Double bond nucleophilicity

Vinylation of dienes in the presence of piperidine or morpholine yields aminodienes as major products. Sometimes trienes are minor products. The reaction is believed to proceed by way of a ir-allylpal-ladium complex formed by addition of the vinylpalladium halide to the least-substituted diene double bond. Nucleophilic attack of the amine upon the ir-allylic complex gives the aminodienes, while hydridopalladium halide elimination yields trienes (Scheme 6).97... 

Palladium coordinates to one face of the diene promoting intramolecular attack by the alcohol on the opposite face. The resulting <7-alkyl palladium can form a 71-allyl complex with the palladium on the lower face simply by sliding along to interact with the double bond. Nucleophilic attack of chloride from the lithium salt then proceeds in the usual way on the face opposite palladium. The overall addition to the diene is therefore cis. 

Because the elementary reactions of cationic alkene polymerizations are directly related to the organic chemistry of carbocations, Chapter 2 will investigate electrophilic additions to double bonds, nucleophilic substitution, electrophilic aromatic substitution, and elimination reactions. 

Palladium(II) catalysts have been found to be more effective than platinum(II), presumably owing to the lesser stability of the olefin complexes of the former resulting from minimized back-bonding in palladium. By reducing the electron density around the double bond, nucleophilic attack on the double bond is facilitated. 

These were readily converted into (Z)-keteneaminothioacetals by selective ds-de-protonation with t-BuLi followed by S-aUylation, which proceeded with retention of configuration. The [3,3] sigmatropic shifts of the N,S-ketene acetals occurred smoothly with excellent dia- and enantioselectivities (Scheme 9.25). These highly diastereoselective transformations are assumed to proceed under orbital rather than steric control. The postulated transition state is a pseudocycHc chair, where the best donor substituent of the sulfinyl group - the lone pair - is oriented anti to the attack of the allylic chain (electrophile) towards the N,S-ketene acetal double bond (nucleophile). The oxygen atom occupies the inside position. 

Nucleophilic substitution, electrophilic addition to the double bond, nucleophilic addition to the double bond in which atoms differ in their electronegativity, intramolecular conversions, and reduction are the most important reactions that may result in the appearance of asymmetric carbon atoms [41]. Unfortunately,... 

This thesis has been completely devoted to catalysis by relatively hard catalysts. When aiming at the catalysis of Diels-Alder reactions, soft catalysts are not an option. Soft catalysts tend to coordinate directly to the carbon - carbon double bonds of diene and dienophile, leading to an activation towards nucleophilic attack rather than to a Diels-Alder reaction . This is unfortunate, since in water, catalysis by hard catalysts suffers from a number of intrinsic disadvantages, which are absent for soft catalysts. 

Two efficient syntheses of strained cyclophanes indicate the synthetic potential of allyl or benzyl sulfide intermediates, in which the combined nucleophilicity and redox activity of the sulfur atom can be used. The dibenzylic sulfides from xylylene dihalides and -dithiols can be methylated with dimethoxycarbenium tetrafiuoroborate (H. Meerwein, 1960 R.F. Borch, 1968, 1969 from trimethyl orthoformate and BFj, 3 4). The sulfonium salts are deprotonated and rearrange to methyl sulfides (Stevens rearrangement). Repeated methylation and Hofmann elimination yields double bonds (R.H. Mitchell, 1974). 

Epoxide opening with nucleophiles occurs at the less substituted carbon atom of the oxlrane ting. Cataiytic hydrogenolysis yields the more substituted alcohol. The scheme below contains also an example for trons-dibromination of a C—C double bond followed by dehy-drobromination with strong base for overall conversion into a conjugated diene. The bicycKc tetraene then isomerizes spontaneously to the aromatic l,6-oxido[l0]annulene (E. Vogel, 1964). 

Glycosidic thiol groups can be introduced into glycosyl bromides by successive reactions with thiourea and aqueous sodium disulfite (D. Horton, 1963 M. Cemy, 1961, 1963). Such thiols are excellent nucleophiles in weakly basic media and add to electrophilic double bonds, e.g., of maleic esters, to give Michael adducts in high yields. Several chiral amphiphiles have thus been prepared without any need for chromatography (J.-H. Fuhrhop, 1986 A). 

Nucleophilic addition of 2-aminothiazole to the double bond of di-maleic acid hydrazine has been reported (206). No spectroscopic proof, however, is given to establish the proposed structure (60) for the resulting product (Scheme 41). 

Addition of water to the double bond of an alkene takes place in aqueous acid Addition occurs according to Mar kovnikov s rule A carbocation is an in termediate and is captured by a mole cule of water acting as a nucleophile... 

As an electron withdrawing group on a carbon-carbon double bond a carbonyl group renders the double bond susceptible to nucleophilic attack... 

Normally carbon-carbon double bonds are attacked by electrophiles a carbon-carbon double bond that is conjugated to a carbonyl group is attacked by nucleophiles... 

Writing the bromine addition step m this way emphasizes the increased nucleophilicity of the enol double bond and identifies the source of that increased nucleophilicity as the enolic oxygen... 

With certain other nucleophiles addition takes place at the carbon-carbon double bond rather than at the carbonyl group Such reactions proceed via enol intermediates and are described as conjugate addition ox 1 4 addition reactions... 

Ordinarily nucleophilic addition to the carbon-carbon double bond of an alkene is very rare It occurs with a p unsaturated carbonyl compounds because the carbanion that results IS an enolate which is more stable than a simple alkyl anion... 

FIGURE 18 7 Nucleophilic addition to a p unsaturated aldehydes and ketones may take place either in a 1 2 or 1 4 manner Direct addition (1 2) occurs faster than conjugate addition (1 4) but gives a less stable product The product of 1 4 addition retains the carbon-oxygen double bond which is in general stronger than a carbon-carbon double bond... 

The carbon-nitrogen triple bond of nitriles is much less reactive toward nucleophilic addition than is the carbon-oxygen double bond of aldehydes and ketones Strongly basic nucleophiles such as Gngnard reagents however do react with nitriles in a reaction that IS of synthetic value... 

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