General coupling reactions

The E) alkene (21) was obtained from 2-iodoselenophene as an example of a general coupling reaction between iodoarenes and vinylsilanes <86JOC5286>. 

The general coupling reaction of a haloalkane, containing a positively polarized carbon, with an alkylmetal, containing a negatively polarized carbon, is quite exothermic. 

Via intermediates Synthesis of hydrocarbons from ethylene derivatives via boranes A new general coupling reaction... 

Syntheses of alkenes with three or four bulky substituents cannot be achieved with an ylide or by a direct coupling reaction. Sterical hindrance of substituents presumably does not allow the direct contact of polar or radical carbon synthons in the transition state. A generally applicable principle formulated by A. Eschenmoser indicates a possible solution to this problem //an intermolecular reaction is complex or slow, it is advisable to change the educt in such a way. that the critical bond formation can occur intramolecularly (A. Eschenmoser, 1970). 

Reaction that can be carried out by the oxidative coupling of radicals may also be initiated by irradiation with UV light. This procedure is especially useful if the educt contains oleflnic double bonds since they are vulnerable to the oxidants used in the usual phenol coupling reactions. Photochemically excited benzene derivatives may even attack ester carbon atoms which is generally not observed with phenol radicals (I. Ninoraiya, 1973 N.C. Yang, 1966). 

Pyridine-2- and -4-diazonium ions are far less stable than benzenediazonium ions. Azolediazonium salts generally show intermediate stability provided diazotization is carried out in concentrated acid, many of the usual diazonium reactions succeed. Indeed, azolediazonium salts are often very reactive in coupling reactions. 

In general, imines are too reactive to be used to protect carbonyl groups. In a synthesis of juncusol, however, a bromo- and an iodocyclohexylimine of two identical aromatic aldehydes were coupled by an Ullman coupling reaction modi-fied by Ziegler. The imines were cleaved by acidic hydrolysis (aq. oxalic acid, THF, 20°, 1 h, 95% yield). Imines of aromatic aldehydes have also been prepared... 

The true, all-aromatic system (see 18, below) described by Kime and Norymberski is unusual in the sense that all of the ether linkages bridge aromatic carbons ". Synthesis of 18, therefore, required extensive use of copper mediated coupling reactions. As expected for such reactions, yields were generally low. The aromatics such as 18 were ineffective at binding either alkali metal or ammonium cations ". ... 

Reductive coupling reaction of fluonnated vinyl iodides or bromides has been used as a route to fluorinated dienes [246, 247, 248, 249, 250. Generally, the vinyl iodide is heated with copper metal in DMSO or DMF no 1 ntermediate perfluorovmy I-copper reagent is detected. Typical examples are shown m equations 163-165 [246, 247, 249. The X-ray crystal structure of perfluorotetracyclobutacyclooctatetraene, prepared via coupling of tetrafluoro-l,2-diiodocyclobutene with copper, is planar... 

In general the reaction is conducted in aqueous solution under basic conditions—e.g. in the presence of KOH. The 1,3-dicarbonyl substrate is deprotonated to give the corresponding anion 5, which then couples to the arenediazonium species 1, to give the diazo compound 3 ... 

The first unequivocal evidence for the AE + DE mechanism came in three papers by Zollinger (1955 a-c) dealing with general base catalysis and primary kinetic hydrogen isotope effects in azo coupling reactions of various types. Three classes of reactions were identified i) reactions with no isotope effects (ArH/A D - 1.0) and no general base catalysis, ii) others with large isotope effects (k /k — 6.5) and (practically) linear base catalysis, and iii) intermediate cases with isotope effects of around 3.0 and less-than-linear base catalysis. 

The original investigations on the general base catalysis of azo coupling reactions were made with nucleophilic substrates in which the reacting carbon atom was... 

Kishimoto et al. (1974, 1981) found a general acid catalysis by protonated pyridines in coupling reactions of the 1-naphthoxide ion if weakly electrophilic diazonium ions were used. In this case it is likely that the general acid protonates the carbonyl oxygen of the o-complex, with a concerted or stepwise deprotonation at the 4-position (transition stage 12.150). 

A true intramolecular proton transfer in the second step of an azo coupling reaction was found by Snyckers and Zollinger (1970a, 1970b) in the reaction of the 8-(2 -pyridyl)-2-naphthoxide ion (with the transition state 12.151). This compound shows neither a kinetic deuterium isotope effect nor general base catalysis, in contrast to the sterically similar 8-phenyl-2-naphthoxide ion. Obviously the heterocyclic nitrogen atom is the proton acceptor. 

There are two cases in which the general base catalysis observed for an azo coupling reaction is due not to a rate-limiting proton transfer from the o-complex (Scheme 12-66) but to deprotonation of the coupling component when the species involved in the substitution is formed. These reactions are shown in Schemes 12-71 H I... 

In the azo coupling reaction of acetoacetanilide (Dobas et al., 1969b) the reaction steps of Schemes 12-71 and 12-72 constitute a steady-state system, i.e., Arx [B] < Ar [HB+] == 2[Ar —NJ] A 2 — 0 with a fast subsequent deprotonation (Scheme 12-73). As with nitroethane, this reaction is general base-catalyzed because the ratedetermining step is the formation of the anion of acetoacetanilide (Scheme 12-71). In contrast to the coupling of nitroethane, however, the addition of the diazonium ion (Scheme 12-72) is rate-limiting. The overall kinetics are therefore between zero-order and first-order with respect to diazonium ion and not strictly independent of [ArNJ ] as in the nitroethane coupling reaction. 

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