Oxidative decomplexation

The product distribution depends on the isocyanide used. Only with aromatic (R = o-tolyl or 2,6-xylyl), and not with aliphatic isocyanides (R = Bu , Bu or benzyl) are isonitrile insertion products 24 formed. Aryl isocyanide insertion is obviously very fast since the competing formation of 23 from CO insertion is not observed complexes 22 are only minor products. With aliphatic isocyanides, the thpp complexes 22, from double cycloaddition of dmad cf. Section 3.1.1), are the major products (70 to >95% of the product mixture) and indicate a strongly increased 1,3-dipolar reactivity, i.e. the intermolecular second cycloaddition is preferred to the intramolecular CO insertion. Compared with the ruthenium compound 17, the thpp in 22 is strongly bound to the metal and can only be decomplexed oxidatively with cerium(iv), or under 80 bar of CO. 

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. 

Free carbenes based on 1,2,4-triazole are not as numerous as those based on imidazole (70ZN(B)1421, 95AGE1021, 97JA6668, 98JA9100). The carbene complex 169 (Ar = Ph, p-Tol) is prepared by the [3 + 2] cycloaddition route from [W(CO)j(C+=NC-HCOOEt)]- and aryldiazonium (930M3241). Oxidative decomplexation causes tautomerization of the 1,2,4-triazole ligand, the products being 170 (Ar= Ph, i-Tol). 

The synthetic value of the Dotz reaction has for example been demonstrated by the synthesis of vitamin Ki(20) 10 (simplified structure). This natural product has been prepared synthetically from the chromium carbene complex 8 and the alkyne 9 in two steps the second step being the oxidative decomplexation to yield the free product 10 ... 

An analogous reaction occurs with dibenzoylacetylene, whereas the reaction fails with dimethyl acetylenedicarboxylate, benzoyl chloride and tosyl chloride. Decomplexation of compound 29 to 3-(cyclohepta-2,4,6-trienyl)-3//-azepine (73%) has been achieved with trimethylamine N-oxide. 

Tricarbonyl[t/M-(ethoxycarbonyl)-l//-azepine]iron(0) (30) with the 2-oxyallyl cation derived from 2,4-dibromo-2,4-dimethylpentan-3-one and nonacarbonyldiiron(O) yields a mixture of adducts which, after oxidative decomplexation with tetrachloro-l,2-benzoquinone (o-chloranil), affords the tetrahydrofuro[2,3-A)azcpine derivative 33 and the 3-substituted 1H-azepine-l-carboxylate 34.227... 

The isomeric pyridazino[4,5-6]azepine 19 is obtained directly during the decomplexation of the [4 + 2] adduct 17 formed from tricarbonyl(ethyl +17/-azepine-l-carboxylate)iron and 1,2,4,5-tetrazine-3,6-dicarboxylate, with trimethylamine A-oxide.113 Surprisingly, decomplexation of adduct 17 with tetrachloro-l,2-benzoquinone yields only the dihydro derivative 18 (71 %), aromatization of which is achieved in high yield with trimethylamine A-oxide in refluxing benzene. 

Oxidative decomplexation of iron acyl complexes in the presence of alcohols provides the corresponding carboxylates 7. Usual conditions employ ca. 7% alcohol in dichloromethane or dichloromethane/carbon disulfide as the solvent with bromine as the oxidant. 

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. 

Since the FeCp(C6H6)+ unit is robust towards oxidation even in concentrated sulfuric acid, oxidation of alkyl substituents upon boiling in aqueous KMn04 solution can be achieved and leads to carboxylic substituents. The mesitylene complex can be oxidized to the mono-, di-, or tri-carboxylic add depending upon the reaction conditions. In the latter case, the decomplexed trimesic acid is obtained [106, 107] Scheme XXII ... 

Extractable caffeine levels increase on storage, presumably because of decomplexation from theaflavins as the latter decrease. Since caffeine binding with thearubigens is weaker than that with theaflavins, more caffeine may become available.94 An increase in free caffeine may be responsible for decreased value because of the bitterness imparted to the beverage. Caffeine in combination with the normal tea complement of oxidized polyphenolic matter does not exhibit bitterness.95... 

Nickel and palladium complexes also catalyze the formation of the carbon-phosphorus bonds in phosphorus(V) and phosphorus(III) compounds. Indeed, this chemistry has become a common way to prepare phosphine ligands by the catalytic formation of phosphine oxides and subsequent reduction, by the formation of phosphine boranes and subsequent decomplexation, or by the formation of phosphines directly. The catalytic formation of both aryl and vinyl carbon phosphorus bonds has been accomplished. 

Dimethyl 7-10-tetrahaptotricyclo[4.2.2.02 5]deca-3,7,9-triene-7,8-dicarboxy-late tricarbonyliron reacted readily with several 1,3-dipoles nitrile oxides, at the cyclobutene double bond, to give adducts from which the tricarbonyliron group could be easily removed by oxidative decomplexation with trimethylamine N-oxide (237). 

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). 

Widdowson expanded his hexacarbonylchromium chemistry to the synthesis and lithiation of Cr(CO)3-Af-TIPS indole (29), leading to 4-iodoindole 30 after oxidative decomplexation [37]. Stannylation at C-4 could also be achieved using this method (62% yield), and comparable chemistry with 3-methoxymethylindole leading to C-4 substitution was described. 

