Catalyst, nucleophilicity

That the formation of molecular complexes (especially EDA complexes) can catalyse the decomposition of the cr-adduct has been discussed in Section n.E. Another possibility is that the substrate and catalyst (nucleophile or added base) form a complex which is then attacked by a new molecule of the nucleophile in this context catalysis need no longer be associated with proton removal. Thus, Ryzhakov and collaborators183 have recently shown that the N-oxides of 4-chloropyridine and 4-chloroquinoline act as jt-donors toward tetracyanoethylene and that the reactions of these substrates with pyridine and quinoline are strongly catalysed by the jr-acceptor. Similarly, the formation of a Meisenheimer complex between 1,3,5-trinitrobenzene and l,8-diazabicyclo[5,4,0]undec-7-ene in toluene has been assumed to take place via an association complex to explain the observed second-order in tertiary amine184. 

Dicarbonates of enediols have been converted to conjugated dienes on treatment with Pd° catalysts. Nucleophilic displacement of the allyl carbonate by an V-allyl complex may be responsible (equation 122).321... 

Substitution reactions of allylic substrates with nucleophiles have been shown to be catalyzed by certain palladium complexes [2, 42], The catalytic cycle of the reactions involves Jt-allylpalladium as a key intermediate (Scheme 2-22). Oxidative addition of the allylic substrate to a palladium(o) species forms a rr-allylpal-ladium(n) complex, which undergoes attack of a nucleophile on the rr-allyl moiety to give an allylic substitution product. The substitution reactions proceed in an Sn or Sn- manner depending on catalysts, nucleophiles, and substituents on the substrates. Studies on the stereochemistry of the allylic substitution have revealed that soft carbon nucleophiles represented by sodium dimethyl malonate attack the TT-allyl carbon directly from the side opposite to the palladium (Scheme 2-23). 

In this chapter, we will look at some of the most common catalysts— nucleophilic catalysts, acid catalysts, base catalysts, and metal-ion catalysts—and the ways in which they provide an energetically more favorable pathway for an organic reaction. We will then see how the same modes of catalysis are used in enzyme-catalyzed reactions. 

Nucleophilic catalytic reactions are usually addition and substitution reactions. A diverse array of Lewis bases (e.g., tertiary phosphines, tertiary amines, pyridines, and imidazoles) have been shown to serve as nucleophilic catalysts. Nucleophilic reactions typically occur at C=X and activated C=C multiple bonds. In a general form for a reaction... 

Shimizu. S. Kito, K. Sasaki. Y. Hirai. C. Water-soluble calixarenes as new inverse phase-transfer catalysts. Nucleophilic substitution of alkyl and arylalkyl halides in aqueous media. Chem. Conimun. 1997, 1629—1630. 

Hyperbranched polyethers can be synthesized via the A2+B3 approach, when diepoxides (3-24) are reacted with triols, such as TMP. Emrick et al. used 1,2,7,8-diepoxyoctane as the A2 monomer and TMP as the B3 monomer with tetra-n-butylam-monium chloride as the nucleophilic catalyst. Nucleophilic attack of the chloride ion on an epoxide at the less-hindered terminal carbon led to the formation of secondary alkoxide. Due to the equilibrium between primary and secondary alk-oxides via proton exchange, nucleophilic attack of primary alkoxides on the epoxide rings resulted in the formation of aliphatic hyperbranched polyether. As the feed ratio of the diepoxide and TMP was varied from 1.5 to 3, the resulting hb polyether contained two types of terminal units (T), one type of dendritic unit (D), and linear units (L) as shown in Scheme 4. The polydispersity index (PDI) of the polyether increased with the increase of molecular weight (from 1.5-1.8 at M = 1000 up to 5.0 at Mw = 7000). The products are viscous liquids with glass transition temperatures below room temperature. 

Imine reactions catalysed by BlNOL-phosphoric acids have been modelled, using DFT calculations the model can predict the correct enantioselectivity for a wide range of reactions in the literature, based on the E/Z preference of the transition state, and the catalyst-nucleophile orientation. ... 

Palladium-catalyzed allylation of nucleophiles proceeding in an Sn2 or Sn2 fashion depending on the catalyst, nucleophile, and substituents on the substrate ... 

Catalyst Nucleophile Temp (°C) Reaction time (h) Yield (%)... 

Apart from base catalysis, Michael addition of thiols can also be performed using nucleophUic catalysis. Primary and secondary amines and certain phosphines are the most commonly used catalysts. Nucleophile mediated thiol-Michael addition reactions have extensively been studied. The nucleophiles attack the Michael acceptors to generate a carbanion, which abstracts protons from thiols to generate thiolate anions, which in turn propagate the reaction (Scheme 1.16) [56]. NucleophUicity of the catalyst plays a crucial role in the kinetics of the nucleophile-based thiol-Michael addition reactions, as stronger the nucleophile, more easily the thiolate anion will be generated. 

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