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Synthesis and Catalytic Activity of Gold Chiral Nitrogen Acyclic Carbenes and Gold Hydrogen Bonded Heterocyclic Carbenes in Cyclopropanation of Vinyl Arenes and in Intramolecular Hydroalkoxylation of Allenes

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posted on 01.11.2010, 00:00 by Camino Bartolomé, Domingo García-Cuadrado, Zoraida Ramiro, Pablo Espinet
Mononuclear and dinuclear chiral gold(I) carbene complexes with carbene ligands of the type HBHC (hydrogen bonded heterocyclic carbenes) and NAC (nitrogen acyclic carbenes) have been prepared by reaction of isocyanide gold(I) complexes and chiral amines or diamines. The reaction of [AuCl(CNPy-2)] (1) (Py = pyridyl) with the corresponding chiral primary amines afforded the chiral HBHC complexes (R)-[AuCl{C(NH(CHMePh))(NHPy-2)}] ((R)-2), and (S)-[AuCl{C(NH{CHMe(1-naphthyl)})(NHPy-2)}] ((S)-3), while the reaction of 2 equiv of 1 with diamines produced (S)-2,2′-bis[NH{C(AuCl)(NHPy-2)}]2-binaphthyl ((S)-4), (1R,2R)-1,2-bis[NH{C(AuCl)(NHPy-2)}]-diphenylethane ((1R,2R)-5), and (1R,2R)-1,2-bis[NH{C(AuCl)(NHPy-2)}]-cyclohexane ((1R,2R)-6). On the other hand the addition of alkyl amines to (S)-2,2′-[NCAuCl]2-binaphthyl ((S)-8) gave the chiral NAC complexes (S)-2,2′-bis[NH{C(AuCl)(NMe2)}]2-binaphthyl ((S)-9) and (S)-2,2′-bis[NH{C(AuCl)(NiPr2)}]2-binaphthyl ((S)-10), while the addition to (S)-2,2′-[NCAuCl]2-3,3′-Ph2-binaphthyl ((S)-12) yielded (S)-2,2′-bis[NH{C(AuCl)(NMe2)}]2-3,3′-Ph2-binaphthyl ((S)-13) and (S)-2,2′-bis[NH{C(AuCl)(NEt2)}]2-3,3′-Ph2-binaphthyl ((S)-14). All the complexes are active catalysts in the cyclopropanation of vinyl arenes and in the intramolecular hydroalkoxylation of allenes, providing good yields and modest or poor enantioselectivity. The results show that all these ligands are compatible with different functions and reaction conditions and are worth considering as alternative systems to NHCs or phosphines in gold catalyzed reactions.