The Behavior of Cp*MoBr/Br2 Redox Systems: The Unusual Structure of [(CpMo2Br4)2(Cp*MoBr4)3], a Compound Containing MoIII, MoIV, and MoV
✍ Scribed by Dr. Jahanvi U. Desai; Dr. John C. Gordon; Dr. Heinz-Bernhard Kraatz; Dr. Beth E. Owens-Waltermire; Prof. Rinaldo Poli; Prof. Arnold L. Rheingold
- Publisher
- John Wiley and Sons
- Year
- 1993
- Tongue
- English
- Weight
- 375 KB
- Volume
- 32
- Category
- Article
- ISSN
- 0044-8249
No coin nor oath required. For personal study only.
✦ Synopsis
it must be assumed that heterodiene 1 and Lewis acid 4 form at least two diastereomeric transition structures in which the ratio of 1 to 4 is unknown. The pressure dependence of the enantioselectivity implies that the transition structure that leads to 2 has a smaller (more negative) activation volume and thus is more compact. Therefore the preferential formation of 2 at normal pressure is enhanced at increased pressure. However, this advantageous result for the enantioselective transformation of 1 cannot be generalized. Preliminary investigations[' on the intermolecular cycloaddition of 5 and 6 in the presence of chiral Lewis acid 4 showed a decrease in enantioselectivity when pressure was increased.
5 6
This is what we expected, because it can be assumed that the diene attacks the complex of the dienophile and the chiral Lewis acid from the less hindered side; the corresponding transition structure should therefore have a larger volume than that leading to the other enantiomer, and will thus be disfavored at high pressure.
The observed increase in the enantioselectivity of the intramolecular reaction of 1 at higher pressure may be due to the formation of complexes with different stoichiometry. This point will be clarified by further investigations with other substrates, and by varying the Lewis acid concentration. In particular, chiral and achiral Lewis acid/substrate complexes will be studied at high pressure by N M R spectroscopy.