ChemInform Abstract: Intermolecular Hydroamination of Allenes with N‐Unsubstituted Carbamates Catalyzed by a Gold(I) N‐Heterocyclic Carbene Complex.

Abstract

Carbamic acid derivatives P 0480 Intermolecular Hydroamination of Allenes with N-Unsubstituted Carbamates Catalyzed by a Gold(I) N-Heterocyclic CarbeneComplex. -The reaction is general for mono-1,3-and 1,1-disubstituted, tri-and tetrasubstituted allenes. Reaction of enantioenriched (XI) results in formation of a racemic product. -(KINDER, R. E.; ZHANG, Z.; WIDENHOEFER*, R. A.; Org. Lett. 10 (2008) 14, 3157-3159; French Family Sci. Cent., Duke Univ., Durham, NC 27708, USA; Eng.) -R. Steudel 48-052

Kinetics and Mechanism of the Gold-Catalyzed Intermolecular Hydroalkoxylation of Allenes with Alcohols

“…The selective formation of the linear allylic ether 3a in the gold-catalyzed hydroalkoxylation of 1 with 2 is intriguing owing both to the selective formation of tertiary allylic ether in the gold-catalyzed hydroalkoxylation of 1 in DMF (eq 5) and also the formation of the tertiary allylic carbamate in the closely related gold(I)-catalyzed hydroamination of 1 with benzyl carbamate (eq 12). 36,52 Our experimental data argue strongly against isomerization of 4a to 3a under reaction conditions, and we can also safely rule out regiochemically determining formation of gold π-allene complex 5•OTf owing to the facile (ΔG ⧧ ≤ 11 kcal/mol) unimolecular interconversion of gold π-allene complexes 5 and 5′ via η 1 -allene intermediate IV (Scheme 4). 19 The simplest rationale for the observed regioselectivity of the gold-catalyzed hydroalkoxylation of 1 with 2 is to invoke selective, irreversible attack of 2 at the less substituted allene terminus of 5.…”

“…However, we cannot rule out a mechanism involving rapid and reversible attack of 2 on 5′ that is superimposed on slower, irreversible attack of 2 on 5, and indeed, this mechanism might better account for all the observations made in the context of gold-catalyzed allene hydrofunctionalization. 4,36,52 Supporting the feasibility of such a mechanism, gold vinyl complex II′ generated via attack of 2 on 5′ is expected to be less stable than II owing to the diminished olefinic substitution, and likewise, experimental and computational analysis of protodeauration supports the more facile protodeauration of II relative to II′ owing to the presence of the electron-releasing olefinic methyl groups of II. 53 Role of Activated η 1 -Allene Species in Hydroalkoxylation.…”