Bis arene derivatives

The cocondensation of Nb atoms with toluene has yielded bis( -toluene)niobium, which serves as a useful intermediate for the preparation of a wide series of mono- and bis-arene derivatives. While Ti, V, Nb, Cr, Mo, and W form (Ar)2M sandwich complexes, Mn, Tc, and Re appear less promising, because Mn-arene complexes have already been shown to be unstable, Tc is radioactive, and Re remains to be investigated. As already mentioned, Ti forms fairly labile bis(arene)Ti complexes. The unstable 16-electron sandwich complexes from elements of group 4 can be stabilized with phosphine ancillary ligands, yielding bis-arene derivatives as in Table 6. 

Neutral Bis(Arene) Derivatives of Zero- Valent Rare Earths... 

Synthetic utility of stereoselective alkylations in natural product chemistry is exemplified by the preparation of optically active 2-arylglycine esters (38). Chirally specific a-amino acids with methoxyaryl groups attached to the a-carbon were prepared by reaction of the dimethyl ether of a chiral bis-lactam derivative with methoxy arenes. Using SnCl as the Lewis acid, enantioselectivities ranging from 65 to 95% were obtained. 

Capsules are instantly formed by simple addition of two equivalents of Pd(dppp)(OTf)2 (dppp = l,3-bis(diphenylphosphino)propane) to acetone, CH2C12 or CHC13 solutions of resorcin [4]arene derivatives having four pyridine units as pendent groups, see Figure 64.637... 

The coordination chemistry of tertiary phosphine-functionalized calix[4]arenes have been described.279 Treatment of a bis(diphenylphosphino) or bis(dimethylphosphino) derivative of calix[4]arene with [PtCl2(COD)] leads to the formation of the corresponding dichloroplatinum(II) complex. The related diplatinum(II) species has also been reported with the tetrafunctionalized calix[4]arene.280 The mononuclear derivative is susceptible to oligomerization if the two free phosphine ligands are not oxidized or complexed to another metal center such as gold(I).279 The platinum(II) coordination chemistry of a mono-281 and diphosphite282 derived calix[ ]arene (n = 4 and 6, respectively) has also been described. 

Co-condensation of transition metal atoms with arenes such as benzene and toluene is well known to yield bis-arene-metal compounds. However, in many cases the yields based on the metal atoms are less than 40%. Evidence that competing reactions such as carbon-hydrogen activation can occur is provided by the isolation of non-metal-containing products such as biaryl derivatives (2JL). ... 

V,/V-dimethylaniline, especially when those strong donors are paired with the relatively electron-poor MES derivative of the bis(arene)iron(ll) acceptor. As such, the dark reactions arise via essentially the same multistep mechanism as that for charge-transfer de-ligation, the difference arising from an adiabatic electron transfer (10) as the initial step that is thermally allowed when the driving force -AGET is sufficient to surmount... 

Perhaps most dramatically of all, for the first time, bis(carbene)-substituted iridium complexes, such as [Ir(cod)(NHC)2] (NHC = 1,3-dimethyl- or 1,3-dicyclohexylimidazolin-2-ylidene] were successfully used by Herrmann and coworkers as C—H-activation catalysts in the synthesis of arylboronic acids starting from pinacolborane and arene derivatives [46]. 

The Ullmann coupling is the classical example of Cu-catalyzed biaryl coupling, wherein (a) a phenol and arylhalide substrate are converted to a bis-arylether or (b) two arenes are coupled to form a bis-arene species. These coupling reactions are of great importance for general organic synthesis as well as pharmaceutical and fine chemicals. The copper-catalyzed phenol coupling to arrive at chiral biphenol derivatives is used extensively as a test reaction for the catalytic activity of new copper complexes [254,255]. 

Bis(arene) [M(jj6-C6H3R3)2] niobium (R = H3, H2Me or 1,3,5-Me3), and less stable tantalum derivatives (R = H3) were prepared in a similar way.714 They are extremely sensitive to oxygen, but relatively stable to water. The photoelectron spectra of the volatile bis(i76-arene) niobium display low values for the first ionization potentials (5.18-5.57 eV), indicating that the compounds are electron rich. The IR spectra of [Nb(C6H3R3)2] (R = H, Me) in an argon matrix at 80 K are in agreement with a symmetrical sandwich structure. 

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... 

One of the most remarkable boronic acid derivatives is the very rigid bis(bora)-p-rt>utylcalix[4]arene derivative 4.78 developed by Matt Davidson at the University of Bath, UK. The host binds fluoride very strongly in chloroform with log Kn = 6.3. The fluoride is exclusively chelated in between the two boron atoms since fluoride binding to the outside of the host would result in unfavourable steric repulsions between the phenyl groups, Scheme 4.6. While relatively weak, the fluorescence of the phenyl groups may be used to monitor binding addition of tetrabutylammonium fluoride almost completely quenches the fluorescence.58... 

Scheme 4.6 Fluoride binding by the bis(bora) p I butylt alix 41 arene derivative 4.78.58...

Taking into account om calix[4]tube ligand design10, the novel bis-calix[4]diquinone receptors were prepared via the initial synthesis of various bis-calix[4]arene derivatives connected by alkylene chain units and subsequent oxidation of the remaining phenolic... 

---