Alkyl isocyanides oxidation

Many papers devoted to electrochemical processes of oxidation and reduction of metal isocyanide complexes have also appeared. It has been found that the ease of oxidation increases when the number of carbonyl ligands in the coordination sphere decreases and as a result of substitution of aryl isocyanides by alkyl isocyanides. Oxidation of [Cr(CNR)6] to [Cr(CNR)6] proceeds more easily in the case of alkyl isocyanides while reduction of Cr(II) complexes to Cr(I) compounds and Cr(I) complexes to Cr(0) compounds takes place more readily for aryl isocyanide ligands. " " " >... 

Still, the ease of oxidation is obviously a property of considerable chemical importance. These differences require that the choice of aryl vs. alkyl isocyanides not be taken lightly. 

The metal-free eyclobutane-1,2-dioxime can be generated by oxidative displacement. It is interesting to note that, unlike ketene dimerization, head-to-head dimerization takes place here. The chromium ketenimine complex 20 is prepared by reaction of the Fischer-type chromium carbene complex with alkyl isocyanides.60 A cyclobutane-1,2,3,4-tetraimine 24 has been reported from the reaction of the ketenimine phosphonium ylide 22.61 Bisimine 23 has been proposed as the intermediate in this transformation. 

The lability of the N2 ligand towards alkyl and aryl isocyanides has been demonstrated by equation (2) which occurs readily with both alkyl and aryl isocyanides.40 The resulting complexes are electron rich , the alkyl isocyanide derivatives ReCl(CNR)(dppe) (R = Me or Bul) undergoing electrophilic attack upon treatment with HBF4 to form the carbyne complexes trcms-[ReCl(CNHR)(dppe)2]BF4.41 Other noteworthy features of the series of complexes ReCl(CNR)(dppe)2 and ReCl(CNAr)(dppe)2, are the low value of v(CN), the presence of a bent M—CNR coordination mode, and their ease of oxidation (as measured by cyclic voltammetry) to the corresponding rhenium(II) monocations.40... 

It has been reported that [1+ 4]-cycloaddition between alkyl isocyanides and 3-benzylidene-2,4-pentadione followed by tautomerisation of the resulting iminolac-tone gives the /V- a 1 k y 1 - 2 - a m i n o fu r a n s 71 (R = cyclohexyl, Bu, PhCH2) (Scheme 14) (97MC697). However, subsequent studies have shown that the isolated products are the pyrrol-2-ones 72, which are probably formed by oxidation of the intermediate 2-aminofurans 71 (04TL1413) (see also Section Il.C.l.a). 

A similar transformation of 2-furylcarbamates (e.g. 42) to A-carboxy-5-hydro-xypyrrolin-2-ones has been reported (78H1603, 80H1073). This type of oxidative rearrangement probably also accounts for the failure to isolate the 2-aminofurans 71 formed from 3-benzylidene-2,4-pentanediones and alkyl isocyanides (Scheme 14) (97MC697, 04TL1413). 

Preparation and properties of ferrous chloroperoxidase complexes with dioxygen, nitric-oxide, and an alkyl isocyanide—spectroscopic dissimilarities between the oxygenated forms of chloroperoxidase and cytochrome-P-450. J. Biol. Chem. 260, 5530-5535. 

The interaction of a phosphinothioic acid (or a dialkyl phosphorothioic acid) with an alkyl isocyanide yields the corresponding A-alkylthioformamide together with a monothio anhydride (reaction 21) ". A further interesting sequence commences with the interaction of a diarylphosphine oxide, R2P(0)H (R = Ph or p-Tol) with HCNS. It is known that such... 

Exocyclic bis-silylated olefins have been constructed through the Pd(OAc)2-catalyzed reaction of alkynes with a tethered disi-lanyl group. The reactions are carried out in the presence of a tert-alkyl isocyanide, although the precise role of this ligand is unclear. Diimide reduction of the disilylated alkene so-formed followed by Fleming-Tamao-type oxidation of the two C-Si bonds in the saturated product then affords 1,2,4-triols in a stereoselective manner (eq 83).l ... 

