Oxidative demetalation

Arenes, on complexdQon with Cr, Fe, Mn, and so forth, acquire strongly electrophilic character, such complexes m reactions with nucleophiles behave as electrophilic tutroarenes. Synthesis of aromatic tutnles via the temporary complexanon of rutroarenes to the catiotuc cyclopentadienyhron moiety, cyarude addition, and oxidative demetalation v/ith DDQ has been reported fEq. 9.43 ... 

The use of a stereogenic carbon centre allowed an efficient asymmetric induction in the benzannulation reaction towards axial-chiral intermediates in the synthesis of configurationally stable ring-C-functionalised derivatives of al-locolchicinoids [51]. The benzannulation of carbene complex 52 with 1-pen-tyne followed by oxidative demetalation afforded a single diastereomer 53 (Scheme 33). 

Hexacarbonyldicobalt-stabilized propargyl cation is captured by a nucleophile. Subsequent oxidative demetallation then gives propargylated product. 

VII. Successful Approach to Strychnine Part 3 - Formation of the E Ring by Oxidative Demetallation/Intramolecular Conjugate Addition... 

Iron carbonyls have been used in stoichiometric and catalytic amounts for a variety of transformations in organic synthesis. For example, the isomerization of 1,4-dienes to 1,3-dienes by formation of tricarbonyl(ri4-l,3-diene)iron complexes and subsequent oxidative demetallation has been applied to the synthesis of 12-prostaglandin PGC2 [10], The photochemically induced double bond isomerization of allyl alcohols to aldehydes [11] and allylamines to enamines [12,13] can be carried out with catalytic amounts of iron carbonyls (see Section 1.4.3). 

Electron-donating substituents direct the incoming nucleophile predominantly to the meta-position and electron-withdrawing substituents to the ortho-position. Oxidative demetallation (DDQ, iodine) is applied to reoxidize the cyclohexadienyl ligand, releasing a substituted arene. Addition of nucleophiles to halobenzene-FeCp complexes leads to nucleophilic substitution of the halo substituent (Scheme 1.34). Demetallation of the product complexes is achieved by irradiation with sunlight or UV light in acetone or acetonitrile. 

Based on these reactivities various derivatives of carbenes, such as the aminocarbene 238, are prepared by displacement of the OR group in 237 with amine via addition elimination, analogous to transesterification [74,75], As an example the carbanion 240, generated by deprotonation of 239, attacks ethylene oxide to give the lactone equivalent 241, which is further alkylated by chloromethyl methyl ether, again at the -position. Finally the oc-methylene-y-lactone 242 is obtained by oxidative demetallation with a Ce(TV) salt [76],... 

I he cycloaddition reactions generally give exo adducts with 1 -unsubstituted pyrroles, but 1 -methyl and 1 -trimethylsilyl substituents favor the endo stereoisomers. Cycloaddition has been achieved with. V-phenylmaleimide, methyl acrylate, methyl crotonate, dimethyl maleate, acrylonitrile and crotonitrile. The rate of cycloaddition is increased by 2,5-dimethyl substitution and this is attributed to the affect on the equilibrium between the 2,3 and 3,4-complexes. The isolation of azanorbornenes from the adducts by oxidative demetallation frequently leads to cycloreversion but azanorbornanes can be isolated by hydrogenation immediately after oxidation. <95JA3405>... 

Treatment of epoxy-alkyne derivatives 314 with dicobalthexacarbonyl [Co2(CO)6] and a catalytic quantity of Lewis acid, followed by oxidative demetallation of the intermediate cobalt complex 315 affords 2-alkynyl-3-acetoxy tetrahydropyrans 316 in excellent yield. Syn- or tf /7-tetrahydropyrans are obtained by selection of either the anti- or syn- starting epoxides 314 respectively (Scheme 79, Table 10) <1998T823, 2000JOC6761>. 

A Friedel-Crafts type alkylation of 1,1,2,3,3-pentamethylindane with (R)- or (.S )-ructhylcpoxide followed by acid-catalyzed condensation of the resulting alcohols with paraformaldehyde affords two diastereomeric pairs of isochro-mans 586, 587 (Scheme 127) <1999HCA1656>. Separation of the enantiomers of (4.S, 7/ /.S )-isochromans 586 is achieved by flash chromatography of their corresponding tricarbonylchromium complexes and subsequent oxidative demetallation the result of this synthetic work establishes that (4,y,71J)-isochroman is found to be responsible for the intense musk odour of the perfume galaxolide (Scheme 127) <1999HCA1656>. 

