Transition-Metal-Catalyzed Enantioselective Synthesis of Compounds with Non-Centrochirality

Ogasawara Masamichi. Watanabe Susumu. Synthesis  2009,  1761 

After publication of the above review article, the authors became aware that the majority of research works on ‘the catalytic asymmetric synthesis of planar-chiral chromium(0)-(h6-arene) complexes’ were erroneously omitted. The sections 2.1a-c given below should be added after the Section 2.1 in the review article. The authors apologize for the oversight.

2.1a Synthesis of Planar-Chiral Chromium(0)-(η⁶-arene­) Complexes by Palladium-Catalyzed Methoxycarbonylation

Palladium-catalyzed methoxycarbonylation was applied to the preparation of planar-chiral chromium(0)-arene complexes by Schmaltz and co-workers. A palladium complex coordinated with (R)-(S)-ppf-pyrrolidine was effective in desymmetrizing 5, and the monoester (1S)-A1 was obtained in 95% ee (31% yield) together with 48% of the achiral diester A2 (Scheme A1).A1a Although the ee value of the initially formed A1 was not so high (ca. 60% ee), (1S)-A1 of higher optical purity could be obtained because the minor enantiomer (1R)-A1 was consumed more rapidly than its antipode in the second methoxycarbonylation leading to A2.

Scheme A1

The same catalytic system was also applied to the desymmetrization of the tricarbonyl(h6-1,3-dichloroarene)chromium(0) complexes A3.A1b A substituent at the 2-position in the h6-arene moiety affected some enantioselectivity in the palladium-catalyzed reaction. Although the 2-unsubstituted substrate A3a gave a nearly racemic product, A4b and A4c were obtained in 90 and 41% ee, respectively (Scheme A2). In the second methoxycarbonylation of A4b giving A5b, a kinetic resolution with a selectivity factor of S = 4.3 was observed.

Scheme A2

2.1b Palladium-Catalyzed Intramolecular Mizoroki-Heck Reaction of Planar-Prochiral Chromium(0)-(η⁶-arene) Complex

In 2005, Uemura described that a palladium complex coordinated with a phosphoramidite (S)-A9 gave rise to asymmetric intramolecular Mizoroki-Heck reactions of (h6-1,3-bisalkenyl-2-chlorobenzene)chromium(0) complexes A6 with moderate enantioselectivity (Scheme A3).A2 While the bis(3-butenyl) substrate A6a afforded the cyclized product A7a in 67% ee (40 ˚C) and 73% ee (60 ˚C), the bis(4-pentenyl) homologue A7b provided A7b in lower enantioselectivity of 40% ee at 60 ˚C.

A palladium intermediate, which was generated after the carbopalladation/cyclization step in the Mizoroki-Heck reaction, could be trapped with an aryl- or alkenylboronic acid nucleophile. The Mizoroki-Heck/Suzuki-Miyaura cascade process proceeded with excellent diastereoselectivity and A8 was obtained in 54% yield from A6a as a single diastereomer, for which the enantiomeric excess was 68% ee.

Scheme A3

2.1c Palladium-Catalyzed Enantioselective Hydrogenolysis of (η⁶-5,8-Dibromonaphthalene)Cr(CO)₃

Enantioposition-selective hydrogenolysis of planar-prochiral (h6-5,8-dibromonaphthalene)Cr(CO)3 (A10) was accomplished with excellent enantioselectivity by Kündig and co-workers in 2006.^A³ A palladium complex coordinated with the bulky phosphoramidite (S,R,R)-A13 was found to be an excellent catalyst for the asymmetric reaction, and the planar-chiral (h6-arene)tricarbonylchromium(0) (pR)-A11 was obtained in 97% ee in 78% yield under the optimized conditions (Scheme A4).

Scheme A4

References

(A1) (a) Gotov, B.; Schmalz, H.-G. Org. Lett. 2001, 3, 1753.
(b) Böttcher, A.; Schmalz, H.-G. Synlett 2003, 1595.

(A2) Kamikawa, K.; Harada K.; Uemura, M. Tetrahedron: Asymmetry­ 2005, 16, 1419.

(A3) Kündig, E. P.; Chaudhuri, P. D.; House, D.; Bernardinelli, G. Angew. Chem. Int. Ed. 2006, 45, 1092.