Norbornadienes homo-Diels-Alder cycloaddition

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. 

Enantioselective homo-Diels-Alder cycloaddition can also be achieved with Co(II)/Zn catalysts chirally modified with amino acid derived phosphite ligands63, Stereoselective cycloaddition of norbornadiene has also been observed with enones64 to give the [2 + 2 + 2] adducts, while with dienes in the presence of chirally modified Co(II)/Et2AlCl catalysts optically active [4 + 2 + 2] adducts were obtained in up to 66% yield and 79% ee65. 

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). 

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 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%. ... 

Phenyl- and 4-methyl-4H-l,2,4-triazole-3,5-dione are often used as reactive dienophiles with subtituted norbornadienes and other polycyclic 1,4-dienes. 2-Trimethylsilyl-, 2-chloro-, 2-cyano- and 2-methoxycarbonylnorbornadiene react with 4-phenyl-4i/-l,2,4-triazole-3,5-dione in dichloromethane at room temperature to give the corresponding homo-Diels-Alder adducts 18, accompanied by [2 + 2] cycloaddition, rearrangement, insertion and polymerization products. 

The main product in the reaction of norbornadiene with buta-1,3-diene resulted from a 1,4-cycloaddition onto the conjugated diene (for extended homo-Diels-Alder reactions, vide infra). On the other hand, the 1,2-homo-Diels-Alder adduct 4 was obtained when bisfcycloocta-tetraenejiron [or iron(III) chloride/isopropylmagnesium chloride] was used as the catalyst. ... 

The use of transition metal complexes as catalysts allows 1,4-cycloadditions to be involved as the major pathway in several cases when conjugated dienes are reacted with norbornadiene. No normal homo-Diels-Alder reaction was observed by reaction of the latter with buta-1,3-diene in the presence of an iron complex catalyst, the main product obtained was such a 1,4-adduct 2f the same adduct 2 was obtained in good yield and selectivity when a catalyst formed from cobalt(II) chloride, diethylaluminum chloride and bis(l,2-diphenylphos-phinojethane was used. ... 

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

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]. 

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