Mechanisms 5n2

The reduction ofsec-, and /-butyl bromide, of tnins-1,2-dibromocyclohexane and other vicinal dibromides by low oxidation state iron porphyrins has been used as a mechanistic probe for investigating specific details of electron transfer I .v. 5n2 mechanisms, redox catalysis v.v chemical catalysis and inner sphere v.v outer sphere electron transfer processes7 The reaction of reduced iron porphyrins with alkyl-containing supporting electrolytes used in electrochemistry has also been observed, in which the electrolyte (tetraalkyl ammonium ions) can act as the source of the R group in electrogenerated Fe(Por)R. ... 

A kinetic study of the previously reported substitution of aromatic nitro groups by tervalent phosphorus has established an aromatic 5n2 mechanism. Similarities in values of activation energies, and in relative reactivities of phosphite and phosphonite esters, between this displacement and the Arbusov reaction suggest a related mechanism (31), while the lack of reactivity of p-dinitrobenzene is attributed to the need for intramolecular solvation (32). The exclusive formation of ethyl nitrite, rather than other isomers, is confirmed from the decomposition of triethoxy-(ethyl)phosphonium fluoroborate (33) in the presence of silver nitrite. A mechanism involving quinquevalent phosphorus (34) still seems applicable, particularly in view of the recent mechanistic work on the Arbusov reaction. ... 

Fig. 7.2. a) The most common mechanism of base-catalyzed ester hydrolysis, namely specific base catalysis (HCT catalysis) with tetrahedral intermediate and acyl cleavage. Not shown here are an W mechanism with alkyl cleavage observed with some tertiary alkyl esters, and an 5n2 mechanism with alkyl cleavage sometimes observed with primary alkyl esters, particularly methyl esters, b) Schematic mechanism of general base catalysis in ester hydrolysis. Intermolecular catalysis (bl) and intramolecular catalysis (b2). c) The base-catalyzed hydrolysis of esters is but a particular case of nucleophilic attack. Intermolecular (cl) and intramolecular (c2). d) Spontaneous (uncatalyzed) hydrolysis. This becomes possible when the R moiety is... 

The regio- and stereo-chemistry of the nucleophilic attack of (5)-trani -hex-3-en-2-ol and (5)-trani-hex-4-en-3-ol on the corresponding O-protonated or -methylated derivatives have been examined in the gas phase at 40 °C and 720 Torr. Firm evidence of various kinds was obtained for the concerted 5ivr2 pathway accompanying the classical 5n2 mechanism. Competition between the two processes is essentially governed by the orienting properties of the oxonium intermediate towards the approaching nucleophile. Many other details were elucidated. 

Decarboxylation of 1,3-dimethylorotic acid in the presence of benzyl bromide yields 6-benzyl-1,3-dimethyluracil and presumably involves a C(6) centered nucleophilic intermediate which could nonetheless have either a carbene or ylide structure. Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry has been used to explore the gas-phase reactions of methyl nitrate with anions from active methylene compounds anions of aliphatic ketones and nitriles react by the 5n2 mechanism and Fco reactions yielding N02 ions are also observed nitronate ions are formed on reaction with the carbanions derived from toluenes and methylpyridines. 

Lee s group has published extensive results on aminolysis of sulfonates.278 281 Thus the reactions of anilines witii 2-cyano-2-propyl and 1-cyanocyclooctylarenesulfonates in acetonidile have been stiidied.278 A dissociative, S k2 mechanism with a loose TS is supported from die usual LFERs. An 5k2 mechanism is also found for the reaction in acetone of (Z)-benzyl (X)-benzenesulfonates witii (Y)-pyridines.279 Nucleophilic substiditions witii die cycloalkyhnetiiylsulfonates (306) and anilines in MeOH were also sfridied.280 Finally die reaction of tiiiopheneethyl arenesulfonates (307) with anilines and AvA -dimethylanilines in MeCN has been reported on.281 Frontside-atiack in an 5n2 mechanism witii a four-centre TS is supported. 

A study of the photo-sensitized ring-opening reactions of the radical cations (76) of arylcyclopropanes (75) with methanol, water, and cyanide nucleophiles suggests a three-electron 5n2 mechanism (Scheme 11).185 The isolated products are methyl propyl ethers, derived from nucleophilic attack of methanol on the radical cation (76). They were detected by UV-VIS spectroscopy and shown to react with nucleophiles by transient kinetic methods. The benzyl radical (77) reacts with the DCB radical anion to afford monoaromatic ether (78) by oxidation and protonation or the disubstituted ether (79) by addition of DCB. Regio- and stereo-selectivity of the substitution were complete regiochemistry and rate constant were profoundly effected by the electronic nature of the aryl substituents.186 Elsewhere, a combined ab initio and CIDNP study... 

Nucleophilic alkylation with Grignard reagents (mediated by Et2AlCl) on chiral perhydropyrido(2.1 -/>]pyrrolo[l, 2-d [, 3,4]oxadiazinc (25) proceeds via an 5n2 mechanism below —80 °C, forming the inversion product (26) with high stereoselectivity.22 At higher temperatures the stereoselectivity shifts in favour of retention. 

IV-Substituted imidazoles have been synthesized quickly in high yields and with high stereoselectivity from the adducts formed from Baylis-Hillman acetates134 and DABCO in 20% aqueous THF at room temperature.135 The substitution reaction with imidazole appears to proceed via an 5N2,-5N2/ mechanism (Scheme 18). 

