W bonds formation

Some selected reactions of 21b were investigated [32]. In the reaction with [(Ph 0)4W=0j the dinuclear compound 24 is formed (Eq. 16) containing an almost planar W2OP four-membered ring system. The structure of 24 reveals that after the formal cycloaddition reaction a reductive W-W bond formation occurs under loss of OPh moieties. 

The products are those of the pure electron transfer steps that occur without solvent or ligand redistribution and without W-W bond formation. The occurrence of electron transfer alone is the feature that makes the reaction so efficient. The diagram in Scheme II details the various reactions... 

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

FIGURE 27 13 The mechanism of amide bond formation by W W dicyclohexylcarbodiimide promoted condensation of a carboxylic acid and an amine... 

The apparent inertness of the noble gases gave them a key position in the electronic theories of valency as developed by G. N. Lewis (1916) and W. Kossel (1916) and the attainment of a stable octet was regarded as a prime criterion for bond formation between atoms (p. 21). Their monatomic, non-polar nature makes them the most nearly perfect gases known, and has led to continuous interest in their physical properties. 

R. O. Duthaler, A. Hafner, M. Riediker in Asymmetric C-C Bond Formation with Ti-, Zr-, and Hf-Carbohydrate Complexes in Organic Synthesis via Organometallics, K. H. Dotz, R.W Hoffmann, Eds., pp 285-310, Vieweg, Braunschweig 1990. 

The possibility that only some of the outer electrons enter into bond formation needs to be considered. There is a close relation between interatomic distance and bond type, and the values of the interatomic distances in the sequence6 K, A2, 4.62A Ca, Al,A3, 3.93-3.95A Sc, not yet investigated Ti, A3, 2.92-2.95A V, A2, 2.63A Cr, A2, 2.49A, as well as those in the similar sequences from Rb to Mo and Cs to W, indicate that the number of bonds resonating among the available positions increases from one to nearly six. 

Li, D., Che, C.-M., Kwong, H.-L. and Yam, V.W.-W. (1992) Photoinduced C-C bond formation from alkyl halides catalysed by luminescent dinuclear gold(I) and copper(l) complexes. Journal of the Chemical Society, Dalton Transactions, 3325-3329. 

A T structure with the strongest ct-donor D trans to the empty site (I in Scheme 1) is preferred in the case of three pure cr-donor ligands. The presence of a ir-acceptor ligand also makes the T structure more stable. When one of the ligands is a tt-donor, X, a Y structure of type II (Scheme 1) is observed. This structure permits the formation of a w bond between the empty metal d orbital and the lone pair of X. No such tt bond is present in the T structure since all symmetry adapted d orbitals are filled. This partial M—X multiple bond stabilizes Y over T. 

The shift in the C=C frequency, vi, for adsorbed ethylene relative to that in the gas phase is 23 cm-1. This is much greater than the 2 cm-1 shift that is observed on liquefaction (42) but is less than that found for complexes of silver salts (44) (about 40 cm-1) or platinum complexes (48) (105 cm-1). Often there is a correlation of the enthalpy of formation of complexes of ethylene to this frequency shift (44, 45). If we use the curve showing this correlation for heat of adsorption of ethylene on various molecular sieves (45), we find that a shift of 23 cm-1 should correspond to a heat of adsorption of 13.8 kcal. This value is in excellent agreement with the value of 14 kcal obtained for isosteric heats at low coverage. Thus, this comparison reinforces the conclusion that ethylene adsorbed on zinc oxide is best characterized as an olefin w-bonded to the surface, i.e., a surface w-complex. 

Odom, O.W., Picking, W.D., and Hardesty, B. (1990) Movement of tRNA but not the nascent peptide during peptide bond formation on ribosomes. Biochemistry 29, 10734-10744. 

Winkler, T. and Bencze, W.L., Perkow reaction induced C,C-bond formation, Helv. Chim. Acta, 63, 402, 1980. 

Figure 4.25 (a) DNA polymerase-catalysed phosphodiester bond formation typically requires two metal ions, usually Mg2+. (b) A model of the transition state for phosphodiester-bond formation in RNA polymerase. (From Berg et al., 2002. Reproduced with permission from W.H. Freeman and Co.)... 

Recently, the C—C bond formation through an electroreduction of a diazoalkane group, incorporated as a ligand into W complexes, has been reported. One-electron reduction of trans-[W(N2CH2)L2F]+ in an MeCN-Bu4NBF4-(Hg) system affords W(N2CH2CH2N2) L4F2 (L = dppe) in 55% yield [500]. 

As noted previously, linear gramicidin" " and anabaenopeptilid" synthetases begin with modules containing formylation (F) domains in an F-A-PCP initiation module. In vitro experiments have revealed that formylation domains act upon aminoacyl-S-PCP intermediates before amide bond formation occurs within the downstream C domain. These F domains likely utilize w °-formyltetrahydrofolate as a cofactor. ... 

Fig. 4j6 Stepwise eomplexing of Cu(OH)4 by a tetradentate macrocyclic ligand. The first Cu(II)-N bond is formed by replacement of an axial solvent molecule (k ) followed by a Jahn-Teller inversion (Ar, ) which brings the coordinated nitrogen into an axial position. Second-bond formation follows a similar pattern (k2 and 2b)- Reproduced with permisson from J. A. Drumhiller, F. Montavon, J. M. Lehn and R. W. Taylor, Inorg. Chem. 25, 3751 (1986). (1986) American Chemical Society.

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