Homo-Diels-Alder

The Diels-Alder reactants as shown in Scheme 1.1 can consist of only hydrocarbon fragments (homo-Diels-Alder reaction) but can also contain one or more heteroatoms on any of the positions... 

Nickel-catalyzed [27t + 27t + 27r] (homo-Diels-Alder) and [27r-i-27r] cycloadditions of bicyclo[2.2.1 ]hepta-2,5-dienes [96]... 

Keywords bis-homo-Diels-Alder reaction of 1,4-cyclooctadiene... 

Brunner H. Enantioselective Catalysis With Transition Metal Componnds. Right or Left - This Is the Qnestion Adv. Chem. Ser. 1992 230 143-152 Keywords homo-Diels-Alder reaction of norbornadiene... 

This last reaction is known as the homo-Diels-Alder reaction. 

The geometry of the molecule favours the process. This reaction is also called homo-Diels-Alder reaction because a cyclopropane ring is formed rather than a n bond. 

Most metal mediated [2 + 2 + 2] cycloadditions involve two triple bonds which coordinate to a metal center to form a reactive metallocyclopentadiene species (vide infra). The corresponding reactions involving at least two double bonds and an intermediate met-allocyclopentane species are almost completely limited to norbornadiene systems. These reactions can be considered as homo Diels-Alder reactions. 

The most efficient catalysts for the homo Diels-Alder reactions of norbornadiene were found to be cobalt327 and nickel328 complexes. The general mechanistic pathway that has been proposed for these reactions has been depicted in equation 161329. According to this mechanism, co-ordination of norbornadiene and the olefin or acetylene to the metal center gives 557, which is in equilibrium with metallocyclopentane complex 558. Then, insertion of the olefin or acetylene in the metal-carbon bond takes place to form 559. Reductive elimination finally liberates the deltacyclane species. 

Unlike thermal homo Diels-Alder reactions in which endo adducts predominate330, the nickel catalyzed reactions of acyclic electron-deficient dienophiles afford the exo isomers as the major cycloadducts. This has been explained by unfavorable steric interactions within intermediate 559 leading to the endo adduct. Cyclic dienophiles, on the contrary, give predominantly the endo isomer, which has again been explained by unfavorable steric interactions within exo 559. The preferred conformation of the dienophile, s-cis or s-trans, has also been suggested to play a role328. 

The cobalt mediated homo Diels-Alder reaction of norbomadiene (560) with phenyl acetylene (568a), affording a phenyl substituted deltacyclene, demonstrated the potential of low-valent cobalt complexes as catalysts332. Lautens and coworkers327 extended the scope of this reaction and were able to synthesize a wide range of substituted deltacyclenes from alkynes 568 (equation 164, Table 33). The low-valent cobalt or cobalt(O) species to be used was prepared in situ by reduction of Co(acac)3 with Et2AlCl. Monosubstituted... 

Norbornadiene adds typical dienophiles in a homo-Diels-Alder reaction whereby, for example, the adduct 86 is obtained with tetra-cyanoethylene. Dimethyl acetylenedicarboxylate and dicyanoacetyl-ene add to barrelene (bicyclo[2.2.2]octa-2,5,7-triene) in a similar way. °... 

A combined system formed from Co(acac)3, 4 equiv of diethylalu-minum chloride, and chiral diphosphines such as (S,S)-CHIRAPHOS or (/ )-PROPHOS catalyzes homo-Diels-Alder reaction of norbomadiene and terminal acetylenes to give the adducts in reasonable ee (Scheme 109). Use of NORPHOS in the reaction of phenylacetylene affords the cycloadduct in 98.4% ee (268). It has been postulated that the structure of the active metal species involves noibomadiene, acetylene, and the chelating phosphine. The catalyzed cycloaddition may proceed by a metallacycle mechanism (269) rather than via simple [2+2 + 2] pericyclic transition state. 

A new and readily available catalytic system has been developed to open the cyclopropane ring in 4 + 2-1- 2-homo-Diels-Alder cycloadducts formed by reaction of norbornadienes and buta-1,3-diene.47 The cobalt-mediated homo-Diels-Alder reaction followed by this PtCB-promolcd isomerization is a key step in the efficient route to bicyclo[5.3.0]decanes. 

The homo-Diels-Alder reactions of norbornadiene with dimethylacetylene dicarboxylate (DMAD) and TCNE are reported to have activation volumes of about - 30 cm3 cm-1 and classical examples have already been reviewed [10]. 

Scheme 35 Homo-Diels-Alder reactions of dimethyl oxanorbornadiene-2,3-dicarboxylate and related compounds 128 with cyclooctyne [81]...

Dienophiles like phosphaalkyne 44a or activated alkynes and triphosphadewarbenzene 45 undergo a [2+2+2] homo-Diels-Alder reaction that leads to C3P4 cage 46. Cyclic dienophiles yield pentacycles (Scheme 13) <1997JOM(529)215, 1999S1363>. 

An unusual nickel-catalyzed [2+2+2] homo-Diels-Alder reaction of norbomadiene (140) with (S)-(-)-p-tolyl vinyl sulfoxide (141), which yielded deltacyclane (142), has been reported (Scheme 38).85 A high degree of stereoselectivity was observed, with mainly the exo-product being formed. 

Cyclohexadiene reacts with diazenedicarboxylates and triazolediones to give ene-type products as the primary products, whereas bicyclic 1,4-dienes, e.g., norbornadiene derivatives, give homo-Diels-Alder adducts 96-37-39. However, if a longer tether links C-l and C-4 of the... 

The reaction of norbornadiene with diethyl diazenedicarboxylate affords a mixture of two 1 1 adducts 2 and 3 in approximately equal amounts25 one of the adducts 3 is produced by homo-Diels-Alder cycloaddition, as demonstrated by further conversion to quadricyclene, and the other is the oxadiazine 2, not previously identified. 

The Homo Diels-Alder Reaction of Norbomadiene with Acetylenes. [2 + 2+2] Cycloadditions of dienes such as norbomadiene with the double bonds in 1,4-position are called homo Diels-Alder reactions. Using an in situ catalyst (consisting of Co(acac)3-Et2AlCl-bis(diphenylphosphino)ethane) the products obtained with monosubstituted acetylenes, such as phenyl, i-propyl-, n-butyl-, t-butyl-, and trimethylsilylacetylene, are 4-substituted deltacyclenes. - In the formation of the polycyclic deltacyclene skeleton, six new stereo centers are generated in one step. Thus enantiocontrol by using optically active phosphine ligands as cocatalysts allows the synthesis of optically active cycloadducts, as shown for the reaction of norbomadiene with phenylacetylene to give 4-phenyldeltacyclene (eq 1). 

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