Substituent effects cooperative

Substituent Effects and Cooperative Helical Order Effect... 

One approach that has overcome this dichotomy of substituent effects is the use of a combination of statistical effects and cooperativity in dendrimeric catalysts. 

Cunningham CT, Cunningham KLH, Michalec JF, McMillin DR. Cooperative substituent effects on the excited states of copper phenanthrolines. Inorg Chem 1999 38 4388-92. 

The same is true for Cope rearrangements in general. Most substituents at C-1 and C-3 accelerate the reaction, and with more than one of them, their effects are cooperative. Substituents at C-2, however, shift the balance of the transition structure towards a biradical-like intermediate in which the new a bond is formed ahead of the old one breaking. Substituent effects at this site are not cooperative with substituent effects at C-l and C-3, because they change the nature of the transition structure rather than contribute to it in the same way. 

These results suggest a competitive interaction between the active and nodal substituents. The geometries of these transition states support this competition their values are quite similar to the distance found in the parent 1,5-hexadiene. Computational examinations of the substituent effects on the Cope rearrangement conclude that the centauric model does not apply. The chameleonic model makes a better accounting of the cooperative and competitive ways the substituents affect the Cope rearrangement. Borden has proposed a simple mathematical model that allows for the prediction of the stabilization of the transition state by substituents solely on the change in... 

Hrovat, D. A. Chen, J. Houk, K. N. Thatcher, B. W. Cooperative and competitive substituent effects on the Cope rearrangements of phenyl-substituted 1,5-hexadienes elucidated by Becke3LYP/6-31G calculations, 7. Am. Chem. Soc. 2000, 722, 7456-7460. 

In principle, two (possibly cooperative) effects could be responsible for the observed pronounced substituent effect on the automerization rate different stabilization (or destabilization) of the metallacyclic topomeriza-... 

The experimental values of A// in Table 30.1 show that phenyl groups at Cl, C3, C4, and C6, which stabilize structure C, also give rise to a strongly cooperative substituent effect. When a pair of phenyl groups is attached to Cl and C3 of 1,5-hexadiene, the barrier to the Cope rearrangement is decreased by 3.0 kcal/mol from that for the unsubstituted molecule. If the phenyl substituent effects on the Cope rearrangement were additive, augmentation of a pair of phenyl substituents at Cl and C3 by another pair at C4... 

In contrast to the cooperative substituent effects described above, placement of phenyl groups at Cl, C3, and C5 of 1,5-hexadiene results in a competitive substiment effect. By stabilizing structure A, a single phenyl group at C2/C5 lowers A// by 4.2 kcal/mol and, by stabilizing structure C, phenyl groups at Cl and C3 lower A// by 3.0 kcal/mol. However, the simultaneous presence of phenyl groups at Cl, C3, and C5 lowers A// by... 

As the discussion above demonstrates, the cooperative substituent effects, which have been both observed in and computed for the Cope rearrangements of 2,5-diphenyl- and... 

The ability of radical stabilizing substituents to have a large effect on R in the TS geometry is what makes the Cope TS chameleonic and the chameleonic nature of the TS is responsible for the cooperative and competitive phenyl substituent effects that have been both calculated [27,28] and observed [5,29,30]. Multiple phenyl substituents, attached where they either all stabilize structure A or all stabilize structure C, distort the TS geometry toward one of these two diradical extremes, thus enhancing the ability of all the substituents to stabilize the TS. In contrast, when one set of substituents stabilizes one of these resonance structures and another substituent stabilizes the other, the result is a compromise TS geometry, with a value of R that is optimal for neither set of substituents. 

Decreasing the temperature from 40 to 25 C led to a slight increase of enantiomeric excess at the cost of a much lower conversion. Diphosphites with the same spiro backbone, and 1, l -bisnaphthol substituents of different chirality, afforded the opposite prevailing enantiomer in the product. This shows that the terminal bisnaphthol groups dictate enantioselection. The catalytic efficacy of these diphosphite systems is mainly controlled by the large bulky substituents. No cooperative effect between central and axial chirality was observed in this case [30]. 

Stereoelectronic effects and nonbonded interactions are non-cooperative in the reactions of (E)-allylboronates and x-heteroatom-substituted aldehydes. Thus, while transition state 8 experiences the fewest nonbonded interactions (gauche pentane type, to the extent that X has a lower steric requirement than R3), transition state 9 is expected to benefit from favorable stereoelectronic activation (Felkin-type)58f. This perhaps explains why the reaction of 2,3-[iso-propylidenebis(oxy)]propanal and ( >2-butenylboronate proceeds with a modest preference (55%) by way ol transition state 9. This result is probably a special case, how ever, since C-3 of 2.3-[isopropylidenebis(oxy)]propanal is not very stcrically demanding in 9 owing to the acetonide unit that ties back the oxygen substituent, thereby minimizing interactions with the... 

Fig. 15 Predicted cooperative effects on activation energies (in kcal/mol) at the B3LYP/ 6-31G level for model enediynes ( push and pull denote through-space repulsive (steric) and attractive (H-bonding) interactions of ort/zo-substituents with in-plane 71-orbitals of an adjacent acetylene moeity).

---