Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Deprotonation catalytic

Fig. 10.1 Selected chiral sulfides and results obtained using alkylation/ deprotonation catalytic methodology for the asymmetric synthesis of trans-stilbene oxide, dr = trans cis solvents and additives vary. Fig. 10.1 Selected chiral sulfides and results obtained using alkylation/ deprotonation catalytic methodology for the asymmetric synthesis of trans-stilbene oxide, dr = trans cis solvents and additives vary.
The term Knoevenagel reaction however is used also for analogous reactions of aldehydes and ketones with various types of CH-acidic methylene compounds. The reaction belongs to a class of carbonyl reactions, that are related to the aldol reaction. The mechanism is formulated by analogy to the latter. The initial step is the deprotonation of the CH-acidic methylene compound 2. Organic bases like amines can be used for this purpose a catalytic amount of amine usually suffices. A common procedure, that uses pyridine as base as well as solvent, together with a catalytic amount of piperidine, is called the Doebner modification of the Knoevenagel reaction. [Pg.176]

If the starting ester has more than one acidic a hydrogen, the product /3-keto ester has a highly acidic, doubly activated hydrogen atom that can be abstracted by base. This deprotonation of the product requires that a full equivalent of base rather than a catalytic amount be used in the reaction. Furthermore, the... [Pg.889]

The first attempt at a catalytic asymmetric sulfur ylide epoxidation was by Fur-ukawa s group [5]. The catalytic cycle was formed by initial alkylation of a sulfide (14), followed by deprotonation of the sulfonium salt 15 to form an ylide 16 and... [Pg.5]

Metzner and co-workers reported a one-pot epoxidation reaction in which a chiral sulfide, an allyl halide, and an aromatic aldehyde were allowed to react to give a trons-vinylepoxide (Scheme 9.16c) [77]. This is an efficient approach, as the sulfonium salt is formed in situ and deprotonated to afford the corresponding ylide, and then reacts with the aldehyde. The sulfide was still required in stoichiometric amounts, however, as the catalytic process was too slow for synthetic purposes. The yields were good and the transxis ratios were high when Ri H, but the enantioselectivities were lower than with the sulfur ylides discussed above. [Pg.327]

Oxalamidinate anions represent the most simple type of bis(amidinate) ligands in which two amidinate units are directly connected via a central C-C bond. Oxalamidinate complexes of d-transition metals have recently received increasing attention for their efficient catalytic activity in olefin polymerization reactions. Almost all the oxalamidinate ligands have been synthesized by deprotonation of the corresponding oxalic amidines [pathway (a) in Scheme 190]. More recently, it was found that carbodiimides, RN = C=NR, can be reductively coupled with metallic lithium into the oxalamidinate dianions [(RN)2C-C(NR)2] [route (c)J which are clearly useful for the preparation of dinuclear oxalamidinate complexes. The lithium complex obtained this way from N,N -di(p-tolyl)carbodiimide was crystallized from pyridine/pentane and... [Pg.307]

In principle, numerous reports have detailed the possibility to modify an enzyme to carry out a different type of reaction than that of its attributed function, and the possibility to modify the cofactor of the enzyme has been well explored [8,10]. Recently, the possibility to directly observe reactions, normally not catalyzed by an enzyme when choosing a modified substrate, has been reported under the concept of catalytic promiscuity [9], a phenomenon that is believed to be involved in the appearance of new enzyme functions during the course of evolution [23]. A recent example of catalytic promiscuity of possible interest for novel biotransformations concerns the discovery that mutation of the nucleophilic serine residue in the active site of Candida antarctica lipase B produces a mutant (SerlOSAla) capable of efficiently catalyzing the Michael addition of acetyl acetone to methyl vinyl ketone [24]. The oxyanion hole is believed to be complex and activate the carbonyl group of the electrophile, while the histidine nucleophile takes care of generating the acetyl acetonate anion by deprotonation of the carbon (Figure 3.5). [Pg.69]

In contrast to oxoesters, the a-protons of thioesters are sufficiently acidic to permit continuous racemization of the substrate by base-catalyzed deprotonation at the a-carbon. Drueckhammer et al. first demonstrated the feasibility of this approach by performing DKR of a propionate thioester bearing a phenylthiogroup, which also contributes to the acidity of the a-proton (Figure 4.14) [39a]. The enzymatic hydrolysis of the thioester was coupled with a racemization catalyzed by trioctylamine. Owing to the insolubility of the substrate and base in water, they employed a biphasic system (toluene/H2O). Using P. cepacia (Amano PS-30) as the enzyme and a catalytic amount of trioctylamine, they obtained a quantitative yield of the corresponding... [Pg.99]

Cl—Al Cly) intermediate or a carbocation C AICI4 This intermediate electrophilically attacks the benzene ring to generate a benzenonium ion intermediate which gives alkylated benzene through deprotonation by aluminum tetrachloride anion. Finally the hydrogen aluminum tetrachloride complex affords aluminum chloride and hydrogen chloride gas. This aluminum chloride is recycled in the catalytic cycle of alkylation. [Pg.176]

