Diels-Alder reactions homo-----

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... 

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. 

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). 

This last reaction is known as the homo-Diels-Alder reaction. A similar reaction has been reported with alkynes, using a mixture of a cobalt complex, Znl2 and tetrabutylammonium borohydride as catalysts. ... 

Two homo Diels-Alder reactions and a hetero Diels-Alder reaction, each using a silyloxydiene, have been used in a total synthesis of vinemycinone methyl ester (236) by Danishefsky (84JA2453 85JA1285). Ketoaldehyde 231, prepared by two Diels-Alder cycloadditions, reacted with diene 230 in chloroform in the presence of Eu (fod)j to produce the silyl enol ether... 

The more or less accidental discovery that fluorinated y9-diketonates of europium both effect the Lewis acid catalysis of Diels-Alder reactions [10] and interact with carbonyl functionalities of the substrates [102] emerged in a very prolific research. Despite the ready feasibility of homo-Diels-Alder reactions [103], hetero-Diels-Alder cycloadditions are preferentially mediated [104]. The endo select vities... 

Gugelchuk, M. M., Wisner, J. Density Functional Study of Intermediates in the Nickel-Catalyzed Homo-Diels-Alder Reaction of Norbornadiene with Alkenes. Organometallics 995, 14,1834-1839. 

The homo-Diels-Alder reaction is the thermally allowed [27ts -f 2ng -l- 2jts] cycloaddition, which results in the formation of a cyclopropane. ... 

Some related processes, such as the extended homo-Diels-Alder reaction (e.g. [lug + lug -I- 47Ts] cycloadditions), and the homocheletropic reactions, will also be discussed. 

Homo-Diels-Alder Reactions ([2tig-(-2ng-j-2ng] Cycloadditions)... 

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