Anisole, chromium complex

OC Hs, Phenol, rhodium complex, 27 292 OC,Hj, Anisole, chromium complex,... 

Staggered configurations have also been observed for the tricarbonyl chromium complexes of phenanthrene 294, 295), 9,10-dihydro-phenanthrene 293, 295), anthracene 202), naphthalene 262), and 1-aminonaphthalene 58). The eclipsed configuration has been observed for the tricarbonyl complexes of anisole (57, 229), toluidine 60, 61), methylbenzoate (59), o-methoxyacetylbenzene, o-hydroxyacetylbenzene 101), 2-methoxy-[l-hydroxy-ethyl]benzene (99), and 2-methyl-[l-hydroxy-l-phenylpropyl]benzene (97). It is apparent that the orientation of the chromium tricarbonyl moiety is in many cases controlled by the substituents on the ring to which it is coordinated, and this has been attributed to mesomeric electron repulsion or withdrawal by the substituents 374). 

Nucleophilic substitution reactions involving free arenes are very difficult to carry out. The complexation causes a decrease of electron density on the arene molecule and therefore considerably facilitates nucleophilic attack. Thus, the chlorobenzene chromium complex [Cr(PhCl)(CO)3] easily reacts with sodium methanolate to give anisole chromium tricarbonyl [Cr(PhOMe)(CO)3]. The complex [Cr(PhF)(CO)3] reacts in a similar way. 

Lithiation. With anisole, fluorobenzene, and chlorobenzene chromium complexes, lithiation always occurs at an ortho position of the substituents under mild conditions (eqs 14 and 15). Protected phenol or aniline chromium complexes with sterically bulky substituents produce meta lithiation exculsively. The lithi-ated position of some (arene)chromium complexes (3) differs from that of the corresponding chromium-free compounds (4). ... 

The chromium tricarbonyl complex 429 of triphenylphosphine oxide is hthiated and silylated (in situ quench) by 360 and McsSiCl to give 430 (X = Si) in 90% yield and 73% ee (Scheme 175) °. Interestingly, the sense of asymmetric induction is reversed from the anisole result. With two equivalents of chiral base, bis-sUyl compounds 432 could be formed. 

Methyl benzoate, anisole, and diphenyl ether each give sandwich compounds with chromium vapor, although in rather low yield (32, 55, 110). Chromium appears to attack alkyl ethers and this deoxygenation probably competes with complexation with the aromatic oxygen compounds. No simple product has been isolated from chromium atoms and aniline, but bis(7V,7V-dimethylaniline)chromium has been prepared (32). The behavior of molybdenum and tungsten vapors closely resembles that of chromium in reactions with oxygen- and nitrogen-substituted arenes (113). 

Arene-chromium tricarbonyl complexes. Seinmelhack and Yamashita have used (he activating and meta-directing effect of the CKCO)3 group to obtain the spiro-[4.5]decenone system of acorenone (3) and acorenone B (4) from anisole. Treatment... 

Activation of aromatic compounds by transition-metal complexes was initially studied with Cr(CO)3 complexes. Nucleophilic addition of 2-lithio-l,3-dithianes to arene-chromium(O) complexes 185 followed usually by iodine-promoted decomplexation affords the corresponding 2-arylated 1,3-dithianes 186. The reaction of //-(toluene)- and (anisole)tricarbonylchromium (185) with compound 161 gave mixtures (52 46 and 10 90, respectively) of ortho and meta substituted derivatives (186) (Scheme 54)244. The meta directing effect was also observed (mainly better than 95%) with amino and fluoro substituted complexes245. 

Several arene chromium tricarbonyl complexes form 1 1 adducts with Lewis acids such as tetracyanoethylene and 1,3,5-trinitrobenzene (TNB) 162, 227, 229, 259). The TNB adducts have been isolated as crystaline solids and the structure of the anisole derivative determined by X-ray analysis (57, 229). The plane of the TNB ring was found to be parallel to the anisole ring with an average separation of 3.41 A. This is a somewhat larger separation than that observed in the charge transfer complexes of TNB with aromatic molecules, and the increased separation was attributed to the strong electron-withdrawing capacity of the tricarbonyl chromium moiety which decreases the w-electron donor capacity of the anisole molecule 229). 

One of the first eye-catching synthetic applications of arene-chromium chemistry was the synthesis of the sp/ro-sesquiterpenes ( )-acorenone and ( )-acorenone B (rac-7) disclosed by Semmelhack and Yamashita in 1980 [14]. These authors twice exploited the meta-selective nucleophile addition to anisole-Cr(CO)3 derivatives (Scheme 1). Starting from complex rac-1, such a reaction is first used for the regioselective introduction of an acyl sidechain to give 2 after oxidative workup. A few steps later, the nitrile rac-4 (obtained from rac-3 by complexation and separation of the diastereomeric products by preparative HPLC) is deprotonated to form the spiro addition product rac-5, from which the enone rac-6 is obtained after protonation and hydrolysis of the initially formed dienol ether. The final conversion of rac-6 into acorenone B (rac-7) efficiently proceeds over five steps and involves a diastereoselective hydrogenation of an exo-methylene group. 

The use of 7r-arene-Cr(CO)3 complexes in organic synthesis continues to attract attention. Carbanion attack on 7r-anisole- and 7r-toluene-chromium tricarbonyl complexes gives, after oxidative work-up, meta-substituted aromatics as the major product [equation (24)]. With the anisole complex the me/a-substituted product... 

For monosubstituted arenes, kinetically controlled discrimination between the two enantiotopic ortho hydrogens of the planar chiral benzene chromium tricarbonyl complex leads to nonracemic products. Asymmetric lithiation is more efficient when one or more oxygen atoms, such as ether linkages, are present in the starting prochiral complex (Scheme 26.14). Treatment of Cr(CO)j-anisole complex 52 with the chiral lithium amide 53, in the presence of TMSCl under ISQ conditions, affords (+)-orfho-silylated complex 54 with good chemical yield and ee value [143-145]. The isobenzofuran system 55 reacts as well to give a-sUylated product 56 [146]. 

Arene-metal complexes have again featured in approaches to spirocyclic ring systems. Thus, Semmelhack s group has extended their studies of chromium hexacarbonyl complexes of anisoles to effect new total syntheses of the spiro-sesquiterpenes, acorenone (169) and acorenone B (170) (Scheme 25), In... 

Let s now look at a recent approach to the problem. It is actually a study wherein the problem was used to learn something about the stereochemical course of a new reaction. The Pearson group was studying reactions of ester enolates with arene chromium tricarbonyl complexes. As part of this study, the reaction of anisole derivatives (85) with the enolate derived from terf-butyl propionate was examined. The result was formation of a chromium tricarbonyl complex of a l-methoxy-l,3-cyclohexadiene of type 86. Conversion of the chromium-diene complex into a cyclohexenone would provide material that could serve as an intermediate in a juvabione syntheses if the reaction took place with good diastereoselectivity. 

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