Substituents of alkynes

The substituents of alkynes and hydrosilanes affect the reaction rate. The reactivity of the alkynes decreased in the order phenylacetylene > 1-hexyne > (trimethylsilyl)acetylene 3,3-dimethyl-1 -butyne, whereas for the hydrosilane was MePh2SiH, MeiPhSiH >Et2MeSiH, EtsSiH > tBuMe2SiH iPrjSiH. Thus, the rate of silylformylation largely depends on the combination of alkynes and hydrosilanes. 

A cobalt carbonyl complex can be available for the synthesis of pyridinophanes, 2-oxopyridinophanes and cyclophanes in the reaction of diynes with nitriles, isocyanates, alkynes, respectively. Meta/para selectivity was varied by the tether length of diynes and the substituents of alkynes. 

The stereospedfic and regioselective hydrobromination of alkynes with chlorobis(T -cyclopentadienyl)hydrozirconium and NBS produces ( )-vinylic bromides in good yields. The bromine atom usually adds regioselectively to the carbon atom that bears the smaller substituent and stereoselectively trans to the larger substituent (D.W. Hart, 1975 M. Nakatsuka,... 

Substituents affect the heats of hydrogenation of alkynes m the same way they affect alkenes Compare the heats of hydrogenation of 1 butyne and 2 butyne both of which give butane on taking up two moles of H2... 

Furans, thiophenes and pyrroles have all been obtained by addition of alkynic dienophiles to a variety of other five-membered heterocycles, as illustrated in Scheme 104. As the alkynic moiety provides carbons 3 and 4 of the resulting heterocycle, this synthetic approach provides an attractive way of introducing carbonyl containing substituents at these positions, especially as many of the heterocyclic substrates are readily generated. Such reactions do... 

All of the ethynylated cyclobutadienes are completely stable and can be easily manipulated under ambient conditions, as long as the alkyne arms carry substituents other than H. For the deprotected alkynylated cyclobutadiene complexes, obtainable by treatment of the silylated precursors with potassium carbonate in methanol or tetrabutylammonium fluoride in THF, the stability is strongly dependent upon the number of alkyne substitutents on the cyclobutadiene core and the nature of the stabilizing fragment. In the tricarbonyUron series, 27b, 27c, 29 b, and 28b are isolable at ambient temperature and can be purified by sublimation or distillation under reduced pressure. The corresponding tetraethynylated complex 63 e, however, is not stable under ambient conditions as a pure substance but can be stored as a dilute solution in dichloro-methane. It can be isolated at 0°C and kept for short periods of time with only... 

The participation of complex substrates was illustrated in an elegant total synthesis of (-)-terpestacin (Scheme 8) [24,25]. Coupling of alkyne 12 and aldehyde 13 with Ni(COD)2, phosphine 14, and Et3B afforded product 15 as a 2.6 1 mixture of regioisomers, and a 2 1 diastereoselectivity in a combined 85% yield. Whereas alkynes that possess two aliphatic substituents generally... 

The effect of stoichiometry, substituent, and temperature were investigated in reactions between the hydride HCo(tdppep) (19) and a number of alkynes.175 The cr-acetylide complex (20) and the (7-alkenyl (21) are formed from the stoichiometric reaction with ethyl propiolate. However, when a ten-fold excess of ethyl propiolate is used, the acetylide complex is formed quantitatively and one equivalent of alkyne is hydrogenated to alkene. Forcing conditions and a large excess of... 

The benzoquinone 66 is similarly prepared by the regioselective cycloaddition of 64, derived from 63. The cyclization reaction is based on the electronic effect of the substituent of 65 [34]. The maleoylcobalt complex 67, substituted by PPh3, is unreactive towards terminal alkynes. The reaction course is altered... 

In the course of investigation into new C-C bond formation processes, Hiyama has developed an efficient nickel-catalyzed arylcyanation of alkynes.67 The addition reaction of an aryl-CN bond to alkyne affords aryl-substituted alkene nitrile in good yield. Good regioselectivity is reported in the case of unsymmetrical alkynes with two sterically different substituents. 

In the study of substituent effect on the thermal electrocyclic ring-closing reactions of vinylallenes, two stereo-isomeric boryl-substituted vinylallenes 13 and 14 are synthesized by means of the palladium-catalyzed stannaboration of alkynes (Schemes 59 and 60).248... 

The reaction mechanism was considered to be oxidative cyclization, and pal-ladacyclopentene 32 was formed. Reductive elimination then occurs to give cyclobutene 33, whose bond isomerization occurs to give diene 28. The insertion of alkyne (DMAD) into the carbon palladium bond of 32 followed by reductive elimination occurs to give [2+2+2] cocyclization product 27. Although the results of the reactions of E- and Z-isomers of 29 with palladium catalyst 26a were accommodated by this pathway, Trost considered the possibility of migration of substituents. Therefore, 13C-labeled substrate 25 13C was used for this reaction. 

Efforts to prepare germanium analogs of alkynes which contain Ge-Ge triple bonds have also met with some success.382-385 Reduction of the germylene 163 with a slight excess ( 5%) of potassium metal affords the germyne 172 (Equation (322)), which contains only one substituent at each germanium and a Ge-Ge distance of 2.2850(6) A... 

These isomerization reactions are of great interest to theoreticians because the role of many factors (metal, substituents on the organic fragment, ancillary ligands) on the outcome of the reaction can be studied through computations. The purpose of this chapter is to describe the theoretical studies carried out on the isomerization of alkyne to vinylidene and alkene to carbene in the presence of transition metal fragments. 

Low-temperature photochemical cyclization of alkynes bearing a bulky substituent, mediated by CpCo(CO)2, proceeds with CO insertion to give cyclopentadienone complexes. Higher reaction temperatures lead to cyclotrimerization. The intramolecular variant of this reaction gives the bicyclic cyclopentadienones 139 and 139 (equation 19)142. Cyclization of unsymmetrically substituted diynes with the chiral R CpCo(CO)2 (R = 8-phenylmenthyl) leads to the formation of a mixture of diastereomers modest diastere-oselectivity was found. 

Arene(alkoxy)carbene chromium complexes react with aryl-, alkyl-, terminal, or internal alkynes in ethers or acetonitrile to yield 4-alkoxy-1-naphthols, with the sterically more demanding substituent of the alkyne (Rl Figure 2.24) ortho to the hydroxy group. Acceptor-substituted alkynes can also be used in this reaction (Entry 4, Table 2.17) [331]. Donor-substituted alkynes can however lead to the formation of other products [191,192]. Also (diarylcarbene)pentacarbonyl chromium complexes can react with alkynes to yield phenols [332]. 

In the last case, the a-isomers are the main products and the reactivity is practically insensitive to the nature of the substituents of the alkynes. 

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