Chiral arenechromium tricarbonyls

Diastereoselective lithiation of chiral arenechromium tricarbonyl complexes... 

A few examples of stereoselective additions of organomagnesium or -cerium reagents to imines bearing a chiral arenechromium tricarbonyl or dieneiron tricarbonyl residue have been described [539, 1197], Reactions of allylmetals with an oxime bearing a chiral ether functionality can be stereoselective provided that the oxime displays the E configuration [1198],... 

A problem with (—)-sparteine 362 is its lack of availability in both enantiomeric forms. Reversed selectivity in the generation of planar chirality has been achieved by second lithiations (see Schemes 163 and 171) and a remarkable modification of this strategy works with arenechromium tricarbonyls. By using excess BuLi (sometimes f-BuLi is required) in the presence of sparteine 362, a doubly lithiated species 450 may be formed from 448. The formation of the doubly lithiated species may be confirmed by double deuteriation with excess D2O. However, other electrophiles react selectively only once and give products of opposite absolute stereochemistry from those formed after monolithiation, if in rather low yield. Presumably, the first lithiation, which is directed by (—)-sparteine, produce an organolithium 448 whose complexation with (—)-sparteine remains favourable. The second lithiation must produce a less stable organolithium—one which cannot form a... 

Siwek and Green have deprotonated arenechromium tricarbonyls bearing acidic ben-zylic protons with chiral bases in which the organolithium itself is chiral. Menthyllithium 452 (R = H) performed variably in terms of ee, but 8-phenyhnenthyllithium 452 (R = Ph) gave good yields and ee s (in the opposite enantiomeric series from 452 (R = H)) in the deprotonation of 412 (Scheme 181). Unlike most chiral bases, 452 (R = Ph) gives better results in THF than in ether. 

The chirality of lithiated arenechromium tricarbonyls offers the opportunity for asymmetric lithiation. This has been achieved either with auxiliaries350 353 or using chiral bases.354 359... 

Transition-metal-stabilized carbocations can be generated from functionalized butadieneiron carbonyl or arenechromium tricarbonyl complexes [92], Reactions of such carbocations formed from chiral complexes have been studied, but low selectivities are usually observed [526, 528, 535]. However, chromium tricarbonyl complexes derived from ephedrine 5.66 suffer cyclization in acidic medium. After decomplexation, c/s-tetrahydroquinolines are formed with a high diastereo-and enantioselectivity [540,542] (Figure 5.44). 

The use of chiral sulfoxitnines 1.136 (R = Ph, Tol, Y = ArCH=N) has allowed the enantioselective synthesis of p-aminoesters after cleavage of the S-N bond by CF3COOH [510], Preliminary studies showed that the reaction of C-arenechromium tricarbonyl imines and the lithium enolate of Me2CHCOOEt gave chiral p-lactams after decomplexation with an excellent enantiomeric excess [549, 1291]. 

Among the other acrylates of chiral alcohols used in cycloadditions with cy-clopentadiene, reactions of the monobenzoate of 2,2,6,6-tetramethyl-3,5-hep-tanediol [1398] under TiCl4 catalysis are highly selective, as are reactions of dural arenechromium tricarbonyl complexes 9.61 mediated by stoichiometric amounts of ZnCl2 [548], In this last case, the R group must be bulky (R = 1-Np, 2,4,6 (Figure 9.30). 

Planar chiral compounds usually (and for the purpose of this review, always) contain unsymmetrically substituted aromatic systems. Chirality arises because the otherwise enantiotopic faces of the aromatic ring are differentiated by the coordination to a metal atom - commonly iron (in the ferrocenes) or chromium (in the arenechromium tricarbonyl complexes). Withdrawal of electrons by the metal centre means that arene-metal complexes and metallocenes are more readily lithiated than their parent aromatic systems, and the stereochemical features associated with the planar chirality allow lithiation to be diastereoselective (if the starting material is chiral) or enantioselective (if only the product is chiral). 

Arenechromium tricarbonyls are considerably more acidic than ferrocenes. Complexation to Cr(CO)3 allows even electron-rich ring systems such as anisole to be deprotonated by lithium amides [66]. In 1994, Simpkins showed that chiral lithium amides [83] could be used to achieve this transformation enantioselec-tively [84]. Anisole complex 125 was treated with the chiral base 83 in the pres-... 

An alternative approach to the asymmetric synthesis of arenechromium tricarbonyls is to use achiral alkylHthiums in the presence of a chiral ligand - the diamine (-)-sparteine 85, for example. In a study of the relative efficiencies of a range of diamines, Uemura showed that the best Hgand for introducing enantio-selectivity into the hthiation of 158 and 161 was the diamine 159 (Scheme 42) [101]. (-)-Sparteine 85 performed relatively poorly with 158. 

In the field of nucleophilic additions to arenechromium tricarbonyls [50], the chromium complex of phenyloxazohne adds various organohthium reagents re-gio- and enantioselectively in the presence of (-)-sparteine (or chiral 1,2-di-ethers). The hthium adduct thus formed has been trapped with propargyl bromide, leading to a frans-disubstituted cyclohexadiene of high ee [51] (Scheme 30). 

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