Jencks diagrams

They were able to infer p for the identity reaction in which Ar = Ar, and interpreted the results in terms of a More O Ferrall-Jencks diagram of the type described in Section 5.3. 

Fig. 9 A hypothetical More-O Ferrall Jencks diagram for the attack of methoxide on O-aryl phosphate triesters (20) and 5-aryl phosphorothioates (21). Note that the diagram for attack of a metal-coordinated methoxide would be similar, but Mx +-coordination would push the TS toward the S-corner, possibly stabilizing the pentacoordinated intermediate to the point that the reaction occurs stepwise with the likely rate-limiting step being breakdown.

O Ferrall-Jencks diagram and ab initio calculations with the 6-31G basis set. It is concluded that the transition state is slightly iilcB-like for (27) and more symmetrical for (28). 

A depiction of a hypothetical potential energy surface for a reacting system as a function of two chosen coordinates (c.g., the lengths of two bonds being broken). Such diagrams are useful in assessing structural effects on transition states for stepwise or concerted pathways. An example of More O Ferrall-Jencks diagrams for j8-elimina-tion reactions is shown below. 

Monosialoganglioside sialyltransferase, SIALYLTRANSFERASE MONTE CARLO METHODS MORE O FERRALL-JENCKS DIAGRAM... 

Fig. 1 More O Ferrall-Jencks diagram for the deprotonation of a nitroalkane. The curved line shows the reaction coordinate with charge delocalization lagging behind proton transfer.

Fig. 2 Modified More O Ferrall-Jencks diagram for the CH3N02/CH2=N02 system. The curved lines represent the reaction coordinates through the optimized and constrained transition state, respectively. The constrained transition state is less imbalanced as indicated by its location to the left of the optimized transition state.

The general base catalysis mechanism, (b) in Scheme 11.4, has a More O Ferrall-Jencks diagram (Fig. 11.4B) which shows that decreasing the nucleophilicity/basicity of the nucleophile (decreasing the energy, , of just the top-left corner) would cause movement of the transition structure coordinates towards the top-left. Resolving this into its components indicates an increase in /9. The value of /3 is related to that of a by a = ft — 1, so that the value of a should become less negative, which is not consistent with the observed results. 

Fig. 11.4 More O Ferrall-Jencks diagrams for nucleophilic addition to an aldehyde. (A) For general acid catalysis. (B) For general base catalysis. The symbol indicates an imposed decrease in energy of the state indicated.

Fig. 37 More O Ferrall-Jencks diagram for the Menschutkin reactions of 1-phenylethy] and benzyl chlorides with pyridine. The structures of transition states were optimized by ab initio MO calculation (RHF/b-Sf G ). O, substituted 1-phenylethyl chlorides with pyridine , benzyl chlorides with pyrindine , with 4-nitropyridine O, methyl and A, ethyl chlorides with pyridine (Fujio et al, unpublished).

This indicates a similar transition state in terms of leaving group bond fission for the reactions in water and in r-butanol. Fig. 6 shows a More-O Ferrall-Jencks diagram with the loose transition state for the aqueous hydrolysis denoted in the lower right-hand region. The reaction in t-butanol proceeds along the bottom axis via the intermediate in the lower right corner. 

Fig. 6 More-O Ferrall-Jencks diagram illustrating the mechanisms for the reactions of aryl phosphates with water (hydrolysis) and /-butanol. The hydrolysis reaction is concerted but not synchronous in the transition state, bond fission to the leaving group is ahead of bond formation to the nucleophile. In /-butanol, p-nitrophenyl phosphate undergoes reaction by a two-step Dn + An mechanism. The transition state for the rate-limiting first step is similarly late with regard to leaving group bond fission.

Together the Bronsted LFER and the KIE data indicate that transition states for phosphodiesters lie more toward the central area of the More-O Ferrall-Jencks diagram of Fig. 4 than the monoester transition state. The precise location is dependent upon nucleophile and leaving group, but the reactions of diester anions are concerted and exhibit more nucleophilic participation in the transition state, and less bond fission to the leaving group, than monoesters. 

Summary. In summary, the transition states for the uncatalyzed phosphoryl transfer reactions of the three classes of phosphate esters can be generally represented on the More-O Ferrall-Jencks diagram in Fig. 13. It has been noted that phosphoryl transfer reactions follow a trend for the phosphoryl group to bear a... 

Fig. 13 More-O Ferrall-Jencks diagram depicting the general location of transition states of the uncatalyzed phosphoryl transfer reactions of phosphomono-, di-, and triesters. In general, the transition states become tighter as the alkylation state of the ester increases.

The curvature of the surface and the orientation of the saddle-point can be deduced from the angles of the two level lines (which join points of the same energy as that of the transition structure - see Figure 4) through the saddle point. The coordinates g,g2, of a point on one of the level lines are relative to the coordinates of the transition structure, x, y. The coordinates of the point on the level line relative to the 0,0 corner of the More O Ferrall-Jencks diagram are (.x" + gi),(y + 2)- The slopes of the level lines gi/g2 and 2/ 1 relative to the y and x axes respectively are given by equations (18) and (19). 

Figure 11 More O Ferrall-Jencks diagram for the identity reactions of substituted phenolate ions with substituted phenyl esters (data from reference 24). See text for the identities of the three identity reactions...

Figure 5 More O Ferrall-Jencks diagram for the elimination reaction (Equation 17). A, El-like E2 mechanism B, synchronous E2 mechanism C, Elcb-like E2 mechanism. The vector (a.) corresponds to a Hammond (parallel) effect and (b) to a Thornton (perpendicular) effect...

The change in a and P can be understood from the More O Ferrall-Jencks diagrams (Figures 8 and 9) where the vector resulting from increasing nucleophilicity of HNu causes a shift to smaller a and smaller P for mechanisms A and B respectively. 

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