Tricarbonylchromium arene complexes

So far, chiral lithium amides for asymmetric deprotonation have found use only with a few types of substrates. The following sections deal with deprotonation of epoxides to yield chiral allylic alcohols in high enantiomeric excess, deprotonation of ketones, deprotonation of tricarbonylchromium arene complexes and miscellaneous stereoselective deprotonations. These sections are followed by sections in which various chiral lithium amides used in stereoselective deprotonations have been collected and various epoxides that have been stereoselectively deprotonated. The review ends with a summary of useful synthetic methods for chiral lithium amide precursors. 

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. 

Excellent enantioselectivity and double-bond regioselectivity can be achieved in an asymmetric Heck reaction between 2,3-dihydrofuran and aryl triflates by using a combination of chiral diphosphine-oxazoline ferrocenyl ligand and Pd catalyst <03CEJ3073>, as shown below. Chiral diphosphine-containing (arene)tricarbonylchromium(O) complexes were also used as ligands for this reaction to obtain the 2,3-isomer, however, both the yield and enantioselectivity were modest <03TA1455>. 

Evidence has been provided that adsorption of several (t] -arene)tricarbonylchromium(O) complexes on to silica gel can perturb the electronic structure of these complexes and the quantum efficiencies with which they undergo photodecomposition. The steric constraint imposed by the support also seems to influence the photoreactivity of these complexes. M(CO)4(bipym) (where M = Cr or W, bipym = 2,2 -bipyrimidyl) and (W(CO)4]2(bipym) both have a lowest MLCT state, and on irradiation CO is lost. Wavelength-dependent quantum yields have been obtained for this transformation and at 366 nm = 2.4x 10 quanta/min for Cr-bipym, 2.5 x 10 quanta/min for W-bipym, and 1.1 x 10 quanta/min for W-bipym-W." Correlations have been described between the MLCT absorption energies of a series of M(CO)4 diimine complexes and Reichardt s solvent parameters (Et), and Kamlet s and Taft s solvent polarity scale Paramagnetic complexes... 

The chromium-templated coupling of alkenyl- or arylcarbene, aUcyne and carbonyl ligands generates arene tricarbonylchromium complexes as primary benzannulation products which - based on their unsymmetric substitution pattern - bear a plane of chirality. Chiral arene complexes are powerful reagents in stereoselective synthesis however, the preparation of pure enantiomers is a lengthy and often tedious procedure, and thus diastereoselective benzannulation appears to be an attractive alternative. In order to lure the chromium fragment to one or the other face of the arene formed, chiral information may be incorporated in the carbene complex or the aUcyne. 

Calix[n]arenes are a class of cavity-shaped macrocycles composed of n molecules of phenol and n molecules of formaldehyde. X-ray crystallographic studies of calix[/i]arene derivatives have been continuously reported by Atwood et al [1-4], Andreetti et al. [4-8], and others [9-11]. We have also carried out the structure determination of partial-cone conformers and a 1,3-altemate conformer of calix[4]arene derivatives [12-14]. More recently, we have synthesized arene-tricarbonylchromium complexes from cone, 1,2-altemate and 1,3-altemate conformers of 25,26,27,28-tetrapropoxycalix[4]arene (1) [15]. The purpose of the study was to selectively introduce the desired functional group into the desired benzene nucleus because tricarbonylchromium [Cr(CO)3] forms stable rj -arene complexes and the complexed benzene nucleus becomes extraordinarily "reactive [16-20]. 

The carbonyl complexes in Table XIV are of three types, tricarbonylchromium r] -arene complexes, pentacarbonyltungsten o-pyridine complexes, and car-bonylruthenium o-alkenyl complexes, the first two possessing low y values. For the first two types, lengthening the 7i-system or replacing acceptor by donor substituent on the tricarbonylchromium-coordinated arene ring results in... 

The arene groups in (r/ -arcnc)tricarbonylchromium complexes are typically electron poor and display poor reactivity toward electrophiles. In the case of mercuration reactions, this lack of reactivity can be overcome by attachment of Lewis-basic substituents to the arene ring. For example, in the case of 75a-c, the presence of a pyridyl, oxazolyl, or methyl-A,A-dimcthylami no group promotes ortho-mercuration, leading to the formation of the bimetallic complexes 76a-c (Equation (28)). 07... 

