Chiral cluster cations

This volume seeks in a small way to bridge the wide gap between organic chemistry in the gas and condensed phases. The same types of chiral ion-dipole complexes that form as intermediates of solvolysis may be generated in the gas phase by allowing neutral molecules to cluster with chiral cations. The reactions of these chiral clusters have been characterized in exquisite detail by mass spectrometry. The results of this work are summarized by Maurizio Speranza in a chapter that is notable for its breadth and thoroughness of coverage. This presentation leaves the distinct impression that further breakthroughs on the problems discussed await us in the near future. 

Another way to obtain chiral clusters is to use mixed-metal systems (see Section II,A). Addition of acid to one of the two diastereomers of [WCo(/i-HC2CH(OH)Et)(CO)5Cp] leads to formation of two propargylium cations which do not isomerize in solution on the NMR time scale.75 Similarly, protonation of the separate diastereomers of [MoCo(/i-2-propynylborneol) (CO)5Cp] leads to formation of propargylium cations which are nonflux-ional. It has been shown that the diastereomer ratios for metal-stabilized cationic clusters can be directly correlated with the ionization process, and it is proposed that the elimination of water is anchimerically assisted by... 

A wide range of thermochemical properties can be measured, including not only proton affinity or gas-phase basicity, but also electron affinity, ionization energy, gas-phase acidity and cation affinity Entropy changes upon attachment of an ion to a molecule are also accessible and provide information on both the nature of the bonding and fragmentation mechanisms in cluster ions, especially in biological compounds. Thermochemical determinations by the kinetic method also provide very useful structural information e.g., two-electron three-center bond has been observed in the gas phase by means of the kinetic method. " In the last years, the kinetic method has been also applied to characterize chiral ions in the gas phase. 

FIGURE 99 Structures of the cationic clusters, [Pr4 (+)-L7)9(/i3-OH)] (upper left) with its core motif (upper right), and [Pr3 (-l-)-L7 6(/i3-0H)(H20)3] (bottom), wherein L is a chiral bipyridinecarboxylate ligand (redrawn after Mamula et al., 2006). 

Similarly, a [Pd2(dba)3]/dppb catalyst in toluene at 100 °C isomerizes PhC(O)—C=C—Bu into the dienone PhC(0)CH=CH—CH=CHEt in good yields. A variety of dienones has been prepared in this way. " There can be little doubt that all these reactions proceed via Pd Tr-allyl intermediates. The asymmetric isomerization of 4-hydroxycyclopent-2-en-l-one is catalyzed by [(R)-BINAP)Rh(MeOH)2] and gives 4-hydroxycyclopent-3-en-l-one as the primary product. With racemic starting material, the 5 enantiomer is consumed faster, to give a 5 1 enantiomeric discrimination. The primary product can tautomerize to 1,3-cyclopentadione. The same chiral cationic rhodium catalyst is also effective in the asymmetric isomerization of allylamines. Water impurities deactivate the catalyst. One such deactivation product is the air-stable trinuclear cluster (20), which was characterized by X-ray diffreaction. "" ... 

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