Isocyanide complexes nucleophilic reactions

B. Nucleophilic Reactions with Metal Isocyanide Complexes... 

The subjects of structure and bonding in metal isocyanide complexes have been discussed before 90, 156) and will not be treated extensively here. A brief discussion of this subject is presented in Section II of course, special emphasis is given to the more recent information which has appeared. Several areas of current study in the field of transition metal-isocyanide complexes have become particularly important and are discussed in this review in Section III. These include the additions of protonic compounds to coordinated isocyanides, probably the subject most actively being studied at this time insertion reactions into metal-carbon bonded species nucleophilic reactions with metal isocyanide complexes and the metal-catalyzed a-addition reactions. Concurrent with these new developments, there has been a general expansion of descriptive chemistry of isocyanide-metal complexes, and further study of the physical properties of selected species. These developments are summarized in Section IV. 

The reactions of nucleophilic reagents with cationic and uncharged metal carbonyl complexes have received much attention in the past, and it is not surprising that these studies have now been extended to isocyanide metal complexes. Different products in these reactions can arise by three general routes these include ligand substitution, reactions involving attack at a ligand, and reduction of the metal complex. All have been observed in reactions with metal isocyanide complexes. 

The susceptibility of a metal complex to nucleophilic attack is enhanced by a positive charge on the complex. This fact, and the fact that most metal isocyanide complexes are cationic, probably explains why no nucleophilic reactions of uncharged isocyanide complexes have yet been reported. It is... 

The reaction of isocyanide complexes with nucleophiles gives metal-carbene complexes [49], which constitute an important branch of organometallic chemistry and are effective catalyst systems for a variety of processes [50, 51]. 

Transition metal isocyanide complexes can undergo reactions with nucleophiles to generate carbene complexes. Pt(II) and Pd(II) complexes have been most extensively investigated, and the range of nucleophilic reagents employed in these reactions has included alcohols, amines, and thiols (56) ... 

The most common reaction of platinum(II) isocyanides is their reaction with nucleophiles. This reaction is a useful one for the formation of platinum carbene complexes, and this will be discussed in the next section. 

A cyclopentadienylcopper-fcr/-butyl isocyanide complex catalyzes the Michael addition of dimethyl methylmalonate to acrylonitrile at room temperature to give an S6% yield of the adduct 249). As the CU2O—BNC complex can also catalyze the addition of indene to methyl acrylate, the intermediate is most likely an organocopper complex. The reactions and kinetic data support the mechanism given by Eq. (118) to (120), involving metalation and nucleophilic attack by the carbanion on the olefin within the complex. Displacement of a solvent ligand by the olefin and coordination of the latter to the copper species are essential features of the mechanism. The rate of reaction is decreased if the compound with the... 

There are several other methods for the synthesis of carbene complexes of osmium. The reaction of a nucleophile with isocyanide complexes can result in the formation of osmium carbene complexes. Certain electron-rich alkenes add to osmium to form carbene complexes. For example. 

Palladium isocyanide complexes react with amines and alcohols to give palladium Carbene Complexes (equation 15) in a reaction that is a nucleophilic attack on the isonitrile carbon. These complexes easily lose a halide if it is trans to the carbene ligand, since the carbene carbon has a high trans influence. The structures of these compounds have the carbene carbon and its two heteroatom groups in one plane, which is perpendicular to the coordination plane of the Pd. 

Isocyanides, being nucleophilic, and having a greater proton affinity than the corresponding cyanides, add readily to carbocations, though the acid sensitivity of isocyanides limits the reaction to those carbocations which can be prepared under non-acidic conditions. The complex of cyclobutadiene with AICI3 provides a useful example of this type of reaction. The isocyanide can also insert into the C—Cl bond of 143. 

Just like the isoelectronic carbon monoxide, an isocyanide is an excellent ligand to metal ions. The chemistry of metal isocyanide complexes has been reviewed by Singleton and Oosthuizen. Only a few examples will be given here. Insertion of an isocyanide into a metal-carbon bond frequently occurs. It is not always clear whether the key step is electrophilic or nucleophilic attack on the coordinated isocyanide or whether the reaction is concerted. Insertion into metal-carbene and metal-carbyne complexes have been reviewed by Aumann. Coordination to the metal considerably affects the chemistry of the isocyanide. If the metal is electron-donating, as in nitrogenase-like centres, the coordinated isocyanide is apt to electrophilic attack at nitrogen cf. Section III. 

Organogold carbene compounds [Au(carbene)(CN) j iR ] can be obtained from com-plexed cyanides [Au(CN)m(C6F5) ] (n = 1 or 3, m = 1 and n = m = 2) by sequential alkylation (to form isocyanide complexes) and nucleophilic attack of amine" . Gold(III) diisocyanides [Au(CNR)2(CgF5)2]+ (R = Ph or p-tolyl), which can also be obtained by isocyanide substitution of ether in [Au(C6F5)2(OEt2)2] > react with hydrazobenzene NH(Ph)NHPh and hydrazine or phenyUiydrazine according to the reactions in Scheme 32 to furnish cyclic bis(carbene) and cyclic carbene-imidoyl compounds . ... 

XPS spectra " show that coordination of the RNC ligand causes a decrease of the electron density on the isocyanide carbon atom. Therefore, addition of nucleophiles to the carbon atom and addition of electrophiles to the nitrogen atom represent characteristic reactions of the isocyanide ligands. Reactions of metal isocyanide complexes with compounds of the type RXH (RX = RO, RNH, RS, etc.) lead to the formation of metal carbene complexes [Section 5.8.b, reactions (5.13)-(5.19)]. 

The reactions of isocyanide complexes with nucleophiles parallel the reactions of metal carbonyls. Alcohols were found to add across the carbon-nitrogen bond of complexed isocyanides (Badley et al., 1969, 1971). Amines... 

Generally, isocyanides can be attacked by proton bases HX (X = OR, RNH) in a nucleophilic reaction that leads to acylic heterocarbene complexes [27, 28] The use of functionalized isocyanides containing both the isocyanide group and the nucleophile in the same molecule gives access to complexes with heterocyclic carbene ligands via an 1,2-addition across the C=N triple bond [29]. A number of research groups have been active in the development of nucleophile-functionalized isocyanides, which could subsequently be cyclized in metal template-controlled reactions. 

A copper-isocyanide complex functions as a selective catalyst in the Michael reaction, preferentially activating acrylonitrile and its homologues to nucleophilic attack. 

In pioneering work Chatt et al studied the reaction of several isocyanide complexes with nucleophiles yielding carbene complexes (45). We have studied the reaction of (CO)5Cr(CNCF3) and Cp Co(CNCF3)2 with nucleophiles The reaction... 

The stereocontrol observed in these reactions is rationalized on the basis of zinc complexes formed with the Schiff bases, as described in Section 1.4.4.2.2. The isocyanide, being a free and potent nucleophile, attacks this complex from the sterically less-hindered side, that is. from the side of the ring oxygen. 

---