Diels-alder dimerization reaction intermolecular

In 2005, Nikaido et al. [9] reported the first example of cobalt-phosphine-catalyzed intermolecular Diels-Alder dimerization reaction (Scheme 16.7). The dimerization of piperine derivative (28) in the presence of cobalt-phosphine complexes appear to be more efficient than that of using only cobalt, which, combined with the result under purely thermal conditions, indicates that the addition of phosphine ligand changes the inhibition of cobalt to the reaction into promotion. Triphenylphosphine (PPhs) is the most favorable phosphine ligand of the reaction. 

The structural similarities between S-alkylthiophenium salts and thiophene sulfoxides have prompted the suggestion that both classes are antiaromatic (70JA7610, 70JCS(C)1764). Thiophene sulfoxides are exceptionally reactive, undergoing spontaneous Diels-Alder dimerization unless stabilized by bulky substituents at the 2- and 5-positions. This type of reactivity is reminiscent of cyclobutadiene. In stark contrast to the sulfoxides, S-alkylthiophenium salts show no tendency to take part in Diels-Alder reactions either in an intra- or intermolecular sense. Pentamethylthiophenium hexafluorophosphate (11, X = PFg ) was shown to be completely unreactive toward both electron-rich and electron-deficient dienophiles under conditions where thiophene sulfoxides undergo efficient cycloaddition reactions. 

An increase of char yield is generally reflected as an improvement in oxygen index. In the styrylpyridine based polyesters and polycarbonate an intermolecular thermally induced Diels-Alder reaction has occurred through the double bond, this increased the char yield and decreased the flammability. The Fries rearrangement, as well as dimerization and isomerization, occurred simultaneously during the UV irradiation of p-VPPB, but no dimerization or isomerization occurred for p,p -BVPDPC, probably due to steric effects. 

For the structure determination of flavoskyrin (556), a yellow metabolite of Penicillium islandicum, a biosynthetic pathway involving Diels-Alder cycloaddition (Scheme 70) has been proposed by Shibata and co-workers (73T3721). The dimerization of l-oxo-l,2,3,4-tetrahydroanthraquinone derivative 551 has been elucidated by intermolecular Diels-Alder reaction with exo approach of the monomers in the model experiment as formulated in 553. Eventually, an enolized form of tetrahydroemodin (552) was considered as a monomeric precursor which could be dimerized by the intermolecular Diels-Alder reaction to form flavoskyrin (556). 

A v/c-dihaloadamantane undergoes the intermolecular reductive dimerization via unstable adamantene species to give stereoisomeric products in 80-90% yields, as in Eq. (34) [240]. Cathodic reduction of or, or -dibromo-l,2-dialkylbenzenes in the presence of dienophiles (maleic anhydride, etc.) and hinderd dienophiles provides diels-Alder adducts stereoselectively [241,242]. In this reaction, the dienophiles play a duel role as mediators... 

Generally, on PhI(OAc)2-promoted oxidation of o-methoxyphenols in MeOH containing a large excess of electron-rich dienophiles, the resulting o-quinone monoketals may undergo an intermolecular Diels-Alder reaction with the dienophiles to afford the corresponding dimers. 4-Methoxycarbonyl-2-methoxyphenol (421) was submitted to PhI(OAc)2-promoted oxidation in MeOH containing benzyl vinyl ether (BVE) or dihydrofuran (DHF)... 

Cycloadditions only proceeding after electron transfer activation via the radical cation of one partner are illustrated by the final examples. According to K. Mizono various bis-enolethers tethered by long chains (polyether or alkyl) can be cyclisized to bicyclic cyclobutanes using electron transfer sensitizer like dicyanonaphthalene or dicyano-anthracene. Note that this type of dimerization starting from enol ethers are not possible under triplet sensitization or by direct irradiation. Only the intramolecular cyclization ci the silane-bridged 2>. s-styrene can be carried out under direct photolysis. E. Steckhan made use of this procedure to perform an intermolecular [4+2] cycloaddition of indole to a chiral 1,3-cyclohexadiene. He has used successfully the sensitizer triphenylpyrylium salt in many examples. Here, the reaction follows a general course which has been developed Bauld and which may be called "hole catalyzed Diels-Alder reaction". 

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