Hydrogenation of electron-deficient alkenes

Until now, hydrogen sources other than formates have been rarely reported in microwave-assisted transfer hydrogenations of carbon-carbon multiple bonds. An exception is a transfer hydrogenation of electron-deficient alkenes where a series of 1,4-dihydropyridines supported on silica gel were used as the hydrogen source (Scheme 4.6). The influences of electronic effects of the alkene, steric effects of the dihydropyridine and type and power of the microwave irradiation were studied24. 

Desai, B. and Danks, T.N., Thermal- and microwave-assisted hydrogenation of electron-deficient alkenes using a polymer-supported hydrogen donor, Tetrahedron Lett., 2001,42, 5963. 

Desai and Danks (2001) investigated that a combination of formate bound to an ion- exchange resin and Wilkinson s catalyst can be used in the transfer hydrogenation of electron deficient alkene. Reactions were completed upon microwave irradiation in 30 sec and the products were obtained in quantitative yields (80-95%). 

The hydroaminations of electron-deficient alkenes with aniline or small primary alkylamines proceed at high conversions (85-95%, nnder mild conditions, 5 mol%, rt), giving exclnsively the anh-Markovnikov addition product. Secondary dialkyl or bnlky primary amines require longer reaction times. With amines containing P-hydrogens, no imine side-products were observed. 

Direct phase-transfer catalysed epoxidation of electron-deficient alkenes, such as chalcones, cycloalk-2-enones and benzoquinones with hydrogen peroxide or r-butyl peroxide under basic conditions (Section 10.7) has been extended by the use of quininium and quinidinium catalysts to produce optically active oxiranes [1 — 16] the alkaloid bases are less efficient than their salts as catalysts [e.g. 8]. In addition to N-benzylquininium chloride, the binaphthyl ephedrinium salt (16 in Scheme 12.5) and the bis-cinchonidinium system (Scheme 12.12) have been used [12, 17]. Generally, the more rigid quininium systems are more effective than the ephedrinium salts. 

A bimetal redox couple, zinc/cobaloxime, promotes hydropcrfluoroalkylation of electron-deficient alkenes, such as acrylates, acrylonitrile and methyl vinyl ketone, by perfluoroalkyl iodides and bromides, hydrogen replacing iodine or bromine. A typical reaction is the formation of2. ... 

Reaction of chromic anhydride (CrOg) with t-butanol yields t-butyl hydrogen chromate, a powerful oxidant suitable for allylic oxidation of electron-deficient alkenes. Oxidations using t-BuOCr03H in CCI4 are highly exothermic and should be performed with caution. 

The epoxidation of electron deficient alkenes such as methyl methacrylate has also been carried out using reaction conditions similar to those shown in Eq. (15), and with a,p-unsaturated ketones alkaline hydrogen peroxide has been generated from UHP and affords good yields of epoxides. Pulegone gave a 50% yield of the epoxide and the result obtained with isophorone is shown in Eq. (16). 

Nucleophilic oxidation of electron-deficient alkenes is another route to epoxides. For example, reaction of enones with hydrogen peroxide and sodium hydroxide provides epoxides in good yield. The first attempt to turn this into an asymmetric transformation utilised the benzylchloride salt of quinine as a chiral phase transfer catalyst but only moderate enantioselectivity was obtained (55% with... 

Shibasaki has recently described a process for epoxidation of electron-deficient alkenes catalyzed by chiral lanthanoid-BINOL complexes (5-8 mol %) using ferf-butyl hydrogen peroxide [or cumene hydroperoxide (CMHP)] [56]. Epoxides were obtained in excellent yields and enantioselectivities as shown in Scheme 21. 

The most attractive oxidant is hydrogen peroxide being cheap and environment friendly thus it has found wide application in many processes. There are two major variants of its use under PTC conditions direct transfer of OOH anions as TAA salts into the organic phase or extraction of molecular H2O2 by hydrogen bonding to the catalyst anion. Both of these approaches assure efficient conversion of electron-deficient alkenes such as chalcones, unsaturated nitriles, or carbonyl compounds into oxiranes (eq. 173 for review see Ref. 79). 

Alkaline hydrogen peroxide is used for epoxidation of electron-deficient alkenes such as acrylic acids, and also for oxidation of alkylboranes to alcohols, the second step of hydroboration-oxidation. 

The asymmetric epoxidation of electron deficient alkenes like a,p-unsaturated esters, ketones and nitriles often is not efficient with the reagents suitable for electron rich systems. Prominent examples for the successful epoxidation of a,P-enones are the well-known Weitz-Scheffer epoxidation using alkaUne hydrogen peroxide or... 

Electron-deficient alkenes add stereospecifically to 4-hydroxy-THISs with formation of endo-cycloadducts. Only with methylvinyl-ketone considerable amounts of the exo isomer are produced (Scheme 8) (16). The adducts (6) may extrude hydrogen sulfide on heating with methoxide producing 2-pyridones. The base is unnecessary with fumaronitrile adducts. The alternative elimination of isocyanate Or sulfur may be controlled using 7 as the dipolarenOphile. The cycloaddition produces two products, 8a (R = H, R = COOMe) and 8b (R = COOMe, R =H) (Scheme 9) (17). Pyrolysis of 8b leads to extrusion of furan and isocyanate to give a thiophene. The alternative S-elimi-nation can be effected by oxidation of the adduct and subsequent pyrolysis. 

The Homer - Emmons reagent (52) is effective in the one carbon homologation of ketones possessing acidic a-hydrogen atoms <96SL875> and electron-deficient alkenes add to 2-phenylseleno-l,3-dithiane in a photo-initiated heteroatom stabilised radical atom transfer process, giving products of considerable synthetic potential <96TL2743>. 

Recently, several studies have been made of the photolysis of disilanes or polysilanes in the presence of an electron-deficient alkene using a photosensitizer (such as phenanthrene) and acetonitrile as solvent. These conditions result in the addition of silyl groups to one end of the alkene double bond and hydrogen to the other end (equation 18) and evidently involve the reaction of the radical anions of the electron-deficient silene with silyl radicals67 (see also Section VIII.A). 

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