Cieplak effect

Halterman et al. reported that 5-aryl-5-phenylcyclopentadienes 23-25 reacted with dienophiles to favor the reactions on the anti side of the more electron rich aromatic system [19]. The orbital mixing rule failed to predict this selectivity, since orbital mixing is expected to take place mainly by mediation of the JtAr-HOMo of more electron rich aromatic system (Scheme 13). Destabilization due to the orbital phase environment or stabilization due to Cieplak effects can be responsible for the selectivity (See Sects. 2.1.2 and 2.1.3). 

We pointed out that these results can be attributable to the a-n interaction. At the transition state, the o orbital at C5 on the anti side of the dienophile is parallel with the n orbital, the o bond electrons are able to delocalize much more effectively than that on the syn side. Since the electron donating o bond on the anti side stabilizes the transition state, the a-n interaction can contribute to rr-facial selectivity. These results suggested that the bond lengthening cannot necessarily be convincing evidence for the Cieplak effect, but can be explained in terms of the a-n interaction without assuming the incipient a bonds at the transition state (Scheme 30). 

Ab initio calculation of Diels-Alder reactions of a series of 5-heteroatom substituted cyclopentadienes Cp-X (65 X = NH, 50 X = NH, 64 X = NH3, 67 X = O", 54 X = OH, 68 X = OH3% 69 X = PH, 51 X = PH, 70 X = PH3% 71 X = S, 55 X = SH, 72 X = SH/) with ethylene at HF/6-31++G(d)//HF/6-31-i i-G(d) level by BumeU and coworkers [37] provided counterexamples of the Cieplak effect. The calculation showed that ionization of substituents has a profound effect on the n facial selectivity deprotonation enhances syn addition and protonation enhances anti addition. The transition states for syn addition to the deprotonated dienes are stabilized relative to those of the neutral dienes, while those for anti addition are destabilized relative to those of the neutral dienes. On the other hand, activation energies for syn addition to the protonated dienes are similar to those of the neutral dienes, but those for anti addition are very much lowered relative to neutral dienes (Table 6). 

Overman, Hehre and coworkers reported anti rr-fadal selectivity in Diels-Alder reactions of vinylcyclopenten 73, 74 and 4,5-dihydro-3-etliynylthiophen S-oxide 75 [38] (Scheme 31). These results are not in agreement with the Cieplak effect, at least in Diels-Alder reactions of the dienes having unsymmetrical rr-plane. Yadav and coworkers reported that the reactions between the vinylcyclohexene 76 and dienophiles favor the reactions syn to oxygen, while 77 and 78 favor the reaction anti to oxygen substituents [39], They discuss the Cieplak effect but the reactions are not suitable. 

The Jt-facial selectivity was explained by the Cieplak Effect due to back-donation of lone pair electrons on sulfur (Scheme 49). Mansuy and coworkers reported that in situ generated thiophene 1-oxide 99 could be trapped by 1,4-benzoqui-none to afford the corresponding syn attack product [58]. Tashiro and coworkers also reported that in situ generated thiophene 1-oxide derivatives 98,100-103 and... 

Some further examples of the reduction of adamantanones have highlighted that increasing the positive dipole on the C=0 using Lewis acids, or placing charged substituents at C(5) within the adamantyl framework, enhances face selectivities in borohydride and aluminium hydride reductions due to Cieplak effects. 

Aggarwal, V. K., Ford, J. G., Fonquerna, S., Adams, H., Jones, R. V. H., Fieldhouse, R. Catalytic Asymmetric Epoxidation of Aldehydes. Optimization, Mechanism, and Discovery of Stereoelectronic Control Involving a Combination of Anomeric and Cieplak Effects in Sulfur Ylide Epoxidations with Chiral 1,3-Oxathianes. J. Am. Chem. Soc. 1998, 120, 8328-8339. 

Thia-Claisen rearrangements of N-benzylpyrrolidine-2-thione and chiral allylic bromides derived from D-mannitol have been reported. Introduction of a bromine atom 0 onto the double bond of the allyhc bromide reverses the sense of diastereoselectivity in the [3,3]-sigmatropic rearrangement and DFT calculations rationalize the selectivity in terms of a Cieplak effect (Scheme 34). 

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