Transition state substitution reactions

Section 4 9 The potential energy diagrams for separate elementary steps can be merged into a diagram for the overall process The diagram for the reac tion of a secondary or tertiary alcohol with a hydrogen halide is charac terized by two intermediates and three transition states The reaction is classified as a ummolecular nucleophilic substitution, abbreviated as SnI... 

Another such effect is the intervention of cyclic transition states in reactions of organometallic compounds (Section II, B, 5) with azines or in intramolecular nucleophilic substitutions (Section II, F). 

Reactions that take place through dipolar transition states Menschutkin reaction (10-44), electrophilic aromatic substitution. 

Y is a strongly pi electron donor group. As previously noted in the results section, examples of Y from Table VI include centers of high pi electron charge density at carbon, sulfur, nitrogen, and oxygen. Also included in Table VI are examples of nucleophilic substitution transition states (cf. reactions 21 and 22) of the type... 

Honeyman147 reported the synthesis of methyl 2,3-anhydro-/3-L-lyxopyranoside, and claimed that, on alkaline hydrolysis, this gives a 2 1 ratio of L-xyloside L-arabinoside, but these results could not be substantiated by Buchanan and R. Fletcher,68 who recorded different constants for the epoxide and for the disulfonic ester claimed147 to be the starting material for its synthesis. The same epoxide has been synthesized by Reist and coworkers,160 and its properties are in agreement with Buchanan and Fletcher s results. Ethyl or methyl 3,4-anhydro-/3-L-ribopyranoside undergoes substitution at C-4 in all the reactions thus far studied. Neither half-chair conformation would seem to be clearly favored, but the specificity observed can be rationalized by considering the steric and polar interactions that may arise in the transition state substitution at C-3 in conformation 58 would involve marked interactions between the nucleophile and... 

Co(NH8) + is stable in aqueous solution whereas fCo(NH3)5 X)2+ Ihdeigoes moderately rapid substitution of water for the acid group X, f e rates of aquation reaction depends strongly on basicity of X-, for Sample, nitrate acetate is 103 1. The difference in behaviour between Cr(III) and Co(III) ammines may be due to necessity for energy of activation m the transition state for reaction with the latter. 

Some organic reactions can be accomplished by using two-layer systems in which phase-transfer catalysts play an important role (34). The phase-transfer reaction proceeds via ion pairs, and asymmetric induction is expected to emerge when chiral quaternary ammonium salts are used. The ion-pair interaction, however, is usually not strong enough to control the absolute stereochemistry of the reaction (35). Numerous trials have resulted in low or only moderate stereoselectivity, probably because of the loose orientation of the ion-paired intermediates or transition states. These reactions include, but are not limited to, carbene addition to alkenes, reaction of sulfur ylides and aldehydes, nucleophilic substitution of secondary alkyl halides, Darzens reaction, chlorination... 

Bromination of l,3,6-triazacycl[3,3,3]azine (11.112, R = X = H) occurs at the 4-position and then at the 7- and 9-positions. Calculations predict this (73ACS242I) but the reason can be easily seen by comparison of the transition states for 4- and 7-substitution (11.113 and 11.114). Aromaticity is created in both transition states, so reaction should be rapid. However, the triazanaphthalene lOir-ring in 11.113 is more stable than the tetrazanaphthalene lOir-ring in 11.114. Bromination of the 4-cyano-3-methyl-substituted cyclazine occurs at the 7- and 9-positions (72ACS624), and of9-ethoxycarbonyl-2-methyl-l,3,4,7-tetrazacycl[3,3,3]azine (11.115] at the 6-position (73ACS2421). 

Normally the less substituted double-bond isomer (la) is the more reactive since reaction at the more substituted position It engenders severe allylic interactions (A1,3-strain5) in the transition state. Further reaction therefore occurs preferentially to give the / ,/ -disubstituted enamine or iminium salt (3) and hence the a,a -disubstituted ketone (5) on hydrolysis. However, there are exceptions to this (see Sections III.B, V and VI.D). A recent development uses the observation6 that such steric interactions do not apply to secondary enamines (Section VIII) since a conformation (2) can be adopted... 

There have been both experimental and theoretical studies to probe the degree of concertedness in gas-phase substitutions as shown in Scheme 1. Is (2) an intermediate with a finite lifetime, or are the addition and elimination steps concerted so that (2) is a transition state Experimental molecular beam studies on the femtosecond time-scale have been reported for the reaction of chloride ions with the iodobenzene cation to yield chlorobenzene and iodine. The results show an 880 fs reaction time for the elimination process, indicating a highly non-concerted process, so that here the (x-complex is an intermediate rather than a transition state. The reactions of halobenzene cations with ammonia have been interpreted in terms of the formation of an addition complex which may eliminate either halogen, X , or hydrogen halide, HX, depending on the nature of the halogen. ... 

The products of the substitution of a ligand G for ligand A of the species [M ABCDEF]" (in which M is a metal ion and A, B, C, D, E, and F are monodentate ligands or donor atoms of chelating ligands of an octahedral complex) are shown in Figure 1. Letters under the arrows indicate positions made adjacent by the loss of A, or the insertion position of G. The products are based on the principle that minimal atomic motion accompanies the attainment of the transition state, the reaction intermediates (if any), and the products. The results of a large number of studies support this principle, which was assumed by Werner, and which recently has been used by Pearson and Basolo (81) and by Kyuno, Boucher, and Bailar (67). 

The mercurinium ion is attacked by the nucleophilic solvent—water, in the present case— to yield the addition product. This attack is back-side (unless prevented by some structural feature) and the net result is anti addition, as in the addition of halogens (Sec. 7.12). Attack is thus of the Sn2 type yet the orientation of addition shows that the nucleophile becomes attached to the more highly substituted carbon— as though there were a free carbonium ion intermediate. As we shall see (Sec. 17.15), the transition state in reactions of such unstable three-membered rings has much SnI character. 

Another study utilizing more than one type of KIE to model the Sn2 transition states for reactions with a different leaving group was done using the reactions between meta-chlorobenzyl-para-substituted benzenesulfonates and cyanide ion 85... 

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