Diels repetitive

In a second novel approach, Mullen and co-workers further developed an inter-molecular repetitive Diels-Aldcr procedure for the generation of dendritic and hy-perbranched poly(phenylcnc)s. Hereby, they applied the concept of reacting A2B-type monomers, in this case monomers containing both cyclopcnladienone (dien-... 

Synthesis of sterically rigid macrocycles by the use of pressure-induced repetitive Diels-Alder reactions [84]... 

Tetramethylenebicyclo[2.2.2]oct-2-ene as bisdiene in repetitive Diels-Alder reactions [93]... 

Beside the polymerization routes of 1,3-cyclohexadiene derivatives repetitive Diels-Alder polyadditions were widely used to prepare arylated PPPs. Stille et al. developed a set of suitable monomers (1,4-diethynylbenzene and 1,4-phenyl-ene-bis(triphenylcyclopentadienone) derivatives) to generate phenylated PPPs (e.g. 17) with molecular weights of 20,000-100,000 [31]. Unfortunately, the repetitive polyadditon does not proceed regioselectively polymers containing para-as well as mefa-phenylene units within the main chain skeleton are formed. 

Higher hexaphenylbenzene homologues were also prepared following a repetitive Diels-Alder procedure. The synthesis of a bis(hexaphenyl)benzene was first described by Ogliaruso et al. [55]. 

In line with a second novel synthetic principle, the authors further developed the repetitive Diels-Alder procedure, in which monomers containing cyclopentadienone (dienophile) units were reacted with protected/deprotected ethynylene functions (see [31]). In this way, they generated a novel class of highly arylated phenylene dendrimers 46, starting from a 3,3, 5,5 -tetraethynyl-substituted biphenyl core [60]. 

The synthesis of the macrocycles 43 (Scheme 9) is an example of repetitive, highly stereoselective Diels-Alder reaction between bis-dienes 41 and bis-dienophiles 42, containing all oxo or methano bridges syn to one another. The consecutive inter- and intramolecular Diels-Alder reactions only succeed at high pressure. Obviously, both reactions are accelerated by pressure. The macrocycles are of interest in supramolecular chemistry (host-guest chemistry) because of their well-defined cavities with different sizes depending on the arene spacer-units. 

If the oxo (or methano) bridges are not exclusively syn to one another in either the bis-dienophiles or bis-dienes, then the pressure-induced repetitive Diels-Alder reactions (proceeding again highly stereoselectively) produce rigid ribbon-type oligomers on a nanometer scale (Scheme 10 entry 1). Bis-diene 45 reacts less stereoselectively than bis-diene 44 and forms with bis-dienophiles such as 46 the ribbon-type oligomers 47... 

SCHEME 9. Repetitive Diels-Alder reactions in the synthesis of macrocycles having cavities of different size93-94... 

Reacting 126 with dienophiles 283 produces Diels-Alder adducts 284, which, as cydobutenes, can thermally be ring opened to the conjugated dienes 285, ready for a repetition of the cycloaddition step to provide the 2 1 adducts 286. Hence the whole sequence represents a diene-transmissive Diels-Alder protocol [118]. 

The cydopropanated version of 3, l,3-di(cyclopropyl)allene (18), has been used as a coupling partner in Heck-type reactions. For example, with iodobenzene the conjugated cyclopropylhexatriene 330 is obtained whereas repetition of the experiment in the presence of dimethyl maleate yields the Diels-Alder adduct 331 [58]. 

Homologation of 1,3-dienes.1 The reagent is particularly useful for cyclohomologation of 1,3-dienes. Thus the adduct a of a Diels-Alder reaction of 1 with 1,2-bis(methylene)cyclohexane undergoes Ramberg-Backlund rearrangement to the tri-ene 2. Repetition of the sequence provides the tetraene 3. The sequence is applicable to cyclopentadiene, furan, and 2,3-dimethyl-l,3-butadiene. 

The synthetic method applied is that of a repetitive Diels—Alder reaction whereby ethynyl-substituted tetraphenylcyclopentadienones 105 and 106 serve as branching reagents, since they contain one diene and two to four dienophile functions, respectively. The... 

