Anion donor

The methanol can be removed by heating gently in vacuo. Similar compounds can be made with other carboxylate groups, either by using this method or by heating the acetate with excess carboxylic acid. Treatment of the anhydrous carboxylate with various neutral ligands (L) or anionic donors (X-) forms Rh2(OCOR)4L2 and [Rh2(OCOR)4X2]2-, respectively. The colour of the adduct depends on the donor atom in L (or X) ... 

Monodentate or multidentate thiols as the ligand coordinate are anionic donors in form of thiolates. The metal core of their complexes is normally [M =0] 3 +. The charge requirements of this core are easily met by the ligands. In the case of pure thiolato coordination, anionic complexes result. In a few cases, neutral binuclear oxo complexes and oxo-free species also occur. 

Likewise, cationic acceptors afford mixed (positively) charged complexes with electron-rich donors,11 i.e., [D, A+] and anionic donors associate with electron-poor acceptors to form mixed (negatively) charged complexes,12 i.e., [D-, A]. In each case, the intermolecular (ionic) complexation or association represents the highly oriented organization of the donor/acceptor pair (independently of whether they bear positive, negative or no charge) that is often sufficient to afford crystalline complexes amenable to direct X-ray structure elucidation.13... 

Transfer of the acyl group from the acylzirconocene chloride to aluminum (transmetala-tion) by treatment with aluminum chloride has been reported to give an acylaluminum species in situ, and the possibility of the acylaluminum acting as an acyl anion donor has been suggested (Scheme 5.5) [7]. However, the acyl anion chemistry through this trans-metalation procedure appears to be limited since only protonolysis to the aldehyde proceeds in good yield, which could be achieved by direct hydrolysis of the acylzirconocene chloride. 

Scheme 5.11. Acylmetal as an unmasked acyl anion donor.

To replace the aforementioned acyl-main group and acyl-transition metal complexes, the natural course of events was to search for a stable and easy-to-handle acyl-metal complex that reacts as an unmasked acyl anion donor. Thus, the salient features of acylzirconocene chlorides as unmasked acyl anion donors remained to be explored. In the following, mostly carbon—carbon bond-forming reactions with carbon electrophiles using acylzirconocene chlorides as acyl group donors are described. 

Synthetic routes to a-ketol through the reactions of an unmasked acyl anion with carbonyl compounds are not numerous. The first practical application of an acylzirconocene chloride as an unmasked acyl anion donor was reported in the reaction with aldehydes in 1998 (Scheme 5.12 and Table 5.1) [19]. 

Although the reactivity of acylzirconocene chlorides towards imine derivatives under Yb(OTf)3/TMSOTf (20 mol%, l l)-catalyzed conditions is not necessarily very high, the direct access to a-amino ketone derivatives indicates the usefulness of acylzirconocene chlorides as unmasked acyl anion donors. 

R)-product. Although there is still room for further improvement of the enantioselectivity, this first example of an enantioselective reaction with a,(S-unsaturated ketones reveals the potential of acylzirconocene chlorides as unmasked acyl anion donors. 

The cited reactions may be considered as ligand exchange reactions, in which either a neutral donor D replaces an anion donor X at M or a neutral acceptor A replaces the cation at X. 

In Table 5 the values for the free standard enthalpies for the reactions of neutral donors and anion donors with vanadyl acetylacetonate are listed. It can be seen that towards the reference molecule iodide ion is a somewhat weaker ligand than propanediol carbonate, whereas the bromide ion is between tri-methylphosphate and acetone, and the chloride ion between DMF and DMSO 22>. The fluoride ion and the NCS -ion are stronger donors than all neutral donors but are somewhat weaker than the azide and the cyanide ion. 

This reaction proceeds with a variety of aryloxy ligands and a spectrum of substituents R and R at the phosphorus centre. Alkalimetal fluorides, tetrabutylammonium fluoride (TBAF), hypervalent silicon Ph3Sip2 and tin Ph3Snp2 difluorides are suitable fluoride anion donors. The exchange of aryloxy ligand proceeds efficiently in solvents such as THF, MeCN and CH2CI2. 

A special case of complexes containing the [Tc=0] moiety and exclusive coordination to sulfur donors is that of thioether coordination. Although the [Tc=0] " " core is thiophilic, thioether coligands are not sufficient to stabilize the +V state and the 3-f charge. Several attempts in that direction have failed and complexes with additional anionic donors were formed. Reaction of [TcOClJ with bidentate thioethers (S2) leads to a class of complexes containing the... 

Figure 4.4. Homogeneous electrocatalytic redticiion of 2-bromopyridine in ditnethylfomiamide. Forward electron transfer rate constant vs. standard potential of the aromatic radical-anion donor. Data from ref, [29],...

In ETD, an anion donor collides with the positively charged peptide and transfers an electron to the peptide subsequent to this intermolecular electron transfer, the peptide undergoes cleavage at one of its N-Ca or S-S bonds to form fragment ions. The mass to charge ratios and intensities of the fragment ions are the raw data that is then used to infer the primary sequence of the original... 

Figure 1 Prototypical polypeptide showing disulfide (SS) linkage, one of many N-Ca bonds, amino acid side chains (wavy lines), protonated amines on side chains (wavy lines), and one of many peptide bonds. Also shown is an anion donor (H3C ) colliding with the peptide.

In both ECD and ETD, the initial conditions appropriate to the experiments do not correspond to the ground electronic state of the electron/peptide (ECD) or anion/ peptide (ETD) system. In both cases, there are a myriad of lower-energy electronic states, and this fact presents major challenges to the theoretical study of these processes. In Figure 2, we show qualitative plots of energies as functions of the distance R between a H3C anion donor and a polypeptide having total charge Z. 

Families of Rydberg states in which the excess electron has moved from the anion donor to reside in a Rydberg orbital (ground 3s, or excited 3p, 3d, 4s, etc.) on one of the polypeptide s protonated amine side chains. These curves (at least... 

A more recent investigation has been carried out on the homogeneous ET to an extended series of diaryl disulfides (X = NH2, OMe, H, F, C02Et, CN. NO2) in DMF. The redox catalysis approach was applied extensively. The mechanism of the homogeneous reaction between electrogenerated radical anion donors D and (ArS)2 takes place according to the sequence (equations 80-83) ... 

Niobium and tantalum compounds form adducts with virtually all types of neutral ant anionic donors. The coordination chemistry of the higher halides is widely developed, and thei activity as Friedel-Crafts catalysts is another manifestation of their Lewis acidity. The stron acceptor capacity of the high valent metal compounds tends to favor the formation of dimers and sometimes of higher condensation products, which competes with coordination with othe donor molecules. Numerous simple anionic or heteropolyanionic species, but little cationi chemistry, and no simple metal salts, are known. 

Compounds of formula [M BgLj], where B is a bridging anionic donor and L a neutral one, may be considered as formally derived from M3X8 adducts by solvolysis. The reduction of Nbv in acid solutions has been used for its qualitative and quantitative determination, but the exact nature of the reduced species remains unknown. The red-brown salt obtained as early as 1912 by reduction of Nbv in aqueous H2S04 has been formulated as [K4(H502)][Nb302(S04)6(H20)3] 5H20 (48) on the basis of a recent X-ray structure... 

The dissociation constants of MX2 (M = Zn, Cd or Hg X = Cl, Br, I or C104) in DMF, acetone and ROH solvents have been measured.972 The order of anion donor strength is found to be 01O4Br>Cl a small-angle neutron scattering study of D20 solutions of zinc chloride has been reported.973... 

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