Metal alkynylating agents

Scheme 10.7. Preparation of metal alkynyls via metal halides with alkynylating agents.

Fig. 16.30. Pd(0)-catalyzed arytation of a copper acetytide at the beginning of a three-step synthesis of an ethynyt aromatic compound. Mechanistic details of the C,C coupling Step 1 formation of a complex between the catalytically active Pd(0) complex and the arylating agent. Step 2 oxidative addition of the arylating agent and formation of a Pd(II) complex with a cr-bonded aryl moiety. Step 3 formation of a Cu-acetylide. Step 4 trans-metalation the alkynyl-Pd compound is formed from the alkynyl-Cu compound via ligand exchange. Step 5 reductive elimination to form the -complex of the arylated alkyne. Step 6 decomposition of the complex into the coupling product and the unsaturated Pd(0) species, which reenters the catalytic cycle anew with step 1.

For the preparation of conjugated alkynes, one can alkenylate or arylate alkynes according to Section 13.3.4. Alternatively, metallated alkenes or metallated aromatic compounds also may be alkynylated, but this option will not be pursued further. We merely mention in passing that bromoalkynes and iodoalkynes are suitable alkynylat-ing agents and that these can be obtained in a one-step reaction from terminal alkynes ... 

A common route to alkoxy(l-alkynyl)carbene complexes 1 involves addition of a 1-lithio alkyne to a metal carbonyl to give an acyl metallate, which on alkylation affords an alkoxy(l-alkynyl)carbene complex (vide supra). Several transformations of acyl metallates with electrophiles other than alkylation agents have been reported. 

Cleavage of the alkynyl-metal bond in organometallic derivatives initiated by such agents as H+, Ag+, RCO+ and halogens may be considered formally as electrophilic substitutions. 

Lockwood, R. F., Nicholas, K. M. Transition metal-stabilized carbenium ions as synthetic intermediates. I. a-[(Alkynyl)dicobalt hexacarbonyl] carbenium ions as propargylating agents. Tetrahedron Lett. 1977, 4163-4166. 

---