Chromium demetallation

Most mordant dyes are monoazo stmctures. The most important feature of this class of dyes is excellent fastness to light and washing. Mordant dyes are available ia aU shades of the spectmm with the exceptioa of bright violets, blues, and greens. To be useful, the metal complexes must be stable, ie, must not demetallize when subjected to dyebath conditions and aU aftertreatment processes, especially repeated washings. Chromium forms stable chelate rings with mordant dyes which are not affected by treatment with either weak acid or alkaU (see Coordination compounds). 

Merlic developed a new variation of the thermally induced benzannulation reaction. The dienylcarbene complex 132 was reacted with isonitrile to give an orf/zo-alkoxyaniline derivative 135 [76] (Scheme 56). This annulation product is regiocomplementary to those reported from photochemical reaction of chromium dienyl(amino)carbene complexes. The metathesis of the isocyanide with the dienylcarbene complex 132 generates a chromium-complexed di-enylketenimine intermediate 133 which undergoes electrocyclisation. Final tau-tomerisation and demetalation afford the orf/zo-alkoxyaniline 135. 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

Aminothiophenol 37 and 277 formed an isolable adduct 278 (X = S) that on heating was transformed to a mixture of complex 283 (10%) and the demetallated benzothiazepinone 284 (51%) (Scheme 50) <2003CEJ4943>. Similarly, monocyclic 1,4-oxazepinone derivatives of tungsten and chromium have been prepared by a domino [4+2]/[2+2] cycloaddition reaction of 1-alkynyl Fischer carbenes with oxazolines <20050M302>. 

The tetrafunctional alcohol pentaerythritol is a popular core in dendrimer chemistry it has been modified into a tetrakis chromium phenylcarbene 85, which underwent a quadruple benzannulation upon reaction with 3-hexyne. The reaction proceeded with only moderate diastereoselectivity in terms of the planes of chirality formed demetalation by mild oxidative work-up gave the tetrakis-hydroquinone derivative 86 (Scheme 33) [76]. 

The photochemistry of transition carbonyl complexes of chromium or iron with phosphirane or l//-phosphirene ligands is studied with time-dependent DFT theory to explore the propensity of the excited metal-phosphine-carbonyl complexes to expel their ligands <2003JA3558>. The complexes of these phosphorus heterocycles show similar behavior as carbonyl complexes with the PH3 ligand and they differ mainly in their enhanced a-donating ability. The calculations point that the excited complexes prefer the expulsion of the phosphorus substituents, which can be an alternative method to demetalate transition metal complexes of organophosphorus compounds. 

Condensation of [Co(tame)2]3+ cation with propanal is described in more detail in Ref. 152. The resultant [Co(/ ac-Me5tricosanesar)]3+ sarcophaginate was demetallated, and the free ligand was employed for the synthesis of a chromium(III) sarcophaginate with unusual spectral characteristics (Scheme 68) [153]. 

Organometallic reagents were used for the synthesis of bicyclic aromatic compounds via Moore-type cyclization. Rahm and Wulff described the new synthesis of 5-hydroxyindolines with the use of a chromium carbene complex bearing alkynyl substituent 22." The amino-tethered bis-alkynyl carbene complex 22 was transformed into indoline 23 by thermolysis in the presence of a hydrogen source. The low yield of product 23 was improved when the reaction was carried out in the presence of the electrophile, added to protect the phenol function. This process involves the insertion of one carbon monoxide group from the chromium complex into the skeleton of an eneyne compound 24. The resulting enyne-ketene 25 undergoes a cycloaromatization reaction to afford the 1,4-diradical intermediate 26. Subsequent demetalation yields product 23."... 

Alternatively, demetallation to give enol ethers 839 can be achieved by treatment with pyridine (Scheme 8.14). Under these conditions, as pyridine is a weak base, an equilibrium is established between the carbene 8.35 and its anion 837. The anion, however, can also reprotonate on chromium to give a chromium hydride 838. This is followed by reductive elimination. The enol ether 839 is obtained as its Z-isomer, a consequence of the carbene anion having S-geometry to keep the alkyl group away from the bulky Cr(CO)s moiety the chromium is converted into a pyridine complex. 

The single electron transfer (SET) reduction of Fischer alkynyl carbene complexes 95 with potassium graphite (CgK) results in the formation of cyclopentadienes 100 (Scheme 5.20) [31], The radical anion intermediates 96, generated by the SET reduction of Fischer alkynyl carbene complexes 95, undergo tail-to-tail dimerization to form bis-carbene anions 97. The intermediates 97 are protonated with a strong acid to produce the intermediates 98, which convert in situ into chromium cyclopentadienylcarbene 99. After the demetallation of the cyclopentadienylcarbene 99, cyclopentadienes 100 are obtained. 

Similar to aminolysis, the alcoholysis reaction can be applied to the synthesis of higher nuclearity metal carbenes as well. Low-temperature addition of 0.4 equivalents of pentaerythritol in DMF to a solution of chromium acetoxycarbene generated in situ from benzoyl chromates 133 (or 134) affords moderate yields of tetra-kischromium carbene 135 (or 136) which undergoes complete benzannulation with 3-hexyne to give an 80% yield of the spherical tetrakishydroquinone 137 after in situ protection of the naphthol intermediate with TBDMSCl and subsequent oxidative demetallation (Scheme 11.32) [57]. 

O-Glycosidic styrylcarbene complexes 298-302 prepared from chromium methyl-carbenes 239-243 and ben2aldehyde in a trans-selective aldol condensation undergo a chromium-templated benzarmulation upon reaction with per-benzyl-protected ethynyl glucopyranose 303. Oxidative demetallation results in the formation of hydroquinoid C- and O-biphenyl disacdiarides 304-308 (Scheme 11.62) [117]. 

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