Non-Cp titanium alkoxide-based homolytic ring-opening of epoxides by an intramolecular hydrogen abstraction in β-titanoxy radical intermediates

Abstract

A low-valent titanium species derived in situ from Ti(O-i-Pr)(4), Me(3)SiCl and Mg powder in tetrahydrofuran reacted with epoxides to selectively provide less hindered alcohols via a homolytic ring-opening of epoxides, in which the intermediate β-titanoxy radical intramolecularly abstracted a hydrogen atom from an alkoxy moiety in the titanium complexes.

1,n‐Hydrogen‐Atom Transfer (HAT) Reactions in Which n≠5: An Updated Inventory

“…In connection with the diastereoselective method developed by Gansauer for the reduction of epoxides (see Scheme ),48 it is worth citing the procedure reported by Okamoto et al. (Scheme ) 93. A low‐valent titanium species, generated in situ from [Ti(O‐ i Pr) 4 ], Me 3 SiCl, and Mg powder in THF, is responsible for the regioselective reductive cleavage of the CO bond.…”

“…In connection with the diastereoselective method developed by Gansauer for the reduction of epoxides (see Scheme 19), [48] it is worth citing the procedure reported by Okamoto et al (Scheme 46). [93] A low-valent titanium species, generated in situ from [Ti(O-iPr) 4 4 ] to afford 46.4. The preference for the less hindered cis-fused seven-membered transition state controls the diastereoselectivity of the reaction.…”

Synthetic Reactions Using Low-valent Titanium Reagents Derived from Ti(OR)4or CpTiX3(X = O-i-Pr or Cl) in the Presence of Me3SiCl and Mg

“…Sections 3.3.1 and 3.3.2 disclose radical ring-opening reactions of epoxides and oxetanes mediated by titanium alkoxide 1 or 2 with Me 3 SiCl and Mg powder. [22,23] In the reduction of epoxides to give the corresponding alcohols, reactions with metal hydrides typically proceed at less hindered positions to afford more-substituted alcohols. On the other hand, reductions via single-electron transfers (SETs) proceed at more-substituted positions due to the increased stabil-ity of the intermediate radicals, yielding less-substituted alcohols.…”

Nachhaltige stereoselektive Radikalreduktion durch katalytischen H‐Atom‐Transfer (HAT)

“…Im Falle cyclischer Radikale werden daher hoch selektiv über eine syn-selektive Radikalreduktion die trans-Produkte erhalten, [9] was sowohl die experimentellen als auch die theoretischen Untersuchungen belegen (Schema 2). Der postulierte intramolekulare HAT sollte zu geordneten cyclischen Übergangszuständen führen.…”

“…Der postulierte intramolekulare HAT sollte zu geordneten cyclischen Übergangszuständen führen. Im Falle cyclischer Radikale werden daher hoch selektiv über eine syn-selektive Radikalreduktion die trans-Produkte erhalten, [9] was sowohl die experimentellen als auch die theoretischen Untersuchungen belegen (Schema 2).…”