Dimerization and Related Reactions

As another route, formation of 1,3,7-octatriene (7) proceeds at higher temperature in the absence of nucleophiles by Pd-catalyzed elimination of acetic acid or phenol via a 7r-allylpalladium complex from their telo-mers[l4,17]. 

The linear dimerization of substituted conjugated dienes is difficult, but the Pd-catalyzed intramolecular dimerization reaction of the 1,3,9,11-tetraene 13 gives the 3-propenylidene-4-allylpiperidine derivative 14, which has the 1,3,7-octatriene system. The corresponding 1,3,8,10-tetraene also affords the 3-pro-penylindene-4-allylcyclopentane derivative[18]. 

Dimerization is the main path. However, trimerization to form 1.3,6,10-dodecatetraene (15) takes place with certain Pd complexes in the absence of a phosphine ligand. The reaction in benzene at 50 C using 7r-allylpalladium acetate as a catalyst yielded 1,3,6,10-dodecatetraene (15) with a selectivity of 79% at a conversion of 30% based on butadiene in 22 h[ 19,20]. 1,3,7-Octatriene (7) is dimerized to 1,5,7,10.15-hexadecapentaene (16) with 70% selectivity by using bis-rr-allylpalladium. On the other hand. 9-allyl-l,4,6.12-tridecatetraene (17) is formed as the main product when PI13P is added in a 1 1. ratio[21]. 

The dimerization of isoprene is possible, but the reaction of isoprene is slower than that of butadiene. Dimerization or telomerization of isoprene, if carried out regioselectively to give a tail-to-liead dimer 18 or a head-to-tail 

The dimerization and addition of butadiene to allyidimethylamine takes place to afford 6-allyl-2,7-octadienyldimethylamine (24) by the mechanism shown. The triene 24 is a useful starting material for some natural products. 

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Selective Linear Dimerization and Related Reactions 3.1 Cationic Oligomerization... 

Dimerization and Telomerization of Conjugated Dienes and Related Reactions... 

The nitrogen and oxygen analogs of cycloheptatriene offer attractive potential substrates for higher-order cycloadditions. Unfortunately, the reactions of these compounds with a variety of dienes usually result in low yields of [6 + 4] adducts. The mechanistic details of many of these processes are intriguing, however, and merit some attention. The dimerization of IV-substituted azepines, for example, has provided numerous clues as to the actual pathways involved in these and related reactions. " Thermolysis of A -ethoxycarbonylazepine at 130 C gave initially an exo [6 + 4] cycloadduct (90) and a small amount of a dimer ostensibly derived from a thermally forbidden [6 + 6] cycloaddition process. At 200 °C the azepine was converted into the [6 + 6] product in 83% yield. Apparently the [6 + 4] product isolated under milder conditions is an intermediate in the formation of the [6 + 6] dimerization prc uct. 

The reaction between pheiianthroline and ferric chloride in glacial acetic acid gives the yellow complex Fe(phen)Cl3, which has been formulated to contain six-coordinate iron(III), possibly Fe2(i)hen)2CI(i. The direct reaction between aquo-ferric ions and pheiianthroline gives a brown material containing two iron atoms per molecule, whereas oxidation of the deep red tris(phenanthroline)iron(II) ion affords the pale lilue Fe(III)(phen)3 oii ( )- The brown dimer and related species have been the subject of many investigations (16, 96, 205, 222, 416). They were... 

Since isoprene was the principal desired product from methyl pentene pyrolysis it was essential to determine its stability under reaction conditions. Isoprene degradation proceeds via second-order rate processes. Isoprene dimer and related compounds formed during the pyrolysis are dependent on the isoprene partial pressure in the reaction system. 

One of the arene ligands in Mo(CgHg)2 is easily displaced, for example by phosphines. The products Mo(CgHg)(PR3)3 are strong bases which can be protonated once or twice by acids. The addition of allyl chloride (3-chloropro-pene) with elimination of one benzene ligand gives a chloro-bridged allyl dimer. These and related reactions provide an entry into mono-arene molybdenum chemistry (Fig. 10.3). 

