The Homo-Diels-Alder Reaction

This related reaction, [9] illustrated in Fig. 7.4, differs from +,r 2]-cycloaddi- 

Only the three tt bonds of the reactant and the three newly formed a bonds of the product need be included in the diagram. Bicycloheptadiene and the dienophile, characteristically an electronegatively polysubstituted ethylene, are set up in the tt orbital of the latter lying in the zx plane. This orbital is 

The one mismatch in Fig. 7.4 is between 7r(ai) and r b2). Correspondence between them is induced by a 62 displacement motion of the dienophile parallel to the y axis so that it can bond concertedly either to Ci and C4 or to C2 and C3. 

If the reactants and product are set up in C, all of the occupied MOs would correlate across the diagram. Alternatively, a correspondence diagram, in which the reactants are set up in C2V and the product is anasymmetrized to that symmetry point group, would show that formal desymmetrization of the pathway to C, - i.e. to the true molecular symmetry of the product -is called for. The methylene bridge of cyclopentadiene is innocuous so, as it turns out, is the bridging carbonyl group in tropone. It will become evident from subsequent examples, however, that the presence of heteroatoms and/or multiple bonds can make a substantial difference to the conclusions drawn from an orbital symmetry analysis. 

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

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. 

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]. 

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. ... 

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

An investigation of the homo-Diels-Alder reaction between norbornadiene and different dienophiles showed general endo selectivity in the products 1 for R = H however, when... 

Tetracyanoethene, dimethyl acetylenedicarboxylate, methyl propynoate, maleic anhydride, 4-methyl- and 4-phenyl-4//-l,2,4-triazole-3,5-diones, are the most frequently used dienophiles in the application of the homo-Diels-Alder reaction. 

The use of zero-valent transition metal (mainly nickel and cobalt) complexes as promoters of the homo-Diels-Alder reaction has been an important development. These catalysts allow 1,4-dienes to react with nonactivated alkenes and alkynes, broadening the scope of the homo-... 

Yield improvements and changes in selectivity were observed by using nickel, cobalt or manganese catalysts in this cycloaddition. Noteworthy is the formation of a new isomer, 8-ethylidenetetracyclo[4.3.0.0 . 0 ]nonane, shown to be the primary product (8%) of the homo-Diels-Alder reaction when a nickel catalyst was used." ... 

The homo-Diels-Alder reactions of norbornadiene and quadricyclane with acrylic esters in the presence of nickel catalysts and phosphorus ligands gives, with different kinetics, the same endojexo mixtures of adducts, leading to the conclusion that a common intermediate and stepwise mechanism are involved.A systematic investigation of the effects of several phosphorus ligands on the stereoselectivity of the nickel-catalyzed homo-Diels-Alder reaction of norbornadiene with acrylonitrile shows a clear correlation between ligand bulk and the ratio of exoj endo-Komers ... 

The use of nickel catalysts allows the homo-Diels-Alder reaction to occur between norbornadiene and a-unsaturated ketones and sulfoxides to give the products 8 often with excellent yield and stereoselectivity. Whereas the exo preference, usual in the catalytic homo-Diels-Alder reactions, is here evident for the open-chain dienophiles, the endo selectivity observed with cyclopentenone could be due to its s-trans conformation." ... 

The homolog of prismane, quadricyclane (29) was obtained through the homo-Diels-Alder reaction between diethyl azodicarboxylate and norborna-2,5-diene, ° although the deazetization has to be carried out photochemically. Thermal deazetization of the diazene 28 (and of closely related diazenes ) results in a reversion to the homodiene. 

Albeit not as popular as the homo-Diels-Alder reaction, the hetero [4+2] cycloaddition, possessing all of the features of the homo version, offers a significant potential for the synthesis of the six-membered hetero rings constituents of several natural products e.g., carbohydrates and alkaloids. 

Catalyzed and uncatalyzed homo-Diels-Alder reactions behave mechanistically in the same way as Diels-Alder reactions. However, in the presence of a Lewis catalyst two competing processes, the homo-Diels-Alder reaction and the [2 + 2] cycloaddition, take place. The smaller absolute value of AV observed for the [2 + 2] cycloaddition is good evidence of a stepwise process. In this case, the size of the pressure effect depends on the possibility of electrostriction due to the polarity of the transition state [14],... 

Cobalt(II) iodide-triphenylphosphine-zinc systems catalyse the homo-Diels-Alder reaction of norbornadiene with alkynes R C=CR (R = Bu, Ph or SiMe3 R = H, Me or Et) to give the tetracyclic adducts 512. Asymmetric induction has been reported for the cobalt acetylacetonate-catalysed homo-Diels-Alder addition of 1 -hexyne to norbornadiene in the presence of the diphosphine (R)-(+)-Ph2PCH2CHMePPh2 to yield the dextrorotatory product 513 in 78% enantiomeric excess. Six new stereocentres are created in this... 

It seems that the homo-Diels-Alder reactions proceed markedly more rapidly than the Diels-Alder reactions since the primary [4-1-2] adducts carmot be detected spectroscopically. Furthermore, it is noteworthy that all 1 -alkynes react regiospecifically in the initial [4-1-2]-cycloaddition process. This addition proceeds in such a way that, independent of the polarity situation of the acetylene, the carbon atom bearing the substituent is positioned in the immediate vicinity of the phosphorus atom of the phosphirane increment. This suggests that steric factors are responsible for the direction of the addition. This is further supported by the observation that 11a does not participate in cycloaddition reactions with any disubstituted triple bond system, except for cyclooctyne, which possesses the necessary reactivity for a cycloaddition on account for the cisoid-dis-... 

Homo-Diels-Alder reactions have also been achieved with, in some cases, very high selectivity. In 1990, two groups independently reported achieving the homo-Diels-Alder reaction between norbornadiene (8.88) and phenylacetylene (8.89), affording the deltacyclene product (8.90). ° When the phosphine used in this reaction was Norphos, the product was formed with remarkable selectivity. 

Cycloaddition Reactions with Norbomadienes (The Homo Diels-Alder Reaction).55... 

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

Cyclic enones have not been previously examined as dienophiles in the homo Diels-Alder reaction. We found that this class of compounds is moderately reactive in the presence of Ni(COD)2/PPh3. One feature of the cycloaddition is that pentacyclic compounds 13 are created with high stereoselectivity in a single step (> 20 1 ratio of stereoisomers). However, in contrast to reactions with acyclic enones, the stereochemistry of the newly formed rings in 13a and 13b is endo (Eq. 9). The change... 

The homo Diels-Alder reaction gives high levels of stereoselectivity, but which isomer would predominate is not easily predicted given the... 

Use of a monosubstituted acetylenes, such as phenylacetylene, in the homo Diels-Alder reaction results in chiral deltacyclenes, which contain a total of 6 asymmetric carbon atoms. However, as a consequence of the two and only two different addition modes of a monosubstituted acetylene to norbomadiene, the deltacyclene is formed as a pair of enantiomers. 

An improved procedure has been reported for performing the homo-Diels-Alder reaction between allyl cations and cyclopentadiene. In the new method the allyl cations are generated by reaction of aa -dibromoketones with triethylborate and zinc. A typical example is illustrated by the conversion of (233) into bicyclo[3,2,l]oct-6-en-3-ones (234a—c), and the tentative reaction pathway is outlined (Scheme 1). 

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