Copper(i)-catalysed regio- and diastereoselective intramolecular alkylboration of terminal allenes via allylcopper(i) isomerization

Abstract

We report the first copper(i)-catalysed intramolecular alkylboration of terminal allenes with an alkyl halide moiety. The reaction provides alkenylboronates bearing a four-membered ring structure with high regio- and diastereocontrol. A possible reaction mechanism is proposed, involving the facile isomerization of an allylcopper(i) intermediate. A DFT study explains the experimental regio- and diastereoselectivity.

Regio- and Stereoselective Synthesis of Multi-Alkylated Allylic Boronates through Three-Component Coupling Reactions between Allenes, Alkyl Halides, and a Diboron Reagent

“…In these reactions, an allylcopper­(I) species generated in situ via the regioselective 2,1-borylcupration of allenes is the key intermediate toward the alkenyl boronates. The groups of Tsuji, Hoveyda, Brown, and Liu have reported α-carbo-functionalization reactions from allylcopper­(I) intermediates to synthesize linear ( Z )-alkenyl boronates I . − On the other hand, reaction at the γ-position of the allylcopper­(I) intermediates produces the branched alkenyl boronates II , which has been reported independently by several research groups including our own. − …”

“…However, the high-valent copper(III) species could be a transient species or a transition state of the concerted mechanism. 49,71 Finally, reductive elimination forms the new C−C bond to produce the alkylboration product 4xy with concomitant restoration of copper(I) alkoxide Int1.…”

“…36−39 On the other hand, reaction at the γ-position of the allylcopper(I) intermediates produces the branched alkenyl boronates II, which has been reported independently by several research groups including our own. 40−58 During our previous study of the intramolecular alkylboration of allenes, 49 we became interested in the regiodivergence of the borylcupration. We anticipated that the multisubstituted allylic boronates III could be synthesized by generating the alkenyl copper(I) intermediate through the 1,2-borylcupration of the allene substrates and applying the intermediate to the following C−C bond formation reaction (right arrow in Figure 2A).…”

“…During our previous study of the intramolecular alkylboration of allenes, we became interested in the regiodivergence of the borylcupration. We anticipated that the multisubstituted allylic boronates III could be synthesized by generating the alkenyl copper­(I) intermediate through the 1,2-borylcupration of the allene substrates and applying the intermediate to the following C–C bond formation reaction (right arrow in Figure A).…”

One-Pot Preparation of Tricyclo[5.2.2.04,9]undecanes via Cu-Catalyzed Asymmetric Carboboration of Cyclohexadienone-Tethered Allenes

“…Because of their structural complexity, efficient synthetic approaches to such intriguing bridged tricyclic skeletons are extremely rare. Recently, significant progress has been made in borylcopper (Cu–B)-catalyzed borylation of allenes, especially carboboration of allenes (Scheme a). − The reactions of Cu–B species with allenes result in useful β-borylated allylcopper intermediates, which subsequently undergo a variety of transformations with different carbon electrophiles: aldehydes, ketones, imines, and nitriles for 1,2-addition; allyl phosphates for boroallylation; aryl iodides for boroarylation; dienoates for 1,6-addition; formate esters, carboxylic anhydrides, and acyl fluorides for boroacylation; bromoalkynes for boroalkynylation; and alkyl halides for alkylboration . Despite the above notable advances, the 1,4-addition of β-borylated allylcopper especially to α,β-unsaturated ketones has never been uncovered, due to the fact that the six-membered chairlike transition state makes the 1,2-addition to ketone more favorable .…”