Aldehydes from alkyne hydration

By the example of 34 different alkynes, it was convincingly demonstrated that the product of the treatment of [PtCLJ with CO at 40-110 °C is a very powerful alkyne hydration catalyst some of the reactions are shown on Scheme 9.7 [25], The best medium for this transformation is THF containing 5 % H2O. The reaction can also be performed in a water-organic solvent two-phase system (e.g. with 1,2-dichloroethane), however in this case addition of a tetralkylammonium salt, such as Aliquat 336, is required to facilitate mass transfer between the phases. After the reaction with CO, the major part of platinum is present as H2[ Pt3(CO)6 n], but the catalytic effect was assigned to a putative mononuclear Pt-hydride, [PtHCl(CO)2], presumably formed from the cluster and some HCl (supplied by the reduction of [PtCU]). The hydration of terminal acetylenes follows Markovnikov s mle leading exclusively to aldehyde-free ketones. 

D Ketones and Aldehydes from Hydration of Alkynes (Section 9-9F)... 

Alkynes undergo hydroboration to give alkenylboranes, which can be oxidized to give carbonyl compounds with hydrogen peroxide. The net result of the two-step sequence is hydration, which gives aldehydes from terminal alkynes. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Alkenes and alkynes obviously don t fit easily into these categories as they have no bonds to heteroatoms. Aikenes can be made from alcohols by dehydration without any oxidation or reduction so it seems sensible to put them in the alcohol column. Similarly, alkynes and aldehydes are related by hydration/dehydration without oxidation or reduction. 

In addition to hydrogenation, alkynes can be hydrated into either a ketone or an aldehyde form. A (Markovnikov) ketone can be created from an alkyne using a solution of aqueous sulfuric acid (H2/H2SO4) and HgSO4, whereas the anti-Markovnikov aldehyde product requires different reagents and is a multi-step process. 

Hydrogenation of aromatic nitro compounds to aromatic amines Hydrogenation of aldehydes and ketortes to alcohols Hydrogenation of atomic olehnic groups Hydrogenation of unsaturatcd nitriles to unsaturated amines Hydrogenation of diolefins artd alkynes to monoolehns Hydration of ethylene oxide alkylation of aromatics with olehns oxidation of alcohols to aldehydes Oxidation of ethylene to ethylene oxide Synthesis of HCN from NH, and CH4 Oxidation of CH3OH to HCHO... 

Oxidation of organoboranes to alcohols is usually effected with alkaline hydrogen peroxide. The reaction is of wide applicability and many functional groups are unaffected by the reaction conditions, so that a variety of substituted alkenes can be converted into alcohols by this procedure. Several examples have been given above. A valuable feature of the reaction is that it results in the overall addition of water to the double (or triple) bond, with a regioselectivity opposite to that from acid-catalysed hydration. This follows from the fact that, in the hydroboration step, the boron atom adds to the less-substituted carbon atom of the multiple bond. Terminal alkynes, for example, give aldehydes in contrast to the methyl ketones obtained by mercury-assisted hydration. 

Thus far, we have learned two ways to make aldehydes (i) a/ift-Maikovnikov hydration of a terminal alkyne or (ii) ozonolysis of an alkene, either of which is potentially reasonable here. However, in order to produce botii of these compounds from a single synthetic protocol, a key recognition is that they can be produced fiom ozonolysis of the following (hsubstituted alkene. 

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