5n2 pathway

While y-alkylations did not occur with saturated and benzylic halides, y-selectivities in the 62-99% range were observed in reactions of dicopper(I) dianions of a variety of a, -unsaturated acids with allylic halides. y-Unsubstituted allylic halides reacted by an Sn2 mechanism, y-disubstituted compounds underwent direct 5n2 displacement and y-monosubstituted systems reacted by both 5n2 and 5n2 pathways. Scheme 72 provides an example of the dramatic reversal in regioselectivity that was observed in the allylation of the dianion (147) of tiglic acid when the metal cation was changed from lithium to copper(I). The y-alkylation product from the latter species was exclusively the ( )-isomer. 

Me3CCH2CH2, [Fe-R]" escapes and undergoes homolysis [equation (50)]. (The radicals Fe and R abstract halogen from solvent.) Which mechanism is preferred seems to depend on the tendency of R to react by an 5n2 pathway. [FeCH2CH2Ph]" gives a phenonium ion as in equation (51). " ° ... 

The Fujisawa group have continued their study of the Cu -catalysed coupling of Grignard reagents with /8-propiolactones. Studies this year have centered upon propiolactones with a /3-olefinic or acetylenic substituent, and their reaction through an 5n2 pathway to give 3,7-dienoic acids,3-alenyl acids/ and 3,7,11-trienoic acids (Scheme 18). The stereochemistry of the newly... 

Full details have appeared on the process that has been termed transfer alkylation . Generally, polyenolates are alkylated at the a-site reaction of the polyeno-late with a source of positive bromine leads to the formation of the a-bromo-derivative which reacts with alkylating agents (by 5 n2 or 5N2" pathways) to give the products resulting from overall y- or e-alkylation. - ... 

The catalytic enantioselective desymmetrization of meso compounds is a powerful tool for the construction of enantiomerically enriched functionalized products." Meso cyclic allylic diol derivatives are challenging substrates for the asymmetric allylic substitution reaction owing to the potential competition of several reaction pathways. In particular, S 2 and 5n2 substitutions can occur, and both with either retention or inversion of the stereochemistry. In the... 

Pathway b is the specific base catalyzed (HO -catalyzed) hydrolysis. This bimolecular 5n2 reaction leads to the alcohol and nitrate. A peculiar pathway is carbonyl elimination (Fig. 9.1,c). This bimolecular reaction is catalyzed by strong bases and produces a dismutation of the two moieties, the organic group being oxidized to a carbonyl compound and nitrate being reduced to nitrite. Note that proton-catalyzed hydrolysis does not appear in Fig. 9.1 since this mechanism either does not occur or is negligible. 

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. 

The reduction of alkyl halides by LiEtsBH exhibits typical 5n2 characteristics, but it has been suggested by Ashby et al. that the reductions of secondary alkyl iodides and bromides proceed in part by an SET pathway. ... 

In summary, while alkyl sulfonates react exclusively by the 5N2-like mechanism of Scheme 2 (or equation 4), evidence for alternative SET processes in reactions of both alkyl iodides and alkyl bromides has recently accumulated. The relative contributions of Sn2 and SET mechanisms in reactions of alkyl halides is difficult to quantify, however, since the normal substitution product can arguably arise via either pathway. 

Vinyl epoxides, like allylic halides and carboxylates, have available both Sn2 and 5n2 reaction pathways. For the most part, regio- and stereo-selectivity follow patterns similar to those discussed in Section... 

Support for the intermediacy of a homoallylic cholecalciferyl cation (381) in the formation of the 3,5-cyclo-products (375) and (376) came from the solvolysis of cholecalciferyl tosylate (374) under a variety of conditions. Depending upon the solvent, and any nucleophiles supplied, products showed compositions varying from total retention of configuration to 89% inversion, interpreted as evidence of competing pathways through either the homoallylic cation (381) or an 5n2 substitution, respectively. 

Stereoselective 5n2 additi(Hi of cuprates has been documented for vinyloxiranes such as (147a) and (147b). The pairs (148) and (149) arc enantiomeric, so this selectivity also reflects a controlled access to all four enantiomers of these alcohols. The normal 5n2 opening or elimination pathways were suppressed in these systems. The selectivity increased with (Z)-isomers in all cases studied, lire stereochemistry of the cuprate additions to optically pure macrocyclic (and exocyclic) vinyloxiranes has also been investigated (equation 48). ... 

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. 

Primary alcohols prefer an 5n2 mechanism. When this mechanism is hindered by steric bulk, the Wag-ner-Meerwein rearrangement competes with substitution (Scheme 14). Secondary and tertiary alcohols follow an 5n1 pathway. The intermediate carbonium ion has the tendency to isomerize to a more stable cation. 

The aminolysis of 0,0-diethyl 4-nitrophenyl phosphate (Paraoxon) by piperidine in 10 ILs was compared to results in MeCN, dioxane, and DMSO. Generally, rates and selec-tivities in the ILs were similar to those in DMSO. P NMR analysis of the aminolysis products in the ILs showed that piperidine attacked P=0 (5n2(P)), the C( 1) aromatic carbon ( nAt) and the ethyl group (5 2(0)), the latter pathway not having been observed previously. A theoretical study of the effect of sulfur substitution on the methanolysis... 

In general, iron carbonyls react with organic halides by two pathways. However [Fe(CO)3(AsPh3)2] and CCI4 follow only one route, which is believed to involve radicals since there is an induction period, no reaction in the dark, and a decrease in rate in the presence of duroquinone. On the other hand, [Fe(CO)3(PMe3)2] and Mel react only by the alternative pathway, which has no characteristics of a radical process. This reaction is first order in each reactant, A5 being -33 cal deg mol . A concerted nucleophilic displacement (two-center 5n2) is proposed as in (77), in which the first step is rate determining. 

The reactions of methyl, allyl, and benzyl halides, as well as chloromethyl methyl ether, do not show features of chain reactions. While this does not conclusively exclude a radical chain process, it is consistent with the S 2 pathway proposed several years ago on the basis of kinetic studies. However, a possible objection is that the rate of such an 5n2 process for methyl bromide would have to be at least 10 times faster than that for ethyl bromide, which is an unusually high factor. 

The third mechanism in Fig. 1.4 is a type of radical mechanism known as nonchain radical mechanism. This non-chain variant is generally believed to operate in oxidative additions of certain alkyl halides [92] and consists in the one-electron oxidation of the metal by the A-B molecule giving rise to the radical species [L M-A] and B-, which rapidly recombine to yield the oxidative addition product [L M(A)(B)]. Similarly to the 5n2 mechanism, the rate of this radical mechanism increases as more basic is the metal, and the more easily the ligand A is transferred to the metal. Experimental evidences for this type of mechanism can be the significant changes in the reaction rate produced by the introduction of slight modifications of the substrate, the metal complex, or the solvent. Another alternative to confirm this mechanism is to use radical scavengers, such as RNO-. This alternative, however, has been somewhat criticized on the basis that the presence of such species may initiate a radical pathway for a reaction that otherwise would have followed a non-radical mechanism. 

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