Hydride complex, solutions

Reduction of tosylhydrazone derivatives to the corresponding methylene analogs with metal hydride complexes provides an excellent solution to this problem. Soon after the discovery of this reaction it was recognized... 

In synthesis (b), the initial product is a 5-coordinate (sp) iridium(III) hydride complex, which is rapidly oxidized in solution to the planar iridium(II) complex. Both of the compounds are paramagnetic with one unpaired electron, as expected for square planar d7 complexes. 

The chemistry of [Rh(OEP)h in benzene is dominated by Rh—Rh bond homolysis to give the reactive Rh(Il) radical Rh(OEP)-. This contrasts with the reactivity of fRh(OEP)] in pyridine, which promotes disproportionation via the formation of the thermodynamically favorable Rh(IlI). ct complex [RhjOEPKpy) ] together with the Rh(l) anion, Rh(OEP)J The hydride complex Rh(OEP)H shows NMR chemical shift changes in pyridine consistent with coordination of pyridine, forming Rh(OEP)H(py). Overall, solutions of Rh(OEP)l in pyridine behave as an equimolar mixture of [Rh(OEP)(py ) and (Rh(OEP). For example, reaction... 

Transfer hydrogenation of aldehydes with isopropanol without addition of external base has been achieved using the electronically and coordinatively unsaturated Os complex 43 as catalyst. High turnover frequencies have been observed with aldehyde substrates, however the catalyst was very poor for the hydrogenation of ketones. The stoichiometric conversion of 43 to the spectroscopically identifiable in solution ketone complex 45, via the non-isolable complex 44 (Scheme 2.4), provides evidence for two steps of the operating mechanism (alkoxide exchange, p-hydride elimination to form ketone hydride complex) of the transfer hydrogenation reaction [43]. 

A Cd2 complex with one of the tris(pyrazolyl)borate ligands mentioned in Section 6.9.4.2.3(ii) has been identified in solution by its large coupling constant 1/(113Cd,111Cd) of 20.626 kHz also a Cd11 hydride complex has been detected by NMR.410... 

P Pr3)2. The protonation of OsHMe(CO)2(P Pr3)2 with a diethyl ether solution of HBF4 in the presence of acetone leads to the quantitative formation of the cationic hydride complex [OsH(CO)2 ii1-OCMe2 (P,Pr3)2]BF4. If water instead of acetone is used as a Lewis base, the aquo hydride compound [0sH(C0)2(H20) (P Pr3)2]BF4 is obtained also in excellent yield. 

The next step involved cooling the reaction mixture to -196°C, removing the H2 at low pressure, and sealing the tube. This sealed tube was then used in the equilibrium measurements. When it warmed up, a fraction of the hydride complex reacted with benzene, yielding H2 and the phenyl complex, according to equilibrium 14.12. Therefore, the total amount of substance of H2 in equation 14.18 is given by the sum of the initial amount of substance of H2 (no) and the amount of substance of Sc(Cp )2Ph in equilibrium. The latter is easily calculated from the relative concentrations of Sc(Cp )2Ph and Sc(Cp )2H determined by H NMR, and the known initial concentration of Sc(Cp )2H (5.4 x 10-5x 1000/0.5 = 0.108 mol dm-3). To evaluate the initial amount of substance of H2, consider the experimental procedure before and after reaction 14.19 takes place. When this reaction occurs (at 25 °C) a certain amount of H2 remains in solution, and it can be calculated by an equation similar to 14.17. This amount will be equal to no, by assuming that (1) there is no further H2 solubilization when the tube is rapidly cooled to — 196 °C, and (2) only the H2 dissolved in the frozen reaction mixture is not removed by the evacuation procedure. 

