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

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.