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Nucleophilic monoanions

Solvolysis of both a- and p-2-deoxyglucopyranosyl-isoquinolinium salts allowed generation of oxocarbenium ions with a poorly nucleophilic leaving group (Scheme 7). It was observed that solvolysis of the p-anomer in the presence of the series of nucleophilic monoanions AcO-, Cl-, Br-, and N+ gave a Swain-Scott parameter of 0.03 + 0.05. Similarly, solvolysis of the a-compound in the presence of the same series of salts afforded a Swain-Scott parameter of 0.03 + 0.10. The results indicate little or no sensitivity to anion nucleophilicity and therefore point to a mechanism that does not involve an AnDn transition state.27,28... [Pg.286]

In stereoselective antitheses of chiral open-chain molecules transformations into cyclic precursors should be tried. The erythro-configurated acetylenic alcohol given below, for example, is disconnected into an acetylene monoanion and a symmetrical oxirane (M. A. Adams, 1979). Since nucleophilic substitution occurs with inversion of configuration this oxirane must be trens-conilgurated its precursor is commercially available trans-2-butene. [Pg.204]

Reduction Conversion of Nitriles into Amines Reduction of a nitrile with LiAIH4 gives a primary amine, RNH . The reaction occurs by nucleophilic addition of hydride ion to the polar C=N bond, yielding an imine anion, which still contains a C=N bond and therefore undergoes a second nucleophilic addition of hydride to give a dianion. Both monoanion and dianion intermediates are undoubtedly stabilized by Lewis acid-base complexafion to an aluminum species, facilitating the second addition that would otherwise be difficult Protonation of the dianion by addition of water in a subsequent step gives the amine. [Pg.769]

Phosphate esters have a variety of mechanistic paths for hydrolysis. Both C-O and P-0 cleavage are possible depending on the situation. A phosphate monoanion is a reasonable leaving group for nucleophilic substitution at carbon and so 8 2 or SnI reactions of neutral phosphate esters are well known. PO cleavage can occur by associative (by way of a pentacoordinate intermediate), dissociative (by way of a metaphosphate species), or concerted (avoiding both of these intermediates) mechanisms. [Pg.21]

Scheme 14-1. General in-line monoanionic mechanism of phosphodiester cleavage transesterification catalyzed by hairpin ribozyme the first proton transfer (PT1), the nucleophilic attack (Nu), and the exocyclic cleavage (Cl) steps are shown, and the Oip and O2p pathways are indicated by blue and red colored hydrogens, respectively. For the uncatalyzed model reaction in solution, the Ojp and O2p pathways are energetically equivalent... Scheme 14-1. General in-line monoanionic mechanism of phosphodiester cleavage transesterification catalyzed by hairpin ribozyme the first proton transfer (PT1), the nucleophilic attack (Nu), and the exocyclic cleavage (Cl) steps are shown, and the Oip and O2p pathways are indicated by blue and red colored hydrogens, respectively. For the uncatalyzed model reaction in solution, the Ojp and O2p pathways are energetically equivalent...
Figure 14-7. Snapshots of the active site structures near the transition state of (top) the nucleophilic attack and (bottom) the exocyclic cleavage for the in-line monoanionic O2p mechanism of cleavage transesterification in the hairpin ribozyme. The yellow and red colored cartoon is for the substrate and ribozyme strands, respectively, and water molecules interacting with non-bridging oxygens and O5/ are shown... Figure 14-7. Snapshots of the active site structures near the transition state of (top) the nucleophilic attack and (bottom) the exocyclic cleavage for the in-line monoanionic O2p mechanism of cleavage transesterification in the hairpin ribozyme. The yellow and red colored cartoon is for the substrate and ribozyme strands, respectively, and water molecules interacting with non-bridging oxygens and O5/ are shown...
Monoanionic carborane anions based around [c/ow-l-CBnHi2]- are among the most inert and least coordinating anions currently known. The high chemical stability and low nucleophilicity allows the stabilization of cationic species not isolable with other anions. The silver salt of this compound was crystallized as Ag(CB11H12) 2C6H6 (236), in which the silver is bonded to two... [Pg.981]

Full details on the phosphorylation of water and alcohols by 4-nitrophenyl dihydrogen phosphate and the NfC H ) - and N(CH3) -salts of its mono- and dianion have been published 146>. Phosphoryl group transfer from the monoanion and dianion is thought to proceed via the monomeric POf ion. Addition of the sterically unhindered amine quinuclidine to an acetonitrile solution containing the phosphate monoanion and tert-butanol produces t-butyl phosphate at a faster rate than does the addition of the more hindered diisopropylethylamine. This nucleophilic catalysis of the phosphorylation reaction is also explained by the intermediacy of the POf ion. [Pg.121]

The process is assumed to take place by a chemoselective attack of the dianion 2-223 at the bromomethyl group of 2-221 and subsequent nucleophilic attack of the resultant monoanion 2-224 onto the epoxide moiety to give 2-225. Use of the sodium-lithium-salt 2-223 of the dicarbonyl compound 2-220, the reaction temperature as well as the Lewis acid LiC104, are crucial. The reaction seems to be quite general, since various 1,3-dicarbonyl compounds can be converted into the corresponding furans. [Pg.81]

