Cationic Chiral Dirhodium Carboxamidates Are Activated for Lewis Acid Catalysis

Abstract

Dedicated to Professor Henri Brunner on the occasion of his 72nd birthday Chiral dirhodium(II) carboxamidates have high potential for enantioselective Lewis acid catalyzed reactions because they hold the Lewis base, which is activated for reaction, at the axial coordination site in close proximity to the ligand attachments for chiral differentiation. As has already been demonstrated for the hetero-Diels-Alder reaction (Scheme 1) [1,2] and for trimethylsilylketene/glyoxal cycloaddition, [3] the chiral environment around the axial coordination site strongly influences enantiocontrol and also pushes the product off the rhodium axial coordination site to provide turnover numbers (TON) as high as 10 000. However, the Lewis acidity for dirhodium(II) carboxamidates is low compared to that of many other catalysts for these reactions.[4] Suitable Diels-Alder, ene, and dipolar cycloaddition reactions, for example, show no catalytic activity with chiral dirhodium(II) carboxamidates, even with a,adifluoro analogues of the mepy and meaz catalysts that were developed to enhance Lewis acid association with Lewis bases.[5] We have prepared cationic Rh II /Rh III counterparts to the moderately active chiral dirhodium(II) carboxamidates to enhance the Lewis acidity of these chiral dirhodium catalysts. Cationic metal complexes are now commonly used to achieve rate and selectivity enhancements for those transformations suitable to catalysis by the cationic metal complex.[6] We anticipated that cationic chiral Rh II /Rh III compounds could increase the closeness of association of the catalyst with Lewis bases, increase the rate of reaction with selected substrates, and enhance enantiocontrol.Oxidation of dirhodium(II) (Rh 2 4+ ) compounds is well known ,[6] but their Rh 2 5+ counterparts have been produced in the presence of either a less labile ligand such as halide [7] or by using another transition metal such as Ag I , Ce IV , or Cu II for the oxidation, [6,8] none of which are amenable to the use of Rh 2 5+ complexes as catalysts without laborious separation or further catalyst manipulation. However, we have recently discovered that nitrosonium salts effect facile oxidation of dirhodium(II) carboxamidates at room temperature to form the corresponding Rh II /Rh III salts quantitatively, evolving nitric oxide in the process. These complexes exhibit a characteristic electronic absorption near 1000 cm À1 . A crystal structure for the bis(acetonitrile) complex of [Rh 2 {(4S)-meox} 4 ]BF 4 is shown in Figure 1.To test the ability of chiral Rh 2 5+ carboxamidates to enhance selectivity in Lewis acid catalyzed transformations we turned our attention to the hetero-Diels-Alder reaction and to [Rh 2 (mepy) 4 ] as the catalyst. As has been reported, [1a] the use of 1.0 mol % [Rh 2 (mepy) 4 ] with p-nitrobenzaldehyde and the Danishefsky diene (see Scheme 1) resulted in the corresponding hetero-Diels-Alder product (53 % yield) in 73 % ee; use of the corresponding Rh 2 5+ complex, [Rh 2 {(5S)-mepy} 4 ]BF 4 , produced the same product in 93 %...

Iridium-Catalyzed 1,3-Dipolar Cycloadditions

“…Cycloadducts were obtained as 3,4-endo/3,5-endo mixtures with molar ratios ranging from 13/87 to 90/10. The enantioselectivity of the 3,4 isomers is higher than that of the 3,5 adducts and increases when the steric bulk of the ligand ester group increases [36]. NN donor atoms) efficiently catalyze the DCR of methacrolein with a series of nitrones (Scheme 12).…”

5.10 Hetero-Diels–Alder Reactions

“…[31][32][33] Although 35 has high catalytic activity, the use of the cationic dirhodium(II) carboxamidate [Rh 2 (5S-mepy) 4 ]BF 4 (36), which is prepared by the oxidation of 35 with NOBF 4 , further improves the catalytic activity and enantioselectivity ( Table 5). 34 With 36, the rate is estimated to increase by at least a factor of 10. In its use as an LA catalyst, coordination of the cationic rhodium complex with water is expected to produce a protic acid.…”

Metal complexes versus organocatalysts in asymmetric 1,3-dipolar cycloadditions

“…This interaction blocked the approach of the dipolarophile by the Re-face of the a-carbon-carbon double bond. 57 The new generation of cationic chiral dirhodium carboxamidates 89 ( Figure 7) were tested as catalysts in the reaction depicted on reaction a in the Scheme 20. The yields were good (50-90%) and the enantioselections were, in some cases, very important (up to 94% ee).…”