Coupling reactions terminal

CgHgCHg-CHgCgHs Various radical coupling reactions terminate the chain. 

The first method has serious limitations because well-defined end groups can be observed only if not more than one type of primary radical is formed that does not cause side reactions such as transfer. Moreover, a propagating radical will readily react with another radical, primary or macroradical, either through disproportionation or through coupling reactions (termination) (Fig. 2). The former will produce monofunctional telechehcs with both a saturated and an imsaturated chain end, while only the latter will yield bifunctional telechelics. 

TrialkyIboranes (p. 9), which can be synthesized from olefins and diborane, undergo alkyl coupling on oxidation with alkaline silver nitrate via short-lived silver organyls. Two out of three alkyl substituents are coupled in this reaction. Terminal olefins may be coupled by this reaction sequence in 40 - 80% yield. With non-terminal olefins yields drop to 30 - 50% (H.C. Brown, 1972C, 1975). 

The formation of disubstituted alkynes by coupling of terminal alkynes, followed by intramolecular attack of an alcohol or amine, is used for the preparation of benzofurans and indoles. The benzo[il)]furan 356 can be prepared easily by the reaction of o-iodophenol with a terminal alkyne[262]. The 2-substituted indole 358 is prepared by the coupling of 2-ethynylaniline (357) with aryl and alkenyl halides or triflates, followed by Pd(ll)-catalyzed cycliza-tion[263]. 

A similar phenomenon was observed for 3-amino- and5-amino-4-iodopyrazoles. The anomalous reaction in which the products of oxidative coupling of terminal acetylenes (up to 90%) are present along with the products of deiodination (up to 90%) has been described for the first time [99JCS(P1 )3713] and will be considered below in the part related to cross-coupling of 4-iodopyrazoles. 

The same type of reactions includes the Chodkiewicz-Cadiot reaction, i.e., a coupling of terminal acetylenes with bromoacetylenes, which is performed in... 

The Glaser reaction is an oxidative coupling of terminal alkynes 1 to yield a symmetrical Z -acetylene 2 the coupling step is catalyzed by a copper salt. Closely related is the Eglinton reaction, which differs from the Glaser reaction mainly by the use of stoichiometric amounts of copper salt as oxidizing agent. 

ABA type poly(hydroxyethyl methacrylate) (HEMA) and PDMS copolymers were synthesized by the coupling reactions of preformed a,co-isocyanate terminated PDMS oligomers and amine-terminated HEMA macromonomers312). Polymerization reactions were conducted in DMF solution at 0 °C. Products were purified by precipitation in diethyl ether to remove unreacted PDMS oligomers. After dissolving in DMF/toluene mixture, copolymers were reprecipitated in methanol/water mixture to remove unreacted HEMA oligomers. Microphase separated structures were observed under transmission electron microscope, using osmium tetroxide stained thin copolymer films. 

A cross-coupling reactions of terminal alkynes with terminal alkenes 32 supported on Merrifield-resin (Scheme 4.5) in the presence of Grubs ruthenium initiator [Cl2(PCy3)2Ru = CHPh] provided efficient access to supported 1,3-dienes 33 which were transformed into octahydrobenzazepinones 34 via MeAlCl2 catalyzed Diels-Alder reaction [27]. 

In these reactions a new carbon-carbon bond is formed, and they may be given the collective title coupling reactions. In each case, an aUcyl or aryl radical is generated and then combines with another radical (a termination process) or attacks an aromatic ring or alkene to give the coupling product. ... 

In 1988, Linstrumelle and Huynh used an all-palladium route to construct PAM 4 [21]. Reaction of 1,2-dibromobenzene with 2-methyl-3-butyn-2-ol in triethylamine at 60 °C afforded the monosubstituted product in 63 % yield along with 3% of the disubstituted material (Scheme 6). Alcohol 15 was then treated with aqueous sodium hydroxide and tetrakis(triphenylphosphine)palladium-copper(I) iodide catalysts under phase-transfer conditions, generating the terminal phenylacetylene in situ, which cyclotrimerized in 36% yield. Although there was no mention of the formation of higher cyclooligomers, it is likely that this reaction did produce these larger species, as is typically seen in Stephens-Castro coupling reactions [22]. 

Related to these strategies, Fe(OTf)3/TfOH cocatalyzed the coupling reaction of terminal alkynes with benzylic alcohols in the absence of base by means of a sp-sp C-C bond formation (Scheme 16) [32]. 

Scheme 17 Iron-catalyzed three-component coupling reactions of aldehydes, terminal alkynes, and amines...

To check the scope of this coupling reaction, a study with different combinations of aldehydes, amines, and terminal alkynes was performed. Aromatic alkynes turned out to be more reactive than aliphatic ones. This study included aliphatic... 

The coupling of terminal alkynes with aryl or alkenyl halides catalysed by palladium and a copper co-catalyst in a basic medium is known as the Sonogashira reaction. A Cu(I)-acetylide complex is formed in situ and transmetallates to the Pd(II) complex obtained after oxidative addition of the halide. Through a reductive elimination pathway the reaction delivers substituted alkynes as products. 

The ODNs used here were prepared on an automated DNA synthesizer and with the standard j6-cyanoethyl phosphoramidite coupling reaction. The ferrocenyl ODN (T12Fc) was synthesized by the coupling of amino-terminated ODN with the activated ester of ferrocenecarboxylic acid. A 16 mer ODN (el6S), which has five successive phosphorothioate units on its 5 -terminus, was synthesized using Beaucage s reagent... 

There are a number of procedures for coupling of terminal alkynes with halides and sulfonates, a reaction that is known as the Sonogashira reaction.161 A combination of Pd(PPh3)4 and Cu(I) effects coupling of terminal alkynes with vinyl or aryl halides.162 The reaction can be carried out directly with the alkyne, using amines for deprotonation. The alkyne is presumably converted to the copper acetylide, and the halide reacts with Pd(0) by oxidative addition. Transfer of the acetylide group to Pd results in reductive elimination and formation of the observed product. 

The additive coupling of terminal alkynes catalyzed by copper is a classical reaction known as the Strauss coupling. Unfortunately, under the... 

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