Lithiation arene complexes

The advantage of using lithiated arene complexes of chromium as starting materials to synthesize o-,7r-bimetallic complexes of chromium and manganese was clearly demonstrated by reacting (77 -o-C6H4XLi)Cr(CO)3 (X = H, F) with Mn(CO),Br (305). The products, (77< -C6H4XC(0) Mn-... 

As an example, reaction of a lithiated arene complex with dibromoethane or diiodoethane permits the efficient introduction of bromo and iodo substituents (Scheme 6.4). The direct complexation of the parent free arene would be unsuccessful, giving rise instead to a dehalogenation reaction. ... 

By coordinating to arenes, transition metals can facilitate ring lithiation by decreasing the electron density in the ring and acidifying the ring protons. We shall consider briefly the two most important metal-arene complexes in this regard—arenechromium tricarbonyls and ferrocenes. 

Desymmetrisation by enantioselective ortholithiation has been achieved with ferrocenylcarboxamides 434,187 and also (with chiral lithium amide bases) a number of chromium-arene complexes.188 The chromium arene complex 435, on treatment with s-BuLi-(-)-sparteine, gives 436 enantioselectively, and reaction with electrophiles leads to 437. However, further treatment with r-BuLi generates the doubly lithiated species 438, in which the new organolithium centre is more reactive than the old, which still carries the (-)-sparteine ligand. Reaction of 438 with an electrophile followed by protonation therefore gives ent-431.m... 

Other aminations of more substituted arene complexes allow the regiospedfic synthesis of polysubstituted aromatics. For example, p-fluoroanisoletricarbonylchromium complex can first be lithiated and quenched with chloroformate to give 33b (R = OMe, R = CChMe). After substitution of the fluoride by pyrrolidine, complex 31 is obtained in 89 % yield (Scheme 18) [29]. 

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. 

Gibson and coworkers have examined the possibility of using chiral base methodology to enantioselectively lithiate the benzylic position of arene complexes (Scheme 46). (R,R)-3 has been used in combination with LiCl and followed by quenching with... 

Keywords Lithiation Arene-chromium complexes Aryl-lithium Regioselectivity in arene lithiation... 

Semmelhack MF (1995) Transition metal arene complexes ring lithiation. In Abel EW, Stone EGA, Wilkinson G (eds) Comprehensive organometallic chemistry II. Pergamon... 

The synthesis of an enantiomerically enriched chromium complex via asymmetric lithiation of a prochiral tricarbonyl(ri -arene)chromium complex using a chiral lithium amide base was first demonstrated in 1994 by Simpkins [88]. Arene complex 44 was treated with C2-symmetric chiral base ent-39 in the presence of TMSCl as an internal quench and silylated complex 45 was obtained in 84% ee (Scheme 24). 

SCHEME 26.14 Asymmetric lithiation of chromium arene complexes. 

SCHEME 26.15 Asymmetric lithiation of arene complexes bearing a sugar residue. 

A 1,2 or 1,3 unsymmetrically disubstituted arene is prochi-ral and therefore the corresponding chromium tricarbonyl compounds are chiral. (Substituted arene) complexes with amine, carboxyl, and formyl groups at the ortho position are resolved into optically active chromium complexes through corresponding diastereomeric adducts (eq 25). Biocatalysts also perform the kinetic resolution of racemic chromium complexes (eq 26). The optically active chromium complexes can be prepared by di-astereoselective ortho lithiation of the chiral benzaldehyde or acetophenone acetal complexes, and diastereoselective chromium complexation of the chiral ort/io-substituted benzaldehyde am-inals (eq 27). Catalytic asymmetric cross-coupling of meso (1,2-haloarene)chromium complex produces chiral monosubstituted complexes. The chiral (arene)chromium complexes can be used as ligands in asymmetric reactions. ... 

Semmelhack ME, Chlenov A (2004) (Arene)Cr(Co)3 Complexes Arene Lithiation/Reaction with Electrophiles. 7 21-42... 

Since different reactivity is observed for both the stoichiometric and the catalytic version of the arene-promoted lithiation, different species should be involved in the electron-transfer process from the metal to the organic substrate. It has been well-established that in the case of the stoichiometric version an arene-radical anion [lithium naph-thalenide (LiCioHg) or lithium di-ferf-butylbiphenylide (LiDTBB) for using naphthalene or 4,4 -di-ferf-butylbiphenyl (DTBB) as arenes, respectively] is responsible for the reduction of the substrate, for instance for the transformation of an alkyl halide into an alkyllithium . For the catalytic process, using naphthalene as the arene, an arene-dianion 2 has been proposed which is formed by overreduction of the corresponding radical-anion 1 (Scheme 1). Actually, the dianionic species 2 has been prepared by a completely different approach, namely by double deprotonation of 1,4-dihydronaphthalene, and its X-ray structure determined as its complex with two molecules of N,N,N N tetramethylethylenediamine (TMEDA). ... 

Tetrahydrofuran itself can be opened using either the stoichiometric or the catalytic version of arene-promoted lithiation, but both cases need the activation by boron trifluoride. The catalytic reaction was performed by treating the solvent THF 324 with the complex boron trifluoride-etherate and a catalytic amount (4%) of naphthalene. The intermediate 325 was formed. Further reaction with carbonyl compounds and flnal hydrolysis yielded the expected 1,5-diols 326 (Scheme 95), which could be easily cyclized to the corresponding substituted tetrahydropyrans under acidic conditions (concentrated FlCl). 

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