Metal decomplexation

The [5 + 2]-cycloadditions of air-stable 7]3-pyranyl and 7]3-pyridinyl molybdenum 7T-complexes (4758 and 48,59 respectively) with alkenes reported by Liebeskind and co-workers provide a novel method for the construction of oxabicyclo[3.2.1]octenes and highly functionalized tropanes (Scheme 20). This process involves the formation of a TpMo(CO)2 complex which in the presence of EtAlCl2 reacts with an alkene in a stereoselective [5 + 21-cycloaddition to give metal-complexed cycloadducts 50 and 51 (Tp = hydridotrispyrazolylborato). Metal decomplexation via protiodemetalation or oxidation affords the products in good to excellent yields (Scheme 21). 

The cycloaddition of several diazoalkanes with (2-arylvinyl)-[(—)-(8-phenyl-menthyloxy] methylene chromium complexes 72 gave the A -pyrazolines 73 with high diastereoselectivity. These compounds were converted into pyrazolinecarbox-ylates 74 by A-protection and metal decomplexation (98) (Scheme 8.17). It is... 

Cycloheptadienyl-iron complexes. Cyclohcptadienyl-Fe(CO), reacts with nucleophiles to give mixtures of products, usually in low yield. However, complexes 1, in which one CO ligand is replaced by triphenylphosphinc or triphenyl phosphite, react with most nucleophiles at the dienyl terminus and in high yield. Hydride abstraction followed by reaction with a second nucleophile results in a regio- and stereocontrol led substitution at the opposite terminus and again anti to the metal. Decomplexation affords ri.v-5,7-disub-stituted cycloheptadienes. 

Apart from raising the pH level, precipitation can be induced in a variety of other ways. Anionic species, for example, can be deposited on the surface of suspended carriers by decreasing the pH level. This procedure has been used for vanadium(V) and Mo(Vl). Oxidation at a pH level where the ions of the lower valency are soluble and the oxidized species insoluble, can also be utilized to precipitate from a homogeneous solution. Iron(lI/lll) and Mn(IlI/rV) are cases in point. Oxidation can be effected by dissolved agents like nitrate, or electro-chemically. Reduction to insoluble ionic compounds has been applied with Cr, Cu, and Mo. Reduction to the metal has also been practised, and appears to work especially well with noble metals. Decomplexing is another possibility e.g. destruction of complexing EDTA by hydrogen peroxide has been employed to produce supported catalysts. 

Irradiation of a solution of thiepine 1,1-dioxide chromium complex (15) in the presence of diene (99), followed by metal decomplexation with oxygen provided [6 -I- 4] cycloadducts (100) <92TL5873, 93JA1382). The relative stereochemistry of the products (100) was shown to be derived from an endo transition state. The electronic character of the diene (99) appears to offer a moderate influence on reaction efficiency. For example, yields were uniformly lower for reactions involving electron-deficient diene partners relative to those employing electron-rich dienes (Equation (17)). 

MO calculations suggest that, in nucleophilic addition reactions of substituted benchrotrenes [( -PhX)Cr(CO)3] (see Vol. 9, ref. 412), the preferred orientation of addition is influenced more by the conformation of the Cr(CO)a residue than by the electronic character of the substituent X. Whereas lithiation (with BuLi-TMED) of 3-methoxybenzyl alcohol occurs very predominantly at the 2-position of the arene ring, similar treatment of the corresponding n-Cr(CO)3 complex gives a mixture of the 2- and 4-lithio derivatives in the ratio 23 77, respectively these products were characterized by isolation of carboxylic acid derivatives following carbonation (COg) and metal decomplexation hv 0. Experimental details for the preparation of [(iy-PhX)Cr(CO)3] (X=I and SiMes) from [( -PhLi)Cr(CO)3] have been reported. This lithio derivative has also been used to prepare the carbene complexes [(OC)3Cr(i7-Ph)C(OEt)M(CO)5] (M=Cr, Mo, and W), which react with BX, (X=C1 and Br) to give the carbyne products [(OC)3Cr( Ph)C=M(CO)4X] (M=Cr and W only). ... 

