Imidation cobalt

Nomura H, Richards CJ (2007) An investigation into the allylic imidate rearrangement of trichloroacetimidates catalyzed by cobalt oxazoline palladacycles. Chem Eur J 13 10216-10224... 

Complex 165 reacts with CO at room temperature to give />-tolyl isocyanate and the golden dicarbonyl compound 166. The latter can also be generated by reaction of 164 with excess of CO and converted back to imide 165 by the addition of 2 equiv of /> toly 1 azide (2003JA322). Reaction of cobalt(m) imide 167 analogous to 165, available from [PhB(CH2PPh2)3]CoI 156 by the same procedure as for 165, with CO proceeds similarly <2002JA11238>. 

Reduction of low-spin Fe(m) imide 165 with hydrogen (1 atm, 20 °C) proceeds stepwise leading first to anilido complex 168 and then to 77-cyclohexadienyl complex 169 via hydrogenolysis of the Fe=NR linkage (Scheme 64). It should be mentioned that the similar low-spin cobalt complex [PhB(CH2PPh2)3]Co N-/>-Tol is stable to hydrogen pressure (1-3 atm) up to 70 °C <2004JA4538>. 

Oxidation of 158 and 159 with sodium amalgam as well as of 170 and 171 with ferrocenium (Cp2Fe+) leads to dinuclear N2-bridged products 173, converted to iron(m) and cobalt(m) imides 174 under action of adamantyl and p-tolyl azides (Scheme 66) <2003JA10782>. 

Cobalt(II) chloride was dissolved in poly(amide acid)/ N,N-dimethylacetamide solutions. Solvent cast films were prepared and subsequently dried and cured in static air, forced air or inert gas ovens with controlled humidity. The resulting structures contain a near surface gradient of cobalt oxide and also residual cobalt(II) chloride dispersed throughout the bul)c of the film. Two properties of these films, surface resistivity and bullc thermal stability, are substantially reduced compared with the nonmodified condensation polyimide films. In an attempt to recover the high thermal stability characteristic of polyimide films but retain the decreased surface resistivity solvent extraction of the thermally imidized films has been pursued. 

Reactions involving the [4 + 1 + 1] principle, an example of which is shown in equation (136), are rather uncommon and of strictly limited utility [3 + 2 + 1] and [2 + 2 + 2] processes, on th,e other hand, are well known. Representative [3 + 2+1] three-bond formation processes are given in equations (137)—(141), from which it can be seen that the common situation is where ammonia, a substituted amine or formamide constitutes the one-atom fragment. Many [2 + 2 + 2] atom fragment syntheses are known and some are familiar reactions. Thus, the cobalt(I)-catalyzed condensation of nitriles and isocyanates with alkynes gives pyridines and 2-pyridones, often in excellent yield (e.g. equation 142), while the cyclotrimerizations of nitriles, imidates, isocyanates, etc., are well established procedures for the synthesis of 1,3,5-triazine derivatives (e.g. equation 143). Further representative examples are given in equations (144)-(147), and the reader is referred to the monograph chapters for full discussion of these and other [2 + 2 + 2] processes. Examination of the... 

Hydrolysis of peptides,84 amides,85 phosphate esters,86 sulfonate esters87 and acetals88 can also be metal catalyzed. The hydrolysis of a phosphate ester coordinated to cobalt(III) also occurs at an increased rate (Scheme 19).89 A rather similar reaction occurs in the amine exchange of coordinated dithiocarbamates (equation 21).90 The conversion of imidates to amidines has been mentioned previously and is a similar type of reaction (see Section 7.4.2.2.1). 

Initial cyclizadons were effected by the addition of an enamine to an imidate ester, both groups being suitably located by ligand coordination.263 An analogous process can be carried out on a thioimidate but a sulfide is formed and removal of sulfur with consequent ring contraction yields the corrin (100).264 These two complementary routes can be effected with different metal ions, nickel(II), palladium(II) and cobalt(III) in the first route, zinc(II) in the second. Removal of zinc ions easily provides the free corrin macrocycle. These two routes are summarized in Scheme 64. The sulfide contraction route was used in the Eschenmoser-Woodward total synthesis of vitamin Bn-265... 

In many of the above cyclization reactions it is not clear to what extent the metal ion is important. In most cases a geometrical effect is significant but electronic activation is probably not always of importance. The corphin synthesis (see Scheme 68) is a good example of precise geometrical requirements. This cyclization succeeds for the palladium complex but not for nickel or cobalt complexes. The imidate condensation seems to require steric acceleration and more is provided by the desire of palladium to attain planarity than the urge for nickel or cobalt to do so. 

Cobalt Azoimide, Co(N3)2, and a Basic Azoimide, N3-Co.OH, have been obtained in mixture by Curtius3 as a violet powder. Potassium cobalto-azoimide, KN3.Co(N3)2, is obtained as a bright blue crystalline precipitate when concentrated solutions of the two azo-imides are mixed. It explodes at 225° C. The ammonium analogue, NH4N3. Co(N3)2, has also been prepared. 

Macrocycle 177 in which the 2,6-pyridino and the 1,4-piperazino moieties were incorporated into the macrocyclic framework has been reported. The synthesis of 177 (10%) was accomplished by treatment of 2,6-dichloropyridine with the dianion of Ar,Ar -W. (2-hydroxyethyl)piperazine in refluxing xylene. Attempts to prepare the cobalt(II) complex of 177 resulted in diprotonation of the macrocycle. The X-ray crystal structure determination analysis has been performed for both 177 and 178. According to the crystal structure analysis of 178, the piperazine rings are in the chair conformation in the solid state and the molecule is fairly rigid due to the imposed steric constraints of the imidate moieties l39). 

The principal impetus to the study of zinc thiolates has undoubtedly been the search for structural models for the metal coordination in zinc metalloproteins such as the Zn(S-Cys)2(His)2 center in the transcription factor IIIA and other zinc fingers that feature in protein-DNA interactions (93, 94). The 2 1 complexes [M(SR)2l (M = Zn, Cd SR = TIPT, TEMT) were used as the principle precursors for the synthesis of such models for the Zn(S-Cys)2(His)2. Thus complexes of the types [Cd(TIPT)2(l-CH3-imid)2], [Zn(TIPT)2(bipy)], and [Zn(TEMT)2(l-CH3-imid)2l (imid = imidazole, bipy = bipyridine) were ssmthesized by addition of a nitrogeneous base to the precursor in MeCN. All of them have been characterized by X-ray crystallography and were found to have distorted tetrahedral geometries. It was noticed that the cobalt analogue binds a solvent molecule MeCN to form five-coordinate [Co(TIPT)2(bipy)(CH3CN)], whereas [Zn(TIPT)2(bipy)] does not 96,97). 

Activated aziridines should be as useful as epoxides for carbon-carbon bond formation, with the advantage that the product will already incoiporated the desired secondary aminated stereocenter. To date, a general enantioselective method for the aziridination of alkenes has not been developed. Eric. lacobsen of Harvard University Angew. Chem. Int. Ed. 2004, 43, 3952) has explored an interim solution, based on the resolution of racemic epoxides such as 1. The cobalt catalyst that selectively hydrolyzes one enantiomer of the epoxide also promotes the addition of the imide to the remaining enantiomerically-enriched epoxide. As expected, the aziridine 4 is opened smoothly with dialkyl cuprates. 

The catalysed rearrangement of allylic imidates including allylic trichloroace-timidates such as (12.12) has been reported by Overman using palladium catalysts with some of the highest ees obtained using the cobalt oxazoline palladacycle (12.13). 

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