Olefination base-catalyzed elimination

Conversion of sugar phenylhydrazones into olefinic azo-sugars on treatment with acetic anhydride and pyridine was shown by Wolfrom and co-workers (28) (The acetylated forms of the acyclic phenylhydrazones of D-glucose, D-mannose, and D-galactose readily lose the elements of acetic acid to yield 1-phenylazo-frans-l-hexenetetrol tetraacetate when treated with warm aqueous ethanol (28, 30). It is assumed that atmospheric oxygen partakes in this elimination reaction.) This is a special case of base catalyzed -elimination reactions of the type proposed by Isbell in 1943 ( 31), involving consecutive electron displacement (which actu-... 

If substituent R has no stabilization effect on carbanion 6, there exists a possibility of isolating /f-hydroxysilanes 2. An olefin can be formed from them both by acid as well as base catalyzed elimination. 

In the past the products from the addition of dinitrogen totroxide to olefins have been hydrolyzed and then converted to 1-nitro olefins by various methods, e.g., acetylation of the isolated nitro alcohol and elimination of acetic acid with potassium carbonate,5 6 dehydration of the nitro alcohol with phthalic anhydride7 or potassium hydrogen sulfate,8 and base-catalyzed elimination of nitrous and nitric acid from dinitro compounds and nitro nitrates, respectively.2 Besides representing longer syntheses, these routes require separation of the... 

Nitromercuration of olefins followed by base catalyzed elimination of mercury gives the product of olefinlc nitration. 1-Nitrocyclohexene obtained by this scheme readily undergoes conjugate addition of nucleophiles and serves as a dlenophile. 

Such reactions are formally the reverse of the alkylation of cobalt(I) nucleophiles by suitably activated olefins (Eqn. 8). Indeed, Schrauzer et al. [53] have presented spectroscopic as well as other evidence that for cobaloximes where X = —CN or —COOCH2CH3 these reversible reactions proceed via intermediate formation of a cobaloxime(I)-olefin w complex, i.e. the microscopic reverse of Eqns. 10 and 11. However, Barnett et al. [73] have studied the kinetics of the analogous base-catalyzed elimination of 2-cyanoethylcobalamin to produce cob(I)alamin and acrylonitrile. These authors found no general base catalysis and a rate law which was first order in organocobalamin and first order in hydroxide ion and determined a second-order rate constant of 230/M/min. As these authors pointed out, this rate constant is several orders of magnitude greater than the second-order rate constant for ionization of acetonitrile so that the mechanism must either by a concerted E2 elimination (or possibly direct elimination of hydridocobalamin) or, if stepwise, the rate of /8-proton dissociation must be substantially enhanced by the cobalt-containing substituent. 

We have previously seen (Scheme 2.4, entry 5) that the dehydrohalogenation of alkyl halides is a stereospecific reaction requiring an anti orientation of the proton and halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.5, entry 1), in the sense that the more stable olefin is formed preferentially. Base-catalyzed elimination of 2-iodobutane affords three times as much tran5-2-butene as cw-2-butene. 

It was demonstrated some years ago that crown ethers complex cations and enhance the specific activity of the associated anions [27—34]. Considerable attention has been given to base catalyzed elimination reactions where the presence of crowns significantly affects the association of the base in solution and the differences in base species is reflected not only in the rates of elimination, but in the geometries and orientation of resulting olefins [35]. 

Solvent for Base-Catalyzed Reactions. The abihty of hydroxide or alkoxide ions to remove protons is enhanced by DMSO instead of water or alcohols (91). The equiUbrium change is also accompanied by a rate increase of 10 or more (92). Thus, reactions in which proton removal is rate-determining are favorably accompHshed in DMSO. These include olefin isomerizations, elimination reactions to produce olefins, racemizations, and H—D exchange reactions. 

Two disadvantages are associated with the use of S-acetyl or 5-benzoyl derivatives in peptide syntheses (a) base-catalyzed hydrolysis of 5-acetyl- and 5-benzoylcys-teine occurs with /S-elimination to give olefinic side products, CH2=C-(NHPG)CO—(b) the yields of peptides formed by coupling an unprotected amino group in an 5-acylcysteine are low because of prior S-N acyl migration. ... 

In the present chapter, no explicit discussion or review of the acid-and/or base-catalyzed isomerization of olefins will be included. The discussion will be confined to isomerizations achieved with soluble transition metal complexes. However, it will be seen that addition and elimination reactions and allylic intermediates figure prominently in discussions of the mechanisms. 

Since the basic or carbanion intermediate can continue to go to product by Steps 2 and 3, we have a chain reaction which is consistent with the rapid isomerizations which may be obtained using these catalysts. This mechanistic interpretation was proposed in one of the first papers published on this subject (5) it and similar interpretations have been very helpful in bringing about an understanding of base-catalyzed reactions. The chain-reaction sequence may be terminated by reaction with a formation of a material which is not basic enough to metallate the olefin. Such compounds may be polyunsaturated hydrocarbons which may be formed by elimination of hydride ions from a carbanion. 

R = R = Me) which are obtained by photooxidation of the corresponding olefin.72 The acid-induced cyclization of the same alcohols has been used they are prepared from coumarins 87,88,185,186 or by base-promoted elimination from epoxides (50).150,164 The acid-catalyzed equilibrium between these alcohols and the corresponding... 

