Carbonyl compounds alkynylation reactions

In recent years, Hoppe s group has considerably extended the isomerization-addition methodology, especially for the highly regio- and stereoselective synthesis of 1,2-alkadienyl carbamates. It involves deprotonation of alkynyl carbamates, transme-talation into a titanium species and subsequent reaction with carbonyl compounds [26-30]. This group recently described the preparation of enantiomerically enriched 4-hydroxyallenyl carbamates 22 by sparteine-mediated lithiation of alkynyl carbamates 20 [29]. Impressive examples of these transformations are summarized in Scheme 8.8. 

Carbonylative cross-coupling reaction between triarylbismuth(V) compounds and organostannanes is smoothly catalyzed by PdCl2 under 1 atm CO at room temperature (Scheme 48) [34]. Not only arylstannanes, but also heteroaryl-, alkynyl-, and vinyltributylstannanes were employed efficiently. 

Addition of enolizable carbonyl compounds to the C=C bond of (1-alkynyl)carbene complexes can proceed by C- or O-addition of the enol unit. The C/O ratio depends very much on the substrate as well as on the reaction conditions. Reaction of cyclic 1,3-diketones 72 in diethyl ether in the presence of catalytic amounts of Et3N results in the formation predominantly of (red) divinyl ethers 73 (by O-alkylation) together with smaller amounts of (blue) pyranylidene complexes 74 by (C-alkylation) (Scheme 23).26103 For related reactions, see Scheme 67. 

Acetyl- and 3-benzoylisoxazoles 389 (and isoxazolines) have been prepared by one-pot reactions of alkynes (and alkenes) with ammonium cerium(iv) nitrate (CAN(lv)) or ammonium cerium(lll) nitrate tetrahydrate (CAN(m))-formic acid, in acetone or acetophenone. These processes probably involve 1,3-dipolar cycloaddition of nitrile oxides produced via nitration of the carbonyl compound by cerium salts. The existence of nitrile oxides as reaction intermediates was proved by the formation of the dimer furoxan 390 when the above reaction was carried out in absence of any dipolarophile (Scheme 95) <2004T1671>. An analogous improved procedure has been applied to alkynyl glycosides as dipolarophiles for the preparation of carbohydrate isoxazoles <2006SL1739>. 

Until recently, a-substitution of carbonyl compounds was largely restricted to those cases where alkyl groups, such as Me, allyl, and benzyl, were introduced via classical enolate alkylation. a-Aiylation of ketones with aryl halides can be effected using K in NH3 [120-123]. However, this reaction does not appear to be satisfactory for selective -alkenylation and a-alkynylation. -Arylation of ketones has also been catalyzed by Ni [124] and Pd [125,126]. The Pd-catalyzed version employed enolstannanes. This reaction has also been applied to a-alkenylation [127]. However, these reactions appear to be of very limited scope. Thus, the Pd-catalyzed procedures appear to be satisfactory mainly for a-substitution of methyl ketones. Furthermore, none of them addresses the critical question of how to control the regiochemistry of c-substitution, i.e., a vs. a. Critically needed from the viewpoint of selective synthesis were... 

The alkynyl moiety of the adduct may subsequently undergo further reactions for example, by the partial reduction to a double bond by using hydrogen with a Lindlar catalyst, which results in the overall formation of an allylic alcohol from a carbonyl compound. 

---