Diels-Alder reaction BINOL-boron

The enantioselection depends greatly on the nature of the R2 group at the boron atom, and the ee values were as high as 97 %. High enantioselectivity was observed in the synthesis of 4-dihydropyranones, based on the Diels-Alder reactions of aldehydes 74 and Danishefsky s diene, catalyzed by a BINOL-Ti(0-i-Pr)4-derived catalyst [75] (Equation 3.23). 

Enantioselective Diels-Alder reactions of acrolein are also catalyzed by 3-(2-hydroxy-3-phenyl) derivatives of BINOL in the presence of an aromatic boronic acid. The optimum boronic acid is 3,5-di(trifluoromethyl)benzeneboronic acid, with which >95% e.e. can be achieved. The transition state is believed to involve Lewis acid complexation of the boronic acid at the carbonyl oxygen and hydrogen bonding with the hydroxyl substituent. In this transition state, re,re-interactions between the dienophile and the hydroxybiphenyl substituted can also help to align the dienophile.65... 

Diphenyl-BINOL-derived chiral aluminum reagents are prepared in situ by addition of Ethylaluminum Dichloride or Diethylaluminum Chloride to 3,3 -diphenyl-BINOL. These chiral aluminum reagents promote the enantioselective Diels-Alder reaction of cyclopentadiene with the oxazolidone dienophile (eq 14). Endo products are obtained with a high level of asymmetric induction (>90% ee) however, a stoichiometric amount of the Lewis acid is required. The preparation and use of a C3 symmetric BINOL-derived boronate has been reported (eq 15). BINOL-B(OAr)3 complexes have recently been developed for the asymmetric Diels-Alder reaction with imines (eq 16). ... 

In the presence of a Zr complex of (J )-6,6 -dibromo-BINOL, A -(2-hydroxyphenyl)-aldimines participate in asymmetric hetero-Diels-Alder reactions It is an improvement to the reactions involving stoichiometric chiral boron reagents. 

Their 3,3 -substituents are utilized not only for their steric bulk, but also for the coordination to metals. Yamamoto and coworkers employed a boron complex of 3,3 -bis(2-hydroxyphenyl) BINOL in the asymmetric Diels-Alder reaction of cyclopentadiene and acrylaldehyde (equation 70) . The ligand possesses two additional hydroxy groups and forms a helical structure on coordination. The catalyst is considered to function as a chiral Brpnsted acid and a Lewis acid. The complex was also used in the Diels-Alder reactions and aldol reactions of imines. Although addition of diethylzinc to aldehydes gives low ee using BINOL itself or its 3,3 -diphenyl derivative, the selectivity can be increased when coordinating groups are introduced at the 3,3 -positions. Katsuki and... 

Asymmetric aza-Diels-Alder reactions.1 This reaction can be effected in 72-90% cc by catalysis with the chiral boron reagent 1, prepared by reaction of (R)-BINOL with triphenyl borate in CH2CI2 at 25°. Thus reaction of an aldiminc and the Danishefsky... 

Hetero-Diels-Alder reactions. The cycloaddition of Danishefsky s diene (and analogs) to chiral imines catalyzed by boronates derived from commercially available BINOL ligands proceeds with good diastereoselectivity. The double asymmetric induction operates for matching pairs, which exhibit fast reaction rates. On the other hand, the reaction of achiral substrates in the presence of chiral binol-boronates gives rise to products in good ee. ... 

The development of a catalytic enantioselective process has received much attention and, in common with the all-carbon Diels-Alder reaction, most success has been achieved using metal-based Lewis acids and, most recently, organocatalysts/ The earliest work on an enantioselective variant of this reaction was performed by Danishefsky/ The enantiomerically pure europium complex Eu(hfc)3 provided moderate enantioselectivities for the hetero-Diels-Alder reaction between derivatives of diene (8.123) and benzaldehyde (8.125), performed under solvent-free conditions. Higher ees have been obtained using the hindered BINOL ligand (8.126), as its aluminium complex. Yamamoto s CAB catalysts (see Section 8.1) such as (8.129), with the appropriate substituent on boron, have also been used to good effect. 

---