From this, it can be seen that the amount of KOH within the hydroxide mixture would probably be critical in removing organo-sulfur from coal. While the particular role of KOH has not been determined, evidence from the literature has shown that the size of the cation may be important in stabilizing intermediate carbanions. Wallace et al. (J ) conducted a series of base- catalyzed, beta-elimination reactions with isopropyl sulfide and measured the amount of olefin production. The proposed mechanism involved initial abstraction of a proton by the t-butoxide base, and formation of a carbanion, with subsequent elimination of the sulfur moiety (which can be considered a good leaving group) to form the olefin (Equation 5). 

On the basis of this palladium-mediated Michael addition cyclization process, a novel two-step synthetic entry into functionalized furan derivatives 67 has also been devised (Scheme 28). Substitution of benzylidene (or alkyli-dene) malonates for their ethoxymethylene analog (65) as activating olefins gave rise to the formation of the corresponding 2-ethoxy-4-arylidene tetrahy-drofurans 66. An in situ addition of potassium ferf-buloxidc induced a decar-boxylative elimination reaction which was followed by an isomerization of the exocyclic double bond. The entire process successively involved a conjugate addition, a palladium-catalyzed cyclization-coupling reaction, a base-induced eliminative decarboxylation, and finally, a double bond isomerization [73]. 

Another 8-elimination to give an olefin observed by Schriesheim et al. is the base-catalyzed decomposition of aliphatic sulfoxides and sulfones. With potassium ( butoxide in DMSO the reaction proceeds easily at 55° ... 

Aziridines2 )3-Iodoazides, available by the addition of iodine azide to olefins, undergo reduction of the azide function followed by base-catalyzed ring closure to aziridines. The most satisfactory reagent for this purpose is lithium aluminum hydride, which can accomplish both reaction steps since it is a Lewis acid as well as a reducing agent. Competing side reactions are elimination of the elements... 

The various methods of generating o-quinone methides,4-5 including the thermal or (Lewis) acid-catalyzed elimination of a phenol Mannich base,149 150-160-161163 the thermal or (Lewis) acid-catalyzed dehydration of an o-hydroxybenzyl alcohol (ether),147-149-151-153-156-157-162-163-165-168 171-175-178-183 the thermal 1,5-hydride shift of an o-hydroxy styrene,171-173 175 178-183 the thermal dissociation of the corresponding spirochromane dimer,158 163-164,166 oxidation of substituted o-alkylphenols,152-170 and the thermal or photochemical-promoted cheletropic extrusion154-155 159 of carbon monoxide, carbon dioxide, or sulfur dioxide (Scheme 7-III), as well as their subsequent in situ participation in regiospecific, intermolecular [4 + 2] cycloadditions with simple olefins and acetylenes,147 149-151 152 153159 162-164... 

Acetylenic aldehydes 96 have been utilized as electrophiles in MBH reactions to give allyl propargyl alcohols 97 in moderate to good yields. Aldehydes 98, as a masked formybutadiene, react with activated olefins to give the eorre-sponding MBH adducts 99 via a tandem sequence base-induced elimination-MBH reaction catalyzed by DABCO (Seheme 1.47). ° ... 

The problem of directing Wittig and Horner reactions to both the ( )- and the (Z)-olefins has found a variety of solutions. One is the use of phosphine oxides as the donor component (Scheme 3.85) [137]. Thus, 445 was deprotonated and then treated with acetaldehyde to give the erythro-adduct 446 with d.r. 3 1. After separation, base-catalyzed syn-elimination of phosphinate gave (Z)-olefin 449. On the other hand, 445 was deprotonated and acylated to give ketone 447, which was then reduced with sodium borohydride to give the threo-isomer 448 with d.r. 3.5 1. Separation/syn-elimination generated the ( )-olefin 450 selectively. 

Oxidative Carbonylation of Ethylene—Elimination of Alcohol from p-Alkoxypropionates. Spectacular progress in the 1970s led to the rapid development of organotransition-metal chemistry, particularly to catalyze olefin reactions (93,94). A number of patents have been issued (28,95—97) for the oxidative carbonylation of ethylene to provide acryUc acid and esters. The procedure is based on the palladium catalyzed carbonylation of ethylene in the Hquid phase at temperatures of 50—200°C. Esters are formed when alcohols are included. Anhydrous conditions are desirable to minimize the formation of by-products including acetaldehyde and carbon dioxide (see Acetaldehyde). 

The unsubstituted quinazolidine system 5 was constructed from mesylate 173. The key feature in this synthesis is based on a cyclohydrocarbonylation of the protected 4-amino-l,6-heptadiene 169 catalyzed by Rh(acac)(CO)2-BIPHEPHOS. Formation of the hemiamidal-aldehyde 171 took place by hydroformylation of the two olefin moieties and cyclization. Elimination of water gave 172, which, after treatment with NaBFE, subsequent mesylation to 173, and catalytic hydrogenation, afforded 5 (Scheme 29) <1998TL4599>. 

Figure 1.. The two proposed reaction pathways based on experimental results for hydroboration reactions of olefins catalyzed by the Wilkinson catalyst. (O.A. Oxidative Addition Olefin Migratory Insertion R.E. Reductive Elimination)...

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