Grignard reagents-catalyzed hydroboration of aldehydes and ketones
Introduction
Since their discovery by Victor Grignard in 1900, Grignard reagents have proved to be an extremely powerful and ubiquitous synthetic tool in synthetic chemistry due to their ease of preparation and broad application in organic and organometallic synthesis [1,2]. Over the past 100 years and more since their discovery, considerable efforts have been made to investigate the formation, structure, reactivity, application, and mechanism of Grignard reagents [[3], [4], [5], [6]]. Grignard reagents, especially the classical magnesium-based Grignard reagents (RMgX), are probably the most widely used organometallic reagents in inorganic, organic and organometallic chemistry to date. Nevertheless, the development of organomagnesium chemistry is still vital and full of surprises for their easy accessibility, high reactivity, improvement of atom economy and widespread application. Not only do they find extensive use on a small scale in many research laboratories worldwide but they also have been prepared and utilized on a larger scale in diverse industrial processe [[7], [8], [9], [10], [11], [12]]. For example, one of the most important uses of the Grignard reagent is the reaction with aldehydes and ketones to form alcohols. When reacted with another halogenated compound in the presence of a suitable catalyst, they can also be used as transfer reagents for alkyl and aryl moieties in the C–C cross-coupling reaction to increase the carbon chain length of products [[13], [14], [15], [16]], and even reacted with carbon dioxide to generate the corresponding carboxylic acid containing one more carbon atom than that of the starting material [17]. However, to our surprise, the amount of Grignard reagents employed in these protocols are stoichiometric and very few catalytic applications of Grignard reagents have been reported to date[18,19].
Hydroboration of carbonyl compounds is an attractive organic transformation among the various carbonyl reduction methodologies available because boron hydrides are relatively stable and helps to circumvent the use of highly flammable and pressurized hydrogen gas and stoichiometric amounts of metal hydride reagents. In addition, the resultant borate esters are versatile synthetic intermediates which can be further hydrolyzed to various functional alcohols [[20], [21], [22], [23]]. Until now, the catalytic hydroboration of aldehydes and ketones is achieved with numerous catalysts based on transition metals and main group elements [24]. Among main group elements, literature reports about the earth-abundant and inexpensive alkaline earth metal catalyzed hydroboration of carbonyls are relatively few [[25], [26], [27], [28], [29], [30], [31], [32], [33]]. In most instances, these catalysts stabilized by sterically hindered ligands are expensive and relatively difficult to synthesize. Recently, Hreczycho and co-workers reported catalyst-free and solvent-free hydroboration of aldehydes. However, this protocol requires higher temperature and longer reaction time to get better conversions. More importantly, this method is completely ineffective for ketones and gave only trace amounts of the desired boronic esters even at elevated temperature [34]. Quite recently, Ma et al. reported hydroboration of ketones in the absence of a catalyst, however, it also requires excessive HBpin and higher temperature [35]. Therefore, it is essential to develop a convenient and easily attainable alkaline earth metal-based catalyst for hydroboration of both aldehydes and ketones at ambient temperature which will offer a powerful and sustainable alternative.
Herein, we report the highly efficient hydroboration of aldehydes and ketones with HBpin catalyzed by simple, commercially available and inexpensive Grignard reagents in truly catalytic amounts.
Section snippets
Results and discussion
We began our investigations into a plausible catalytic application of Grignard reagents with the addition of only 0.05 mol% of MeMgI as a catalyst to the mixture of benzaldehyde and HBpin under neat condition. To our surprise, the corresponding borate ester product was observed in a quantitative yield after 10 min at room temperature (Table S1, entry 1). Other commercially available Grignard reagents have been screened as well. All tested Grignard reagents showed excellent catalytic activity
Summary
In summary, we have demonstrated that a series of simple, inexpensive, commercially available Grignard reagents can be employed in truly catalytic scale as efficient precatalysts for the hydroboration of a wide range of aldehydes and ketones with HBpin. The reaction proceeded rapidly with very low catalyst loading under neat condition at room temperature. It demonstrated high functional group tolerance for pyridine, ester, halides and nitro group etc. High chemoselectivity for aldehydes over
Experimental section
General Information. All reactions were performed under an atmosphere of nitrogen using glovebox technique. 1H, 13C{1H}, 11B{1H}, and 19F{1H} NMR spectra were recorded at 25 °C on Bruker Avance III 600 MHz spectrometer in deuterated solvents and were referenced to the resonances of the solvent used. Chemicals were purchased from Sigma-Aldrich, Alfa Aesar, and Acros and used without further purification.
Experimental procedure. General procedure for catalytic hydroboration of aldehydes. To the
Notes
The authors declare no competing financial interest.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We are thankful for financial support from the National Natural Science Foundation of China (21772093) and the Natural Science Foundation of Jiangsu Province, China (BK20181421).
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