The styryl iodonium salt (7a) reacts slowly with acetic acid to give the E and Z isomeric products (8a) and (9a) in the ratio 85 15. The decenyl system (7b) is much more reactive and gives only the inversion product, the Z isomer (9b). It was suggested that the styryl system reacts in a two-step mechanism via the vinylene phenonium ion (10), whereas the decenyl system follows a one-step vinylic 5n2 mechanism. In contrast, 2-bromo-l-decenyIiodonium salt reacts with bromide ion in MeCN with complete retention." The observed rate constants show an unexpected dependence on [Br ], in accordance with a mechanism involving ligand coupling within a bromoiodane intermediate. 

Stereochemical results for the alkylation of two anions by optically active a-chloro-p-nitrophenylethane have been interpreted in terms of competition between 5rn 1 and 5n2 mechanisms. The ambident anion of 2-nitropropane gives C-alkylation with complete racemization by SrnI and (9-alkylation by 5n2. The other anion studied was diethy Idithiocarbamate. 

The kinetics of the hydrolysis of A/ -(2,4-dinitrophenyl)benzhydrazonyl bromide and its derivatives have been studied over a wide range of pH. Roles for both 5n1 and 5n2 mechanisms were inferred. 

The perchloric acid-catalyzed methanolysis of 2-methyloxetane gives a mixture of 4-methoxy-2-butanol and 3-methoxy-l-butanol, with the former somewhat predominating (equation 21). The effect of solvent on the product distribution and the reaction rates indicated that protonated 2-methyloxetane was reacting by a borderline 5n1-5n2 mechanism (67MI51302). Similar studies with the acid-catalyzed methanolysis of oxetane itself indicated that methanol reacted with protonated oxetanium ion by the 5n2 process. The same type of studies with a series of 2-aryloxetanes indicated that methanolysis of these compounds involved the borderline mechanism for the protonated oxetanium ions (69MI5 loi, 72MI5102, 73MI5100). 

In Mannich reactions performed in basic media, the active aminomethylating species is less well defined. It may be a methylenediamine (9), a hydroxymethylamine (8), or an alkoxymethylamine (ROCH2NR2 ) (in alcoholic solvents). Aminomethylation in base-catalyzed Mannich reactions, as shown for cyclohexanone, is believed to occur by an 5n2 mechanism in which the enolate (13) displaces either NR2", 0H , or OR from one of the intermediates above (Scheme 2). 

Mechanism of Ligand-displacement Reactions.41 For square complexes, the mechanistic problem is more straightforward and hence better understood. One might expect that four-coordinate complexes would be more likely than octahedral ones to react by an 5N2 mechanism, and extensive studies of Pt11 complexes have shown that this is so. 

Mg is associated with a large number of enzymes involving the hydrolysis and transfer of phosphates. The MgATP complex serves as the substrate in many cases. As noted in Section 62.1.2.2.2, the interaction of Mg " with the ATP enhances the transfer (to a substrate or water) of the terminal phosphoryl group. The results of many studies with model compounds lead to the postulate of an 5n2 mechanism for this reaction. Associative pathways allow greater control of the stereochemistry of the substitution, and the rates of such processes are accelerated more effectively by metal ions. 

Further reductions of 17-hydroxy-17-alkynyl-steroids with LiAlH4-AlCl3 to give the 17(20),20-dienes (allenes) confirmed that the reactions proceeded by a stereospecific c/5-5n2 mechanism. Aromatic fluorides were prepared in high yield by treatment of aryl-triazenes with 70% HF in pyridine and 4-fluoro-oestrone methyl ether was prepared by this method. ... 

The oxidatively assisted nucleophilic substitution of iodine with perchlorate anion in primary and secondary substrates proceeds according to an 5n2 mechanism as indicated by the stereochemistry of this reaction in the polycyclic substrates 640 leading to products 641 with inverted configuration (Scheme 3.251) [668]. 

Treatment of alkyliodides with peracids in the presence of water results in oxidatively assisted hydrolysis and the formation of alcohols [661]. This reaction can be used as a synthetic method for the mild hydrolysis of iodides for example, iodide 642 undergoes facile stereoselective hydrolysis to the protected amino alcohol 643 under neutral conditions upon exposure to ten equivalents of buffered peroxytrifluoroacetic acid (Scheme 3.252) [670]. The oxidatively assisted hydrolysis of iodide 642 results in a complete inversion of configuration, which is consistent with the 5n2 mechanism of substimtion in the hypervalent iodine intermediate [670]. 

Figure 1 Formation of metal complexes in aqueous solution. Route I [(i) (vi) (iv)] represents a limiting mechanism, route Ila [(i) (ii) (iii) (iv)] a concerted water-exchange mechanism, and route Ilb [(i) (ii) (v) (iv)] a limiting 5n2 mechanism...

Krische and co-workers have investigated the tertiary phosphine-catalyzed regjospecific allylic amination of MBH acetates through a tandem 5n2 -5n2 mechanism by using phthalimide derivatives as nucleophiles. When (1 )-C1-MeO-BIPHEP was used as a catalyst, the MBH adduct obtained from p-nitrobenzaldehyde and methyl acrylate reacted with phthalimide to give the allylic substituted product 297 in 80% yield with 56% ee (Scheme 3.126). ... 

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