Alkynes alkyne concentration

Standard conditions acetone (or -cf), water (5 equiv) and 2 mol % catalyst, initial alkyne concentration 0.50 M, reaction temperature 70 2 °C, reaction monitored by ll NMR or GC using appropriate internal standards. b In situ generation of catalyst see text for further details. 

As expected, the decrease in the initial alkyne concentration results in less dimerization and higher initial hydrogenation rates. A similar substrate-inhibition effect has also been observed with PhC CH as substrate, revealing a complex dependence of the hydrogenation rate upon the alkyne concentration. To the best... 

These results can be summarized as follows (1) the cobalt-mediated pyridine formation and alkyne cyclotrimerization depend on the square of the alkyne concentration and are independent of the nitrile concentration (2) a common cobaltacydopentadiene intermediate is responsible for both the pyridine and the benzene formation and may be regarded as a key intermediate for both hetero- and carbocyclic pathways. 

The formation of the different co-oligomers is not only controlled by the butadiene and alkyne concentration or the choice of catalyst, but is also dependent on the nature of the substituents on the alkyne or on the butadiene (96). It has been found that the formation of a ten-membered ring from a substituted alkyne and two butadiene molecules with a nickel-... 

NITRATION OF ALKENES AND ALKYNES WITH CONCENTRATED NITRIC ACID... 

When the alkyne concentration drops almost 100 to zero (10 minutes), the only alkene that has ... 

FIGURE 42 Comparison of mean initial activities per unit silica volume (ry jio ) for the selective hydrogenation of 3-methyl-l-pentyn-3-ol (A) and mean maximum yields of the desired intermediate product 3-methyl-l-penten-3-ol in the hydrogenation of 3-methyl-l-pentyn-3-ol (B) averaged over several runs for the conversion catalyzed by acicular mullite with large pores, acicular mullite with small pores, and cordierite. Conditions hydrogen pressure 2.3 bar alkyne concentration 20 mol/m solvent, ethanol and temperature, 297.4 K. 

The reactions of Fischer carbene complexes with alkynes can under certain conditions lead to products that result from the incorporation of two alkynes, the carbene ligand and a carbon monoxide. In inter-molecular reactions, this is most commonly observed for acetylene itself or for sterically unhindered al-kynes. °2 As can be anticipated by the mechanism in Scheme 36, two-alkyne incorporated products of the type (258) are also favored for high alkyne concentration. Synthetically, the two-alkyne reactions are most useful in intramolecular reactions, two of which have been reported and are exemplified by the reactions in Scheme 43. The typical product from the reaction of a Fischer carbene complex with a diyne, such as (308), is a bicyclic phenol of the type (309). ° These products are apparently the result of the assembly of pieces indicated by (311). Under some conditions, dienones of the type (310) and (314) can be isolated, and it is thought they are the immediate precursors of the phenol products via an in situ reduction by a chromium(O) species. This reaction is completely regioselective with diyne (308) and the phenol (309) results from incorporation of the terminal alkyne of (308) before the disubstituted alkyne. Phenols of the type (309) have also been observed from the reaction of diynes with carbyne complexes. ... 

Exliaust gases from spark ignited engines contain alkynes, essentially acetylene. Alkynes strongly coordinate on precious metal catalysts and inliibit all post-combustion reactions. This occurs even at the low alkyne concentrations characteristic of real exliaust gases. Tliree major consequences have been evidenced ... 

A real example following the H2-decoordination mechanism is provided by the selective hydrogenation of 1-alkynes to alkenes with the ruthenium(II) complex [(PP3)RuH(Ti2-H2)]BPh4 [PP3 = P(CH2CH2PPh2)3l [25]. A kinetic study of the hydrogenation of phenylacetylene at ambient or sub-ambient pressure showed that the reaction is first-order in catalyst concentration, second order in H2 pressure and independent of substrate concentration. At very low 1-alkyne concentration (<0.12 M), a first order dependence with respect to the substrate was observed. 

Under catalytic conditions, the reaction was independent of the alkyne concentration and second order with respect to Cu(I) (163). The first step in the suggested mechanism is the formation of a dinuclear copper complex, an intermediate species that reacts with the azide. 

The mechanism proceeds by two steps. The first step is first-order in both platinum oxygen complex and alkyne, and the rate of the second step is independent of alkyne concentration (R = CF3, C02Me). The intermediate may be an adduct with the alkyne coordinated to platinum, or the reaction may proceed in the unsymmetrical metallocycle as is suggested for the SO2... 

It is interesting that the product distribution from the reaction of the o-methoxyphenyl complex with diethylacetylene was found to be dependent on alkyne concentration. One equivalent of alkyne, slow addition of alkyne, or low absolute concentration of alkyne favors the indene product. Reactions performed with excess or high concentrations of alkyne favor benzannulation. The observation, reffered to as the allochemical effect, was explained as resulting from coordination of reaction intermediates by the alkyne. Thus excess alkyne acts as a ligand to stabilize intermediates along the path to benzannulation. Finally, it was noted that the product distribution was not substantially altered by addition of phosphines, phosphine oxides or sulfides. 

R is hydrogen, alkenyl, or alkyne. In remote tropospheric air where NO concentrations ate sometimes quite low, HO2 radicals can react with ozone (HO2 + O3 — HO + 2 O2) and result in net ozone destmction rather than formation. The ambient ozone concentration depends on cloud cover, time of day and year, and geographical location. 

Hi) Preparation of isoxazoles from nitrile N-oxides The reaction between a nitrile //-oxide and an alkyne is so facile that it is usually sufficient to leave an ether solution of the reactants at room temperature to obtain the desired isoxazole in good yield. The reaction is in general sensitive to the size of the substituent on the alkyne but not on the nitrile -oxide. In the case of poorly reactive alkynes, the difficulty may be overcome by generating the nitrile -oxide in situ and keeping its concentration low. 

Alkynes can be hydrated in concentrated aqueous acid solutions. The initial product IS an enol, which isomerizes to the more stable ketone. 

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