Cationic oxazaborolidine Lewis

Br0nsted-Acid-Assisted Lewis Acid Catalysis (Cationic Oxazaborolidine) [136]... 

Although the Diels-Alder reaction of furan-2(5H)-one itself 140b and its derivatives has been used to synthesize structurally complex polycyclic lactones, lower reactivity of such lactone dienophiles compared to the acyclic dieno-philes sometimes is problematic. Therefore, more reactive synthetic equivalents of y-butenolides were developed [114]. In addition, due to relatively strong Lewis basicity of the lactones, the reaction often requires the use of a stoichiometric amoxmt of Lewis acids [115]. As a recent elegant example of the catalytic Diels-Alder reaction, Corey demonstrated that chiral cationic oxazaborolidine 172 works as one of the effective asymmetric Lewis acid catalysts for the reaction of 140b with cyclopentadiene (Scheme 68) [116]. 

Very recently, Corey and coworkers have introduced the very powerful cationic chiral Lewis acids (14) [15] and (15) [16], which are derived from protonation of the parent oxazaborolidines with triflic acid and triflimide, respectively. Because of this protonation, the boron atom becomes even more acidic. The authors have demonstrated that catalyst (14) is able to efficiently promote the cycloadditions of 2-substituted acroleins with a variety of dienes (Equation 5) [15]. 

The mechanism A very detailed mechanistic study of this phosphoramide-catalyzed asymmetric aldol reaction was conducted by the Denmark group (see also Section 6.2.1.2) [59, 60], Mechanistically, the chiral phosphoramide base seems to coordinate temporarily with the silicon atom of the trichlorosilyl enolates, in contrast with previously used chiral Lewis acids, e.g. oxazaborolidines, which interact with the aldehyde. It has been suggested that the hexacoordinate silicate species of type I is involved in stereoselection (Scheme 6.15). Thus, this cationic, diphosphoramide silyl enolate complex reacts through a chair-like transition structure. 

In most Diels-Alder reactions, no catalyst is needed, but Lewis acids are effective catalysts in many cases, particularly those in which Z in the dienophile is a C=0 or C=N group. A Lewis acid catalyst usually increases both the regioselec-tivity of the reaction (in the sense given above) and the extent of endo addition, °° and, in the case of enantioselective reactions, the extent of enantioselectivity. It has been shown that InCla is an effective catalyst for aqueous Diels-Alder reactions,which is suitable for ionic Diels-Alder reactions, and there are other Lewis acid catalysts that are effective in water. °° Brpnsted acids have also been used to accelerate the rate of the Diels-Alder reaction.Lanthanum triflate [La(OTf)3] has been reported as a reusable catalyst ° ° and Me3SiNTf2 has been used as a green Lewis acid catalyst. ° Cationic Diels-Alder catalysts have been developed, particularly oxazaborolidine catalysts. ° Some Diels-Alder reactions can also be catalyzed by the addition of a stable cation radical, for... 

Chiral oxazaborolidine salts are useful catalysts not only for enantioselective reductions (i.e., CBS reduction), but also for promoting enantioselective D-A reactions. The cationic Lewis acids formed by reaction of chiral oxazaborolidines with triflic acid [TfOH] or trifluoromethanesulfonimide [(Tf)2NH] coordinate to dienophiles to direct subsequent cycloadditions in a highly controlled fashion. The D-A reactions using these chiral salts proceed in CH2CI2 under very mild conditions at temperatures as low as -95 °C (Table 9.4). ... 

Proline-derived oxazaborolidines 45 have shown to be effective pre-catalysts with triflic acid as an activator to generate cationic Lewis acids.18,15 The optimal proportions of 45 and triflic acid was found to be 1.2 1. Protonation of 45 produced a 1.5 1 mixture of 46 and 47 as determined by low temperature H NMR. Their interconversion at low temperature (-80 °C) is slow on the NMR timescale. However, this interconversion increases as the temperature rises and at 0 °C this becomes rapid (Tc). Phenyl or o-tolyl were determined to be the best substituents for the R group in 45. For the Ar group of 45, phenyl and 3,5-dimethylphenyl were determined to be optimal. 

Cationic Lewis acids show improved acidity and hence activity. For instance, even at —94°C, relatively unreactive dienes such as butadiene (8.29) react with bromoacrolein (8.05) with excellent yields and good selectivities in the presence of the oxazaborinane (8.30). The Lewis acidity of oxazoborolidines can also be enhanced by protonation using strong Bronsted acids. The triflate salt (8.32) generated by protonation of the corresponding oxazaborolidine with triflic acid shows high activity and wide scope catalysing the Diels-Alder reaction of acyclic... 

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