Planar chiral arenes

1 Planar Chiral xf-Arene-Cr(CO) Complexes Al-Aryl indoles and related compounds having a chiral C—N bond have received recent attention as novel atropisomeric intermediates for asymmetric synthesis [10, 48]. Al-Aryl indoles 59,60 can be prepared by stereoselective 

TABLE 8.2 Planar Chiral (/ -Arene-CrlCOIj Complexes 

SCHEME 8.13 Planar chiral j/ -aiene-CriCO), complexes. 

To rationalize these results, geometry optimizations for the coupling of 57c with 58b wctc carried out computationally with the RHF level of theory and 3-2 lG(d) basis sets for all atoms [49]. The reaction presumably proceeds via the generally accepted addition-elimination mechanism involving a Meisenheimer complex intermediate (Schane 8.14). In the addition step, to avoid stoic repulsion, the 2-methylindole approaches the arenechromium complex from the less stoically 

SCHEME 8.14 Planar chiral // -arene-CrfCO) complexes mechanism. 

---

Due to the inherent unsymmetric arene substitution pattern the benzannulation reaction creates a plane of chirality in the resulting tricarbonyl chromium complex, and - under achiral conditions - produces a racemic mixture of arene Cr(CO)3 complexes. Since the resolution of planar chiral arene chromium complexes can be rather tedious, diastereoselective benzannulation approaches towards optically pure planar chiral products appear highly attractive. This strategy requires the incorporation of chiral information into the starting materials which may be based on one of three options a stereogenic element can be introduced in the alkyne side chain, in the carbene carbon side chain or - most general and most attractive - in the heteroatom carbene side chain (Scheme 20). 

Mulzer J, Ohler E (2004) Olefin Metathesis in Natural Product Syntheses. 13 269-366 Muniz K (2004) Planar Chiral Arene Chromium (0) Complexes as Ligands for Asymetric Catalysis. 7 205-223 Murai S, see Kakiuchi F (1999) 3 47-79... 

As well as organic chiral auxiliaries, organometallic fragments have found some application as chiral auxiliaries in conjugate addition reactions. Particularly noteworthy are chiral molybdenum allyl complexes [69], chiral iron complexes [70], and planar chiral arene chromium species [71]. 

In a further development of the norbornene/anihne OHA reaction, Salzer and coworkers used planar chiral arene-chromium-tricarbonyl-based diphosphines for the in situ formation of cis-trans mixtures of complexes 9 and 10 that gave enanti-oselectivities of 51% and 70%, respectively, at 333 K and with a 40-fold excess of naked fluoride , but activities were very low. In the same paper complex 6 was shown to be superior in both activity and enantioselectivity (64% ee) to the corresponding Josiphos compound 5 [15]. The activated N-H bond of benzamide was also stereoselectively added across the double bond of norbornene to afford N-benzoyl-e%o-aminonorbornane in up to 50% yield and 73% ee in the presence of 0.5mol% [IrCl((R)-MeO-bipheb)]2 at 373 K [16]. 

Planar chiral arene Cr(CO)3 complexes have been shown to undergo highly diastereoselective cycloadditions and Kiindig has extended this protocol to the [3+2] cycloadditions of azomethine ylides (96). Enantiopure ortho- substituted p -benzaldehyde complex 337 underwent condensation with an ot-amino ester to afford imine 338 in the presence of EtaN. Subsequent treatment with methyl acrylate at ambient temperamre in the presence of LiBr and EtaN delivered cycloadduct 339, with excellent stereoinduction and high material yield. Photoinduced oxidative decomplexation in air furnished the final arylpyrrolidines (Scheme 3.114). 

Uemura and Kamikawa have presented a review on the stereoselective synthesis of axially chiral biaryls utilizing planar chiral (arene)chromium complexes [44]. 

A reaction sequence published recently by KUndig and co-workers also deserves notice [35] in a one-pot reaction the planar chiral arene-Cr(CO)3 complex 10 is first converted (with chirality transfer) to the enyne 11, which then affords the tricyclic Pauson-Khand product 12 in high yield and completely diastereoselec-tively (Scheme 12). 

As (—)-pulegone is not readily available, subsequent work addressed the natural enantiomer by different methods [12], but all completed their synthesis via Snider s enone (17). Asaoka utilised a TMS group as temporary directing volume (Scheme 5) [12a]. Cossy applied a photore-ductive cyclopropane ring opening, which was obtained by a diastereoselective Simmons-Smith reaction [12b]. Schmalz finally used a planar-chiral arene complex to synthesise enone 17 with interesting CrfCO), chemistry [12c]. 

Bolm, C. Muniz, K. Planar Chiral Arene Chromium (0) Complexes Potential Ligands for Asymmetric Catalysis, Chem. Soc. Rev. 1999, 28, 51-59. 

Similarly, ketene generated from acid chloride by treatment with triethyl-amine reacted with tricarbonylchromium-complexed benzaldimines to afford P-lactam derivatives via [2+2] cycloaddition with high diastereoselectivity. Thus, the cycloaddition of benzaldimine chromium complexes with ketenes generated from acid chloride at 0 °C in the presence of triethylamine afforded czs-p-lactam as a single diastereomer 27 (Eq. 20) [19]. Remote positioned imine having the planar chiral arene chromium complex was also reacted with ketene to afford p-lactam complex as diastereomeric mixture (Eq. 21) [19]. 

The synthesis of planar-chiral arene chromium complexes has been reviewed several times and has also been discussed in an earlier article [9,10]. Apart from stereoselective complexation reactions, diastereoselective and enantioselective... 

Planar Chiral Arene Chromium(O) Complexes as Ligands for Asymmetric Catalysis... 

Furthermore, Kiindig et al. [26] investigated intramolecular Mizoroki-Heck reactions at planar chiral arene tricarbonyl chromium complexes 29a—c, giving indanes substituted in the benzylic position in good yield (80% for = Me, 78% for = OH and 85% for R = OMe) (29a—c 30a—c. Scheme 5.5). 

Kiindig, E.P., Ratni, H., Crousse, B. and Bernardinelli, G. (2001) Intramolecular Pd-catalyzed carbocyclization, Heck reactions, and aryl-radical cyclizations with planar chiral arene tricarbonyl chromium complexes. J. Org. Chem., 66,1852-60. 

---