Exceptional Deshielding of59Co Caused by Deuteration of the Hydrogen Bonds in Cobaloximes

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.

Simulation of 59Co NMR Chemical Shifts in Aqueous Solution

“…[70] We have evaluated this secondary isotope effect for 3, employing the (mass-dependent) perturbational zeropoint corrections in the gas phase. With this method, even more subtle H/D isotope effects in cobaloximes [71] have been reproduced and interpreted computationally. [17] On going from 3 to [D 18 ]-3, a shielding of the 59 Co resonance of À105 ppm is computed, in excellent agreement with experiment.…”

Direct detection of nitrogen-14 in solid-state NMR spectroscopy

“…The above publications spurred an increase in the use of 14 N NMR as a probe of LCs, lipid bilayers and related systems, with a number of reports studying 14 N linewidths and relaxation [180][181][182][183][184][185][186][187][188][189][190][191], and others investigating quadrupolar splittings [192][193][194][195][196][197][198][199][200][201][202][203][204][205][206][207][208][209][210].…”