In a different approach, Franck-Neumann et al. [24] utilized the manganese complex 14 (formed by deracemization) to obtain the enantiomerically pure target molecule 12 via Horner-Wadsworth-Emmons olefination and oxidative decomplexation of the intermediate vinylallene complex 15 (Scheme 18.6). 

The reaction of (cyclobutadiene)metal complexes with X2 results in the oxidative decomplexation to generate either dihalocyclobutenes or tetrahalocyclobutanes. In comparison, substitution of (cyclobutadiene)MLn complexes 223 [MLn = Fe(CO)3, CoCp, and RhCp] with a variety of carbon electrophiles has been observed (equation 34)15. Electrophilic acylation of 1-substituted (cyclobutadiene)Fe(CO)3 complexes gives a mixture of regioisomers predominating in the 1,3-disubstituted product and this has been utilized for the preparation of a cyclobutadiene cyclophane complex 272 (equation 35)246. For (cyclobutadiene)CoCp complexes, in which all of the ring carbons are substituted, electrophilic acylation occurs at the cyclopentadienyl ligand. 

The 774-vinylketene complex (85) could be oxidatively decomplexed with Ce(IV) to afford the lactone (87). Although no reaction was observed with methanol (unlike a postulated chromium analogue16,18 26), treatment with sodium methoxide produced the expected /3, y-unsaturated ester (88). Thermolysis of complex 85 afforded no trace of the naphthol that one would expect33 from a proposed chromium vinylketene complex with the same syn relationship between the phenyl group and the ketene moiety. Instead, only the furan (89.a) was seen. Indeed, upon exhaustive reaction of tricarbon-ylcobalt carbenes (84 and 90) with different alkynes, the furans (89.a-d) were isolated as the exclusive products in moderate to excellent yields. 

In further reactivity studies, Franck-Neumann has shown112 that a range of products such as 155 may be isolated by the trimethylamine A-oxide-promoted decomplexation of vinylketene complexes. 

Decomplexation of the vinylketene complex 185.b with trimethylamine A-oxide affords the ethyl ester 190 in 48% yield. 

Shortly after this reaction was reported, Hegedus found46 that imines undergo a similar insertion process. The products (251) are structurally very similar to the alkyne adducts (247), and decomplexation with iodine, followed by treatment with trimethylamine TV-oxide, afforded a variety of substituted pyridones in good yield. 

The Dotz benzannulation reaction yields either arene chromium tricarbonyl complexes or the decomplexed phenols, depending on the work-up conditions. Because of the instability of hydroxy-substituted arene chromium tricarbonyl complexes, yields of the latter tend to be low. High yields of arene complexes can, however, be obtained by in situ silylation of the crude product of the benzannulation reaction [336]. Oxidative work-up yields either decomplexed phenols or the corresponding quinones. Treatment of the benzannulation products with phosphines also leads to decomplexed phenols [272]. 

Lohmeijer BGG, Schubert US (2003) Water-soluble building blocks for metallo-supramo-lecular polymers synthesis, complexation and decomplexation studies of poly(ethylene-oxide) moities. Macromol Chem Phys 204 1072-1078... 

Planar chiral arene Cr(CO)3 complexes have been shown to undergo highly diastereoselective cycloadditions and Kiindig has extended this protocol to the [3+2] cycloadditions of azomethine ylides (96). Enantiopure ortho- substituted p -benzaldehyde complex 337 underwent condensation with an ot-amino ester to afford imine 338 in the presence of EtaN. Subsequent treatment with methyl acrylate at ambient temperamre in the presence of LiBr and EtaN delivered cycloadduct 339, with excellent stereoinduction and high material yield. Photoinduced oxidative decomplexation in air furnished the final arylpyrrolidines (Scheme 3.114). 

Some further examples of stereoselective deprotonation/alkylation reactions of tricarbonyl-chromium complexed (V-methyl tetrahydroisoquinolines have been reported27. Starting with the enantiomerically pure (35)-methyl tetrahydroisoquinoline reaction with hexacarbonyl-chromium led to a mixture of endo- (40%) and exo- (60%) complexes, which were deprotonated with butyllithium and subsequently methylated with iodomethane. In this way methylation occurred firstly at the 4- and secondly at the 1-position. In all cases, the methyl group entered anti to the chromium complexed face. After separation of the alkylated complexes by chromatography and oxidative decomplexation, the enantiomerically pure diastereomers (—)-(l 5,35,47 )-and ( + )-(17 ,35,45)-1,2,3,4-tetrahydro-l,2,3,4-tetramethylisoquinolme were obtained, benzylic amines such as tetrahydroisoquinoline to 2-amino-4,5-dihydrooxazoles. Deprotona... 

Oxidative decomplexation of the dialkylated organic ligand to provide a butyrolactone is accomplished by successive treatment of the cationic carbene complex with hydroxide followed by bromine96. 

A further chiral auxiliary-based tactic exploited tricarbonyl( 76-arene)chromium complexes of aromatic imines 71, which reacted under ultrasound (US) irradiation with a-bromoesters in a predictable stereochemical course to give comparable amounts of /S-aminoesters and / -lactams, as outlined in equation 44127. Chromium decomplexation is eventually achieved by photochemical oxidation under air. 

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