Alkynes aliphatic

Peterson and co-workers have carried out a careful investigation of the electrophilic addition of trifluoroacetic acid to a series of aliphatic alkynes (38) and alkenes (39,40). Of particular interest is the behavior of 3-hexyne. At. 1 M concentrations of 3-hexyne, nearly equal amounts of the cis- and trans-3-hexen-3-yl trifluoroacetates are formed in 98% yield, together with about 2%... 

Terminal aliphatic alkynes (e.g., 1-octyne) react with iron(III) halides (FeCls and FeBrs) to give the corresponding 2-halovinyl derivatives (route A, Scheme 10). The moderate yields were remarkably improved upon addition of stoichiometric amounts of carboxylic acids. 

A catalytic system comprising TiCNMe ), LiNCSilVIej) and IMes has been developed for the intermolecular hydroamination of terminal aliphatic alkynes (1-hexyne, 1-octyne, etc.) with anilines [toluene, 100°C, 10 mol% TiCNMe ) ]. Markovnikov products were dominant. Substituted anilines reacted similarly. High conversions (85-95%) were observed with specific anilines. The optimum Ti/IMes/ LiN(SiMe3)2 ratio was 1 2 1. However, the nature of the active species and especially the role of LiN(SiMe3)2 are unclear [74]. 

Derivatives of aliphatic alkynes (14 and 15) are more thermally unstable than 12, but they show SmA and N phases at low temperatures (below 130 °C). The type of phase and the mesophase stability depend on the length of both the terminal and the lateral chains. When both chains are elongated, the mesomorphism becomes metastable and compounds 14 display monotropic N and SmA transitions. Complexes IS, which contains an ester group with an opposite direction to that of complexes 14, display less stable nematic mesophases. 

Liquid crystals based on aliphatic isocyanides and aromatic alkynyls (compounds 16) show enantiotropic nematic phases between 110 and 160 °C. Important reductions in the transition temperatures, mainly in clearing points (<100 °C), areobtained when a branched octyl isocyanide is used. The nematic phase stability is also reduced and the complexes are thermally more stable than derivatives of aliphatic alkynes. Other structural variations such as the introduction of a lateral chlorine atom on one ring of the phenyl benzoate moiety or the use of a branched terminal alkyl chain produce a decrease of the transition temperatures enhancing the formation of enantiotropic nematic phases without decomposition. 

Another important click reaction is the cycloaddition of azides. The addition of sodium azide to nitriles to give l//-tetrazoles is shown to proceed readily in water with zinc salts as catalysts (Eq. 11.71).122 The scope of the reaction is quite broad a variety of aromatic nitriles, activated and nonactivated alkyl nitriles, substituted vinyl nitriles, thiocyanates, and cyanamides have all been shown to be viable substrates for this reaction. The reaction of an arylacetylene with an azide in hot water gave 1,4-disubstituted 1,2,3-triazoles in high yields,123 while a similar reaction between a terminal aliphatic alkyne and an azide (except 111 - nitroazidobenzcnc) afforded a mixture of regioisomers with... 

Palladium-catalyzed addition of the selenium-silicon bond of PhSe-SiMe3 to arylacetylenes proceeds in a regio- and stereoselective manner to afford (Z)-a-(phenylseleno)-/ -(trimethylsilyl)styrenes (Equation (123)).250 Aliphatic alkynes fails to undergo the addition reaction. Analogous addition of the Se-Ge bond to alkynes occurs under similar conditions. 

Addition of a phosphorus-sulfur bond to a carbon-carbon triple bond is catalyzed by a palladium(O) complex (Equation (130)).298 Terminal aliphatic alkynes having various functional groups undergo the addition with PhS-P(0)(OPh)2 to afford (Z)-adducts in high yield. In contrast to aliphatic alkynes, phenylacetylene gives a mixture of E Z adducts. Internal alkynes and alkenes are unreactive. 

Facile 1,2-addition of the P-Se bond to alkynes occurs in a manner analogous to the addition of the P-S bond (Equation (131 )).299 High regio- and stereoselectivies are observed with phenylacetylene as well as aliphatic alkynes. 

Aliphatic alkynes are also reactive in the addition with HPPh2, but the selectivity is low as compared with phenylacetylene unless a sterically demanding substituent is bound to the triple bond (Table 2). 

A mechanism that involves ytterbium phosphide species has been proposed, similarly to the foregoing intramolecular hydrophosphination. Generation of the phosphide species is supported by the formation of Ph2CDNHPh (after aqueous quench) upon treatment of the imine complex with Ph2PD (Scheme 15). Lanthanide phosphide is known to react with THF, forming a 4-diphenylphosphino-l-butoxyl species [21], which was indeed found as a side product in the catalytic hydrophosphination of disubstituted aliphatic alkynes run in THF, supporting further the ytterbium-phosphide intermediate (Scheme 16). 

Most of these catalytic systems are able to dimerize either aromatic alkynes, such as phenylacetylene derivatives, or aliphatic alkynes, such as trimethylsilylacetylene, tert-butylacetylene and benzylacetylene. The stereochemistry of the resulting enynes depends strongly on both the alkyne and the catalyst precursor. It is noteworthy that the vinylidene ruthenium complex RuCl(Cp )(PPh3)(=C=CHPh) catalyzes the dimerization of phenylacetylene and methylpropiolate with high stereoselectivity towards the ( )-enyne [65, 66], and that head-to-tail dimerization is scarcely favored with this catalyst. It was also shovm that the metathesis catalyst RuCl2(P-Cy3)2(=CHPh) reacted in refiuxing toluene with phenylacetylene to produce a... 

