Dimerization reactions intramolecular coupling

The intramolecular coupling of the terminal amine with the oxazol-idine ring attached to the other end of the polymeric chain involves the C-5 carbonylic group since the C-2 CO group appears to be much less reactive (see, e. g. p. 6). However, for steric reasons such a reaction is prevented in the initiating dimer, hence the amine of the dimer may react only with the 2-carbonyl group, viz. 

The primary adduct 53 (Eq. (117) ) of the anodically generated radical R undergoes a series of follow-up reactions a) hydrogen abstraction to 54, b) dimerization to additive dimers 55, c) coupling with R to 1,2-disubsti-tuted monomers56, d) le-oxidation to a carbonium ion that either solvolyzes to 57 or, when 1,3-dicarbonyl compounds are added cyclizes intramolecularly to tetra (58) - or dihydrofuran derivatives (59). Product control is possible in some cases by suitable choice of the anode potential. With a high anode potential,... 

Not many catalyzed processes involving free radicals are known with these metals. Some vanadium-catalyzed pinacol coupling reactions were developed (reviews [129, 171], [172, 173] and cited ref, [174]). Niobium and tantalum complexes were applied in pinacol coupling reactions [130]. Vanadium(IV) [175-179] and vanadium(V) ([129], reviews [180-186]) complexes are known to catalyze asymmetric oxidative dimerizations of phenols and naphthols in moderate to excellent ees applying oxygen as the terminal oxidant. Biaryls are accessible by intramolecular coupling of sodium tetraarylborates, catalyzed by EtOVOCl2 in the presence of air [187]. 

Dimerization of ketone enolates. Frazier and Harlow1 have noted that ketone enolates (LDA or KH) couple to 1,4-diketones when oxidized by dry FeCl3 in DMF. Yields range from 25 to 70%. Paquette et al.2 have extended this reaction to intramolecular coupling of a bisenolate to a cyclopropane. Thus addition of a... 

In 1977, McMurry and Kees [152] developed a titanium-induced intramolecular coupling procedure to form cycloalkenes from dicarbonyl compounds. Mechanistically, as shown in Scheme 85, the coupling reaction proceeds by an initial pinacol dimerization of the dicarbonyl 253 to 254, followed by titanium-induced deoxygenation to afford alkene 255. 

Various protocols have been developed for the dimerization of imines, providing high yields of the desired 1,2-diamines [46, 48], Unfortunately, diastereoselectivities are quite low in these cases. By contrast, the intramolecular coupling reaction of the Cr(CO)3-biaryl complexes [39a] and dimeric ferrocenyl complexes [39b] proceed with generally high stereoselectivities (Eq. 43). 

This type of reaction is mainly restricted to heme peroxidases and it involves one-electron transfer processes with radical cations and radicals as intermediates (path 2). As a consequence, substrates are usually electron-rich (hetero)aromatics, which upon one-electron oxidation lead to resonance-stabilized radicals, which spontaneously undergo inter- or intramolecular coupling to form dimers or oligomers. This reaction is commonly denoted as the classical peroxidase activity, since it was the first type of peroxidase-reaction discovered. Examples of such reactions are shown in Scheme 2.176. Oxidation of phenols (e.g., guaiacol, resorcin) and anilines (e.g., aniline, < -dianisidine) leads to the formation of oligomers and polymers under mild conditions [1315-1317], In certain cases, dimers (e.g., aldoximes [1318], biaryls [1319]) have been obtained. 

Two approaches were available for the preparation of quaterrylene bisimides, taking advantage of either intramolecular cyclization or intermolecular dimerization reactions. These two methods were developed independently in Mullen s and Langhals groups. As shown in Scheme 7, Mullen s intramolecular approach started from bromination of perylene monoimide 74, which was subjected to Yamamoto coupling reaction to produce precursor 76, and the quaterrylene bisimide 77 was... 

Dimerization Reaction. Dimerization is another commonly encountered reaction of diazo compounds. Alcohol (16) is initially transformed into diazoester (17) with compound (1) and then converted to bisdiazocarbonyl compound (18). The Intramolecular coupling of this mixed diazo compound with a catalytic amount of Rh2 (OAc)4 produces a 1 1 mixture of (19) and (20). Furthermore, treatment of (19) with a catalytic amount of iodine quantitatively converts (19) to (20) (eq 12). ... 

Application of this reaction to a,ct)-bisallylic halides makes the synthesis of medium and large rings possible. Treatment of l,l-bis(chloromethyl)-ethylene (CVII) with nickel carbonyl in tetrahydrofuran gives the dimer CVIII and trimer CIX in 11 and 54% yield, respectively (Corey and Sem-melhack, 1966). These products are derived by intermolecular coupling, followed by intramolecular coupling of allylic chlorides. The nine-membered carbocycle (CIX) was produced by the coupling of CVn and the dichloride... 

The simplest p-alkylphenol is p-cresol (35), which has a methyl substituent. One of the first detailed studies of the HRP-catalyzed oxidation of p-cresol was reported in 1976 [51]. Recently, a detailed in situ NMR analysis revealed details of the coupling mechanism of the p-cresol polymerization. NMR and H- H gCOSY 2D NMR analysis suggested that ortho-ortho coupling (43) is the dominant coupling mechanism at the initial stage of the polymerization. The consumption of dimer accelerated only after the complete conversion of the monomer in the reaction mixture. After a reaction time of about 75 min, the formation of Pummerer s ketone (44) was observed. These ketonic species are formed from ortho-para-coupled dimers by intramolecular Michael addition. They are probably not able to participate in the further polymerization process and remain as side products (Scheme 9) [76]. Experiments with 4-propylphenol have revealed that the formation of Pummerer s ketone may be suppressed at lower temperatures [112]. 

The key step in the synthesis of 168 and 169 is an oxidative acetylenic coupling reaction of open-top precursor cavitands 170 and 171. Intramolecular coupling of 171 affords the basket 169 along with a dimeric structure and the switchable tube 172, in a ratio of 2/6 10 1.The two products can be separated by preparative high-performance gel-permeation chromatography (2007ACE260). 

An example of the construction of dibenzofurans by palladium-catalyzed C—H arylation was reported by Itatani in 1974 (Scheme 3.4) [17]. Compared with previous methods [18], this reaction could obviate the tedious synthetic route and is suitable for the preparation of substituted dibenzofurans with high efficiency. It should be noted that substituents on the benzene nucleus are believed to reduce the interaction between molecules and favor intramolecular coupling rather than intermolecular dimerization. 

In 1973, Yoshimoto and Itatani successfully carried out a biaiyl Pd-cata-lyzed CDC reaction with pressurized O2 as the oxidant.They coupled biphenylethers intramolecularly, but also got an equal amount of inter-molecular dimers as side-products. The experimental conditions were harsh (50 kg cm of 1 1 ojq gen and nitrogen and heated to 150 °C) and were not regioselective. In 1999, Akermark and co-workers published a Pd catalyzed CDC reaction that intramolecularly coupled arylaminoquinones. This reaction worked in only 1 atm. of O2 however, when coupling biphenylethers or biphenylamines, the reaction required additional Sn(ii) acetate additives to prevent bulk Pd from falling out of solution. 

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