Hydride acidity

Dialkyl 4-pyrimidinylaminomethylenemalonates (1426, n = 2,3,5) were prepared in 70-80% yields in the base-catalyzed transesterification of diethyl 4-pyrimidinylaminomethylenemalonates (1426, n = 1) with the appropriate alkanol at ambient temperature for 120 hr in the presence of sodium hydride. Acid-catalyzed transesterification was unsuccessful (84JHC247). 

REDUCTION WITH LITHIUM ALUMINUM HYDRIDE Acidic Quenching. Reduction of Aldehydes and Ketones [ 5]... 

Experimentally, this pathway has been well established from IR spectra of the [CpRuH(C0)(PCy3)]/(CF3)30H system in CH2CI2, where large variations in hydride/alcohol ratios did not affect slow transformation of the H H complexes to hydrogen-bonded ion pairs with k values between 1.4 X 10 and 1.6 X 10 s [25]. Activation parameters for this step (Table 10.3) have been determined in hexane [6]. It is probable that a similar mechanism operates for protonation of the hydrides [ReH2(NO)(CO)(PR3)2] with CF3COOH (Table 10.3) in CD2CI2, where the reaction corresponds to first-order kinetics on the acid at hydride/acid ratios > 1 [7]. 

Trans effect, 543-545 Transferrins, 937 Transition metal hydrides, acidities of. 643... 

An early review by Koelle on transition metal catalyzed proton reduction nicely developed the various chemical steps involved in hydrogen evolution including metal hydride formation, hydride acidity (basicity) and protonation and requisite redox potentials.284 The complexes review here have little structural relevance to the hy-drogenase active sites but many show promising catalytic activity. More recently... 

Preparation of Derivatives. A -Acyl- and A-enoylsul-tam derivatives are routinely prepared in good yields using either sodium hydride-acid chloride or trimethyl-aluminum-methyl ester single-step protocols. A variant of the former method employing in situ stabilization of labile enoyl chlorides with CuCl/Cu has also been reported. A two-step procedure via the A-TMS derivative (1) is useful when a nonaqueous work-up is desirable and for synthesis of the A-acryloyl derivative. A-Enoyl derivatives may also be prepared via the phosphonate derivative (2) by means of an Homer-Wadsworth-Emmons reaction (eq... 

Preparation of Derivatives. IV-EnoyP and IV-acyl sultam derivatives are readily prepared using either Sodium Hydride-acid chloride or Triethylamine-add chloride single-step protocols. Various alternative derivatization procedures that work for the 10,2-camphorsultam auxiliary would also be expected to be effective. 

Table 1. Metal-hydride acidities, oxidation potentials, and bond-dissociation energies in acetonitrile.

The thermochemical cycle in Scheme 4 can be used to estimate the effect of one-electron oxidation on metal-hydride acidities. The method is analogous to one that has been extensively used to investigate organic cation radicals [10c]. Eq. 29 shows that measurements of °ox(MH) and °ox(M ) provide relative p a data for metal hydrides and their cation radicals. Absolute values for p a(MH +) are obtained if the acidities of the neutral hydrides are known. The oxidation potentials of 18-electron hydrides can be readily obtained by cyclic voltammetry. In our experience, the waves that are obtained are frequently chemically irreversible, even at rather high scan rates. Consequently, the oxidation peak potentials will be kinetically shifted and represent minimum values for the true °ox(MH) data, the estimates for p a(MH +) represent maximum values, and calculated Ap a are minimum values. 

Recniisc water is a poor. solvent for most transition metal hydrides, acidities were determined from equilibrium measurement.s in acetonitrile and pAj, value.s in water were estimated from the equtition pAj,(H>0) = pA (CHjCN) - 7.5. 

Compound (248), a relative of [14] annulene containing a hydrazine bridge, has been synthesized by a Knoevenagel condensation of the [l,r-bipyrrole]-2,2 -dicarbaldehyde (247) and r-butyl cyanoacetate, followed by reduction of the exo double bond with diisobutylhaluminum hydride, acid-catalyzed cycliza-tion, and decarboxylation.283... 

ETHYLENE DIACETATE (111-55-7) C.H, 04 Combustible liquid. Forms explosive mixture with air [explosion limits in air (vol %) 1.6 to 8.4 flash point 191°F/88°C autoignition temp 900°F/482°C Fire Rating 2]. Strong oxidizers may cause fire and explosions. Incompatible (may release flammable hydrogen gas) with alkali metals hydrides, acids, strong bases, inorganic hydroxides, caustic solutions, nitrates. On small fires, use dry chemical powder (such as Purple-K-Powder), alcohol-resistant foam, or COj extinguishers. 

Figure 18.11 The effect of atomic and molecular properties on nonmetal hydride acidity. As the electronegativity of the nonmetal (E) bonded to the ioniz-able proton increases left to right), the acidity increases. As the length of the E—H bond increases (fop to bottom), the bond strength decreases, so the acidity increases. (In water, HCI, HBr, and HI are equally strong.)...

It will be remembered that acid esters of sulphuric acid can be prepared by the action of the acid on unsaturated hydrocarbons. The preparation of alcohols from unsaturated hydrocarbons through the esters of sulphuric acid is a convenient method, as the esters of acids which contain oxygen are much more easily hydrolyzed than are the esters of the halogen hydrides. Acid ethyl sulphate is readily hydrolyzed when boiled with water. Ethyl acetate is converted into ethyl alcohol and sodium acetate when warmed with an aqueous solution of sodium hydroxide. 

Our understanding of the factors controlling metal hydride acidity and hydride acceptor abilities for nickel diphosphine complexes, together with the potential... 

Acetonitrile has been used for the determination of metal-hydride acidities by equilibrium measurements and is an excellent solvent for electrochemistry, so a great number of investigations have been performed into this solvent. Table 1 gives a comprehensive list of M-H BDE data for metal carbonyl and cyclopentadienyl hydrides that have... 

DuBois and co-workers introduced the thermochemical cycle in Scheme 4 as a means to determine the hydride donor power (AG°h-) or hydricity, of a cationic metal hydride. The method requires the knowledge of metal-hydride acidity (p- a) data and the electrode potentials for the oxidation of the metal-hydride conjugate base to two-electron oxidized counterpart, either by two successive one-electron processes (Equation (10), Scheme 4), or by one two-electron process (Equation (11), Scheme 4). The thermochemical cycle is derived from one that was introduced by Parker and co-workers for use in organic systems/ The accuracy of values on an absolute scale rests upon the... 

Table 5 Metal-hydride acidities, conjugate base oxidation potentials, and hydricities in acetonitrile...

Scheme 6 Thermochemical cycle for the determination of the effect of one-electron oxidations on metal hydride acidities.

The precise namre of the Pd—H bond is an interesting issue that seems to depend largely on the nature of the other ligands bound to palladium. For a series of [Pd(triphos-phine)(H)](BF4) complexes, the hydridic/acidic nature of the hydride ligand has been investigated via reactivity smdies and extended Hiickel calculations, which showed a... 

Reduction of the cyano function of (434) to aldehyde proceeds in very high yield on employing di-isobutylaluminium hydride. Acid-catalysed cleavage of (435) (Bn = benzyl) leads only to the trans-diaminocyclopropane (436) and not to the... 

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