Showing papers on "Nucleophile published in 1974"

Phase-Transfer Catalyzed Two-Phase Reactions in Preparative Organic Chemistry

Abstract: Quaternary ammonium and phosphonium salts catalyze reactions between substances located partly in an aqueous and partly in an organic phase. Use of such phase-transfer catalysts simplifies and accelerates numerous reactions traditionally conducted in nonaqueous media. These reactions include carbene reactions, nucleophilic substitutions, alkylations of ketones and nitriles, Wittig and Darzens reactions, formation of ethers and esters. Other reactions such as hydrolysis and oxidation can be accelerated.

α-Metalated Isocyanides in Organic Synthesis. New synthetic methods (6)

Abstract: α-Alkali-metalated isocyanides, which can be obtained from isocyanides and bases, can be used for nucleophilic introduction of (masked) α-aminoalkyl groups. Intramolecular ring-closure may then follow if a nucleophile combines at the electron sextet of the isocyanide carbon. Treatment of α-alkali-metalated isocyanides with electrophilic agents permits rapid and efficient synthesis of, inter alia, 2- and 3-amino alcohols, straight-chain, branched, and β-functional α-amino acids, olefins, vinyl isocyanides, and of a large number of mainly five-, but also six- and seven-membered aza-, diaza-, oxa-aza-, and thia-aza heterocycles.

Carbene complexes. Part VIII. Chromium(0), iron(0), rhodium(I), iridium(I), nickel(II), palladium(II), platinum(II), and gold(I) mono- and oligo-carbene species from electron-rich olefins

Abstract: The reaction of the electron-rich olefin 1,1′,3,3′-tetramethyl-2,2′-bi-imidazolidinylidene (I)(L2) or, in one case, of bis(N-methylbenzothiazolinylidene)(II)(L′2) with various transition metal d6 or d8 complexes yields mono- di-, or tri-carbene metal complexes of Cr0, Fe0, RhI, IrI, NiII, PdII, PtII, or AuI, such as cis-Cr(CO)4(L)2, [Ir(CO)(L)3]+, or cis-[PtMe(PPh3)(L)2]+. The reactions involve (i) displacement from the metal of a negative (Cl–, I–) or neutral [CO, tertiary phosphine, or π-allyl (transformed into σ-allyl)] ligand or ligands by one or more nucleophilic carbene moieties L or L′, (ii), for [Rh(CO)2Cl]2, bridge-splitting by L, or (iii) displacement of a halide anion by BF4–. The twenty-four new carbene complexes are stable and their ready formation in high yield serves as a further demonstration of the scope of electron-rich olefins as reagents in transition metal chemistry. They clearly behave as nucleophilic carbene (L or L′) transfer reagents.

The remarkable reactivity of aryl halides with nucleophiles

Abstract: Nucleophilic attack on carbon; nucleophilic attack on hydrogen; aryl formation by halide ion loss from o-halophenyl anions; nucleophilic attack on halogen; acceptance of an electron, and its consequences.

SN2 Reactions with Carboxylic Esters. Selective cleavage of methyl esters

Abstract: Sodium cyanide in hexamethylphosphoric triamide selectively cleaves methyl esters in the presence of ethyl esters with yields of ca. 80%. The mechanism of the reaction has been investigated. It consists of nucleophilic displacement by cyanide of the carboxylate ion from the alcohol carbon atom (BAl2 mechanism).

Reactions of L-ergothioneine and some other aminothiones with2,2'-and 4,4'-dipyridyl disulphides and of L-ergothioneine with iodoacetamide. 2-Mercaptoimidazoles, 2- and 4-thiopyridones, thiourea and thioacetamide as highly reactive neutral sulphur nucleophils.

Abstract: 1. The reactions of 2,2'- and 4,4'-dipyridyl disulphide (2-Py-S-S-2-Py and 4-Py-S-S-4-Py) with l-ergothioneine (2-mercapto-l-histidine betaine), 2-mercaptoimidazole, 1-methyl-2-mercaptoimidazole, thiourea, thioacetamide, 2-thiopyridone (Py-2-SH) and 4-thiopyridone (Py-4-SH) were investigated spectrophotometrically in the pH range approx. 1-9. 2. These reactions involve two sequential reversible thiol-disulphide interchanges. 3. The reaction of l-ergothioneine with 2-Py-S-S-2-Py and/or with the l-ergothioneine-Py-2-SH mixed disulphide, both of which provide Py-2-SH, is characterized by at least three reactive protonic states. This provides definitive evidence that neutral l-ergothioneine is a reactive nucleophile, particularly towards the highly electrophilic protonated disulphides. 4. A similar situation appears to obtain in the reactions of l-ergothioneine and Py-2-SH with 4-Py-S-S-4-Py and in the reactions of the other 2-mercaptoimidazoles, thiourea and Py-4-SH with 2-Py-S-S-2-Py. The nucleophilic reactivity of Py-4-SH suggests that general base catalysis provided by the disulphide in a cyclic or quasi-cyclic transition state is not necessary to generate nucleophilic reactivity in the other amino-thiones whose geometry could permit such catalysis. 5. The existence of a positive deuterium isotope effect in the l-ergothioneine-2-Py-S-S-2-Py system at pH6-7 provides no evidence for general base catalysis but is in accord with a mechanism involving specific acid catalysis and post-transition-state proton transfer. 6. The pH-dependences of the overall equilibrium positions of the various thiol-disulphide interchanges are described. 7. Reaction of thioacetamide with a stoicheiometric quantity of 2-Py-S-S-2-Py at pH1 provides 2 molecules of Py-2-SH per molecule of thioacetamide and elemental sulphur; these findings can be accounted for by thiol-disulphide interchange to provide a thioacetamide-Py-2-SH mixed disulphide followed by fragmentation to provide CH(3)CN, S and Py-2-SH. 8. Provision of high reactivity in the neutral forms of the members of this series of sulphur nucleophiles by electron donation by the amino group is compared with the well known alpha effect that provides enhanced nucleophilicity in compounds containing an electronegative atom adjacent to the nucleophilic atom. 9. The decrease in the u.v. absorption of l-ergothioneine at 257nm consequent on transformation of its aminothione moiety into an S-alkyl-2-mercaptoimidazole moiety provides a convenient method of following the alkylation of l-ergothioneine by iodoacetamide. 10. The pH dependence of the extinction coefficient of l-ergothioneine at 257nm is described by epsilon(257)={8x10(3)/(1+K(a)/[H(+)]} +6x10(3)m(-1).cm(-1) in which pK(a)=10.8. 11. In the pH range 3-11 the reaction is characterized by two reactive protonic states (X and XH). 12. The X state, reaction of the ionized 2-mercaptoimidazole moiety of the l-ergothioneine dianion with neutral iodoacetamide, is characterized by the second-order rate constant 4.0m(-1).s(-1) (25.0 degrees C, I=0.05). The XH state, characterized by the second-order rate constant 0.03m(-1).s(-1), is interpreted as reaction of the thione form of the neutral 2-mercaptoimidazole moiety of the l-ergothioneine monoanion with neutral iodoacetamide. 13. The XH state of the alkylation reaction does not exhibit a deuterium isotope effect.

