Elimination reactions, promotion

The addition of two o-methyl groups to ring-substituted cumyl derivatives to give derivatives (28) causes only a modest change (< 5-fold) in feobs for reaction in 50 vol.% CF3CH2OH-H2O 2,6-Me2/2,6-H2 rate ratios for X = MeO, Me, and H are 0.63, 1.7, and 4.4, respectively. However, for the sterically hindered cation (29, X = Me) there is a 70-fold decrease in and a 60-fold increase in fee the product rate-constant ratios for partitioning of (29) between substitution and elimination reactions promoted by solvent (feg and fee) and by azide ion and fes, respectively) have also been discussed. 

Removal of the Fmoc group initially involves an elimination reaction promoted by piperidine to form the carbamic acid derivative of the amino acid and 9-methyhdenefluorene, which reacts further with piperidine by nucleophilic addition to form the byproduct. Spontaneous decarboxylation of the carbamic acid generates CO2 and the fiee amino acid. 

N,N,N, N -tetramethyl-l,8,-naph-thalenediamiDe M.P. 51 C. A remarkably strong mono-acidic base (pKg 12-3) which is almost completely non-nucleophilic and valuable for promoting organic elimination reactions (e.g. of alkyl halides to alkenes) without substitution. 

It is possible to take advantage of the differing characteristics of the periphery and the interior to promote chemical reactions. For example, a dendrimer having a non-polar aliphatic periphery with highly polar inner branches can be used to catalyse unimolecular elimination reactions in tertiary alkyl halides in a non-polar aliphatic solvent. This works because the alkyl halide has some polarity, so become relatively concentrated within the polar branches of the dendrimer. This polar medium favours the formation of polar transition states and intermediates, and allows some free alkene to be formed. This, being nonpolar, is expelled from the polar region, and moves out of the dendrimer and into the non-polar solvent. This is a highly efficient process, and the elimination reaction can be driven to completion with only 0.01 % by mass of a dendrimer in the reaction mixture in the presence of an auxiliary base such as potassium carbonate. 

The mechanistic possibilities for the imine formation by a base-promoted 1,2-elimination reaction are shown in Scheme 3. 

More O Ferrall diagram for base-promoted, imine forming 1,2-elimination reactions. 

Aldehydes can be obtained by reaction of Grignard reagents with triethyl orthoformate. The addition step is preceded by elimination of one of the alkoxy groups to generate an electrophilic oxonium ion. The elimination is promoted by the magnesium ion acting as a Lewis acid.93 The acetals formed by the addition are stable to the reaction conditions, but are hydrolyzed to aldehydes by aqueous acid. 

Both phase transfer and crown ether catalysis have been used to promote a-elimination reactions of chloroform and other haloalkanes.153 The carbene can be trapped by alkenes to form dichlorocyclopropanes. 

The elimination is promoted by oxidation of the addition product to the selenoxide by f-butyl hydroperoxide. The regioselectivity in this reaction is such that the hydroxy group becomes bound at the more-substituted end of the carbon-carbon double bond. The regioselectivity of the addition step follows Markovnikov s rule with PhSe+ acting as the electrophile. The elimination step specifically proceeds away from the oxygen functionality. 

Such nucleophilic displacements are likely to be addition-elimination reactions, whether or not radical anions are also interposed as intermediates. The addition of methoxide ion to 2-nitrofuran in methanol or dimethyl sulfoxide affords a deep red salt of the anion 69 PMR shows the 5-proton has the greatest upfield shift, the 3- and 4-protons remaining vinylic in type.18 7 The similar additions in the thiophene series are less complete, presumably because oxygen is relatively electronegative and the furan aromaticity relatively low. Additional electronegative substituents increase the rate of addition and a second nitro group makes it necessary to use stopped flow techniques of rate measurement.141 In contrast, one acyl group (benzoyl or carboxy) does not stabilize an addition product and seldom promotes nucleophilic substitution by weaker nucleophiles such as ammonia. Whereas... 

The results of a thorough study of the kinetics, products and stereochemical course for the nucleophilic substitution and elimination reactions of ring-substituted 9-(l-Y-ethyl)fluorenes ([31]-Y, Y = Br, I, brosylate) have been reported (Scheme 19).121,122. The reactions of the halides [31]-Br and [31]-I were proposed to proceed exclusively by a solvent-promoted ElcB reaction or an E2 reaction with a large component of hydron transfer in the transition state .122... 

Additions to quinoline derivatives also continued to be reported last year. Chiral dihydroquinoline-2-nitriles 55 were prepared in up to 91% ee via a catalytic, asymmetric Reissert-type reaction promoted by a Lewis acid-Lewis base bifunctional catalyst. The dihydroquinoline-2-nitrile derivatives can be converted to tetrahydroquinoline-2-carboxylates without any loss of enantiomeric purity <00JA6327>. In addition the cyanomethyl group was introduced selectively at the C2-position of quinoline derivatives by reaction of trimethylsilylacetonitrile with quinolinium methiodides in the presence of CsF <00JOC907>. The reaction of quinolylmethyl and l-(quinolyl)ethylacetates with dimethylmalonate anion in the presence of Pd(0) was reported. Products of nucleophilic substitution and elimination and reduction products were obtained . Pyridoquinolines were prepared in one step from quinolines and 6-substituted quinolines under Friedel-Crafts conditions <00JCS(P1)2898>. 

The elimination from the zirconium alkoxide B (Scheme 8.23) to give the 1,4-diene also proceeds through cationic activation. An independently prepared sample of pure B (X = Cl) would not undergo elimination unless a catalytic amount of AgC104 (or TMSC104, which is the probable chain carrier in this elimination reaction) was added. If AgAsF6 is used as the promoter for the reaction sequence, only the first (addition) step takes place and no elimination to the diene is observed [51],... 

The structure of the substrate also plays an important role. The elimination reaction of compounds [174] in t-butyl alcohol promoted by KOt-Bu at 30°C... 

Addition to Double Bonds and Elimination Reactions. Both Lewis and Br nsted acidity of mineral surfaces can promote addition and elimination reactions (37). Equation 14 shows an example for an addition/elimination equilibrium catalyzed by Br nsted acidity ... 

In a recent paper, Lin and Saunders (1994) reported experiments aimed at identifying the structural factors that promote tunnelling in E2 elimination reactions. In that investigation, the alkyl group of the doubly labelled 2-arylethyl substrates, [22]—[24], were varied while the leaving group was kept constant. 

The difficulty of distinguishing mechanisms at the ElcB-E2 borderline has also been discussed for reactions of secondary halides (1-X and 2-X) which feature a f-hydrogen made acidic by incorporation of an a-indenyl substituent (Scheme 1). 1,2-Elimination reactions of (/f,5 )-l-(l-X-ethyl)indene (1-X, X = Cl, Br, I, OBs) and the corresponding R,R isomers (2-X) promoted by water containing 25 vol.% acetonitrile... 

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