Ethanol olefin elimination

A relatively weak rate enhancement was observed in the biphasic hydroformylation of 1-octene using Rh/tppts catalysts in the presence of cosolvents such as ethanol to enhance the solubility of the olefin in the aqueous phase and with addition of buffers (Na2C03/NaHC03) to eliminate side reactions such as the formation of acetals.31,365,366 Similarly, addition of ethanol improved the yields in the hydroformylation of 1-octene catalysed by Rh2(p-S-tBu)2(CO)2(tppts)2 species in an aqueous/organic two phase system.367 However, the high selectivity to linear aldehyde observed for neat olefin in the biphasic system (97%) decreased (to 83%) in the presence of the cosolvent.367... 

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-... 

When primary alkyl phenyl tellurium or secondary alkyl phenyl tellurium compounds in methanol were treated with an excess of 3-chloroperoxybenzoic acid at 20, the phenyltelluro group was eliminated and replaced by a methoxy group. This reaction, which converts alkyl halides used in the synthesis of alkyl phenyl telluriums to alkyl methyl ethers, produced the ethers in yields as high as 90%3-4 Olefins are by-products in these reactions4 With ethanol as the solvent, ethyl ethers were formed. Other oxidizing agents (hydrogen peroxide, ozone, (ert.-butyl hydroperoxide, sodium periodate) did not produce alkyl methyl ethers. 

In 1968 Peterson published his results about the reaction of a-silylated carbanions with carbonyl compounds to achieve /3-hydroxyalkyl-silanes. He found that instantaneous elimination affords unsaturated hydrocarbons ( Peterson olefination ) ° Thus, the lithiated trimethyl-benzyl-silane (274) plus benzophenone (135) give via lithium l,l,2,-triphenyl-2-TMS-ethanolate (275) 1,1,2-triphenyl-ethene (276) ° ... 

The relative partitioning of the carbonium ion between hydration (i.e. exchange) and elimination to olefin is obviously closely related to the proportion of olefin formed in the SN1 hydrolysis of related alkyl halides. The value of 2-8% olefin formation in water at 25°C found on the basis of the exchange experiments agrees very satisfactorily with the value of 3 % found in the solvolysis of t-butyl bromide in water (as calculated by extrapolation from aqueous ethanol solutions). 

As the name suggests, the reverse Cope elimination reaction involves formation of a tertiary amine oxide from a hydroxylamine and olefin.17,18 For example, reaction of N-methylhydroxylamine (49) with 2,2-diphenyl-4-pentenal (50) in ethanol at room temperature gave amine oxide 51 in 51% yield, together with the expected nitrone 52. It has been suggested the mechanism of the reverse Cope reaction could be a radical chain reaction19,20 however, more recent studies have confirmed the mechanism is analogous to the concerted Cope elimination.17... 

Olefin synthesis. The preparation of olefins by decomposition of alkyl phenyl selenoxides (Diphenyl diselenide, 5, 272-276) is very useful, except in the case of primary alkyl phenyl selenoxides, which usually give low yields of terminal olefins on decomposition. However, the presence of electron-withdrawing substituents on the benzene ring increases both the rate of elimination and the yield of olefins. In instances where use of diphenyl diselenide results in low yields. Sharpless and Young recommend use of o-nitrophenyl selenocyanate (1) or 4,4 -dichlorodiphenyl diselenide, both of which are converted into the corresponding ArSe Na" reagents on reduction with sodium borohydride in ethanol. 

In ethanol containing ethoxide, the threo and erythro isomers of 1,2-diphenyl-1-propyl halides or trimethylammonium ion react in a similar stereochemical fashion , but in r-butyl alcohol containing r-butoxide, both the ammonium salts give the tra/i -olefin under kinetically controlled conditions . As both isomers have similar thermodynamic stabilities and react at similar rates, the anticipated comparable energies of the transition states have been attributed to the possible incursion of the ElcB mechanism . This seems unlikely as in the analogous elimination from the 2-phenylethyltrimethyl-... 

A change to a less basic solvent can influence the stereochemistry of unimolecular elimination. In nitrobenzene or acetic acid, erythro-3-deutcro-2-butyl tosylate yields deuterium-free m-2-butene and monodeuterated /ra 5-2-butene, suggestive of a jy/i-elimination (104). In more basic solvents, such as water, acetamide and 80% aqueous ethanol, the labelling of olefins is reversed and a/i/i-elimination is followed. The change in stereochemistry is attributed to the relative basicities of the tosylate anion and solvent and hence the tendencies of these species to accept a beta proton from the carbonium ion (105). 

Phenyl acetylene and acetophenone are also the reaction products observed when /3-bromostyrene is solvolysed in aqueous ethanol at 100°C . This reaction is also strongly accelerated by electron-releasing aromatic substituents but electron-withdrawing substituents render the substrates almost inert. The possibility that the ketone arises from the addition of water to the olefin followed by an Eco reaction (see Section 4.3) seems unlikely, as the rate of solvolysis is not influenced by varying the concentration of added triethylamine although this reagent induces bimolecular elimination from the strongly acidic p-nitro substrate. Moreover, as silver-ion catalysis is observed, it seems that the mechanism is of the El type, (214). 

Lead-Peterson olefination is achieved similarly to the germanium and tin versions, though the lead version is not advantageous over the latter reactions in most cases. A much simpler and more gentle elimination method applicable to the lead compounds is to pass the solution of the bis(triphenylplumbyl)ethanol derivative 249, which is synthesized from triplumbylmethane 248 and benzaldehyde, through a silica gel column (Scheme 2.152) [399]. 

---