A downfield shift of 25 ppm or more for each deuteration of the hydrogen bonds of cobaloximes (see schematic illustration; L=pyridine, PPh3; R=Me, iPr) indicates an expansion of the coordination environment and is an impressive example of the powerful magnifying effect of the 59Co chemical shift in revealing structural changes. The 1H and 2H NMR data also are consistent with lengthening of the O⋅⋅⋅O distances.

No abstract

No abstract

Transition metal shieldings are useful probes of structure and reactivity for coordination compounds, since they allow even tiny variations at the coordination site to be detected. [1] The isotope 59 Co exhibits the largest known shielding range. Its NMR properties have been extensively studied for tert-Butyl, phenyl, and benzyl cyanocuprates were not reactive enough to take part in the reaction. The phenyl and benzyl groups could nonetheless be introduced starting from phenyl-or benzylacetylene. However, neopentyl alkynes could not be carboaluminated efficiently. Also, the stereoselectivity in the cuprate addition was slightly lower in the case of phenyl-substituted allyl pivalates 6 c and 7 c. This decrease was also observed in our first generation system with aryl-substituted allyl carbonates (Scheme 1, R 1 Ar). [1b] The disubstituted double bonds in adducts 9 were easily cleaved with ozone or with KMnO 4 /NaIO 4 leading to good yields of aldehydes, carboxylic acids, or alcohols depending on the work-up performed. For example, 9 d was cleaved with O 3 in dichloromethane to yield 65 % of the aldehyde 12 (Scheme 4). The chiral auxiliary was recovered in 80 % yield iPr Me iPr (CH 2 ) 3 OTBS Me iPr H O TBSO 9 d a) O 3 , CH 2 Cl 2 -78 °C b) Ph 3 P 12 + 5 Scheme 4. Oxidative cleavage of 9 d. TBS tert-butyldimethylsilyl. after chromatography and could be re-used. The conversion of 9 e into 14 (Scheme 5) was confirmed by comparison of its optical rotation ([a] D ) with that of its enantiomer reported in the literature ([a] D À 9.78, CHCl 3 , c 0.3; literature [a] D 9.88, c 0.3). [20] This observation indicated that the stereochemistry of 9 e results from an anti-selective cuprate addition on the configuration and conformation corresponding to D (Scheme 2). iPr Me Et TBSO iPr Me Et MeO 2 C MeO 2 C Me Et MeO 2 C 9 e 13 14 a) TBAF / THF, 100% b) Jones' c) CH 2 N 2 / Et 2 O 85% for 2 steps a) KMnO 4 / NaIO 4 acetone / H 2 O b) CH 2 N 2 , Et 2 O 40% for 2 steps Scheme 5. Proof of the absolute configuration of 9 e. TBAF tetrabutylammonium fluoride.The present method should prove a valuable addition to the arsenal of asymmetric reactions available for the synthesis of natural or unnatural chiral compounds.