Arenechromium tricarbonyls

In the substituted arenechromium tricarbonyl system 129), the following ion-molecule reactions are observed ... 

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. 

Diastereoselective lithiation of chiral arenechromium tricarbonyl complexes... 

Some headway has been made using sulphoxides to direct the lithiation of arenechromium tricarbonyls in the manner of Kagan s work with ferrocenes . Diastereoselectiv-ities in the lithiation-quench of 392 are excellent, though yields are poor with most electrophiles. Diastereoselectivity reverses on double lithiation, because the last-formed anionic site in 394 is the most reactive (Scheme 163). 

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. 

Couplings can also be carried out by simple nucleophilic substitution reactions of arenechromium tricarbonyls . For example, in the synthesis of biaryl 469, asymmetric lithiation of 463 using in situ silylation provides the complex 466 via 464 and 465. Nucleophilic substitution by the tolyl Grignard 467 yields 468 as a single atropisomer in 68% yield, and decomplexation gives the biaryl 469 in 92% yield (Scheme 184). 

Wilhelm and Widdowson have exploited the asymmetric deprotonation of 470 in a synthesis of a protected version 478 of the biaryl component of vancomycin, actinoidinic acid (Scheme 185) " . One of the rings derives from an arenechromium tricarbonyl with stereochemistry controlled by asymmetric lithiation. The most readily lithiated position of 470, between the two methoxy groups, first needed blocking. Enantioselective... 

Mandelhauni, A., Z. Neuwirth, and M. Cais Organometallic studies VII. The resolution of an arenechromium tricarbonyl racemic mixture. Inorg. Chemistry 2, 902 (1963). 

However, these cations are stabilized by an arenechromium tricarbonyl group in the a-position. Thus benzyl alcohol complexed with chromium tricarbonyl undergoes Ritter reactions rapidly and in high yield (equation I). On the other hand, the... 

Stereoselective benzylic alkylation.6 Benzylic acetates of arenechromium tricarbonyls react with R3A1 or (QHs Zn-TiCU to form exo-alkyl chromium complexes. 

Arenes are inert to nucleophilic attack and normally undergo electrophilic substitution. However, arenes coordinate to Cr(CO)6 to form the i/fi-arenechromium tricarbonyl complex 79, and facile nucleophilic attack on the arene generates the anionic jy5-cyclohexadienyl complex 80, from which substituted arene 81, or cyclohexadiene is obtained by oxidative decomplexation. In this reaction, strongly... 

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... 

In addition to alkenes, arenes can sometimes be used as radical acceptors in Sml2-mediated carbonyl-alkene couplings. For example, Schmalz reported extensive studies on ketyl additions to arenechromium tricarbonyl complexes 66,67 tetralin-Cr(CO)3 complex 49 underwent reductive carbonyl addition to the aromatic ring upon treatment with Sml2 to furnish the skeleton of the naturally occurring aryl glycoside pseudopterosin G (Scheme 5.37).66,67 Here, the bulky metal tricarbonyl group not only serves to control the... 

Anti-Bredt alkenes, 56, 137, 217 Antimony(V) fluoride, 20-21 Aphidicolin, 205, 206, 294 Aplysistatin, 512 Apotphines, 201, 513-514 Arachidonic acid, 242-243 a-Arenechromium tricarbonyl complexes, 21, 117-118... 

Addition of earbanions to ir-arenechromium tricarbonyl complexes. Experimental details for addition of earbanions to n -benzenechromium tricarbonyl and for subsequent transformations (oxidation, reaction with electrophiles) of the resulting addition compounds have been published. ... 

Substituent effects in addition of earbanions to 7r-arenechromium tricarbonyl complexes have been examined. Methyl and chloro substituents give mixtures of ortho- and mefa-substitution, with the amount of mem-isomer increasing with bulk of the anions. Methoxyl and dimethylamino substituents are strongly meta -directing. The more hindered 3-position of 1,2-dimethoxybenzene is substituted even by a tertiary carbanion. Naphthalene shows 99% a -substitution. A trimethylsilyi group is strongly para-directing. 

Complexed Aryllithiums. Arenechromium tricarbonyl complexes are inctiilatcd with n-butyllithium and TMEDA in THE at -78° to afford chromium Iriciirbonyl complexes of aryllithiums. Metalation oeeurs specifically ortho to... 

Relatively extensive studies have been performed on the solvolysis of several arenechromium tricarbonyl tosylates (6, 7). Comparison of the solvolysis data of the complexed and noncomplexed optically active threo-and ervfAro-3-phenyl-2-butyl tosylates led the authors to conclude that complexation of these tosylates inhibited phenyl and hydrogen migration, prevented racemization in the threo case, and reduced direct displacement... 

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). 

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