Copper complexes with alkynes

The reaction of iron-carbonyl complexes with alkynes led to cyclobutenediones, which is formally a [2 + 1 + 1]-cycloaddition process for the formation of a cyclobutene derivative (Scheme 9.22) [49]. Nevertheless, in this reaction the liberation of the ligand is initiated by addition of stoichiometric amounts of copper] 11) salts and the use of various alkynes leads to interesting products such as 30 in good yields. 

AH-1,4-Benzoxazines are available by reaction of the copper complexes derived from o-nitrosophenols with alkynic dienophiles (338 — 339). 

Asymmetric cyclopropanations of alkenes and alkynes with a-diazocarbonyl compounds have been extensively explored in recent years and a number of very effective chiral catalysts have been developed2. Copper complexes modified with such chiral ligands as salicy-laldimines 38202,203, semicorrins 39204 208, bis(oxazolines) 40209-2" and bipyridines 41212 have... 

Alkynyl complexes with copper, 2, 160 with Cp Re(CO) (alkyne), 5, 916 with dicarbonyl(cyclopentadienyl)hydridoirons, 6, 175 with diiron carbonyls, 6, 232-233 donor-free, with gold(I), 2, 255 with gold(I)... 

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.

Scheme 9 Acetylene activation via jr-complexation with copper(I) and proposed mechanism for the formation of diacetylene bond via cuprated alkyne dimers...

Other late transition metals used in special cases of hydrosilation include cobalt, iron, ruthenium (vide infra for reactions with alkynes), osmium, chromium, molybdenum, tungsten and copper. Metallocenes (see Metallocene Complexes) of early transition metals and lanthanides have also been found to catalyze the hydrosilation of a number of unsaturated compounds including alkenes and esters (vide infra). 

Polymer-supported benzenesulfonyl azides have been developed as a safe diazotransfer reagent. ° These compounds, including CH2N2 and other diazoalkanes, react with metals or metal salts (copper, paUadium, and rhodium are most commonly used) to give the carbene complexes that add CRR to double bonds. Diazoketones and diazoesters with alkenes to give the cyclopropane derivative, usually with a transition-metal catalyst, such as a copper complex. The ruthenium catalyst reaction of diazoesters with an alkyne give a cyclopropene. An X-ray structure of an osmium catalyst intermediate has been determined. Electron-rich alkenes react faster than simple alkenes. ... 

The problem with this scenario is that alkynes are more acidic than most hydrocarbons (pK 25), but they are not sufficiently acidic to be deprotonated by amines (p/temperature required for the Sonogashira coupling from >100 °C to room temperature. The Cul may convert the alkyne (RC=CH) to a copper(I) acetylide (RC=C-Cu), a species that can undergo transmetallation with Pd(II). Of course, now the question is, How is RC=C-H converted to RC=C-Cu The alkyne may form a tt complex with Cu, and this complex may be deprotonated (E2-like elimination) to give the Cu acetylide, which can transmetallate with Pd. 

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