Alkynes hydrogen atom

Two types of substituents linked to the steroid rings arc involved in the Mannich reaction, namely, methyl ketone and alkyne residues. The former react on the methyl group, and the latter react chemo.selectivcly on the alkyne hydrogen atom when copper salts are employed as a catalyst. ... 

Bivalent radicals derived from unbranched alkenes, alkadienes, and alkynes by removing a hydrogen atom from each of the terminal carbon atoms are named by replacing the endings -ene, -diene, and -yne by -enylene, -dienylene, and -ynylene, respectively. Positions of double and triple bonds are indicated by numbers when necessary. The name vinylene instead of ethenylene is retained for —CH=CH—. 

Again, two hydrogen atoms are lost when a double bond is converted to a triple bond. Hence the general formula of an alkyne is... 

Employing a molar excess of the alkyne over the active hydrogen of only 12 percent the selectivity (determined by proton NMR after work-up) may be expressed as the ratio of Si-C=CH- to Si-CH2-CH2-CO-, which is about 100 1 [16]. The same reaction, carried out with silicone polymers with 10 or more pendant silicon-bonded hydrogen atoms, proceeds similarly cleanly without noticeable crosslinking by acrylic hydrosilylation. A slight excess of triple bond over Si-H groups is mandatory to suppress this side reaction as well as hydrosilylation of the olefinic group of the 2-silylalkene isomer product (see Table 2.). 

Replacement of the Acetyleneic Hydrogen Atom of Terminal Alkynes... 

An anti addition of hydrogen atoms to the triple bond occurs when alkynes are reduced with lithium or sodium metal in ammonia or ethylamine at low temperatures. 

Alkanes, with the general formula C H2 +2, are saturated hydrocarbons, in which each carbon atom is singly bonded to four other atoms. These atoms are either carbon atoms or hydrogen atoms. Alkenes, C H2 , and alkynes, C H2 2, are unsaturated hydrocarbons in which there is a carbon-carbon double bond or a carbon-carbon triple bond, respectively. 

Oxidative addition of molecular hydrogen was considered to be involved in the alkyne hydrogenations catalyzed by [Pd(Ar-bian)(dmf)] complexes (4 in Scheme 4.4) [41, 42]. Although the mechanism was not completely addressed, 4 was considered to be the pre-catalyst, the real catalyst most likely being the [Pd(Ar-bian)(alkyne)] complex 18 in Scheme 4.11. Alkyne complex 18 was then invoked to undergo oxidative addition of H2 followed by insertion/elimination or pairwise transfer of hydrogen atoms, giving rise to the alkene-complex 19. 

In the transformation of a 1-alkyne to a vinylidene in the coordination sphere of a transition metal, the migrating hydrogen atom plays a key role. Usually, ancillary ligands on the metal are only spectators and do contribute to small modifications of the bonding properties of the metal fragment. However, if a hydride is present as a ligand to the transition metal center, it may interfere with the alkyne to vinylidene transformation. This may open up new selective and efficient routes to vinylidene complexes. 

Previous studies on various aliphatic and aromatic alkynes have evidenced that this type of furan-2-ones could also be produced in the presence of the rhodium carbonyl cluster [Rh4(CO)i2] [39-41]. Labeling experiments using D2O allowed the authors to assign the origin of the hydrogen atoms (Scheme 6). Noteworthy, dicobalt octacarbonyl follows a different catalytic... 

All metal vinylidenes described herein are derived from alkynes. While alkyne-to-vinylidene interconversion typically occurs via the 1,2-shift of a hydrogen atom, the corresponding migration of heavier main group heteroatoms is also possible. 

The reaction is usually carried out in acid solution, but may also be base catalysed. This is the condensation between aldehydes, ammonia or a primary or secondary amine and a compound containing at least one active hydrogen atom e.g., ketones, b-ketoesters, b-cyanoesters, nitroalkanes, alkynes with C°H). For example. 

The alkynes are bonded in essentially the same way as, but less firmly than, the olefins (see Section III,R). In the hex-3-yne series, substitution of an a-hydrogen atom by a methyl group reduces the argentation constant (a measure of the silver-alkyne bond strength) by a factor of roughly 1 /3 this influence of methyl substitution on complex formation is opposite to that observed in the platinum(II) complexes (see Section IV,J). 

A hydrocarbon is a compound that consists only of carbon and hydrogen atoms. Each contains a skeleton of carbon atoms bonded to varying numbers of hydrogen atoms. Hydrocarbons include alkanes, alkenes, alkynes, and aromatic hydrocarbons. 

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