Intermediates 7-coordinate

A finite difference formula is used to estimate the second derivatives of the coordinate vector with respect to time and S is now a function of all the intermediate coordinate sets. An optimal value of S can be found by a direct minimization, by multi-grid techniques, or by an annealing protocol [7]. We employed in the optimization analytical derivatives of S with respect to all the Xj-s. 

Since 5 is a function of all the intermediate coordinates, a large scale optimization problem is to be expected. For illustration purposes consider a molecular system of 100 degrees of freedom. To account for 1000 time points we need to optimize 5 as a function of 100,000 independent variables ( ). As a result, the use of a large time step is not only a computational benefit but is also a necessity for the proposed approach. The use of a small time step to obtain a trajectory with accuracy comparable to that of Molecular Dynamics is not practical for systems with more than a few degrees of freedom. Fbr small time steps, ordinary solution of classical trajectories is the method of choice. 

Fig. 6. Base titration of H A chelant A, free acid without coordinating metal B, in the presence of a metal of intermediate coordinate strength and C, in...

Nakajima reported the use of a chiral bipyridine N,N -dioxide 18 in the desym-metrization of acyclic meso epoxides (Figure 7.3). Although the enantioselectivity was not as high as in the method developed by Fu for meso-stilbene oxide (90% ee vs. 94% ee), it was higher for the same aliphatic epoxide (74% ee vs. 50% ee) [57]. Nakajima showed that mono-N-oxide derivatives 19 and 20 were much less effective than 18 in tenns of both yield and enantioselectivity, and accordingly proposed a unique mechanism for 18 involving a hexacoordinate silicon intermediate coordinated to both N-oxides of the catalyst. 

A Rh—Si bond cleavage has been demonstrated, but an addition-elimination mechanism is unlikely as this would require a seven-coordinate Rh(V) intermediate coordination of HCl through the chlorine atom was postulated [Eq. (64)] 135). 

The solids were used as catalysts in the benchmark cyclopropanation reaction between styrene and ethyl diazoacetate (Scheme 7). As far as the nature of the clay is concerned, laponite was foimd to be the best support for the catalytic complexes. The best enantioselectivity results (Table 7) were obtained with ligand 6b (69% ee in trans cyclopropanes and 64% ee in cis cyclopropanes) but the recovered solid showed a lower activity and enantioselectivity, which was attributed to partial loss of the chiral ligand from the support. In general, the use of the three chiral ligands led to enantioselectivity results that were intermediate between those obtained in homogeneous phase with CuCl2 and Cu(OTf)2 as catalyst precursors. This seemed to indicate that the sohd behaved as a counterion with an intermediate coordinating abihty to the copper centers. 

Normally, the addition of C-nucleophiles to chiral a-alkoxyaldehydes in organic solvents is opposite to Cram s rule (Scheme 8.15). The anti-Cram selectivity has been rationalized on the basis of chelation control.142 The same anti preference was observed in the reactions of a-alkoxyaldehydes with allyl bromide/indium in water.143 However, for the allylation of a-hydroxyaldehydes with allyl bromide/indium, the syn isomer is the major product. The syn selectivity can be as high as 10 1 syn anti) in the reaction of arabinose. It is argued that in this case, the allylindium intermediate coordinates with both the hydroxy and the carbonyl function leading to the syn adduct. 

As before, K20s02(0H)4 is in equilibrium with OSO3. TsNCl adds to Os, which uses a lone pair to displace Cl from N and give the key Os(VHE) intermediate. Coordination of the Sharpless ligand creates a complex that adds rapidly to the alkene. Hydrolysis of the Os(VI) product regenerates 0s02(0H)2 and provides the product. 

The proposed mechanism for allyhc acetoxylation of cyclohexene is illustrated in Scheme 15. Pd -mediated activation of the allyhc C - H bond generates a Jt-allyl Pd intermediate. Coordination of BQ to the Pd center promotes nucleophilic attack by acetate on the coordinated allyl ligand, which yields cyclohexenyl acetate and a Pd -BQ complex. The latter species reacts with two equivalents of acetic acid to complete the cycle, forming Pd(OAc)2 and hydroquinone. The HQ product can be recycled to BQ if a suitable CO catalyst and/or stoichiometric oxidant are present in the reaction. This mechanism reveals that BQ is more than a reoxidant for the Pd catalyst. Mechanistic studies reveal that BQ is required to promote nucleophilic attack on the Jt-allyl fragment [25,204-206]. 

This reaction elucidates the mechanism of the photoreaction of 73 with dienes. In the first step, 73 loses one carbonyl ligand with formation of the reactive 16-electron species [0/5-C5Hj)Mo(CO)2CH3] (87) (109-113), which adds a diene molecule. jj2-Diene complexes [( 5-C5H5)Mo(CO)2CH3-( 72-diene)] (88) are quite likely as intermediates. Coordination of the free C=C double bond of the r 2-diene ligand causes insertion of CO into the Mo—C [Pg.338]

Classical methods for the synthesis of frans-dinitro-bis(ethylenediamine)cobalt(III) nitrate involve conversion of the cis isomer by heat1 or action of sodium nitrite on frans-chloronitrobis(othylenediamine)cobalt(III) nitrate.2 These methods involve isolation of intermediate coordination compounds and consequently are subject to low yields, based upon original cobalt(II) salt. The following procedure for the preparation of the trans compound gives an 84% yield it depends upon oxidation of cobalt(II) to cobalt(III) in the presence of sodium nitrite, ethylene-diamine, and nitric acid. 

