Unusual Carbon−Carbon Bond Formations between Allylboronates and Acetals or Ketals Catalyzed by a Peculiar Indium(I) Lewis Acid

Abstract

In(I)OTf has been uncovered as an effective Lewis acid catalyst for unprecedented nucleophilic substitution of acetals or ketals with allylboronates. A transmetalative S(N)1 mechanism is proposed in which a single In(I) center acts as a dual catalyst to activate both reagents sequentially. Contrary to the classic gamma-selectivity of allylsilanes (Hosomi-Sakurai reaction), this In(I)-catalyzed borono variant displays distinct alpha-selectivity. Substrate scope and functional group tolerance proved to be excellent.

“Design” of Boron‐Based Compounds as Pro‐Nucleophiles and Co‐Catalysts for Indium(I)‐Catalyzed Allyl Transfer to Various Csp3‐Type Electrophiles

“…[7] We have proposed a transmetalative S N 1 mechanism, in which indium(I) plays a dual role, as shown in Figure 1. [6] In the case of allyl borane 2 b, however, we cannot rule out a nontransmetalative pathway, which involves the direct reaction of the more reactive boron-ate complex D with electrophile A. The in situ-formed InOMe then triggers B-to-In transmetalation with borane 2 b (or boronate 2 a) to generate the nucleophilic allyl indium(I) species (B), which subsequently reacts with electrophile A to give the desired allylated product 3.…”

“…This rare a-selectivity with 2 c is a strong indicator for catalytic B-to-In transmetalation prior to CÀC bond formation. [6] Finally, without 4 a, the reaction with allenyl boronate 2 e hardly proceeded (Table 6, entry 5). The low reactivity of these boronates is consistent with our previous work, and may be explained by a particularly slow transmetalation owing to steric congestion at the g-position.…”

“…[2c,d] Moreover, boron-based reagents are attractive because of high functional group tolerance, low toxicity, and convenient handling. [6] To the best of our knowledge, this work represents the first main group metalcatalyzed activation of allyl boronates for C À C bond formation with noncarbonyls. [3] While allylation of Csp 2 -type electrophiles, such as carbonyl compounds and imines, has been extensively studied, [4] the coupling of allyl reagents with Csp 3 -type electrophiles is relatively underexplored.…”

Expanded Substrate Scope and Improved Reactivity of Ether-Forming Cross-Coupling Reactions of Organotrifluoroborates and Acetals

“…24 Furthermore, recent reports have shown that such reactions can be rendered enantioselective. In our own efforts, we recently reported a cross-coupling strategy for the preparation of dialkyl ethers by the BF 3 •OEt 2 -promoted reaction of potassium organotrifluoroborates and O -MOM acetals (Scheme 1).…”

Gallium and Indium Compounds in Homogeneous Catalysis

“…R 1 = alkyl, aryl, heteroaryl (ees: 72-97%)3.3.2 Indium(I) TriflateBorono Variant of Hosomi-Sakurai Reaction with Acetals or Ketals: Synthesis of Homoallyl Ethers The In(OTf)-catalyzed reaction of acetals or ketals with allylboronic acid pinacol esters allows access to various homoallyl ethers(71) 99. Deuterium-labeling experiments support a transmetalative S N 1 mechanism involving the reaction of an In(I)-allyl species with an oxonium triflate intermediate as a key step for product formation.…”