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)]. 

It is possible to replace one isocyanide by triphenylphosphine, or to replace two isocyanides with diphos, giving phosphine analogues of these complexes. These species are not available from analogous reactions of phosphine-palladium(O) and (II) complexes. Reactions with active alkyl halides proceeds with oxidation nitric oxide also oxidizes these complexes. [Eqs. (31, 32)]. 

The aqua ion is not easily reduced nor oxidized. It is the slowest reacting of the bivalent transition metal ions with e " k = 7.7 X 10 M s ) and the product Mn+q is very reactive. However Mn(CNR)5 (R = a variety of alkyl and aryl groups) is stable and the selfexchange in the Mn(l,ll) hexakis(isocyanide) system has been studied by Mn and H nmr line broadening. The effects of solvent, temperature, pressure and ligand have been thoroughly explored. 

The adsorption of alkyl and aryl isocyanides on Au film [26, 32, 33], powder [36, 37] and nanoparticles [39, 41, 42] has been studied using several different techniques IR methods (RAIR, ATR-IR, DRIFT), Raman methods (SERS), X-ray methods (NEXAFS), ellipsometry (OE, SWE) and contact angle measurements (ACA). The gold surface is not oxidized under normal conditions consequently, the experiments were performed in air at room temperature. The gold film was obtained by physical vapor deposition of 100-200 nm of gold on different substrates glass [28, 33], mica [33], silicon [25, 27, 31, 32], ZnSe crystal [26]. A... 

Comparison between the half-wave potentials (equations 2 to 4) of [Cr(CNR)6](PF6)2, e.g. for R = Bu , -1.04, -0.28 and 0.84 V (versus SCE),22 with those for [Cr(CNPh)6](PF6)2, i.e. -0.35, 0.25 and 1.00 V,20 shows that alkyl and aryl isocyanides favour respectively the higher and the lower oxidation states as expected from the greater a-donor and weaker jr-acceptor capabilities of the alkyl over the aryl isocyanides. Similarly, the phosphines in the mixed ligand complexes (Table 3), 23 relative to isocyanide ligands, stabilize the Cr111 oxidation state. The great difference in the relative stabilities of Cr—C bonds in the cyano and phenyl isocyanide complexes is indicated by the magnitude of the shift (ca. 2.0 V) between the Cr(CN) "/Cr(CN)r (-1.130 V) and the Cr(CNPh)i+/Cr(CNPh)i+ reduction potentials.28... 

The isocyanide analogues of the above carbonyls, frans-[Mo(CNR)2(dppe)2] (R = alkyl or aryl) undergo a reversible one-electron oxidation in solution4,5 and oxidation with I2 or Ag1 salts allows isolation of the paramagnetic cation [Mo(CNC6H4Me)2(dppe)2]+ = 2.01 BM). With other isocyanides, substitution at the metal centre occurs on further oxidation.48... 

Low oxidation states are generally stabilized by ligands which have both a donor (lone pairs) and n acceptor (either empty d orbitals or empty antibonding Jt orbitals) capability. Examples of common ligands with these characteristics are carbon monoxide, cyanide ion, alkyl and aryl isocyanides, tertiary phosphines and arsines, and alkyl or aryl phosphites. 

Replacement of ligands in C3H5MoCl(CO)2(NCMe)2 by isocyanides has given the substituted products C3H5MoC1(CO)2(CNR)2 (R = alkyl) and C3H5MoC1(CO)(CNBu )3, and the reduced products [MoC1(CNBu )4]2 and m-Mo(CO)2(CNR)4 (R = Me, Et). No rationale for the loss of allyl and allyl chloride in the latter two cases was proposed (206). These reactions are rare examples of the formation of low-oxidation state metal-isocyanide complexes via reductive elimination of allyl or allyl chloride from metal-allyl species. The potential applications of mono-, bis-, and tris-n-allylic systems as precursors to low-oxidation state compounds remain to be explored. Substitution and simultaneous reduction of Mo(SBu )4 also occurred on reaction with CNBu to give Mo(SBu )2(CNBu )4 (207) (see Section IV,D,2). 

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