Ionic and Organometallic‐Catalyzed Organosilane Reductions

Abstract

This chapter presents a critical review of synthetically useful variations of ionic methods for hydrogenation of organic compounds. In practice ionic hydrogenation involves the formal introduction of hydride from a donor source to an electron‐deficient carbon center. In this chapter, organosilicon hydrides are used as the source of ionic hydride with the goal of completing and updating earlier review work. Organosilicon compounds with at least one Si‐H bond have the ability to serve as mild‐air and water‐stable sources of hydrides and, thus, have reducing properties. In general, organosilicon hydrides do not undergo spontaneous reactions with organic compounds unless the organic substrate is a reasonably strong electrophile or the silane is first activated by the interaction of a nucleophilic species with a nucleophilic center. The organosilicon hydrides are covalent compounds that have little or no nucleophilic properties on their own. The use of these hydrides often provides a means of effecting reductions of organic substrates under very mild conditions and with excellent functional group selectivity. Consideration of the nature of the Si‐H bonds provides insight into the chemical behavior of these hydrides. Enhanced hydridic nature manifests itself in the chemical behavior of hydrosilanes.

Efficient Palladium‐Catalyzed CO Hydrogenolysis of Benzylic Alcohols and Aromatic Ketones with Polymethylhydrosiloxane

“…As can be seen from Table 1, the scope of this Pd‐catalyzed reductive deoxygenation of benzylic alcohols with PMHS appears to be very broad, and the reaction is satisfactory even with versatile secondary alcohols. Since the ability of PMHS to reduce ketones to alcohols has been recognized in the past,13 we hypothesized this reductive system would be expected to achieve the reductive deoxygenation of aromatic ketones. Then the palladium‐catalyzed reductive deoxygenation of aromatic ketones was also tested with PdCl 2 as catalyst and PMHS as reducing agent.…”

“…of PMHS, the generality of the reductive deoxygenation reaction was investigated using a number of substituted benzylic alcohols. The results indicated that secondary aryl alcohols could also be successfully transformed into the corresponding products in good to excellent yields (entries [9][10][11][12][13][14][15][16][17][18][19]. For example, the reduction of o-tolylmethanol (1b), m-tolylmethanol (1c), or p-tolylmethanol (1d), gives o-xylene (3b), m-xylene (3c), or pxylene (3d) in excellent yields, respectively (entries 1-3).…”

1‐Hydrosilatrane: A Locomotive for Efficient Ketone Reductions

“…Organosilicon hydrides, simply referred to as silanes, can act as hydride sources in such reduction reactions. Unlike borohydrides and aluminohydrides, however, silanes are typically weak hydride donors and thus do not react with weak electrophiles such as ketones and aldehydes unless the electrophilicity of the carbonyl group is enhanced . This activation can be achieved by adding a Lewis acid, which can coordinate to the carbonyl oxygen atom; alternatively, the Lewis acid can activate the silicon–hydrogen bond, making the hydride much more nucleophilic .…”

Enzymatic reduction of levoglucosenone by an alkene reductase (OYE 2.6): a sustainable metal- and dihydrogen-free access to the bio-based solvent Cyrene®

“…Cyrene® involve a metal catalyst and dihydrogen. [13][14][15][16][17][18][19][20][21][22][23][24] Despite being efficient in terms of yields and cost, not only these methods can be dangerous to perform -notably at large scale -(i.e., dihydrogen), but the presence of potential metal residues -even in ppm quantities -in Cyrene® may also limit its utilization in certain applications. [25][26][27] These drawbacks could thus limit the potential of Cyrene®, specifically in the food/feed, cosmetic and pharmaceutical sectors.…”