Isocyanide complexes addition reactions

Group IV Donors. A new class of Pt complex of the formula [Pt(PPh3)2-(CNBu )] has been isolated from the reaction between [Pt(PPh3)2(C2H4)] and free isocyanide. Further treatment with CO gave [Pt(PPh3)2(CNBu )(CO)], and comparison of the v(NC) frequencies in these two new complexes suggests that CO is a more effective ji-acceptor than the isoelectronic isocyanide. Oxidative addition reactions were also reported. ... 

At present, this method is often used for the preparation of isocyanide complexes. During reaction, the addition of the ligand may take place without the change of the oxidation state. However, in many cases, reduction of the metal occurs, and complexes in which the transition element has a lower oxidation state than the starting compound are obtained. Copper (I) salts coordinate one to four RNC molecules to give [CuX(CNR) ] and [Cu(CNR)4]X. Copper(II) salts are readily reduced by isocyanides to Cu(I) compounds. However, copper(II) chlorate or tetrafluoroborate and tert-dikyl isocyanide give the cationic complex [Cu(CNR)4]X2. This compound must immediately be separated from the reaction mixture in order to avoid its reduction. 

While on the subject of reviews, attention should also be directed to a very recent collection of articles on isocyanide chemistry edited by Ugi 156). This volume is oriented somewhat toward the organic chemistry of isocyanides, but not with the complete exclusion of metal complexes of these species one is directed in particular to the chapters by Vogler (Chapter 10) on coordinated isocyanides and by Saegusa and Ito (Chapter 4) on a-additions to isocyanides. These latter reactions are often catalyzed by copper(I) compounds and occasionally by other metal complexes as well, and it is believed that this catalysis is accomplished by intermediate formation of 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. 

Substantially more work has been done on reactions of square-planar nickel, palladium, and platinum alkyl and aryl complexes with isocyanides. A communication by Otsuka et al. (108) described the initial work in this area. These workers carried out oxidative addition reactions with Ni(CNBu )4 and with [Pd(CNBu )2] (. In a reaction of the latter compound with methyl iodide the complex, Pd(CNBu )2(CH3)I, stable as a solid but unstable in solution, was obtained. This complex when dissolved in toluene proceeds through an intermediate believed to be dimeric, which then reacts with an additional ligand L (CNBu or PPh3) to give PdL(CNBu )- C(CH3)=NBu I [Eq. (7)]. 

A number of studies have been reported concerning azide-isocyanide condensations to give tetrazoles. Early work by Beck and co-workers 18, 19) describes the addition of various isocyanides to metal azido species [Au(N3)4]", [Au(N3)2]", Au(PPh3)N3, and M(PPh3)2(N3)2, M = Pd, Pt, Hg. The products are carbon-bonded tetrazolato-metal complexes. It is not known whether metal isocyanide complexes are intermediates in these reactions. More recently inverse reactions with azide ion addition to metal isocyanide complexes were carried out, with similar results. From... 

Since the initial report on the addition reactions of palladium(II) and platinum(II) isocyanide complexes by Badley et al. (S), a rather substantial number of further examples have been reported. These are summarized in Table II. 

Various a-addition reactions are observed to be metal- or acid-catalyzed, or to be uncatalyzed. In this review only the metal-catalyzed reactions will be discussed, since it is generally assumed that metal isocyanide complexes are involved in these systems. A number of metal-catalyzed a-addition reactions have been mentioned recently. Copper(I) oxide seems to be the most commonly used catalyst, although other metal complexes sometimes are satisfactory. Table III presents a partial survey of this work. 

The most interesting work on the isocyanide complexes of the elements in this subgroup has been done with rhodium and iridium. For the most part, the work is involved with the oxidative addition reactions of d square-planar metal complexes. 

Virtually all work on nickel isocyanide complexes centers on nickel(O) species. Malatesta and Bonati 90) describe complexes of the formula NiL4 and Ni(CO) L4 jj. The former are formed in a variety of reactions, including reductions of nickel(II) in the presence of isocyanides, and by the replacement of other ligands by isocyanides. The latter are, of course, derivatives of Ni(CO)4. In addition, a few ill-defined nickel(II) complexes are reported, as is the formally nickel(I) species (C5H5NiCNC6Hj)2. 

A large amount of the work on palladium isocyanide complexes has been mentioned earlier, in discussions on insertion reactions 30,74,108,169,170) and on addition reactions of coordinated isocyanides 25, 33, 34, 49) the reactions of [Pd(CNBu )2] with oxygen 107) and with various olefins 29, 110) were noted. 

The colorless, diamagnetic copper(I) complex Cu(acac)(CNPh)2 is formed from copper(I) acetylacetonate and phenyl isocyanide (103). The copper(I) complex (CuCl)2(CNCjH,)2pip was isolated (124) it decomposes to C5H, oNCH=NCgH 11. Primarily on this basis, copper(I) complexes are presumed to be intermediates in various -addition reactions to isocyanides (Section III,D). 

The reaction of the gold(I) pentafluorophenyl isocyanide complexes with primary and secondary amines as well as alcohols leads to the corresponding gold(I) [62, 65] carbenes (Table 3.2). The addition of amines leads to the corresponding carbenes... 

A very interesting synthetic method was published in 1969 by Richards and co-workers (46). They found that, in the reaction of alcohols with certain isocyanide complexes sueh as those of platinum(II) ], an addition of the alkoxy group to the carbon atom as well as of the hydrogen to the nitrogen atom of the isocyanidc ligand occurs, and one thus obtains the corresponding carbene complexes ... 

Addition reactions of the Si-Si cr-bonds of disilanes 121, 131, and 133 to the C=C bonds of various arynes were found to be promoted by a palladium-1,1,3,3-tetramethylbutyl isocyanide complex. Diverse 1,2-disilylated arenes 130, 132, and 134 were obtained from five-membered and benzo-condensed six-membered cyclic disilanes (Equations 21-23). The H, 13C, and z9Si NMR spectroscopic data as well as X-ray crystallographic analysis were used to confirm the above structures <20050M156>. 

An interesting addition reaction has been found with the isocyanide complexes of the nickel triad and dialkylamines [Eq. (142)]. The metal atom is oxidized, with formation of diaminocarbenes (77). The ligand cis to the metal-bonded oxygen is attacked. This is shown in the X-ray structure of 177b (197). 

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... 

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