In another process for the synthesis of PPS, as well as other poly(arylene sulfide)s and poly(arylene oxide)s, a pentamethylcyclopentadienylmthenium(I) TT-complex is used to activate -dichlorobenzene toward displacement by a variety of nucleophilic comonomers (92). Important facets of this approach, which allow the polymerization to proceed under mild conditions, are the tremendous activation afforded by the TT-coordinated transition-metal group and the improved solubiUty of the resultant organometaUic derivative of PPS. Decomplexation of the organometaUic derivative polymers may, however, be compHcated by precipitation of the polymer after partial decomplexation. 

Incorporation of a chiral phosphane allowed resolution of the complex 6 which was obtained in enantiomerically pure form. Reaction of 6 with 2,2-dimethylpropanal provided the adduct 7 as the sole observable aldol product13. Oxidation of the metal center of 7 with ferric chloride induced decomplexation via reductive elimination, to provide the enantiomerically pure cy-clobutanone 8. 

In the context of Scheme 11-1 we are also interested to know whether the variation of K observed with 18-, 21-, and 24-membered crown ethers is due to changes in the complexation rate (k ), the decomplexation rate (k- ), or both. Krane and Skjetne (1980) carried out dynamic 13C NMR studies of complexes of the 4-toluenediazo-nium ion with 18-crown-6, 21-crown-7, and 24-crown-8 in dichlorofluoromethane. They determined the decomplexation rate (k- ) and the free energy of activation for decomplexation (AG i). From the values of k i obtained by Krane and Skjetne and the equilibrium constants K of Nakazumi et al. (1983), k can be calculated. The results show that the complexation rate (kx) does not change much with the size of the macrocycle, that it is most likely diffusion-controlled, and that the large equilibrium constant K of 21-crown-7 is due to the decomplexation rate constant k i being lower than those for the 18- and 24-membered crown ethers. Izatt et al. (1991) published a comprehensive review of K, k, and k data for crown ethers and related hosts with metal cations, ammonium ions, diazonium ions, and related guest compounds. 

The objective of this chapter is to compile work related to the beginning of sonochemical research and its extension to the aqueous solutions of metal ions. Ultrasound propagation in aqueous salt solutions leads to the hydrolysis, reduction, complexation, decomplexation and crystallization. Such works from different laboratories, along with the effect of dissolved gases on the production of free radicals in water and aqueous solutions upon sonication has been reviewed in this chapter. The generation of turbidity, due to the formation of metal hydroxides and changes in the conductivity of these aqueous solutions, carried out in this laboratory, has also been reported, to give firsthand information of the ultrasound interaction of these solutions. 

Decomplexation of some metal complexes calls for drastic conditions. This is true for (/7-arene)(/7-cyclopentadienyl)iron(II) hexafluorophosphates, [FeAr(Cp)][PF6] [54, 55]. Although their chemical decomplexation is known [55 a], the most widely used method is pyrolytic sublimation at high temperatures (>200 °C) [55 b]. To evaluate MW irradiation as the method of decomplexation of such iron complexes, Roberts et al. performed the reaction in the presence of graphite [54]. They discovered that the yield of the free ligand from the [Fe( -N-phenylcarbazole)( -Cp)][PF6] complex (43) depended on the kind (flakes or powder) and amount of graphite, and on the irradia-... 

Tungsten(O) pentacarbonyl-methylene chloride complex and alkynes form vinylidene complexes by photo-irradiation, which react with an imine or a dialkylcarbodimide to afford /3-lactams after decomplexation of the metal.233... 

Sheridan and co-workers reported a novel photo-assisted [5 + 2 + 2]-reaction based on the reactions of 77S-cyclodienyl Mn complexes160,161 or Cr complexes162 and two alkynes. Decomplexation of the metal gives cycloadducts in moderate to good overall yields (Scheme 66, Equation (42), and Scheme 67). It should be noted that the authors refer to this reaction as a [5 + 2]-, [3 + 2]- or a [5 + 2]-, homo-[5 + 2]-reaction. This reaction leads to the formation of impressively complex tricyclic products that would be otherwise difficult to prepare with step economy. 

When the reaction is conducted in the presence of added fumarate, the yield of pyrrolidine (130) increases at the expense of the aziridine. Jacobsen suggests that the aziridines and pyrrolidines arise from a common intermediate, azo-methine ylide (132), Scheme 6, which may also be partly responsible for the poor levels of asymmetric induction in this reaction. Electrocyclic ring closure of the azo-methine while still within the coordination sphere of the metal (131) may provide aziridine with some induction, while decomplexation (132) will lead to the formation of racemic aziridine and pyrrolidine. 

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