Highly diasteroselective and chemoselective reductions may be performed on the hydroxy functions of (r/6-arene)-tricarbonylchromium complexes. Treatment of the chromium-complexed benzylic alcohol 29 with triethylsilane and boron trifluoride etherate in dichloromethane at —78° to 0° gives only diastereomer 30 in 75% yield (Eq. 40).181 In a similar fashion, treatment of the complexed exo-allyl-endo-benzylic alcohol 31 with an excess of Et3SiH/TFA in dichloromethane at room temperature under nitrogen produces only the endo-aflyl product 32 in 92% yield after 1.5 hours (Eq. 41). It is noteworthy that no reduction of the isolated double bond occurs.182... 

The carbonyl complexes listed in Table V are of two types tricarbonyl-chromium rj6-arene 77-complexes, and pentacarbonyltungsten <7-pyridine complexes, with both complex types having relatively low y. Nonlinearities increase on arene or pyridine 77-system lengthening, and on proceeding from acceptor to donor substituent on the tricarbonylchromium-coordinated arene ring. Relative magnitudes and trends thus mirror those observed with quadratic nonlinearities of these complexes (see Ref. 1)... 

Early attempts to utilize I as a donor for a chromium sandwich complex did not meet with success268, but recent studies have now provided several /6-chromium(0) complexes 64- 99-200. Whereas unsubstituted cycloproparenes undergo oxidative addition of the ring to the metal followed by carbon monoxide insertion (Section V.B.4), the l,l-bis(trimethylsilyl) derivatives do not. Instead, reactivity is transferred to the arene and, with tris(acetoni-trile)tricarbonylchromium, -complexes are formed at the ring remote from the cyclo-proparene moiety (equation 28). However, the 1,1 -disilyl derivative of 1 does not react and... 

The isolation of the first halobenzene complex, (q6-chlorobenzene)tricarbonylchromium(0), allowed a test for a direct analog of classical SNAr reactivity.15 The activating effect of the Cr(CO>3 unit was found to be comparable to a single p-nitro substituent in reaction with methoxide in methanol and the substituted arene ligand was detached with mild oxidation (equation 2). 

Bis(arene)hafnium complexes, characteristics, 4, 697 Bis(arene)iron dications, characteristics, 6, 173 Bis(arene)niobium complexes, characteristics, 5, 95 Bis(arene)titanium(0) complexes, characteristics, 4, 243-244 Bis(arene)tricarbonylchromium complex, synthesis, 5, 258... 

The complexation of an arene to the tricarbonylchromium unit promotes the addition of nucleophiles to the arene ring due to the strong electron-withdrawing ability of the Cr(CO)3 group. Other effects of the coordination of the metal on the reactivity of the arene ligand have been well-documented in the literature [1] and concern (Scheme 1) (i) the stabilization... 

Procedures that follow paths b and c (Scheme 2) result in loss of the tricarbonylchromium entity. It is noteworthy that in the case of paths a and d, however (Scheme 2), the Cr(CO)3 unit is not lost, and arene tricarbonylchromium complexes are recovered. These can then be reacted with another nucleophile or be decoordinated under CO atmosphere, which affords the free arene and Cr(CO)6, which can be used to prepare the starting complex. 

Nucleophilic attack by the lithium anion of acetonitrile has been extended to Cr complexes of ring A aromatic steroids, thereby allowing access to more elaborate derivatives. For example, the diastereomeric tricarbonylchromium complexes of 17/ -(tert-butyldimethylsily-loxy)-3-methoxyestra-l,3,5-triene 60a give the corresponding ipso 3-cyanomethyl complexes 60b after displacement of the methoxide group. The free arene was obtained in 46 % yield after h oxidation (Scheme 29) [47]. 

Carbon-boron clusters ( carboranes ) have been shown to react with Cr-coordinated halo-arenes. For example, reaction of 2 equiv. of LiC2BjoHio(CH3) with para-difluorobenzene-tricarbonylchromium complex in refluxing THF results in the displacement of both fluoride substituents from the arene ring to yield the para-phenylene compound, albeit in just 9 % yield owing to the effect of the steric bulk of the carborane [55]. 

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. 

Dihydroxy allenes are generated from ketoenes and ethynyl epoxides. a,P-Epoxy ketones undergo reductive cleavage but the P-hydroxy ketones thus obtained can react further, for example with an e, -double bond to give cyclic 1,3-diols. Note that the double bond does not have to be activated, and furthermore, a silylalkyne moiety and a tricarbonylchromium-complexed arene can play the same role, although in the latter case the net result is a cine-substitution. 

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