In combination, TMGa, a Lewis acid, and NH3, a Lewis base, can form intermediate coordination compounds at low temperatures. The gas phase reactions of TMGa and NH3 at low temperatures have been reported by several groups [15-17], At temperatures <150°C, TMGa and NH3 react rapidly to form the adduct trimethylgalliummonamine (TMGa NH3), as described in EQN (1) ... 

In a similar context Amdtsen developed a new pyrrole synthesis from alkynes, acid chlorides either imines or isoquinolines, based on the reactivity of isocyanides (Scheme 35a) [197]. Although all atoms from the isocyanide are excluded from the final structure, its role in the reaction mechanism is crucial. The process takes place through the activation of the imine (isoquinoline) by the acid chloride to generate the reactive M-acyliminium salt, which is then attacked by the isocyanide to furnish a nitrilium ion. This cationic intermediate coordinates with the neighboring carbonyl group to form a miinchnone derivative, which undergoes a [3+2] cycloaddition followed by subsequent cycloelimination of the isocyanate unit, to afford the pentasubstituted pyrrole adducts 243 and 244 (Scheme 35a, b). 

The use of the PhjCSbFg reagent with our silanes [C5Br5 (SiMe2H) ]Mn(CO)3 (m = 1-5), leads to immediate formation of the corresponding fluorosilanes 12a-e, which were characterized by h, C, and F NMR and IR spectroscopy. There was no indication of an intermediate coordination of the SbF anion. 

Yasuda, H., Yamamoto, H., Yokota, K. et al. (1992) Synthesis of monodispersed high molecular weight polymers and isolation of an organolanthanide(lll) intermediate coordinated by a penultimate poly(MMA) unit. Journal of the American Chemical Society, 114, 4908. 

If the mechanism stated above is correct, the behavior should be differmit when the solute is a cation instead of an anion, because in that case, the primmily coordiiuition site of the solvoit after the abrupt change in charge state of tiie solute, is the oxygen atom. There is no such intermediate coordination structure as in the case of O. The b vior of lnZ(t) for Na in fact shows essentially biexponential decay as shown in Hg.5. 

In Eq. (e), an intermediate, coordinatively saturated monodentate borohydride complex, [h -CjHjVCCOjBHj", is formed, which can either lose CO to give a bidentate borohydride complex or lose BHj to give an anionic hydride Similarly, the borohydride reduction of Mo(CO) in THF proceeds through an [Mo(CO)j(BH )] intermediate that can either lose CO to form [Mo(CO) (BHp] or lose BHj to form [HMOjCCO) ,]". Diethyl ether (a weaker electron-pair donor) instead of THF favors... 

The first attempt to imprint a metal complex with a reaction intermediate coordinated to the metal center was reported by Mosbach and coworkers [51], A Co monomer coordinated with dibenzoylmethane, which is as an intermediate for the aldol condensation of acetophenone and benzaldehyde, was tethered to a styrene-DVB copolymer matrix. After, the template, dibenzoylmethane was removed from the polymer, the resulting molecularly imprinted cavity had a shape similar to the template due to the interaction of the template with the polymerized styrene-DVB monomers through n-n stacking and van der Waals interactions. The rate of aldol condensation of adamantyl methyl ketone and 9-acetylanthracene was lower than the rate of condensation with acetophenone, indicating some degree of increased substrate selectivity. This is the first known formation of a C-C bond using a molecularly imprinted catalytic material. 

In the Cp2TiMe2-catalyzed hydroboration of alkenes, a titanocene bis(borane) complex is responsible for the catalysis. This bis(borane) complex initially dissociates to give a monoborane intermediate. Coordination of the alkene gives rise to the alkene-borane complex, which is likely to be a resonance hybrid between an alkene borane complex and a 3-boroalkyl hydride. An intramolecular reaction extrudes the trialkylborane product, and coordination of a new HBR2 regenerates the monoborane intermediate. 

The set of unknown are the Aj-s, and the goal is to set all the r -s (the residuals) to zero. The same condition is obtained if we first discretize the classical action and consider a stationary condition of the discrete classical action as a function of the intermediate coordinates... 

It is important to emphasize that the stationary solution of the action functional (the classical trajectory) is not necessarily a minimum. This means that a straightforward minimization of S as a function of all the intermediate coordinates is not possible in the general case. The action can be optimized if the matrix of its second derivatives with respect to the intermediate coordinates is definite (positive or negative) to support direct minimization of S or —S. As a simple example that S need not be definite, we consider a one-dimensional... 

The above discussion suggests that we cannot minimize the action directly to compute classical trajectories. We focus instead on the residuals (13). One boundary value formulation that we frequently use minimizes the squares of the residual vectors, i.e., we define a target function T that we wish to minimize as a function of all the intermediate coordinates Xj,j = 2,..., N — 1. 

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