Metal cyanides carbon nucleophiles

The selectivity of the alkenylation reactions and the yields of products can be dramatically improved by carrying out the reaction of alkenyliodonium salts with carbon nucleophiles in the presence of transition metal compounds in stoichiometric or catalytic amounts. Thus, the reactions of bicycloalkenyldiiodo-nium salts 62 with cyanide anion or with alkynyllithium in the absence of transition metals are non selective and lead to a wide spectrum of products, while the same reactions in the presence of the equimolecular amount of copper(I) cyanide afford the respective products of vinylic nucleophilic substitution in good yields (Scheme 29) [52,53]. 

Ordinary carbon nucleophiles such as cyanide or Grignard reagents or organolithium compounds fit the patterns we have described already. They usually give the more stable product by 5 2 or 5 2 reactions depending on the starting material. If we use copper compounds, there is a tendency— no more than that—to favour the 5 2 reaction. You will recall that copper(l) was the metal we used to ensure conjugate addition to enones (Chapter 10) and its use in Sn2 reactions is obviously related. 

Moreover, aryl-oxazoles, -imidazoles [17], or-thiazoles [18], anhydrides [19], and imides [20] are accessible via intramolecular Heck-type carbonylations. In addition to typical acid derivatives, aldehydes [21], ketones [22], aroyl cyanides, aroyl acetylenes, and their derivatives [23] could be synthesized via nucleophilic attack of the acyl metal complex with the corresponding hydrogen or carbon nucleophiles. Even anionic metal complexes like [Co(CO)4] can act as nucleophiles and lead to aroylcobalt complexes as products [24]. 

The most reasonable interpretation (there have been many) is to consider the hydroxide or cyanide as forming first an sp -hybridized carbon atom (a pseudobase or Reissert-type adduct, respectively) and then being transmitted from carbon to metal ion. In other words, the change in reactivity of an N-heterocycle on coordination to a metal ion is akin to that of the same N-heterocycle on classical quaternization by an organic agent such as methyl iodide. The unusual rate equation [Eq. (67) or (68)] involving the nucleophile s concentration in first- and second-order terms arises because the rates of these reactions (apparently hydrolysis or substitution by cyanide at the metal ion) are actually controlled by rates of reaction at the ligand (27 28). 

Nucleophilic addition of the metal-stabilized pyrrolium complexes is readily achieved with borohydride and cyanide ion. The scope of this reactivity is bracketed by the diminished electrophilicity of the iminium carbons and the acidity of the ammine ligands, which prevents the use of strongly basic nucleophiles. Competing deprotonation of the acidic pyrrolium ring protons is observed primarily only with 3//-pyrrolium complexes or when bulky nucleophiles are used. 

Our results indicate that the autoreduction cannot occur by a conventional outer sphere mechanism because of the gross mismatch of the electrochemical potentials. Experimental data available at this time are consistent with homolytic iron-carbon bond cleavage which may or may not involve a simultaneous nucleophilic attack on the coordinated cyanide. The homolytic metal-carbon bond cleavage may serve as a model for similar processes reported for vitamin Bi2 (26). 

The HOMO of the nucleophile will depend on what the nucleophile is, and we will meet examples in which it is an sp or sp3 orbital containing a lone pair, or a B-H or metal-carbon o orbital. We shall shortly discuss cyanide as the nucleophile cyanide s HOMO is an sp orbital on carbon. 

The cyanide ion is an ambident nucleophile (it can react via N or via C) and isocyanides (also called isonitiiles, R—N=C) may be side products. If the preparation of isocyanides is desired, they can be made the main products by the use of reagents with more covalent metal-carbon bonds, such as silver or copper(I) cyanide (p. 515). However, the use on an excess of LiCN in acetoneATHF gave the nitrile as the major product. Tosyl cyanide (T0ISO2CN) has been used in some cases. 

In the area of enantioselective synthesis see Enantio selectivit ), the development of catalytic carbon-carbon bond-forming reactions that proceed under mild conditions in an enantioselective fashion (ee > 95%) remains a challenging objective.Among a great variety of metallic complexes, Zr-containing chiral catalysts can promote efficient and highly enantioselective additions of nucleophilic fragments such as alkylmetals and cyanides to C=0 and C=N bonds. Moreover, Zr-based metallocenes promote additions of alkylmetals to carbon-carbon double bonds, reactions that do not easily occur with other catalysts. One another important feature is that the product of the asymmetric addition of an alkylmetal to an alkene produces a chiral alkylmetal that can be further functionalized. 

Isocyanides are stable nucleophilic carbenes, well able to insert into carbon-hydrogen, carbon-halogenor heteroatom-hydrogenbonds. The reactions may proceed thermally, or can be catalysed by acids or metal salts. Thus heating the isocyanonaphthalene 137 to 235 °C results in isomerization into the benzocycloheptindole 138 and the benzophenanthridene 139, in addition to isomerization into the cyanide. 

Due to the nucleophilicity of both carbon and nitrogen atoms in cyanides, numerous polymers with (M-CN M-) and (M-SCN- M) linkages are found. Examples include AgCN, 43, and AgSCN, 44, which exhibit low conductivities (— 10 i fi- cm" ) (46). One-dimensional cyano polymers with metals indifferent oxidation states have not been reported. Due to valence interchange these materials should exhibit a higher conductivity. 

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