Alkynes metal derivatives

Metal derivatives of terminal alkynes, RC2H. Transition metals form complex acetylides (e.g. (M(C = CR) ]- ) often containing the metal in low oxidation states. 

The compoimd (isomer not stated) and its metal derivatives are explosive. See other alkynes, nitroaryl compounds... 

As discussed in Section 3.1.6, cyclopropenes can react with rhodium complexes [38,585,587-589,1061,1063] or other transition metal derivatives to yield vinylcarbene complexes (see Section 3.1.6). This reaction will proceed particularly smoothly with strained cyclopropenes, because these can already isomerize thermally to vinylcarbenes [1064]. Hence the formation of vinylcarbene complexes from alkynes can proceed by initial cyclopropanation, followed by reaction of the resulting cyclopropene with the complex L,M. 

The reaction of heterocyclic lithium derivatives with organic halides to form a C-C bond has been discussed in Section 3.3.3.8.2. This cannot, however, be extended to aryl, alkenyl or heteroaryl halides in which the halogen is attached to an sp2 carbon. Such cross-coupling can be successfully achieved by nickel or palladium-catalyzed reaction of the unsaturated organohalide with a suitable heterocyclic metal derivative. The metal is usually zinc, magnesium, boron or tin occasionally lithium, mercury, copper, and silicon derivatives of thiophene have also found application in such reactions. In addition to this type, the Pd-catalyzed reaction of halogenated heterocycles with suitable alkenes and alkynes, usually referred to as the Heck reaction, is also discussed in this section. 

Some organometallic compounds are prepared best by the reaction of a strong base or an alkyl metal derivative with an acidic hydrocarbon, such as an alkyne ... 

Protonation or alkylation of several ethynyl-metal derivatives gives the corresponding vinylidene complexes in high yield (14,24). This is a convenient route to the disubstituted vinylidene complexes, as well as the parent compounds, which cannot be obtained from l-alkynes they are formed if MeOS02F or [R30]+ (R = Me, Et) is used as alkylating agent ... 

Bisalkyne bisdithiocarbamate derivatives are both harder to reduce and harder to oxidize than their carbonyl analogs (53). Factors discussed in the Section VI and used to rationalize visible absorption spectra are also applicable here. The potentials required for oxidation and reduction reflect the strength of the alkyne-metal dn interactions. Their properties, as well as chemical behavior, no doubt reflect the complementary nature of the complete set of a and 77 metal-ligand bonds in these happy d4 M(RC=CR)2(S2CNEt2)2 complexes. 

The metal derivatives of alkynes can be added to carbonyl electrophiles as in the following examples. The first (we have reminded you of the mechanism for this) is the initial step of an important synthesis of the antibiotic, erythronolide A, and the second is the penultimate step of a synthesis of the widespread natural product, farnesol. 

SAFETY PROFILE The carbon-carbon triple bond is explosively unstable in many acetylenic compounds. Both the lower alkynes (i.e., propyne, butadyne, etc.) and higher compounds may undergo explosive decomposition. The presence of halogens and heavy metal derivatives may increase these explosive tendencies. See also ACETYLENE, ACETYLIDES, and specific compounds. 

A characteristically different mechanism appears to operate in alkyne trimerization systems based on PdCb." Cationic metal complexes are involved in which initial halide transfer to an alkyne carbon is followed by sequential linear insertion of two more alkyne moieties. Metallacyclopentadiene intermediates are not involved in this sequence. Unique to this mechanism is the subsequent ring-closure to a cy-clopentadienylmethyl metal derivative, which, via halide transfer back to the metal, eventually leads to benzene via a bicyclo[3.1.0] system (Scheme 27). Support for this mechanism comes in part from the isolation of methylcyclopentadienyl-derived structures in several cases, including pentalene derivatives from further alkyne insertion followed by a second ring-closure." ... 

Alkenyl and dienyl groups can be present in Ti and Zr complexes as well. Vinyltitanium compounds cannot be exploited for nucleophilic additions, however, due to their propensity to undergo oxidative coupling reactions,likewise transition metal derivatives. Vinylzirconium compounds (20) can be prepared from alkynes (hydrozirconation equation 12) by action of zirconium derivatives like (19). ... 

The high protic acidity of the acetylenic CH bond makes the alkynylstannanes readily accessible either directly from the terminal alkynes or through their metal derivatives. The alkynes react with distannoxanes in boiling benzene, and the water which is eliminated can be removed azeotropically, or with calcium hydride83 or molecular sieves (e.g. equation 8-44). 

One ingenious and elegant improvement in the standard synthetic procedure for the preparation of alkynes was introduced in 1989 with the use of dimethyl l-diazo-2-oxopropyIphosphonate. - This valuable and efficient reagent is prepared in a single step by reaction of the a-metallated derivative of dimethyl 2-oxopropylphosphonate with tosyl azide. It was observed that the sodium enolate prepared from NaH in 0,1 b,/ I HL solution gives superior results (80-96%) to f-BuOK in... 

---