Alkynes catalytic addition, oxygen

Progress in catalytic additions of oxygen nucleophiles to alkenes (Sect. 3), alkynes (Sect. 4), and allenes (Sect. 5) will now be discussed. 

Palladium is still the most used transition metal for catalytic addition of oxygen nucleophiles toward alkenes and alkynes, and this reactivity is one of the fundamental pathways in the organic reaction promoted by palladium. 

In the Au(I) catalysis of electron-poor alkynes such as 4, the catalytically active species is likely to be a cationic ligand-stabilized gold(I) Jt-complex, as in previously reported additions of oxygen nucleophiles to alkynes [5], Gold catalysts are very soft and thus carbophilic rather than oxophilic. On the basis of this assumption a plausible mechanism can be formulated as shown in Scheme 6. The cationic or strongly polarized neutral Au(I)-catalyst coordinates to the alkyne, and nucleophilic attack of the electron-rich arene from the opposite side leads to the formation of a vinyl-gold intermediate 7 which is stereospecifically protonated with final formation of the Z-olefm 8 [2, 4]. Regioselectivity is dominated by elec-... 

When the catalytic hydrogenation reaction is run under relatively mild conditions (room temperature and a pressure of hydrogen gas of several atmospheres or less), the reaction is very selective. Carbon-carbon double bonds of alkenes and carbon-carbon triple bonds of alkynes react readily, whereas carbon-carbon double bonds of aromatic rings and carbon-oxygen double bonds are usually inert under these reaction conditions. Some examples are provided in the following equations. Note that the stereochemistry of the addition reaction makes no difference in the first two examples. In the last example the major product results from syn addition. 

The Wacker-type addition is the anti-addition of (most commonly) a heteroatom and a Pd(II) species across a C-C double bond. The Wacker-type oxidations are Pd(II)-catalyzed transformations involving heteroatom nucleophiles and alkenes or alkynes as electrophiles.27 In most of these reactions, the Pd(II) catalyst is converted to an inactive Pd(0) species in the final step of the process, and use of stoichiometric oxidants is required to effect catalytic turnover. For example, the synthesis of furan 33 from a-allyl-p-diketone 32 is achieved via treatment of the substrates with a catalytic amount of Pd(OAc)2 in the presence of a stoichiometric amount of CuCh-28 This transformation proceeds via Pd(II) activation of the alkene to afford 34, which undergoes nucleophilic attack of the enol oxygen onto the alkene double bond to provide alkylpalladium complex 35. p-Hydride elimination of 35 gives 36, which undergoes... 

When 1-hexyne is treated with a catalytic amount of sulfuric acid in an aqueous solvent, initial reaction with the acid gives the expected secondary vinyl carbocation 103, and the most readily available nucleophile in this reaction is water (from the aqueous solvent). Nucleophilic addition of water to 103 leads to the vinyl oxonium ion 104. Loss of a proton in an acid-base reaction (the water solvent is the base) generates a product (105) where the OH unit is attached to the C=C unit, an enol. Enols are unstable and an internal proton transfer converts enols to a carbonyl derivative, an aldehyde, or a ketone. This process is called keto-enol tautomerization and, in this case, the keto form of 105 is the ketone 2-hexanone (106). (Enols are discussed in more detail in Chapter 18, Section 18.5.) Note that the oxygen of the OH resides on the secondary carbon due to preferential formation of the more stable secondary carbocation followed by reaction with water, and tautomerization places the carbonyl oxygen on that same carbon, so the product is a ketone. When a disubstituted alkyne reacts with water and an acid catalyst, the intermediate secondary vinyl cations are of equal stability and a mixture of isomeric enols is expected each will tautomerize, so a mixture of isomeric ketones will form. 

The catalytic performance of 1 was examined for hydroformylation of alkyne (as a catalyst additive) and methoxycarbonylation of alkyl halide, and 1 is also used as a stoichiometric reagent for an Ni-catalyzed ketone synthesis from alkyl halide. " Heterogeneous catalysts derived from 1 are used for hydrogenation of GO and coal tar pitch, crude oil upgrading, and coal liquefaction (preparation of the pyrrhotite catalyst). An Fe-Ru-Se catalyst dispersed on a glassy carbon-supported Nafion film prepared from 1 is reported to be effective for electro-catalytic oxygen reduction. ... 

---