Structure–reactivity correlations of the abnormal Beckmann reaction of dihydrolevoglucosenone oxime

Abstract

A structural, spectroscopic and computational study of a series of oximes was undertaken to investigate how geometric and structural changes relevant to the reaction coordinate for the Beckmann reaction (normal Beckmann) and Beckmann fragmentation (abnormal Beckmann) manifest in the ground state. X-ray structures of a range of oximes derived from dihydrolevoglucosan (Cyrene™; which undergoes the abnormal Beckmann reaction), in which the oxygen substituent was systematically varied were determined. As the electron demand of the OR group increased, the major structural changes included lengthening of the N-OR bond distance, and a decrease in the magnitude of the C2[double bond, length as m-dash]N-O bond angle, consistent with the changes seen for cyclohexanone oximes, which undergo the normal Beckmann reaction. However, unique to the Cyrene oximes, an increase in the length of the fissile C1-C2 bond was observed, which correlated with a decrease in the C-C 1-bond coupling constants as the electron demand of the OR substituent increased. Computational studies of Cyrene and cyclohexanone oximes using Natural Bond Orbital analysis support an electronic structure involving n → σ* and σ → σ* localized orbital interactions.

Nitromethane as a nitrogen donor in Schmidt-type formation of amides and nitriles

“…However, mild Beckmann rearrangements are still rare (21). Some specialized active oxime substrates, strong protic acid promotion, or preactivation of the hydroxyl (e.g., by tosylation) were usually required in classic catalytic Beckmann rearrangements (22)(23)(24)(25). Moreover, traditionally, aldoximes are rarely transformed into the corresponding nitriles and N-unsubstituted amides (25).…”

Cyrene: a bio-based novel and sustainable solvent for organic synthesis

“…24 Furthermore, the authors in this case experienced prolonged reaction times and lower yields. They initiated their investigations using iodobenzene (13) and phenylacetylene (14) as a model reaction to find the optimum reaction conditions (Table 2). 24 The employed palladium catalyst was [Pd(PPh 3 )Cl 2 ] and CuI was used as the additive.…”

Bio‐available Solvent Cyrene: Synthesis, Derivatization, and Applications

“…One important area of Cyrene derivatization is the conversion of the bio-available compound into other platform chemicals through cleavage of the acetal moiety( Scheme 1). [33] Owing to the facile enolization of Cyrene( 2)i nt he presence of as trongb ase, several groups have subjected the bicycle to aldol condensation protocols to form enones.F or example, Greatrex and co-workersreported that in the presence of tetramethylguanidine that Cyrene reactedw ith av ariety of aldehydes 10 to afford enones 11 in moderate to good yields (Scheme 3a). [31] Furthermore, treatment of Cyrene with aqueous hydrogen peroxide afforded the unsaturated chiral lactone (S)-g-hydroxymethyl-a,b-butyrolactone (4)i ng ood yield (Scheme1b).…”

“…[36] The stereoselectivity of the process presumably resultsf rom addition of the chloride to the less hindered ether 6.W illiams and co-workersp erformed an abnormal Beckmannr eaction on Cyrene derivative 7 to give nitriles 8 and 9 as an inseparable mixture (Scheme 2b). [33] Owing to the facile enolization of Cyrene( 2)i nt he presence of as trongb ase, several groups have subjected the bicycle to aldol condensation protocols to form enones.F or example, Greatrex and co-workersreported that in the presence of tetramethylguanidine that Cyrene reactedw ith av ariety of aldehydes 10 to afford enones 11 in moderate to good yields (Scheme 3a). [34] In addition, they showedt hat Cyrene was a competent nucleophile for direct alkylation with benzylbromide, although ac omplexm ixture of products was obtained.…”