Aliphatic alkynes, e.g. HC CCjH[ j and HOCCT SiMej react slowly. If a small amount ( 3 ml) of diazabicydoundecene (DBU) is added, a smooth reaction takes place and the diyne CjH j j is obtained in an excellent yield. The yield of Me3SiCH2-... 

Silylstannylation is also observed in the Pd(PPh3)4-catalyzed reaction of alkynes with disilanyl stannanes, in which both Si-Si and Si-Sn bonds are present. The alkynes undergo insertion into the Si-Sn bond, to regio-and stereoselectively yield (/8-disilanylalkenyl)stannanes [Eq. (25)].73 With terminal alkynes, the stannyl group adds regioselectively to the internal carbon atom. Aliphatic alkynes are not reactive in this system. 

A regioselective iodoperfluoroalkylation of terminal alkynes (R—C = CH) has been reported, and is based on photolysis ofthe C—I bond in perfluoroalkyl iodides (Rp-I). Addition of the thus-formed RF" radical onto the alkyne afforded a vinyl radical that in turn abstracts an iodine atom from the starting Rp—I to form the end olefin R-C(I)= CH-Rf. A xenon lamp through Pyrex (hv > 300 nm) was used for the reaction, where aliphatic alkynes gave a better alkylation yield with respect to phenylacetylene [81],... 

It is important to note that even if the addition of Cul as co-catalyst was desirable with deactivated aryl halides, a high yield in coupling product could be obtained under copper-free conditions. Furthermore, the reaction could be smoothly carried out with an aliphatic alkyne in short reaction times but with this catalyst system only 51% of the coupling product was formed from chlorobenzene. 

Tridentate pincer bis-carbene [114] and N-carbamoyl-substituted heterocyclic carbene complexes of Pd(II) [115] have also been used to couple aryl bromides and iodides with aromatic or aliphatic alkynes. Surprisingly, the latter catalytic system requires the use of 1 mol % of PPh3. Its role in the catalytic cycle is still unclear but it might facilitate the initial generation of Pd(0) species. 

Under these conditions, a variety of linear aliphatic terminal alkynes were transformed into aldehydes with good selectivity. The efficiency, regioselec-tivity of the addition and substituent tolerance were improved by using RuCl(Cp)(phosphine)2, where Cp is cyclopentadienyl, or RuCl(Cp)(diphos-phine) as catalyst precursors [30]. The best results were obtained with diphenylphosphinomethane as a ligand, which made possible the preparation of aldehydes from bulky aliphatic alkynes (tert-BuC=CH), aromatic alkynes (PhC=CH), diynes [HC=C(CH2)6C=CH] and functional terminal alkynes [NC(CH2)3C=CH, PhCH20(CH2)2C=CH,...]. The mechanism of this reaction was investigated in details by isolation of intermediates, deuterium-... 

In the Bu3SnH-promoted radical reactions to aliphatic alkynes, using initiators such as AIBN, Et3B, and ultrasound104 furnishes /3-adducts as a mixture of (E)- and (Z)-isomers. Lewis acid catalysts give /3-(Z) isomers,96 whereas transition metal catalysts furnish the predominant formation of (3-(E) isomers.105 The a-stannylation of simple aliphatic alkynes, however, is particularly difficult because of the absence of anchor substituents such as ethers. In the general hydrostannations of aliphatic alkynes, a-adducts are obtained only as minor adducts in the Pd-catalyzed reaction (Equation (35)). 

In a typical reaction, a solution of alkyne in THF is cooled to —20°C for 5 min. An equimolar amount of the dialkylzinc is added in toluene (ratio of THF toluene = 1 3). After 15 min, 10 mol % of the ligand is added, followed by the aldehyde. HPLC analysis shows complete reaction usually within 18 h. Both electron-rich and electron-poor aldehydes have been used along with aromatic and aliphatic alkynes. Yields are normally 70-90% with ee being 65-85%. Once again the optimal ligand structure may involve variation of the amine substitution pattern. 

Very recently, new catalysts precursors derived from [RuCl2(p-cymene)]2 such as the RuCl2(triazol-5-ylidene)(p-cymene) (C, D) (Figure 8.3) [69] or the in-situ-generated catalytic system based on [RuCl2(p-cymene)]2/P(p-C6H4Cl)3/DMAP [50] have revealed their potential to perform the anfi-Markonikov addition of a variety of carboxylic acids to phenylacetylene and terminal aliphatic alkynes. 

Simple internal aliphatic alkynes afford both regio- and stereochemical mixtures of alkenylmercurials, with the trans adducts predominating . At RT, trans-addition of Hg(OAc)2 occurs to MeC=CMe in AcOH however, on heating, the cis-adduct is obtained... 

Internal alkynes yield carboxylic acids and a-diketones when oxidized with the MTO/H2O2 system [22]. Rearrangement products are observed only for aliphatic alkynes. Terminal alkynes give carboxylic acids, derivatives thereof and a-keto acids as the major products. The yields of these products vary with the solvent used [22]. 

The migrating ability decreases from hydrogen to aryl, then to alkyl functions. In the last case, only low yields (<10%) of aliphatic alkynes are obtained. ... 

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