Repetitive Diels-Alder reactions utilizing bis-ort/io-quinodimethane units have been employed[14bl for the construction of polymeric and oligomeric [60]-fullerenes with molecular weights up to 80,000 amu. The potential of using this method for dendrimer construction was suggested. 

Fig. 5. Schematic representation illustrating the basic premise of the repetitive Diels-Alder oligomerization process [21]. The macropolycyclization occurs as a consequence of the reaction between bisdiene and bisdienophilic building blocks whose relative stereochemistries and stereoelectronic characteristics dictate the structural outcome of the reaction...

As part of a research program designed (i) to afford macropolycyclic belts of larger ring sizes and (ii) to construct novel acyclic oligomeric molecular strips, an extended bisdiene 57 was identified as a suitable building block to be used in repetitive Diels-Alder reactions. 

The isolation of 61, as well as the expected syn/endo-Hlsyn/endo-H isomer 59 (Scheme 13), illustrates, for the first time, evidence for the existence of a second reaction pathway operating during these repetitive Diels-Alder reactions. HPLC analysis of the reaction mixture, isolated after a high pressure reaction between 38 and 57, revealed the product ratio of 59 61 to be 18 1. The question of whether 61 owes its origin to the existence of a second, less favorable reaction pathway in a kinetically-controlled process, or is, in fact, the outcome of a degree of thermodynamic control, involving a retro Diels-Alder reaction, followed by recombination, will be discussed in Section 3.2. 

Kinetic Versus Thermodynamic Control Within the Repetitive Diels-Alder Reaction Regime... 

Scheme 18. If the repetitive Diels-Alder reaction adducts reported were being formed under thermodynamic control, heating the hexadecadeutero 2 1 adduct 68 under reflux in toluene in the presence of a 10 molar equivalent of the bisdiene 37 should give rise to a mixture of compounds comprised of 68, 69, and 41. A retro Diels-Alder reaction of 68, followed by scrambling of the labeled bisdiene 66 with the excess of the bisdiene 37 should favor preferential cycloaddition with 37 to afford 69. The excess of 37 should ultimately drive the equilibration process toward the unlabeled 2 1 adduct 41. No evidence for such an equilibration process has been found... 

Another appraisal of the mechanism which operates in these repetitive Diels-Alder reactions, was based directly on the isolation of a minor 2 1 adduct 61 alongside the major 2 1 adduct 59 from the reaction of the syn-bisdienophile 38 with the... 

One of the primary aims of the research program described in this review has been the incorporation of bisdiene and bisdienophilic building blocks into the synthesis of novel macropolycyclic structures by a repetitive Diels-Alder oligo-... 

In summary, the success of the trebly diastereoselective repetitive Diels-Alder oligomerization process for the synthesis of outwardly complex molecular structu-... 

A more recent, and particularly successful, variant of the multifunctional coupling process to ladder polymers is the use of repetitive cycloaddition reactions that start from bifunctional monomers, for instance dienes and dienophiles (Diels-Alder-type cycloadditions) [2-4]. The fact, that both chains are generated simultaneously in a concerted process constitutes the important progress associated with such a route. 

The above methods of synthesis have already used existent ring systems like difunctionalized benzene or cyclohexadienes as starting compounds. In an alternative approach one can generate benzene rings as part of the polymerization reaction, for example, by repetitive Diels-Alder reactions [100] or by valence isomerization [101]. Typical examples of the former include the combination of bisdienes, such as 54 and bisdienophiles, such as 55, whereby the formation of the benzene ring proceeds by extrusion of carbon monoxide from the original Diels-Alder products (scheme 9). The reaction thereby produces a mixture of para- and meto-linked polyphenylenes. 

Both the organometallic methods of phenyl-phenyl coupling (PPP synthesis) and the repetitive Diels-Alder reaction between tetraphenylcyclopentadienones and arylacetylenes toward branched polyphenylenes can be modified for the synthesis of structurally related oligomers (see also section 1.4.). Thus,... 

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