[reaction: see text] The efficient addition of bis(pinacolato)diboron to alpha,beta-unsaturated carbonyl compounds with a copper-diphosphine catalyst has been carried out. A dramatic rate acceleration of the reaction was realized by adding alcohol additives. With use of this procedure, a variety of alpha,beta-unsaturated carbonyl compounds including conjugated substrates at the acid oxidation level such as esters and nitriles were reacted to give to the corresponding beta-boryl carbonyl compounds in high yields.

An existing challenge is the development of efficient regioselective catalytic systems that are compatible with stable hydroboration reagents and can be rendered enantioselective by the use of nonracemic ligands. Copper(I) complexes with chelating phosphines catalyzed the regio‐ and enantioselective hydroboration of styrenes with pinacolborane (PinBH) at room temperature to afford the corresponding branched boronate esters (see example).

Organoboranes are versatile synthetic intermediates for the preparation of a wide range of organic molecules. An increasing effort has been devoted to the efficient synthesis of organoboron compounds. One of the important tools for the synthesis of organoboranes is transition-metal-catalyzed addition of diboron reagents such as bis(pinacolato)diboron to carbon-carbon multiple bonds, which has been the subject of extensive research.[1] In comparison with electron-rich alkene or alkyne substrates, the reaction with a,b-unsaturated carbonyl compounds has not been studied as extensively. Since introducing a boronate group at the b-position to a carbonyl using conventional hydroboration methods is not possible, the metal-catalyzed b-boration of a,b-unsaturated carbonyl compounds provides an interesting approach. Such reactions have been reported using systems based on platinum, [2] rhodium, [3] and copper [4] but with limitations such as high catalyst loading, high temperature, low to moderate yield, and narrow substrate scope.Recently, we reported an efficient copper-catalyzed addition of bis(pinacolato)diboron (B 2 pin 2 ) to a range of a,b-unsaturated carbonyl compounds, the substrate scope of which was extended from enones to more challenging a,bunsaturated esters, nitriles, and phosphonates. [5] In view of the variety of stereospecific transformations available to stereogenic carbon-boron bonds, [6] we envisioned that catalytic enantioselective boration of a,b-unsaturated carbonyl compounds would easily provide functionalized enantioenriched organoboron compounds. However, an asymmetric boration of such compounds has not been reported yet. Herein, we describe the enantioselective b-boration of a,b-unsaturated esters and nitriles catalyzed by a nonracemic copper phosphine complex.In our previous study on the copper-catalyzed b-boration, we found that both a suitable ligand and methanol additive were required for complete conversion. Especially the alcohol was critical to the enhanced rate of reaction; even a reaction with no ligand proceeded with great conversion in the presence of methanol. Because this methanol effect could be deleterious to enantioselectivity, minimizing the background reaction would be the key for the successful development of an asymmetric variant of the b-boration.In preliminary experiments, we chose cinnamonitrile as a model substrate and (R)-(S)-josiphos (L1, Scheme 1) as a nonracemic ligand on the basis of its successful use in the asymmetric reduction of acrylonitriles, [7] and we examined a range of reaction conditions. Variable enantiomeric excesses (50-85 % ee) were obtained without reproducibility when a 1:1 combination of CuCl and NaOtBu was employed. We surmised that insufficient reaction of the two inorganic salts in THF, an inadvertent shortage of the nonracemic ligand relative to copper, or a combination of these two factors might increase the concentration of catalytic species that cause nonselective reactions. Increasing the amount of base to 1.5 equivalents relative to copper...

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The efficient copper-catalyzed addition reaction of bis(pinacolato)diboron to alpha,beta-acetylenic esters has been developed, which produces the corresponding beta-borylated-alpha,beta-ethylenic esters in high yields under mild reaction conditions.