Metallation of alkynes

Substituted pyridines undergo exclusively 1,4-addition, while 4-substituted pyridines give a mixture of products . The metalation of alkynes by n-BuMgCl is often used . 

Metallation of alkyne, nucleophilic addition x to aldehyde (9), oxidation... 

Normant, J. F. and Alexakis, A., Carbometal-lation (C-metallation) of alkynes stereospe-cific synthesis of alkenyl derivatives, Synthe.sis, 841, 1981 see, in particular, pp, 845-846. 

J. F. Normant and A. Alexakis, Carbometallation, C-Metallation of Alkynes, Stereospecific Synthesis of Alkenyl Derivatives , Synthesis, 1981, 841. 

WITH HALIDES CA TALYZED BY Cu AND Pd OR Pd ALONE METALLATION OF ALKYNES IN SITU BEFORE COUPUNG... 

Generally speaking, other cyclopentadienylmetal complex catalyzed carbo-metallations of alkynes are rather rare but a few interesting examples have been reported. The first one is the zirconocene 1 catalyzed ethylmagnesation of dialkyldiynes with ethylmagnesium bromide to afford a mixture of isomeric enynes 34 (Scheme 14). The ethylation proceeds regioselectively on the terminal carbon atom of the diyne moiety. However, the addition is not stereoselective a mixture of cis and trans isomers is obtained [22]. 

Organometallic compounds which have main group metal-metal bonds, such as S—B, Si—Mg,- Si—Al, Si—Zn, Si—Sn, Si—Si, Sn—Al, and Sn—Sn bonds, undergo 1,2-dimetallation of alkynes. Pd complexes are good catalysts for the addition of these compounds to alkynes. The 1,2-dimetallation products still have reactive metal-carbon bonds and are used for further transformations. 

The stereochemistry of metal-ammonia reduction of alkynes differs from that of catalytic hydrogenation because the mechanisms of the two reactions are different The mechanism of hydrogenation of alkynes is similar to that of catalytic hydrogenation of alkenes (Sections 6 1-6 3) A mechanism for metal-ammonia reduction of alkynes is outlined m Figure 9 4... 

Hydrogenation of alkynes may be halted at the alkene stage by using special catalysts Lindlar palladium is the metal catalyst employed most often Hydrogenation occurs with syn stereochemistry and yields a cis alkene... 

The mechanism by which the Birch reduction of benzene takes place (Figure 118) IS analogous to the mechanism for the metal-ammonia reduction of alkynes It involves a sequence of four steps m which steps 1 and 3 are single electron transfers from the metal and steps 2 and 4 are proton transfers from the alcohol... 

CoF is used for the replacement of hydrogen with fluorine in halocarbons (5) for fluorination of xylylalkanes, used in vapor-phase soldering fluxes (6) formation of dibutyl decalins (7) fluorination of alkynes (8) synthesis of unsaturated or partially fluorinated compounds (9—11) and conversion of aromatic compounds to perfluorocycHc compounds (see Fluorine compounds, organic). CoF rarely causes polymerization of hydrocarbons. CoF is also used for the conversion of metal oxides to higher valency metal fluorides, eg, in the assay of uranium ore (12). It is also used in the manufacture of nitrogen fluoride, NF, from ammonia (13). 

The alkynylation of estrone methyl ether with the lithium, sodium and potassium derivatives of propargyl alcohol, 3-butyn-l-ol, and propargyl aldehyde diethyl acetal in pyridine and dioxane has been studied by Miller. Every combination of alkali metal and alkyne tried, but one, gives the 17a-alkylated products (65a), (65c) and (65d). The exception is alkynylation with the potassium derivative of propargyl aldehyde diethyl acetal in pyridine at room temperature, which produces a mixture of epimeric 17-(3, 3 -diethoxy-T-propynyl) derivatives. The rate of alkynylation of estrone methyl ether depends on the structure of the alkyne and proceeds in the order propar-gylaldehyde diethyl acetal > 3-butyn-l-ol > propargyl alcohol. The reactivity of the alkali metal salts is in the order potassium > sodium > lithium. 

Hydrogenation of alkynes to alkanes (Section 9.9) Alkynes are completely hydrogenated, yielding alkanes, in the presence of the customary metal hydrogenation catalysts. 

Catalytic hydrogenation of alkynes on a metal surface provides cis alkenes (see Chapter 7, Problem 13), while treatment with sodium in liquid ammonia nearly always leads to trans alkenes, e.g., hydrogenation of 2-butyne. 

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