Norbornadienes 2 + 2 + 2 cycloaddition reactions

Photoinitiated Quadricyclane and Norbornadiene Cycloaddition Reactions and Rearrangements... 

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. ... 

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). 

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. 

Palladium catalyzed cyclization of l-iodo-2-((propa-l,2-dienyloxy)methyl)benzene 150 occurs with concomitant anion incorporation to furnish 4-substituted-l//-isochromenes 151 (Scheme 49) <1998TL435, 2000J(P1)3799>. In the presence of norbornadiene 152, the initially formed azide can undergo a further 1,3-dipolar cycloaddition reaction and fragmentation to give l-((l//-isochromen-4-yl)methyl)-l//-l,2,3-triazole 153 (Scheme 49) <1998TL435, 2001T7729>. 

Cycloaddition reactions of norbornadiene and its derivatives to give heterocyclic systems 92MI25. 

Cycloaddition reactions of a,/3-unsaturated chromium and tungsten complexes have been studied to a great extent and have been reviewed.3 -6 Our report on cycloaddition of (l-alkynyl)carbene complexes is restricted to a short abstract and an update including more recent results. A most remarkable feature of [4+2] cycloadditions of 1,3 dienes to C=C bonds of (l-alkynyl)carbene complexes, e.g., li, is that such reactions proceed under very mild conditions, compared to those for reactions of propargylic esters, e.g., 41. Thus, formation of a Diels-Alder adduct, e.g., a norbornadiene derivative 42, can be achieved at 25°C via carbene complexes instead of at 190°C via the direct route (Scheme 15).68 Ligand disengagement from compound 40 can be achieved in various ways, e.g., by formation of an ester 43 through oxidation of the Cr=C bond, or by formation of an allyl silane 4369 or a stannane.70 71... 

The vast majority of bicyclic phosphole derivatives with norbornene- and norbornadiene-derived skeletons are prepared via classical [4+2] cycloaddition reactions of phospholes. While Diels-Alder reactions of phospholes with CN 4 (e.g., oxides and sulfides) are common through direct reaction with dienophiles, this type of reaction is comparatively rare for CN 3 phospholes (see Section 3.15.5.1.4). 

In an analogous reaction of (2) with norbornadiene, the tetracyclic adduct (116) is formed which on heating undergoes a retro Diels-Alder reaction with formation of the thiophene derivative (117). Further cycloaddition reactions of mesoionic 1,3-dithiolones have been carried out with cyclopentadiene, 1,3-cyclohexadiene and 1,5-cyclooctadiene (78CB3037). 

The presence of electron-withdrawing groups in the 1,2,4-triazine ring facilitates the cycloaddition reaction. Indeed, ethyl 3-(2-pyridinyl)-l,2,4-triazin-6-carboxylates 170 or 6-cyano-1,2,4-triazines 171 and 172 react with 2,5-norbornadiene on reflux in ethanol or benzene (Scheme 94) <200382400, 2003TL693, 2003JOC2882, 2005TL6111>. 

TRANSITION-METAL-CATALYZED CYCLOADDITION REACTIONS OF BICYCLO[2.2.1]HEPTA-2,5-DIENES (NORBORNADIENES)... 

II. COBALT- AND NICKEL-CATALYZED [2jt + 2n + 2n] CYCLOADDITION REACTIONS WITH NORBORNADIENES (THE HOMO DIELS-ALDER REACTION)... 

III. NICKEL-CATALYZED [27t + 2n CYCLOADDITION REACTIONS WITH NORBORNADIENES... 

Metal-Catalyzed Cycloaddition Reactions of Bicyclo[2.2.1.]hepta-2,5-dienes (Norbornadienes). 

The Ni -catalysed (2+2) cycloaddition of activated olefins such as acrylonitrile or acrylic acid esters to quadricyclane which gives endo- and exo-substituted products is complicated particularly in the case of 1,2-disubstituted olefins by a competing isomerization of quadricyclane to norbornadiene. Norbomadiene gives identical cycloadducts under these conditions with a similar isomer ratio. Kinetic studies of the cycloaddition reactions of quadricyclane gave a plot of log [quadricyclane]t o/ [quadricyclane]i=t against time which exhibited a marked deviation from linearity due to the retarding effect of norbomadiene which is also formed and in turn coordinates to the Ni catalyst. Scheme 11 accounts for these observations. 

At higher temperatures > 90 °C the alternative [4+2] cycloaddition reaction gives (40) endo) and (41) iexo) with norbornadiene and acrylonitrile as substrates and the endo. exo isomer ratio is affected by added phosphines. The ratio shows little dependence on the electronic properties of PRs but bulky phosphines promote the formation of the exo-isomer which can be explained in terms of a preference for intermediate (43) rather than (42). 

The substituent effects of norbornadiene derivatives in the iridium-catalysed asymmetric 2 -I- 2-cycloaddition reactions with arylacetylenes have been investigated. Results have shown that the atom forming the short bridge chain had the greatest effect on the enantioselectivity of the reaction. Heteroatoms such as oxygen and nitrogen resulted in excellent enantioselectivity while a carbon atom on the bridge chain decreased the enantioselectivity. " ... 

Thiosulfines, R2C=S=S, are also not stable and rearrangement to thiocarboxyUc esters seems to be faster than reaction with alkynes or norbornadiene. However, numerous [3-1-2] cycloaddition reactions, in which the C=S=S system reacts as a 1,3-dipolar species are known (see Section 2.3.2). 

As shown in Scheme 6.110, in the [2 + 2 + 2]cycloaddition reactions of nor-bornadiene with electron-deficient acyclic alkenes, Ni(acac)2/Ph3P/Me3Al system is also more effective than simple Ni(cod)2/Ph3P catalyst [130]. Thus, the reaction of norbornadiene with methyl acrylate gave eight-substituted deltacyclane in 69% yield with moderate endo selectivity (exojendo = 23 77). On the other hand, under Ni(cod)2/Ph3P-catalyzed conditions, the desired products were obtained in only 26% yield without stereoselectivity (exojendo = 53 47). 

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... 

A review of recent developments in 1,3-dipolar cycloaddition of nitrones with sila-, thia-, phospha-, and halo-substituted alkenes has been reported. A DFT study of solvent effects on the intermolecular 3-l-2-cycloaddition reaction of norbornadiene with 3,4-dihydroisoquinoline A(-oxide at 398.15 K indicated that the reaction proceeds via a synchronous concerted mechanism. Chiral imidazolidinone salts, in the absence of water, promote the 1,3-dipolar cycloaddition reaction of alanine-derived ketonitrones... 

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. 

Lewis-acid catalysis is effective in intermolecular as well as intramolecular /zomo-Diels-Alder reactions. Thus, complex polycyclic compounds 93 have been obtained in good yield by the cycloaddition of norbornadiene-derived dienynes 92 by using cobalt catalyst, whereas no reaction occurred under thermal conditions [91] (Scheme 3.18). 

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. 

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. 

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