Hence, the electronic reasons for the facilitation of the acid catalyzed dimerization (and trimerization) reactions of EDOT are very closely related to the prerequisites for being oxidizable to a stable conducting state, with special emphasis to the word stable in the meaning of electronically and environmentally stable. The latter means stable to air, including oxygen and humidity, and also to water and solvents like aliphatic alcohols. 

Thus in the dimerization reactions of allenes, or in their cyclo-addition reactions to olefins, non-concerted pathways appear to be favoured. The allene dimers are always of the head-to-head, tail-to-taU, type which suggests that 1,4-diradical intermediates are formed in these reactions. The stereospecific addition that may attend these and related reactions may merely be an indication of the lack of conformational flexibility of the diradical intermediates because of steric crowding. 

In contrast, the photochemistry of uracil, thymine and related bases has a large and detailed literature because most of the adverse effects produced by UV irradiation of tissues seem to result from dimer formation involving adjacent thymine residues in DNA. Three types of reaction are recognizable (i) photohydration of uracil but not thymine (see Section 2.13.2.1.2), (ii) the oxidation of both bases during irradiation and (iii) photodimer formation. 

Two classes of charged radicals derived from ketones have been well studied. Ketyls are radical anions formed by one-electron reduction of carbonyl compounds. The formation of the benzophenone radical anion by reduction with sodium metal is an example. This radical anion is deep blue in color and is veiy reactive toward both oxygen and protons. Many detailed studies on the structure and spectral properties of this and related radical anions have been carried out. A common chemical reaction of the ketyl radicals is coupling to form a diamagnetic dianion. This occurs reversibly for simple aromatic ketyls. The dimerization is promoted by protonation of one or both of the ketyls because the electrostatic repulsion is then removed. The coupling process leads to reductive dimerization of carbonyl compounds, a reaction that will be discussed in detail in Section 5.5.3 of Part B. 

Coupling reactions and related fluoroalkylations with polytTuoioalkyl halides are induced by vanous reagents, among them metals such as copper and zinc, or by an electrochemical cell. More recently, examples of carbon-carbon bond forma tion by coupling of unsaturated fluorides have been reported Both acyclic and cyclic fluoroolefins of the type (Rp)2C=CFRp undergo reducUve dimerization on treatment with phosphines [42] (equation 33) The reaction shown in equation 33 IS also accompbshed electrocheimcally but less cleanly [43]... 

Electronic effects on the reactions of [Rh(Por)h dimers and hydrides were probed by varying the porphyrin macrocycle. OEP and TPP vary considerably in their properties, with OEP being one of the strongest and TPP one of the weakest (7-donors among porphyrin derivatives. However. Rh(Por)]2, Rh(Por)H, and Rh(Por)r showed the same reactivity in a variety of reactions for both OEP and TPP, indicating that electronic effects relating to the porphyrin ligand have... 

In the present study the dimer (salen)CoAlX3 showed enhanced activity and enantioselectivity. The catalyst can be synthesized easily by readily commercially available precatalyst Co(salen) in both enantiomeric forms. Potentially, the catalyst may be used on an industrial scale and could be recycled. Currently we are looking for the applicability of the catalyst to asymmetric reaction of terminal and meso epoxides with other nucleophiles and related electrophile-nucleophile reactions. 

The use of well-defined complexes has been widespread in this reaction, despite intriguing studies by Beller and others that have shown that in situ catalytic systems often give better yields in comparison to isolated carbene-Pd(O) complexes [147-149]. Since the mechanism consists of an oxidative addition on a Pd(0)-monocarbene species, efforts in catalyst synthesis have been directed towards Pd(ll)-monocarbene complexes with other labile groups that can be easily released leading to the formation of Pd(0). This is the case for dimers of the type [Pd( j,-C1)C1(NHC)]2, a family of pre-catalysts effective under aerobic conditions [150], the [Pd(acac)Cl(NHC)] complexes [151] and related palladacycles [152-154],... 

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