Monoanions derived from nitroalkanes are more prone to alkylate on oxygen rather than on carbon in reactions with alkyl halides, as discussed in Section 5.1. Methods to circumvent O-alkylation of nitro compounds are presented in Sections 5.1 and 5.4, in which alkylation of the a.a-dianions of primary nitro compounds and radial reactions are described. Palladium-catalyzed alkylation of nitro compounds offers another useful method for C-alkylation of nitro compounds. Tsuj i and Trost have developed the carbon-carbon bond forming reactions using 7t-allyl Pd complexes. Various nucleophiles such as the anions derived from diethyl malonate or ethyl acetoacetate are employed for this transformation, as shown in Scheme 5.7. This process is now one of the most important tools for synthesis of complex compounds.6811-1 Nitro compounds can participate in palladium-catalyzed alkylation, both as alkylating agents (see Section 7.1.2) and nucleophiles. This section summarizes the C-alkylation of nitro compounds using transition metals. [Pg.138]

The original ion-exchange treatment was developed for competition between reactive and inert monoanions, but Chaimovich, Quina and their coworkers have extended it to competition between mono and dianions (Cuccovia et al., 1982a Abuin el al., 1983a). The ion-exchange constant for exchange between thiosulfate dianion and bromide monoanion is not dimensionless as in (7) but depends on salt concentration, and the formalism was developed for analysing micellar effects upon reaction of dianionic nucleophiles, e.g. thiosulfate ion. [Pg.236]

Nucleophilic Acyl Substitution of 7-Methyl-5H-pyrrolotetrazole Monoanion 313... [Pg.944]

Grafting on the resin was achieved via a nucleophilic substitution of the benzylic chlorine by the deprotonated OH-linker of 52 (Scheme 29) by using a mixture of KO Bu, 18-crown-6 and CsBr. Determining the nitrogen content of solid phase samples by elemental analyses was accomplished, to verify the functionalization of the polymer. This enables calculation of the degree of functionalization. Usually, an occupancy of more than 20 percent of the theoretical sites was achieved. Saponification of the functionalized Merrifield resin P-52 leads to the monoanionic NJ, 0 functionalized solid phase. Subsequent reaction with [ReBrtCOlsJ afforded the polymer mounted tricarbonyl rhenium complex P-52-Re (Scheme 29). [Pg.155]

Morawetz and Oreskes (1958) postulated that intramolecular general acid catalysis occurs in concert with nucleophilic attack by carboxylate anion in the hydrolysis of the monoanion of succinyl salicylate. At pH 4 the observed hydrolytic rate constant was found to be 10 times greater than that for hydrolysis of acetylsalicylate. A... [Pg.78]

In the first proposal of a mechanism for chemiluminescent luminol oxidation, Albrecht postulates a bicyclic endoperoxide as the high-energy intermediate. The endoperoxide is presumably formed by nucleophilic attack of hydrogen peroxide monoanion on one of the diazaquinone 27 carbonylic groups to form 28, followed, after deprotonation to 29, by ring closure to 30 (Scheme 21) . [Pg.1244]

An obvious difficulty arises with this rather elaborate rationale when phosphoramidate and aryl phosphoramidate monoanions are compared for example, the dissimilarity of the dioxan effect yet the identity of product distribution observed in methanol-water competition experiments. Preliminary studies in the author s laboratory have revealed striking differences in the hydrolytic behavior between a series of phosphoramidafes derived from primary aliphatic amines and the above aryl systems. No linear structure-reactivity relationship between the logarithmic rate of hydrolysis of the monoanion species and the pKa of the amine is observed19. Moreover, the rate of hydrolysis of phosphoramidate monoanions derived from aliphatic amines is at least 104 times slower than those formed from aryl amines. In contrast, only a thirtyfold decrease in rate is observed for the corresponding ApKa in the O-phos-phate monoester series. The suspicion that mechanism (1), even with the above proposed modification, is not an accurate description of phosphoramidate monoanion hydrolysis derives some further support from the observation that the monoanion is subject to nucleophilic attack by substituted pyridines al-... [Pg.4]

Other kinetically allowed mechanistic models, i.e. hydroxide ion attack on the monoanion, can be rejected on the grounds that the required rate coefficients far exceed that found for alkaline hydrolysis of phosphate triesters. At pH > 9 two new reactions appear, one yielding a 1,6-a.nhydro sugar by nucleophilic attack through a five-membered transition state of the 1-alkoxide ion upon C-6 with expulsion of phosphate trianion. The second is apparently general-base catalysis by 1-alkoxide of water attack on C-6 or phosphorus through greater than six-membered cyclic transition states. [Pg.35]


See other pages where Nucleophilic monoanions is mentioned: [Pg.399]    [Pg.416]    [Pg.340]    [Pg.399]    [Pg.416]    [Pg.340]    [Pg.530]    [Pg.230]    [Pg.246]    [Pg.138]    [Pg.100]    [Pg.391]    [Pg.98]    [Pg.99]    [Pg.102]    [Pg.106]    [Pg.82]    [Pg.350]    [Pg.190]    [Pg.191]    [Pg.198]    [Pg.202]    [Pg.204]    [Pg.27]    [Pg.27]    [Pg.3]    [Pg.8]    [Pg.9]    [Pg.10]    [Pg.10]    [Pg.11]    [Pg.12]    [Pg.38]    [Pg.202]   
See also in sourсe #XX -- [ Pg.399 ]




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