Reactivity towards copper

NO is very reactive towards copper ions, both Cu(I) and Cu(II). Until very recently only two copper nitrosyl X-ray structures were known, one possessing a bridging Cu"-rNO)-Cu" structure and formulation [v(N-O) = 1460 cm ] [78], the other being a mononuclear copper-nitrosyl complex [(Tp )Cu(NO)j (R= Bu, R = H) ... 

Vanadium is resistant to attack by hydrochloric or dilute sulfuric acid and to alkali solutions. It is also quite resistant to corrosion by seawater but is reactive toward nitric, hydrofluoric, or concentrated sulfuric acids. Galvanic corrosion tests mn in simulated seawater indicate that vanadium is anodic with respect to stainless steel and copper but cathodic to aluminum and magnesium. Vanadium exhibits corrosion resistance to Hquid metals, eg, bismuth and low oxygen sodium. 

A carbene, R2C , is a neutral molecule containing a divalent carbon with only six valence electrons. Carbenes are highly reactive toward alkenes, adding to give cyclopropanes. Nonlialogenated cyclopropanes are best prepared by treatment of the alkene with CH212 and zinc-copper, a process called the Simmons-Smith reaction. 

Divalent Co substitution in copper amine oxidase revealed 19% of the native specific activity (for MeNH2) and 75% of the native reactivity toward phenylhydrazine. The major cause of this was a 68-fold increase in Km for 02. These investigations support the idea that electrons flow directly to bound 02 without the need for a prior metal reduction and that the Cu does not redox cycle but simply provides electrostatic stabilization during reduction of 02 to 02-. 1211... 

The reaction, formally speaking a [3 + 2] cycloaddition between the aldehyde and a ketocarbene, resembles the dihydrofuran formation from 57 a or similar a-diazoketones and alkenes (see Sect. 2.3.1). For that reaction type, 2-diazo-l,3-dicarbonyl compounds and ethyl diazopyruvate 56 were found to be suited equally well. This similarity pertains also to the reactivity towards carbonyl functions 1,3-dioxole-4-carboxylates are also obtained by copper chelate catalyzed decomposition of 56 in the presence of aliphatic and aromatic aldehydes as well as enolizable ketones 276). No such products were reported for the catalyzed decomposition of ethyl diazoacetate in the presence of the same ketones 271,272). The reasons for the different reactivity of ethoxycarbonylcarbene and a-ketocarbenes (or the respective metal carbenes) have only been speculated upon so far 276). 

Due to the poor efficiency of trifluoromethylated organometaUic derivatives as trifluoromethylating reagents, nucleophilic trifluoromethylation has remained unattractive for a long time. Indeed, in the absence of stabilization, the trifluoromethyl anion is very unstable and is quickly transformed into difluorocarbene (cf. Chapter 1). When the carbon-metal bond is relatively covalent, the organometaUic species becomes more stable but it is then less reactive toward an electrophile. On the synthetic level, only zinc and copper derivatives have found real applications in... 

When 303 was directly treated with Me2Cu(CN)Li2, the transmetallation failed to discriminate between the two carbon-metal bonds. By contrast, the allylzincation of the alkynyllithium derived from the propargylic alcohol 309 produced the alkenyl 1,1-dimetallic species 310, in which the two carbon-metal bonds exhibit different reactivities due to the presence of a metal-alkoxide. Indeed, transmetallation with Me2Cu(CN)Li2 led to the alkenyl copper-zinc species 311, which was relatively poorly reactive towards electrophiles but underwent successful 1,4-addition to ethyl propiolate leading to 312 in satisfactory overall yield (equation 145)180. 

In the presence of an equimolar amount of Cu(II) salt, the rate profile shows a steadily increasing reaction velocity as the pH increases, until a practical limit is attained, as a result of the precipitation of copper hydroxide. There is no indication that the rate might level off or decrease at higher pH, as is true for the metal-free ligand. On this basis it seems that the metal ion combines with the substrate in such a way as to increase its reactivity toward the adjacent carboxylate group, as indicated in Figure 3 (formulas XXI and XXIII). Since increasing pH... 

Similar reactivity towards phenylisocyanate is found, while carbodiimide yields a 1 1 adduct in which insertion into the M—-OR bond has not taken place.250,251 Copper(II) methoxide undergoes the reversible insertion of C02 to yield the corresponding carbonate.252... 

The binding of CO to hemocyanin is of interest. The two-coordinate copper centres in deoxyhemocyanin might each be expected to bind CO. However, some two-coordinate complexes of Cu1 with heterocyclic donor ligands show lack of reactivity towards CO in the absence of additional ligands, probably because the metal does not receive enough electron density to back bond to CO.1295... 

Note that no product formation could be obtained in absence of the iron or copper catalyst. Without copper, the iron catalyst generates only stoichiometric amounts (with respect to iron) of an alkenyliron species, which is not reactive towards further transformations such as transmetallation or polymerization. A cuprate, generated by CuBr and ArMgBr, did not undergo carbometallation with the alkyne. These results indicate that the main role of the copper catalyst is most likely to promote the metal... 

Such a simple mechanistic proposal accomodated the observation that highly activated, benzylic alcohols were good substrates due to the enhanced lability of their a-hydrogen atoms. In contrast, aliphatic alcohols are far less reactive towards H-radical abstraction and, accordingly, poor conversions should ensue. However, it was rather disturbing to note that allylic alcohols, such as geraniol and nerol, displayed poor reactivity in this system. Furthermore, it was observed that the aerobic oxidation of aliphatic alcohols invariably resulted in the rapid formation of a green copper(II) salt, with concomitant deactivation of the catalyst. 

The preparation of Copper alkoxydes and their reactivity towards O2 have been reported in the literature. See for example Capdevielle, P. Audebert, P. Maumy, M. Tetrahedron Lett. 1984, 25, 4397 400. 

The protonation equilibrium between ammonia and the ammonium ion is shifted to the right as a result of coordination of unprotonated ammonia to copper. The drop in pH decelerates the complexation reaction, as there is comparatively less free ammonia available. Such protonation equilibria are much more complex if multi-dentate ligands (bases) are involved, but the effect is generally similar a drop in pH is the immediate result of coordination, and this drop increases the protonation of the ligand and thus decreases its reactivity toward the metal ion. 

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