Phosphoric BINOL-derived

Fig. 3 Phosphoric acids derived from BINOL and Hj-BINOL...

In 2008, Toste and coworkers reported the desymmetrization of me o-episulfonium ions 131 generated in situ from ring closure of sulfides 132 featuring a P-trichloro-acetimidate leaving group [76], Chiral BINOL-derived phosphoric acid (5)-3o (15 mol%, R = triggered the formation of the intermediate mera-epi-... 

In 2008, the Ackennann group reported on the use of phosphoric acid 3r (10 mol%, R = SiPhj) as a Brpnsted acid catalyst in the unprecedented intramolecular hydroaminations of unfunctionaUzed alkenes alike 144 (Scheme 58) [82], BINOL-derived phosphoric acids with bulky substituents at the 3,3 -positions showed improved catalytic activity compared to less sterically hindered representatives. Remarkably, this is the first example of the activation of simple alkenes by a Brpnsted acid. However, the reaction is limited to geminally disubstituted precursors 144. Their cyclization might be favored due to a Thorpe-Ingold effect. An asymmetric version was attempted by means of chiral BINOL phosphate (R)-3( (20 mol%, R = 3,5-(CF3)2-CgH3), albeit with low enantioselectivity (17% ee). 

Until 2006, a severe limitation in the field of chiral Brpnsted acid catalysis was the restriction to reactive substrates. The acidity of BINOL-derived chiral phosphoric acids is appropriate to activate various imine compounds through protonation and a broad range of efficient and highly enantioselective, phosphoric acid-catalyzed transformations involving imines have been developed. However, the activation of simple carbonyl compounds by means of Brpnsted acid catalysis proved to be rather challenging since the acid ity of the known BINOL-derived phosphoric acids is mostly insufficient. Carbonyl compounds and other less reactive substrates often require a stronger Brpnsted acid catalyst. 

In 2006, Yamamoto and Nakashima picked np on this and designed a chiral A -triflyl phosphoramide as a stronger Brpnsted acid catalyst than the phosphoric acids based on this concept. In their seminal report, they disclosed the preparation of new chiral BINOL-derived A -triflyl phosphoramides and their application to the asymmetric Diels-Alder (DA) reaction of a,p-unsaturated ketones with sily-loxydienes [83], As depicted in Scheme 59, chiral A-triflyl phosphoramides of the general type 4 are readily synthesized from the corresponding optically active 3,3 -substituted BINOL derivatives 142 through a phosphorylation/amidation route. 

The last few years have witnessed major advances in the use of small organic molecules as organic acid catalysts in asymmetric catalysis [1], Selected examples of such organic acid catalysts include urea and thiourea [2], TADDOL [3], BINOL [4], and phosphoric acid derived from BINOL [5] (Figure 2.1). 

Figure 5.2 Important structural and electronic properties of BINOL-derived chiral phosphoric acids.

While BINOL-derived chiral phosphoric acids have received great attention, a handful of reports implementing alternative chiral backbones have appeared [51]. 

For a related highly enantioselective reduction of a-imino esters with a BINOL-derived phosphoric add diester, see Kang, Q., Zhao, Z.-A. and You, S.-L. (2007) Adv. Synth. Catal., 349,1557-1660. 

A BINOL-derived phosphoric acid derivative has been used as a catalyst in the enantioselective synthesis of a-amino phosphonates via hydrophosphonylation of imines with diisopropyl phosphite.82... 

In this chapter, we focus on recent achievements in the enantioselective synthesis of chiral amines using 1,1 bi 2 naphthol (BINOL) derived monophosphoric acid (1) or related phosphoric acids as chiral Bronsted acid catalysts 2, 3], The contents are arranged according to the type of bond forming reaction, including carbon carbon, carbon hydrogen, and carbon heteroatom bond forming reactions, followed by specific reaction types. 

A number of Bronsted acidic organocatalysts have been applied to the asymmetric hydrophosphonylation of aldimines. Thiourea catalysts related to (6.130) catalyse the asymmetric hydrophosphonylation of a range of aliphatic and aromatic aldimines with high ee and BINOL-derived phosphoric acid derivatives similar in structure to (6.131) are effective catalysts in the asymmetric phosphonylation of cinnamaldehyde-derived aldimines. Asymmetric hydrophosphonylation of aromatic aldimines can also be achieved with high ee using cheap, commercially... 

A chiral Lewis acid catalyst was prepared between the chiral BINOL-derived phosphoric acid (212) and Et2AlCl. In the presence of a catalytic amount of the A1 complex, the reaction between (i )-isocyanoacetamides (209) and aldehydes (210) afforded the corresponding enantio-enriched 2-(l-hydroxyalkyl)-5-aminooxazoles (211) in good yields and enantioselectivities (Scheme 59). ... 

Scheme 19 Coordination of polymerizable phosphorous ligands and (i )-Bu2BINOL to platinum (45) and polymerization with EGDMA creates polymers (P-30 through P-33b) with binding sites selective towards (i )-BINOL derivatives following removal of the template.

In the absence of a catalytic amount of Lewis acids such as Mg(OTf)2, weak Bronsted acids, such as thioureas, BINOL, and BINOL-derived phosphoric acids, could not promote this reaction alone. The reaction could not proceed with phosphoric acid salts, suggesting that the use of free acid is essential for effective catalysis. The combination of chiral phosphoric acid 13e and Lewis acid Mgp2 (4 1 ratio) was identified to be the optimal catalyst for the AFC alkylation reactions of phenols with p,y-unsaturated a-keto esters, affording the alkylation product in good yields with up to 99% ee. Not only free phenols but also indoles could be successfully applied in AFC reactions with P,y-unsaturated a-keto esters under the binary-acid catalysis (82-94% ee). 

Based on acid-base combination chemistry, Ishihara developed a catalytic enantioselective cyanosilylation of ketones by using chiral (J )-Ph2-BINOL-derived lithium phosphate, which was prepared in situ from phosphoric acid... 

Chiral Brpnsted acids are divided into two categories. The first are neutral Brpnsted acids such as thiourea [35b] and TADDOL derivatives, also known as hydrogen-bonding catalysts, and the second are sponger Brpnsted acids such as BINOL derivatives or phosphoric acids. 

Independently, Gong, et.al reported the same reaction with similar chiral Br0nsted acids, a chiral phosphoric acid derived from 3,3-di(4-chlorophenyl)-Hg-binol 122 with fairly good yields and enantioselectivities. Scheme 3.41 [56]. 

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