Metal catalysis in oxidation by peroxides. Vanadium catalysed oxidation of organosulphur compounds by t-butyl hydroperoxide

Abstract: The reaction of di-n-butyl sulphide with t-butyl hydroperoxide proceeds at a convenient rate in ethanol at 25° in the presence of catalytic amounts of bisacetylacetonato-oxovanadium(IV)[VO(acac)2] affording di-n-butyl sulphoxide in quantitative yield. With [ButO2H]0/[VO(acac)2]0 8 and [VO(acac)2]0 0·0001–ca. 0·002M the reaction is first order in sulphide and first order in vanadium catalyst. With an excess of sulphide and varying the initial concentration of hydroperoxide, the kinetics conform to a Michaelis–Menten-type rate law. The relative rates of vanadium catalysed oxidation of some substrates by ButO2H were found to decrease in the order Bun2S (100) > PhSBun(58) > Bun2SO (1.7) > cyclohexene (0·2). The data suggest a mechanism involving rate-determining nucleophilic attack by the sulphur substrate on the O–O bond of a hydroperoxide–vanadium(V) complex.

Intramolecular catalysis of amide hydrolysis by two carboxy-groups

Abstract: In di-isopropylmaleamic acids derived from β-amino-acids (i) the rate constant for hydrolysis of the unactivated amide group is comparable with Kcat for pepsin hydrolysing good syntheic substrates; (ii) a neighbouring carboxy-group participates as a nucleophilic catalyst, but requires a specific structural relationship with the amide group to reach high efficiency; (iii) there is a requirement for a second carboxy-group, also in a special structural relationship with the amide group, but this time in the ionised form, for full catalytic efficiency to be maintained. Remarkably, the ionised carboxy-group acts as a general base to catalyse the interconversion of neutral and zwitterionic forms of the tetrahedral carbinolamine intermediate, a reaction which is catalysed by external general acids. As a result we have developed a system showing a bell-shaped pH–rate profile for hydrolysis, of the sort shown by many enzyme-catalysed reactions, and for the same reasons.

Reactivity of co-ordinated ligands. Part XX. Preparation and reactions of cyclo-octadiene complexes of iron, ruthenium, and osmium

Abstract: The complexes tricarbonyl(η-cyclo-octa-1,5-diene)ruthenium, (I), and tricarbonyl(η-cyclo-octa-1,3-diene)osmium, (II), have been prepared and their reactivities examined and compared to those of the corresponding iron complexes. On reaction with Ph3CBF4 hydride-ion abstraction occurs to give cyclo-octadienylium complexes [(C8H11)M(CO)3]+. These react with anionic nucleophiles to give neutral complexes, some of which contain η-allyl groups and metal–carbon σ-bonds. Catalytic isomerisations of cyclo-octa-1,5-diene have been studied as well as those of the diene complexes, a novel ring contraction being observed in one case. Substitution of a carbonyl ligand by PPh3 has been observed for the η-allyl complexes of iron and ruthenium, and a carbonylation reaction to give bicyclo[4.2.1] non-2-en-9-one is also described.

Urea synthesis from amines and cyanic acid: kinetic evidence for a zwitterionic intermediate

Abstract: The following evdence supports the conclusion that the reaction of amines with cyanic acid and the hydrolysis of urea occur through a stepwise mechanism with the formation of an intermediate zwitterion. (1) The Bronsted type plot of log10k′versus ammonium ion pKa has a slope βnue 0·3 for basic amines and exhibits a break to a slope βnue 0·8 for weakly basic amines; the two segments are assigned to rate-determining nucleophilic attack and protontransfer steps, respectively. (2) General acid-base catalysis is observed only with weakly basic amines, for which the proton-transfer step is rate determining. (3) A change in rate-determining step with increasing buffer concentration is observed as the uncatalysed attack step becomes rate determining.