Isocyanides addition reactions

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)]. 

In the reaction of Ni(CNBu )4 and methyl iodide oligomerization of the isocyanide was observed the only isolable nickel complex was (I), shown below. This product is believed to arise through sequential insertions of three isocyanides into a nickel-carbon bond. Upon further treatment with additional isocyanide at a temperature greater than 60° C one obtains a polymer (RNC) presumably through multiple isocyanide insertion reactions. The addition of benzoyl chloride to Ni(CNBu )4 gave two isolable compounds Ni(CNBu )3(COPh)Cl (74%) and (II) (8.2%). This latter reaction, and the isolation of (II) in particular, suggests that the proposed mechanism for polymerization of isocyanides is reasonable. 

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. 

In addition to a-additions to isocyanides, copper oxide-cyclohexyl isocyanide mixtures are catalysts for other reactions including olefin dimerization and oligomerization 121, 125, 126). They also catalyze pyrroline and oxazoline formation from isocyanides with a protonic a-hydrogen (e.g., PhCH2NC or EtOCOCHjNC) and olefins or ketones 130), and the formation of cyclopropanes from olefins and substituted chloromethanes 131). The same catalyst systems also catalyze Michael addition reactions 119a). 

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. 

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). 

Isocyanides and dialkyl acetylenedicarboxylates in the presence of 2,4-dihydro-3//-pyrazol-3-ones 344 in acetone at ambient temperature undergo a smooth 1 1 1 addition reaction to produce highly functionalized 7-oxo-17/,7H-pyrazolo[l,2-tf]pyrazole derivatives 345 in 69-81% yields (Equation 47) <2005T3963>. 

Addition of isocyanide to dipolarophiles [26]. Energy cost analysis from the terms 1Ex=y — EAHnc and vice versa, shows that HNC acts as a nucleophile in these addition reactions. Thus s for the C atom of HNC and s+ for the X and Y atoms should be considered for determining the preferred reaction site (Scheme 12.3). 

In 1993 the first MCR composed of seven educts was introduced, and it was soon recognized that such higher MCRs are usually unions of the U-4CR and additional reactions. In the first 7-CR, the intermediate 63 was formed by an A-4CR and underwent with the equilibrating product 67 the a-addition of the cations and ions onto the isocyanide 27. Finally, this a-adduct, 69, rearranges into the final product, 71 (Scheme 1.17). 

R = Bu , Pr, p-ClC H, or p-MeCeH4) have been isolated from the reaction between free isocyanide and a variety of Ir substrates. Studies of the oxidative addition reaction ... 

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. ... 

The reaction produces 3,3-disilyl-2,4-disila-l-azacyclobutane derivatives in moderate yields. Although the rearrangement is only observed for aryl isocyanides, addition of a f-alkyl isocyanide to the reaction mixture serves to increase product yields. 

The earliest reports of constrained Ugi adducts derived from bi-functional precursors appeared in the 1960s with the preparation of penicillin derivatives such as 68, involving sequential Asinger and Ugi four-component reactions (Scheme 11.13). As such, the synthesis represents the shortest preparation of a known penicillin derivative [65], The /Mactam ring is formed after isocyanide addition to the cyclic Schiff base, followed by carboxylate nitrilium ion trapping and acyl transfer to give the final penicillin core. In this example, the amine and carboxylic acid inputs may be considered tethered. 

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). 

Noncarbonyl ligands can participate more readily in the hydrogenation of the metal carbonyl as for RUjfCOljjfCNBu-t). Bubbling through the cluster in cyclohexane at reflux for 1 h yields HRu3(ju.3-HCNBu-t)(CO)g (52%) by addition of Hj to the coordinated isocyanide. These reactions require milder conditions than those used for the unfunctionalized RujfCOljj with Hj. 

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