Planar-chiral chromium complexe

Unlike the ferrocenes, the arene complexes of chromium, in particular the arenechromi-umtricarbonyls, have seen much less use in asymmetric catalysis. This is beginning to change, however , and a number of synthetic transformations of arenechromiumtricar-bonyls owe their existence to the formation of planar chiral chromium complexes by... 

The first enantioselective functionalization of tricarbonylchromium arene complexes using chiral bases, to generate planar chiral chromium complexes, was reported by Simpkins and coworkers in 1994 and involved a directed orf/zo-lithiation and subsequent quench with an electrophile78. Both aromatic and benzylic functionalization of tricarbonylchromium arene complexes has been achieved. 

The chromium-complexed benzylic radicals are also known and were employed in a variety of useful organic reactions. One electron reductive coupling of the tricarbonylchromium complexes of o-substituted benzaldehydes and benzaldi-mines with samarium iodide gave 1,2-diols and diamines, respectively, in good yields with extremely high threo diastereoselectivity (Eq. 7) [7]. Therefore, enantiomerically pure 1,2-diols and diamines were easily obtained by using the planar chiral chromium complexes of benzaldehyde and benzaldimine. 

First examples of chiral boratabenzene complexes, which are of interest owing to potential applications as enantioselective catalysts, have recently been reported. The planar chiral chromium tricarbonyl complex (137) was obtained by adduct formation between borabenzene and an (S) -valinol-derived oxazoline ligand. The Cr(CO)3 fragment stereoselectively complexes with this borabenzene adduct. A different approach has been chosen by Herberich, who developed the chiral boratabenzene ligand (138), which is derived from a-pinene. ... 

Planar chiral ortho substituted benzaldehyde chromium complexes are useful compounds for a variety of asymmetric reactions. For example, planar chiral tricarbonylchromium complexes of o-substituted benzaldehydes were reacted with Danishefsky s diene in the presence of Lewis acid at room temperature to afford the chromium-complexed 2,3-dihydro-4-pyranones 25 with high dias-tereoselectivity (Eq. 15) [14]. The high diastereoselectivity of the formation of cycloaddition products 25 is also contributed to an exo-side approach of the diene to anti-oriented carbonyl oxygen of the planar chiral ortho substituted benzaldehyde chromium complexes. When the reaction takes place at lower temperature, aldol-type condensation product 26 was obtained along with the formation of pyranones. The open intermediate 26 was easily transformed to the corresponding cycloaddition product after stirring at room temperature [14]. 

Similarly, the planar chiral tricarbonylchromium complexes of ortho substituted benzaldimines are useful for a variety of stereoselective cycloaddition reactions. For example, the benzaldimine chromium complexes gave aza-Diels-Alder product, 2,3-dihydro-4-pyridinone chromium complexes with high diastereoselectivity by reaction with Danishefsky s diene (Eq. 18) [11,17,18]. The high diastereoselectivity of the cycloadducts is also based on the preferred antz-conformation of the starting benzaldimine chromium complexes as well as the planar chiral benzaldehyde chromium complexes. The cycloaddition of imines having arene chromium complex at the remote position with Danishefsky s diene underwent smoothly in good yields, but the diastereoselectivity was low (Eq. 19) [17]. 

Keywords Arene chromium complexes Arene ruthenium complexes Enantioselective total synthesis Planar-chiral metal complexes Natm-al products... 

Fig. 8 The structure of (a) (5, 5 )-Whelk-01 and the proposed chiral recognition model between (b) the CSP and the most retained enantiomer of amide-type planar tricarbonyl-chromium complexes (adapted from [111])...

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]. 

The enantioselective lithiation of anisolechromium tricarbonyl was used by Schmalz and Schellhaas in a route towards the natural product (+)-ptilocaulin . In situ hthi-ation and silylation of 410 with ent-h M gave ewf-411 in an optimized 91% ee (reaction carried ont at — 100°C over 10 min, see Scheme 169). A second, substrate-directed lithiation with BuLi alone, formation of the copper derivative and a quench with crotyl bromide gave 420. The planar chirality and reactivity of the chromium complex was then exploited in a nucleophilic addition of dithiane, which generated ptilocaulin precnrsor 421 (Scheme 172). The stereochemistry of componnd 421 has also been used to direct dearomatizing additions, yielding other classes of enones. ... 

Ortho-directed metallations also allowed the synthesis of ferrocene-based hydroxamic acids such as 17 and 18 [23] as well as the preparation of planar-chiral carbenes 19 and 21 (which were trapped as chromium and rhodium complexes, 20 and 22, respectively) [24], In this context it is noteworthy that 19 was the first carbene with planar chirality ever reported. 

Three types of reaction systems have been designed and applied for the enantioposition-selective asymmetric cross-coupling reactions so far. First example is asymmetric induction of planar chirality on chromium-arene complexes [7,8]. T vo chloro-suhstituents in a tricarhonyl("n6-o-dichlorobenzene)chromium are prochiral with respect to the planar chirality of the 7t-arene-metal moiety, thus an enantioposition-selective substitution at one of the two chloro substituents takes place to give a planar chiral monosubstitution product with a minor amount of the disubstitution product. A similar methodology of monosuhstitution can be applicable to the synthesis of axially chiral biaryl molecules from an achiral ditriflate in which the two tri-fluoromethanesulfonyloxy groups are enantiotopic [9-11]. The last example is intramolecular alkylation of alkenyl triflate with one of the enantiotopic alkylboranes, which leads to a chiral cyclic system [12], The structures of the three representative substrates are illustrated in Figure 8F.1. 

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

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