When carbon monoxide is bubbled through a methanol solution of (dppp)Pd(triflate)2 a carbomethoxy-palladium species is formed, which can undergo insertion of alkenes and hence this is a feasible alternative initiation route to chain-growth polymerisation (Figure 12.4) [13], To ensure a clean formation of the carbomethoxy species, however, exclusion of water is a prerequisite. If during the preparation water was present the formation of a palladium hydride complex (dppp)PdFT was observed (reaction (1), Figure 12.2). 

Fig. 11.24. Positive-ion ESI spectrum of a cationic dinuclear platinum hydride complex from dichloromethane solution. The insets compare experimental and theoretical isotopic patterns. By courtesy of P. Hofmann, University of Heidelberg.

Until there is a sufficient excess of ethene over [PdH(TPPTS)3] their fast reaction ensures that aU palladium is found in form of tratts-[Pd C(CO)Et (TPPTS)2]. However, at low olefin concentrations (e.g. in biphasic systems with less water-soluble olefins) [PdH(TPPTS)3] can accumulate and through its equihbrium with [Pd(TPPTS)3] (eq. 5.5) can be reduced to metallic palladium. This is why the hydroxycarbonylation of olefins proceeds optimally in the presence of Brpnsted acid cocatalyts with a weekly coordinating anion. Under optimised conditions hydrocarboxylation of propene was catalyzed by PdC + TPPTS with a TOE = 2507 h and l = 57/43 (120 °C, 50 bar CO, [P]/[Pd] = 4, P-CH3C6H4SO3H) [38], In neutral or basic solutions, or in the presence of strongly coordinatmg anions the initial hydride complex cannot be formed, furthermore, the fourth coordination site in the alkyl- and acylpaUadium intermediates may be strongly occupied, therefore no catalysis takes place. 

A robust and highly active catalyst for water-phase, acid-catalyzed THs of carbonyl compounds at pH 2.0-3.0 at 70 °C was disclosed by Ogo and coworkers [60]. The water-soluble hydride complex [Cp lr(bipy)H] (72, Cp = Tl -CsMes, bipy = 2,2 -bipyridine) was synthesized from the reaction of [Cp lr(bipy)(H20)] (73) with HCOOX (X = H or Na) in H2O under controlled pH conditions (2.0 < pH < 6.0, 25 °C). The pH control is pivotal in avoiding protonation of the hydrido ligand of 72 below pH ca. 1.0 and deprotonation of the aquo ligand of 73 above pH ca. 6.0. The rate of the reaction is heavily dependent on the pH of the solution, the reaction temperature, and the concentration of HCOOH. High TOFs of the acid-catalyzed transfer hydrogenations at pH 2.0-3.0, ranging from 150 to 525 h, were observed for a variety of linear and cyclic ketones, as summarized in Table 4.5. 

Since the nature of the hydride chemical shifts, particularly in transition metal hydride complexes, is not simple [32], there is no reliable correlation between Sh and the enthalpy of dihydrogen bonding. Nevertheless, the chemical shifts of hydride resonances and their changes with temperature and the concentration of proton-donor components, for example, can be used to obtain the energy parameters for dihydrogen bonding in solution. As earlier, the enthalpy (A/f°) and entropy (AS°) values can be obtained on the basis of equilibrium constants determined at different temperatures. Let us demonstrate some examples of such determinations. 

PROTON TRANSFER TO A HYDRIDE LIGAND IN SOLUTIONS OF TRANSITION METAL HYDRIDE COMPLEXES THEORY AND EXPERIMENT... 

The first report in this regard described a method for direct formation of the desired optically active (S)-alcohol 32a, via enantioselective reduction with a chiral amine complex of lithium aluminum hydride (Scheme 14.9). Therefore, the necessary chiral hydride complex 38 was preformed in toluene at low temperature from chiral amino alcohol 37. The resulting hydride solution was then immediately combined with ketone 31 to afford the desired (S)-alcohol 32a in excellent yield and enantiomeric excess. In addition to providing a more efficient route to the desired drug molecule, this work also led to the establishment of the absolute configuration of duloxetine (3) as S). 

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

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