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Chiral Brønsted acid may be employed in combination with metal catalysts. Several kinds of combination are reported such as cooperative catalysis and relay catalysis. We already discussed some asymmetric reactions involving the combination of chiral Brønsted acid and metal catalysts in the previous sections, but we will briefly show several representative reactions in this section.

List reported an enantioselective α‐allylation of aldehyde by the combined use of CPA and Pd(PPh₃)₄. Enamine, generated in situ from aldehyde and allylic amine, reacted with π‐allyl palladium intermediate 37 bearing chiral counteranion to afford adducts with 97% ee (Scheme 2.88) [178]. List developed an asymmetric counteranion‐directed catalysis (ACDC) strategy [23], and employed chiral phosphate anion as the chiral counteranion for a range of transformations.

Scheme 2.88. α‐Allylation of aldehyde.

Source: [178].

You reported enantioselective synthesis of polycyclic indoles, such as tetrahydropyrano[3,4‐b] indoles and tetrahydro‐β‐carbolines, through olefin cross‐metathesis and subsequent intramolecular Friedel‐Crafts alkylation reaction using the dual catalysis system of Ru complex 38 and CPA 6o (Scheme 2.89) [179]. The use of readily available starting materials can make the synthesis of polycyclic indoles more practical and economical.

Scheme 2.89. Dual catalysis system of CPA and Ru complex.

Source: Based on [179].

Hu and Gong developed a cooperative catalysis system using CPA 6o and Rh₂(OAc)₄ to realize a three‐component reaction between diazo ester, alcohol, and imine that generated syn‐β‐amino‐α‐hydroxy acid with excellent diastereoselectivity and with 93% ee (Scheme 2.90) [180]. This reaction proceeded through the generation of oxonium ylide intermediate 40 from diazo ester and Rh₂(OAc)₄ by way of Rh carbene species, and the subsequent reaction with imine, which was activated by CPA [181].

Scheme 2.90. Cooperative catalysis with CPA and Rh₂(OAc)_4.

Source: Based on [180].

Hu recently reported the fluoroalkyl‐substituted syn‐diamines by the gem‐difluoro functionalization of 2,2,2‐trifluorodiazoethane, which could be efficiently converted into a series of fluoroalkyl‐substituted structures by the cooperative system of CPA 6b and iron porphyrin complex (FeTPPCl) (Scheme 2.91) [182]. Trifluorodiazoethane reacted with FeTPPCl to give the nonconjugated Fe carbene, which reacted with aniline to give free ylide. Subsequent reaction with imine furnished expected product.

Scheme 2.91. Cooperative catalysis with CPA and Fe complex.

Source: Based on [182].

Hu reported a Pd(II)/CPA 6b catalyzed three‐component reaction between aryldiazoacetate, enamines, and imines to afford α‐amino‐δ‐oxo pentanoic acid derivatives (Scheme 2.92a) [183]. Phenyldiazoacetate reacted with Pd salt to form Pd carbene intermediate, which reacted with aniline to generate Pd enolate. Subsequent aza‐Michael reaction with iminium salt of chalcone derivatives and hydrolysis furnished the adducts (Scheme 2.92b).

Scheme 2.92. Cooperative catalysis with CPA and Pd(II) complex (a) (

Source: Based on [183]

), and its mechanism (b).

Hu reported a reaction between diazoacetamides and various imines using Rh₂(OAc)₄ and CPA 6i to furnish chiral oxindole derivatives. The zwitterionic intermediate was trapped by imines (Scheme 2.93) [184].

Scheme 2.93. Cooperative catalysis with CPA and Rh₂(OAc)₄.

Source: Based on [184].