As O-nucleophiles

Hydroxylamine can act as either a N-nucleophile or O-nucleophile, depending on which of the reactive centers is protected. For all reactions Ph-Pybox has been used as ligand, and moderate to high levels of selectivity have been achieved. Hydroxamic acid derivatives and oximes have also been probed as O-nucleophiles [63]. 

Radical cations of 2-alkylidene-l,3-dithianes can be generated electrochemically by anodic oxidation using a reticulated vitreous carbon (RVC) anode <2002TL7159>. These intermediates readily react with nucleophiles at C-1. Upon removal of the second electron, the sulfur-stabilized cations were trapped by nucleophilic solvents, such as MeOH, to furnish the final cycloaddition products. Hydroxy groups <20010L1729> and secondary amides <2005OL3553> were employed as O-nucleophiles and enol ethers as C-nucleophiles (Scheme 50) <2002JA10101>. 

It is well known that enolates act as O-nucleophiles in Sisr -type cyclizations to furnish alkylidene tetrahydrofurans (Equation 72) <2004JOC6715>. A double intramolecular Sn reaction with O-nucleophiles to yield bis-tetrahydro-furan cores of acetogenins has been achieved <1999JOC2259>. 

SCHEME 1.185 Ketoximes as O-nucleophiles in the reaction with ethoxyacetylene. 

SCHEME 1.190 Addition of acetophenone oxime as O-nucleophile to dimethyl acetylenedicarboxylate. 

The study of the chemistry of carbonyl compounds has shown that they can act as carbon nucleophiles in the presence of acid catalysts as well as bases. The nucleophilic reactivity of carbonyl compounds in acidic solution is due to the presence of the enol tautomer. Enolization in acidic solution is catalyzed by O-protonation. Subsequent deprotonation at carbon gives the enol ... 

Pd(Ph3P)4, dimedone, THF, 88-95% yield. The catalyst is not poisoned by the presence of thioethers such as methionine. Diethyl malonate has also been used as a nucleophile to trap the TT-allylpalladium intermediate and regenerate Pd(O)." ... 

The o. o. o -trihaloketone 7 can further react with the hydroxide present in the reaction mixture. The hydroxide anion adds as a nucleophile to the carbonyl carbon the tetravalent intermediate suffers a carbon-carbon bond cleavage ... 

The cyclization of o-substituted amides 206 was used for the preparation of a series of purine derivatives 207. In this case, the amine behaved as a nucleophile toward the amide function followed by ring closure to the imidazole ring (Scheme 75) [133]. 

Another consequence of application of organic solvents as a reaction medium is associated with the mechanism of enzyme-catalyzed transformations. According to the commonly accepted mechanism, the first product of the interaction of a hydrolytic (serine) enzyme with an ester is an O-acylenzyme (Scheme 5.4). When the reaction is performed in an aqueous solution, water acts as a nucleophile in the next step, to give acid B. If more nucleophilic hydrogen peroxide is present in the reaction mixture, peroxycarboxylic acids C are formed. However, in organic solvents the O-acylenzyme also reacts readily with other nucleophiles, such as... 

The kinetics of formation of phosphonates by reaction of o-dinitrobenzene with phosphites have been examined. The energy of activation for the reaction increases as the nucleophilicity of the phosphite decreases, e.g. ethyl diphenylphosphinite 14kcalmol"S diethyl phenylphosphonite 16 kcalmol S and triethyl phosphite 21kcalmol . An intermediate of the type (61), formed by nucleophilic attack of the phosphite, was proposed. In (61) there is a particularly favourable electrostatic interaction. That p-dinitrobenzene is unreactive, is thought to stem from the fact that this compound cannot form an intermediate with such a stabilizing factor. 

They demonstrated that electron-deficient R groups and electron-rich R substituents at S accelerated the reductive elimination. They proposed 123 (Lj = DPPE, R = Ph, R = Ar) as a transition state, where R acts as an electrophile and thiolate as a nucleophile. The Hammet plot for the reductive elimination showed that the resonance effect of the substituent in R determines the inductive effect of the R group, and the effect in SR showed an acceptable linear relationship with the standard o-values. The relative rate for sulfide elimination as a function of the hybrid valence configuration of the carbon center bonded to palladium followed the trend sp > sp spl... 

Nickel(O) complexes are extremely effective for the dimerization and oligomerization of conjugated dienes [8,9]. Two molecules of 1,3-butadiene readily undergo oxidative cyclization with a Ni(0) metal to form bis-allylnickel species. Palladium(O) complexes also form bis-allylpalladium species of structural similarity (Scheme 2). The bis-allylpalladium complexes show amphiphilic reactivity and serve as an allyl cation equivalent in the presence of appropriate nucleophiles, and also serve as an allyl anion equivalent in the presence of appropriate electrophiles. Characteristically, the bis-allylnickel species is known to date only as a nucleophile toward carbonyl compounds (Eq. 1) [10,11],... 

Recently, the solvolyses of l-chloro-l,3,3-triarylallenes 10 (andof 1-butyl-3,3-diphenyl-allenyl chloride) were carried out in the presence of thiocyanate and o-ethyl dithiocar-bonate anions as nucleophiles and found to give the corresponding allenyl derivatives 11 and 12 in good yield (equation 3)18. However, when potassium cyanate was used as a nucleophile, the cyanate ion attacked at the /-position to give the propargyl amines 14 after decarboxylation of the unstable intermediate 13 (equation 4). 

Fig. 23 Entropy effects on intramolecular reactions of polymethylene chains. Plot of 9AS (e.u.) against number of single bonds for (O) nucleophilic substitutions at saturated carbon ( ) electron-exchange reactions (A) quenching of benzophenone phosphorescence. The straight line has intercept +30 e.u. and slope —4.0 e.u. per rotor. The right-hand ordinate reports the purely entropic EM s calculated as exp(0AS /J )...

Structure B is of most interest. It is responsible for the activity of nitronates as 1,3-dipoles in [3+ 2]-cycloaddition reactions. This is the most important aspect of the reactivity of nitronates determining the significance of these compounds in organic synthesis (see e.g., Ref. 267). In addition, this structure suggests that nitronates can show both, O -nucleophilic properties, that is, react at the oxygen atom with electrophiles, and a-C-electrophilic properties, that is, add nucleophiles at the a-carbon atom. 

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