloaddition with highly reactive dienophiles (maleic imide or strained alkene) in the presence of a nickel (0) catalyst leading to the formation of cyclobutane derivatives. With less reactive dienophiles, the reaction seems to be prone to [2+ 2+ 2] cycloaddition giving homo-Diels-Alder reaction products. These two types of reactions appear to be competitive in certain cases to give a mixture of products [130-133]. 

Cycloaddition of norbornadiene with allene takes place to yield the cyclobutene derivative 10[5], Cyclodimerization of 1,2-cyclononadiene (11) affords a mixture of stereoisomers of the cyclobutane derivatives 12[6,7],... 

The cycloaddition between norbornadiene (23 in Scheme 1.12) and maleic anhydride was the first example of a /mmo-Diels-Alder reaction [55]. Other venerable examples are reported in Scheme 1.12 [56]. Under thermal conditions, the reaction is generally poorly diastereoselective and occurs in low yield, and therefore several research groups have studied the utility of transition metal catalysts [57]. Tautens and coworkers [57c] investigated the cycloaddition of norbornadiene and some of its monosubstituted derivatives with electron-deficient dienophiles in the presence of nickel-cyclo-octadiene Ni(COD)2 and PPhs. Some results are illustrated in Tables 1.4 and 1.5. 

Norbornadienes, norbornenones and their homologs have been prepared [23, 24] by cycloaddition of cyclopentadiene (21) and cyclohexadiene (22) with l-benzenesulfonyl-2-trimethylsilylacetylene (23) and l-ethoxy-2-carbomethox-yacetylene (24). Both were efficient dienophiles in the cycloaddition processes and dienophile 23 acted as an effective acetylene equivalent (Scheme 2.12). Norbornanes and their homologs can also be attained by Diels-Alder reaction... 

The /lomo-Diels-Alder reaction is a [2 + 2 + 2] cycloaddition of a 1,4-diene with a dienophile which produces two new bonds and a cyclopropane ring. This reaction is an example of a multi-ring-forming reaction that to date has found few applications in synthesis, since the use of 1,4-dienes has been limited mainly to bridged cyclohexa-1,4-dienes and almost exclusively to norbornadiene. Lewis-acid catalysts accelerate /lowo-Diels-Alder reactions and increase the selectivity for the [2 + 2 + 2] vs. [2 + 2] cycloaddition. 

Cycloaddition of bicyclo[2.2.1]hept-2-ene-2,3-dicarboxylic anhydride 81 with cyclopentadiene was also studied by Bartlett et al., who found exclusive top addition, the top-endo/top-exo ratio being 3 2 [147]. The endolexo ratio is significantly different from that of 80 (60-70 1). The observed top selectivity in norbornadiene (80) and norbomene (81) derivatives is consistent with the inherent top reactivity of norbomanone 25 and norbomene 57. Orbital unsymmetrization of the dienophile... 

Pyrazoles can be synthesized by thermal cycloreversion of adducts formed in the 1,3-dipolar cycloaddition of alkyldiazoacetates with norbornadiene. The rate of the primary process of cycloaddition is accelerated by iron pentacarbonyl (Scheme 88)155 a similar catalytic effect has been observed during the formation of ethyl 5-phenyl-A2-pyrazoline-3-carboxylate from cycloaddition of ethyl diazoacetate and styrene.155 Reactions of this type are catalyzed presumably because of coordination of one or both reactants to the transition metal, and a wider study of the effect of a variety of complexes on 1,3-dipolar cycloaddition processes would be valuable. 

A convenient synthetic method for 1,2,3-triazoles unsubstituted at C-4 and C-5 utilizes a reaction of azides with norbornadiene, for example, Scheme 29 <2004JOC1081>. The process is performed in refluxing dioxane. In the first step, norbornadiene undergoes 1,3-dipolar cycloaddition to glucose-derived azide 225 to give triazoline 226. The following retro Diels-Alder reaction results in the elimination of cyclopentadiene to furnish triazole derivative 227 in 79% yield. 

Bistrifluoromethyl-l,3,4-thiadiazole 71 undergoes a Diels-Alder reaction with norbornadiene under high pressure to give the unstable cycloadduct 72 which rapidly loses dinitrogen forming the 1,3-dipolar intermediate 73. The [4+2] cycloaddition of the intermediate 73 with a second alkene affords product 74 in 29% yield (Scheme 5) <1997SL196>. 

Starting from the Ni mrao-formyloctaethylporphyrin oxime complex, the meso-cyanooctaethylporphyrin N-oxide complex has been synthesized for the first time. The double addition of the nitrile oxide to 2,5-norbornadiene afford a porphyrin dimer, whose structure has been established by X-ray diffraction analysis (485). The 1,3-dipolar cycloaddition reaction of w< .so-tetraarylporphyrins with 2,6-dichlorobenzonitrile oxide yields isoxazoline-fused chlorins and stereoiso-metric bacteriochlorins. The crystal structure of one of bacteriochlorins has been characterized by X-ray diffraction (486, 487). 

There are two main classes of [4 + 2 + 2]-metal-catalyzed higher-order cycloadditions that have been reported. The first class involves the reaction of 1,3-dienes (the four-carbon component) with norbornadienes (both two-carbon components) and the second involves the reaction of 1,3-dienes with either two alkynes or an alkyne and an alkene as the two-carbon components (Scheme 59). 

Itoh and coworkers111 carried out tandem [2 + 2 + 2]/[4 + 2] cycloadditions catalyzed by a ruthenium catalyst. The reaction of diyne 147 with excess norbomene 148 in the presence of ruthenium catalyst 153, for example, afforded 149. Adduct 150 either dissociated from the catalyst or reacted with another equivalent of norbornene. In the latter case, a ruthenium catalyzed Diels-Alder reaction occurred, affording hexacyclic adduct 152 via 151 (equation 43). Compounds 150 and 152 were obtained in yields of 78% and 10%, respectively. Both cycloaddition reactions proceeded with complete stereoselectivity. When 1,6-heptadiyne was used instead of 147, only trace amounts of a cycloadduct were obtained. Replacing norbornene by norbornadiene, which was expected to result in polymer formation, did not afford any adduct at all. 

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