Conjugate transition state

Finally, Table 8.1 lists three primary alkyl chlorides—allyl chloride, benzyl chloride, and chloroacetone—that react considerably faster than other primary alkyl chlorides. This increase in reaction rate is due to resonance stabilization of the transition state. Each of these compounds has a pi bond adjacent to the reactive site and forms a transition state that is conjugated. The p orbital that develops on the electrophilic carbon in the transition state overlaps with the p orbital of the adjacent pi bond. The stabilization due to the conjugated transition state results in a significantly faster reaction. The transition state for the reaction of allyl chloride with a nucleophile is shown as follows ... 

Because the pi and sigma atomic orbitals that interchange roles are orthogonal, such a reaction does not proceed through a fully CONJUGATED TRANSITION STATE and is thus not a pericyclic reaction and therefore is not governed by the rules that express ORBITAL SYMMETRY restrictions applicable to pericyclic reactions. 

Since the transition state of the Diels-Alder reaction is isoconjugate with benzene, E-type substituents should be able to conjugate with it. If then we have a + substituent, and a — / or — E substituent, attached to atoms taking part in the transition state, mutual conjugation can occur between them if they are onho or para to one another. The resulting stabilization leads to the preferred formation of the product formed via a mutually conjugated transition state. 

Figure 5,30 reprinted from Chemical Physical Letters, 194, Fischer S and M Karplus. Conjugate Peak Refinement An Algorithm for Finding Reaction Paths and Accurate Transition States in Systems with Many Degrees of Freedom. 252-261, 1992, with permission from Elsevier Science. 

The reaction therefore involves nucleophilic displacement on carbon passing through the transition state indicated otherwise expressed, the reaction involves nucleophUic di.splacement in the conjugate acid R—OH2 in which the displaced group is OHj" " ... 

As we saw in Chapter 5 dehydrations and dehydrohalogenations are typically regiose lective m the direction that leads to the most stable double bond Conjugated dienes are more stable than isolated dienes and are formed faster via a lower energy transition state... 

The conjugated stmcture of 1,3-butadiene gives it the abiUty to accept nucleophiles at both ends and distribute charge at both carbon 2 and 4. The initial addition of nucleophiles leads to transition states of TT-ahyl complexes in both anionic and transition-metal polymerizations. 

So far, no ground-state molecule in which the twisted conjugation would exist has been made, so the prediction remains to be tested. It correctness is strongly suggested, however, by the fact that transition states with twisted orbital arrays appear to be perfectly acceptable... 

Conjugated substituents at C-2, C-3, C-4, or C-5 accelerate the rearrangement. Donor substituents at C-2 and C-3 have an accelerating effect. The effect of substituents can be rationalized in terms of the stabilization of the transition state by depicting their effect on two interacting allyl systems. 

The stereochemistry of the 1,4-addition to A -octal-l-one and 1,1-di-methyl-A -octal-2-one has been investigated by House and Marshall, respectively. In summary, steric and stereoelectronic factors play a part in the mechanism of conjugate addition of Grignard compounds. With methylmagnesium iodide, the introduction of an axial methyl group into steroidal 5a-A -3-ketones (3) and 5 -A -3-ketones (6) is favored by stereo-electronic factors in the transition state. 

The Grignard reagent attacks the unsaturated ketones (3) and (6) from the relatively unhindered ot- or jS-side, respectively, perpendicular to the plane of the conjugated system. An analogous transition state (10) leading to axially substituted 1,6-addition products (11) from A -3-ketones (9) with methylmagnesium halide was suggested by Marshall. ... 

Transition state for attack by water on conjugate acid of ethylene oxide... 

Nitro and Other Electron-Withdrawing Substituent. The presence of unsaturated groups such as nitro at conjugative positions should cause strong resonance interaction in the transition state since a nucleophilic reagent is electron-repelling (structures 48-50). At positions of the... 

The negatively charged ring in the transition state and intermediate complex presumably exerts little or no inductive electron attraction on a substituent. So, as one might expect, the transition-state effect of an azine methoxy group can differ from its (conjugated)... 

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