Stereochemistry of the hydroboration reaction

Abstract

the neighborhood of 13-14 kcal/mol can be ascribed to amide torsional barriers. Our results, however, are in striking contrast with those reported for the analagous compound in which the methoxy groups are replaced by fluorine atoms. zb Unless accidental equivalence is responsible for the differences in the two systems, it would appear that replacement of fluorine by methoxy both raises the amide barrier and lowers the barrier to nitrogen inversionzb (or ring reversalzc). Experimental SectionN,iV'-Biscarboethoxy -3,3,4,4-tetramethoxy -1,2 -diazetidene was prepared as previously reported.6The nmr spectra were measured on a Varian A-60A spectrometer equipped with a Varian variable-temperature probe using ca. 10% solutions. Temperatures were determined using methanol spectra as described in the Varian Manual. Rate constants and equilibrium constants were determined by matching experimental spectra with theoretical spectra. The theoretical spectra were generated using Saunder's Many Site NMR Lineshape Program.7 This program allows the calculation of nmr spectra involving exchange between n sites (2 5 n 5 25) which must be uncoupled, but need not have the same population. The two "out" methoxy groups of Ib which are diastereotopic with respect to the "in" methoxy groups were treated as two separate isomers. Exchange was assumed t o be possible between l a and both sites in lb and between IC and both sites in Ib, but not directly between la and IC, nor between the "in" and "out" sites of l b . The chemical shift differences and the relative populations were temperature dependent and were determined by iterating to obtain the best fit.

Stereo- and Regioselectivity of Reactions of Siliranes with Aldehydes and Related Substrates

“…The relative stereochemistry of the deuterated silanes 4 and 6 could not be determined unambiguously. The strategy for proof of relative stereochemistry rested on the oxidation of these silanes to the deuterated alcohols .…”

Transmetalation of Alkylboranes to Palladium in the Suzuki Coupling Reaction Proceeds with Retention of Stereochemistry

“…The isomer derived from cis - 2 possesses a 5.8 Hz coupling constant, whereas the isomer from trans - 5 has a 9.1 Hz coupling constant. , Consequently, the palladium-catalyzed coupling occurred to give the products syn - 7 and anti - 8 from eqs 3 and 4, respectively. Because hydroboration is a syn addition process, and reductive elimination from palladium proceeds with retention of configuration, transmetalation from boron to palladium must therefore occur with retention of configuration.…”

DieB-Alkyl-Suzuki-Miyaura-Kreuzkupplung: Entwicklung, Untersuchungen zum Mechanismus und Anwendungen in der Naturstoffsynthese

“…Eine weitere Hydroborierung von 6 mit 9‐BBN‐H und anschließende Suzuki‐Kupplung mit PhBr lieferten 8 , dessen H a ‐H b ‐Kopplungskonstante von 4.8 Hz für eine syn ‐Konfiguration spricht. Da die Hydroborierung ein syn ‐Additionsprozess50 ist und die reduktive Eliminierung der Pd‐Zwischenstufe bekannterweise unter Retention der Konfiguration verläuft,51 schlussfolgerten Soderquist et al, dass der Transmetallierungsschritt ebenfalls unter Retention der Konfiguration verläuft. Durch Umkehrung der Reihenfolge der Zugabe der Reagentien wurde das anti ‐Isomer 11 hergestellt, wobei die H a ‐H b Kopplungskonstante von 12.5 Hz in Einklang mit der Transmetallierung unter Retention der Konfiguration war (Schema ).…”