Steric and Electronic Effects in the Diastereoselective Catalytic Epoxidation of Cyclic Allylic Alcohols with Methyltrioxorhenium (MTO)

Abstract

The steric effects of allylic substituents in the epoxidations with methyltrioxorhenium/urea/hydrogen peroxide adduct (MTO/UHP) have been assessed for a series of 3‐alkyl‐substituted cyclohexenes. The trans selectivity increases with the size of the substituent and the diastereoselectivities follow an excellent linear correlation with the steric substituent constants of Taft (Es) or Charton (ν). Good cis selectivity is observed in the epoxidation of the cyclic allylic alcohols 2‐cyclopentenol and 2‐cyclohexenol due to a hydroxy‐directing effect through hydrogen bonding; however, for 2‐cycloheptenol and 2‐cyclooctenol hydrogen bonding is ineffective and steric interactions cause a higher trans selectivity. The conformationally fixed cis‐ and trans‐5‐tert‐butyl‐2‐cyclohexenols serve as suitable substrates for gauging the dihedral angle of the transition states for the oxygen transfer in these epoxidations. Thus, the MTO/UHP oxidant favours a quasi‐equatorial arrangement of the hydroxy group for effective hydrogen bonding (hydroxy‐group directivity), in analogy to m‐chloroperbenzoic acid (m‐CPBA) and dimethyldioxirane (DMD), and a dihedral angle of ca. 130° is suggested.

Allylic Alcohol Epoxidation by Methyltrioxorhenium: A Density Functional Study on the Mechanism and the Role of Hydrogen Bonding

“…Our calculations confirm the previous conclusion by Adam et al. on the basis of experimental data. , …”

“…There are four TSs, 21 − 24 , in which OH forms a hydrogen bond to oxygen centers of the reacting peroxo moiety. Two of them have the OH group in outside position (OC 1 C 2 C 3 = 134° and 130° for 21 and 23 , respectively) and are similar to the qualitative transition structures advanced by Adam et al , The other two TSs exhibit an OH group in inside conformation (OC 1 C 2 C 3 = 16° and 13° for 22 and 24 , respectively). Previously, , this conformation was not considered, probably because it does not permit efficient hydrogen bonding with O 5 .…”

“…Two of them have the OH group in outside position (OC 1 C 2 C 3 = 134° and 130° for 21 and 23 , respectively) and are similar to the qualitative transition structures advanced by Adam et al , The other two TSs exhibit an OH group in inside conformation (OC 1 C 2 C 3 = 16° and 13° for 22 and 24 , respectively). Previously, , this conformation was not considered, probably because it does not permit efficient hydrogen bonding with O 5 . In our opinion, the most reasonable description of the hydrogen bonds in TSs 21 − 24 is that of “bridged” hydrogen bonds involving, to some extent, both peroxo oxygens (in particular in 21 and 23 while in 22 and 24 O 4 seems to be favored).…”

“…They considered three types of TSs (Scheme ), namely the dioxirane- ( C ), peracid- ( D ), and perhydrate-type ( E ) TSs; the first one involves a peroxo−Re complex, while the other two exhibit a hydroperoxo−Re complex. The face selectivity observed with chiral allylic alcohols seems to better comply with D -type (or E -type) TSs and with OC 1 C 2 C 3 dihedral angles of ∼120 °C, which, in opinion of these authors, , are typical for TSs of peroxyacid epoxidations of allylic alcohols. However, Adam and co-workers adhered to the generally accepted rhenium−(mono or bis)peroxo complexes as the active epoxidant species via TS C .…”

A Highly Chemoselective, Diastereoselective, and Regioselective Epoxidation of Chiral Allylic Alcohols with Hydrogen Peroxide, Catalyzed by Sandwich-Type Polyoxometalates: Enhancement of Reactivity and Control of Selectivity by the Hydroxy Group through Metal−Alcoholate Bonding

“…In the oxidation of the conformationally rigid cis - and trans -5- tert -butyl-2-cyclohexenols ( cis - 4c and trans - 4c ), more enone is formed from the cis isomer (OH pseudo equatorial , α = 140°) than the trans one (OH pseudo axial , α = 110°), although very high diastereoselectivities were observed in both cases, again in favor of the cis -epoxides (entries 4 and 5). The exclusive trans selectivity for the cyclooctenol 4d (entry 6) speaks for purely steric interactions in the conformer of minimal transannular strain, as reported for other oxidants 4 Catalytic Epoxidation of Cyclic Allylic Alcohols 4 by 30% H 2 O 2 and the Sandwich-Type Polyoxometalate M-POM a a , b For details see Table .…”