The mechanism involves a Pd(0) monocoordinate complex as the active species that undergoes oxidative addition to the aryl halide [141]. Thereafter, coordination of the amine to the palladium centre and deprotonation by the external base results in halide abstraction. After reductive elimination, the coupling product is obtained and the catalytic active species regenerated (Scheme 6.45). [Pg.181]

Some of the details of the mechanism may differ for various catalytic systems. There have been kinetic studies on two of the amination systems discussed here. The results of a study of the kinetics of amination of bromobenzene using Pd2(dba)3, BINAP, and sodium r-amyloxide in toluene were consistent with the oxidative addition occurring after addition of the amine at Pd. The reductive elimination is associated with deprotonation of the animated palladium complex.166... [Pg.1046]

A study of the reaction of chlorobenzene with /V-mclhyl aniline in the presence of Pd[P(r-Bu)3]2 and several different bases indicated that two mechanisms may occur concurrently, with their relative importance depending on the base, as indicated in the catalytic cycle below. The cycle on the right depicts oxidative addition followed by ligation by the deprotonated amine. The cycle on the left suggests that oxidative addition occurs on an anionic adduct of the catalyst and the base, followed by exchange with the amine ligand.167... [Pg.1047]

However, when 2,6-dimethylbenzoquinone with sodium ( >3,5-hexadienoate (generated in situ) was reacted in water in the presence of a catalytic amount of sodium hydroxide, pentacyclic adducts were formed via deprotonation of the Diels-Alder adduct followed by tandem Michael-addition reactions with another molecule of 2,6-dimethylbenzoquinone (Eq. 12.25).83 Similar results were obtained with sodium ( >4,6-heptadienoate. [Pg.394]

The hydrogenation of simple alkenes using cationic rhodium precatalysts has been studied by Osborn and Schrock [46-48]. Although kinetic analyses were not performed, their collective studies suggest that both monohydride- and dihydride-based catalytic cycles operate, and may be partitioned by virtue of an acid-base reaction involving deprotonation of a cationic rhodium(III) dihydride to furnish a neutral rhodium(I) monohydride (Eq. 1). This aspect of the mechanism finds precedent in the stoichiometric deprotonation of cationic rhodium(III) dihydrides to furnish neutral rhodium(I) monohydrides (Eq. 2). The net transformation (H2 + M - X - M - H + HX) is equivalent to a formal heterolytic activation of elemental... [Pg.90]

In order to probe cation occupation in the active site in the absence of Mg2+ ions, we examined Na+ distributions in the reactant and activated precursor (deprotonated 20H nucleophile) states. It has been noted in the recent literature that the modeling of ions in highly charged systems such as HHR affords tremendous challenges with regard to simulation time scales [129], This section presents the results of series of five 300 ns simulations of the full length HHR, in both the reactant and activated precursor states, in order to ascertain the cation occupation requirement of the active site to maintain catalytic integrity. [Pg.397]

The crystal structure of the C-functionalized imidazole derivative of 1,5,9-triazcyclododecane (75) shows a five-coordinate zinc with four /V-donors from ligand and chloride in a distorted trigonal bipyramidal arrangement. The Zn—N imidazole bonds are the shortest at 2.025(3) A.678 Deprotonation of the imidazole group resulted in a bridging imidazolate to form dinuclear zinc complexes. The pATa of 10.3 varies from the pATa of bound water with similar ligands (as low as 7.3) and the complex is not catalytic for the hydrolysis of esters. [Pg.1206]


See other pages where Deprotonation catalytic is mentioned: [Pg.134]    [Pg.134]    [Pg.134]    [Pg.134]    [Pg.46]    [Pg.495]    [Pg.517]    [Pg.520]    [Pg.127]    [Pg.129]    [Pg.130]    [Pg.109]    [Pg.777]    [Pg.264]    [Pg.89]    [Pg.956]    [Pg.14]    [Pg.148]    [Pg.176]    [Pg.177]    [Pg.356]    [Pg.208]    [Pg.956]    [Pg.383]    [Pg.94]    [Pg.261]    [Pg.399]    [Pg.40]    [Pg.108]    [Pg.1161]    [Pg.177]    [Pg.180]    [Pg.238]    [Pg.239]    [Pg.96]    [Pg.59]   
See also in sourсe #XX -- [ Pg.152 , Pg.447 , Pg.448 , Pg.449 , Pg.450 , Pg.451 , Pg.452 , Pg.453 , Pg.454 , Pg.455 , Pg.456 , Pg.457 , Pg.458 ]

See also in sourсe #XX -- [ Pg.282 , Pg.283 , Pg.284 , Pg.285 , Pg.286 ]




SEARCH



Asymmetric catalytic deprotonation

Catalytic Enantioselective Deprotonation

Catalytic methods deprotonation

Catalytic reactions stereoselective deprotonations

© 2024 chempedia.info