Electrochemical synthesis of allenyl silanes and allenyl boronic esters

Abstract

Allenyl silanes and boronates are pivotal building blocks in organic synthesis. Nevertheless, their synthesis requires the manipulation of transition metal or highly reactive species. Hence, the development of more sustainable protocol is highly sought after. Here we show the electrochemical synthesis of allenyl silanes and allenyl boronic esters. This catalyst-free method proceeds under mild reaction conditions. The protocol for the synthesis of allenyl silanes shows an excellent efficiency and a good functional group tolerance. The allenyl silanes are isolated in good yields (28 examples, 45-95% yields) without the use of a transition metal catalyst and under mild reaction conditions. A similar protocol is developed for the synthesis of allenyl boronates, which are obtained in low to moderate yields (13 examples, 5-55% yields). Finally, a mechanism based on an oxidative generation of the silyl and boryl radicals is suggested to access these classes of allenes.

Fig. 1. State of the art and present work.

Eq. (1). Transition metals catalyzed synthesis of allenyl silanes. Eq. (2). Metallated allenes as precursors of allenyl silanes. Eq. (3). Silylated ketene as precursor of allenyl silane (Wittig approach). Eq. (4). Silylated alkynes of propargyl derivatives as precursor of allenyl silanes.

Fig. 2. Electrochemical synthesis of allenyl silanes – scope and limitations.

Reaction conditions: propargyl acetate (0.3 mmol), PhMe₂Si–BPin (0.6 mmol), nBu4NCl (0.4 mmol), CH₃CN/CH₃OH (9:1, 4 mL), 30 mA, r.t., air.^a diasteroisomeric ratio determined by ¹H NMR.

Fig. 3. Electrochemical synthesis of allenyl boranes.

Reaction conditions: propargyl carbonate (0.2 mmol), B₂Pin₂ (0.4 mmol), nBu₄NBF₄ (0.02 mmol), CH₃ONa (0.2 mmol), CH₃OH (4 mL), 25 mA, rAP (150 ms) r.t., air.^a determined by ¹H NMR using CH₃NO₂ as the internal standard.

Fig. 4. Mechanistic investigations and plausible reaction mechanism.

A Study of the stereoretention of the process. B Radical trapping experiment. C Cyclic voltammetry studies. D Plausible reaction mechanism—for the sake of clarity ligand on the boryl radical was omitted.

Fig. 5. Synthetic utility of the products.

^aZ:E ratio determined by ¹H NMR. ^b Diastereoisomeric ratio determined by ¹H NMR.