A model compound from a second group of alkaloids related to colchicine (allocolchici-noid 137) has been synthesized by benzannulation of racemic benzocycloheptenyl carbene complex 135 with 1-pentyne the annulation product 136 was obtained in 48 % yield after oxidative demetalation (Scheme 47) [97]. 

Like its cyclobutadiene analog 39, the 7j4-l,3-diphosphacyclobutadiene complex 38 has high thermal stability, is stable to air, and generally has low reactivity. Reagents that normally release cyclobutadiene from transition metal-cyclobutadiene complexes by oxidative demetallation [i.e., HCN, (NH4)2Ce(N01)6, and alkali metals] do not react with compound 38. 

Fused 2//-pyran-2-ones are formed in excellent yields and under mild conditions through a Ni-catalysed [2+2+2] cycloaddition of diynes and C02 <02JA15188>. Oxidative demetalation of 0i3-allyl)Mo complexes of pyran with pyridinium dichromate (PDC) introduces a carbonyl function at the allylic terminus offering access to dihydropyranones of high enantiopurity <02JOC5773>. 

The same substrate (64) gave the complex (67) (BuLi, THF, -78°C then PhMn (CO)3.BF4 J, -78°C, 30 min 80%), and thence, by oxidative demetalation, 2-isopropyl-3,6-dimethoxy-5-phenyl-2,5-dihydropyrazine (68) [/V-bromosuc-cinimide (NBS), Et20, 20°C, 15 min 60%) also substituted-phenyl analogues likewise.169... 

A second type of mechanism would be the reductive or oxidative demetallation of Intermediate metalloporphyrlns, allowing nickel and vanadium to complex. From the electrochemistry of porphyrins, and solution studies Involving oxidizing and reducing... 

The subsequent transformation of these cycloadducts further illustrates the versatility of this approach in the construction of guaianolide sesquiterpenes (Scheme 1)7 The moiety incorporated in the adduct allows for the introduction of diverse functionalities at C-S and C-2. For example, reduction of the cycloadduct (11) with sodium borohydride produces a good yield of the iron complex (16). Oxidation of this complex with cerium ammonium nitrate gives methyl ester (17). In addition, (11) reacts with nucleophiles such as lithium dimethylmalonate and methanol to give the alkylated complex (18) and the meth-oxylated complex (19), respectively. Oxidative demetallation of (11) with bromine leads to selective replacement of the moiety at C-2 by bromide. ... 

In order for this process to be viable in mainstream organic synthesis, an efficient method for functionalization of the carbon-iron bond is necessary. Oxidative demetalation can be achieved to provide the corresponding methyl ether. 

In contrast to the behavior of free alkynes, 1,3-isomerization of alkyne complexes to produce complexed allenes [140,141) is rather rare. A synthetically useful example [140] is provided by the CpMn(CO)2 complexes of electrophilic alkynes 89 which undergo alumina-promoted isomerization to the corresponding allene derivatives 90 these, in turn, can be oxidatively demetalated to the free allenes 91 (Scheme 4-45). 

This book is a general survey of the nature of trace metals in petroleum and includes analytical methods for their determination. Vanadium, iron, nickel, cadmium, copper, and molybdenum— their nature, determination, chemical aspects, geochemistry, occurrence in new and used petroleum byproducts and their recovery for resource use—are dealt with in detail by the expert authors in this volume. Among the methods covered are instrumental analysis, neutron activation, activation analysis, oxidative demetallation and kinetic studies. Includes 54 figures and 60 tables. 

Cyclobutene-l,2-diones. The reaction of NaFeH(CO) with Mel liberates methane. The simultaneously generated FelCO) species is trapped by various alkynes to provide the cyclobutenediones (7 examples, 27-42%) after oxidative demetallation. 

The reaction of propargyl alcohols with dicobalt octacarbonyl to give the complex salts 148 (X = BF4 or PF6) and synthetic uses of the latter have been reviewed. The salts react with electron-rich aromatic compounds ArH, such as anisole, phenol or N,N-dimethylaniline, to yield substitution products 149 after oxidative demetallation with an iron(III) or cerium(I V) salt with j5-diketones or j -keto esters the corresponding propargyl-substituted compounds 150 are obtained k Acetone reacts in an analogous fashion to give 151. The action of the cobalt complexes 148 on allylsilanes 152 leads to enynes 153. Indole reacts with the complex 148 (R = H R = R = Me) in the presence of boron trifluoride etherate to give 154, which was converted into 155 by the action of iron(III) nitrate " ... 

Hydride nucleophiles can give substitution directly, via reversible addition or by an indirect (addition-protonation) mechanism. In the tetrahydronaphtha-lene complex (31), replacement of a methoxy group by hydride gave (32), using LiAlH4 followed by oxidative demetallation [53]. 

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