Metals, activated dissolving metal reductions

Tetraene 433 was prepared in two steps from the readily available Schroder hydrocarbon 4 0.395 Woelm activity I neutral alumina caused rearrangement of 430 to a mixture of 431 and 432. Thermolysis of 431 at 380 °C in a flow system afforded 4JJ.390 Dissolving metal reduction of433 did not give a rise to a spectroscopically recognizable dianion species. Electrochemical measurements were equally disappointing. 

For reduction, relevant data from polarographic and cyclic voltammetric experiments are summarized in Tables 1 and 2, respectively. For the results in Table 1 the variety of solvents and reference electrodes used makes comparisons difficult. It is clear, however, that even with the activation of a phenyl substituent (entries 6,7,9-14) reduction occurs at very cathodic potentials. In this context it is worth noting that in aprotic solvents at ca. — 3 V vs. S.C.E.) it becomes difficult to distinguish between direct electron transfer to the alkyne and the production of the cathode of solvated electrons. Under the latter conditions the indirect electroreductions show many of the characteristics of dissolving metal reductions (see Section II.B). Even at extreme cathodic potentials it is not clear that an electron is added to the triple bond the e.s.r. spectra of the radical anions of dimesitylacetylene and (2,4,6,2, 4, 6 -hexa-r-butyldiphenyl)acetylene have been interpreted in terms of equal distribution of the odd electron in the aromatic rings . 

There are many synthetic applications for sulfide carbanions. Sulfide 304 required the addition of DABCO to facilitate deprotonation by butyllithium, but the resulting anion coupled readily with allylic chloride 305 under these conditions to give 306. The sulfide moiety was removed by a dissolving metal reduction (for example, see secs. 4.9.D, 4.9.F) to give dendrolasin, 307.The ability to remove the sulfur after activating... 

The metal ion in electroless solutions may be significantly complexed as discussed earlier. Not all of the metal ion species in solution will be active for electroless deposition, possibly only the uncomplexed, or aquo-ions hexaquo in the case of Ni2+, and perhaps the ML or M2L2 type complexes. Hence, the concentration of active metal ions may be much less than the overall concentration of metal ions. This raises the possibility that diffusion of metal ions active for the reduction reaction could be a significant factor in the electroless reaction in cases where the patterned elements undergoing deposition are smaller than the linear, or planar, diffusion layer thickness of these ions. In such instances, due to nonlinear diffusion, there is more efficient mass transport of metal ion to the smaller features than to large area (relative to the diffusion layer thickness) features. Thus, neglecting for the moment the opposite effects of additives and dissolved 02, the deposit thickness will tend to be greater on the smaller features, and deposit composition may be nonuniform in the case of alloy deposition. 

Magnesium in methanol, another dissolving-metal system, is known to be only active in the reduction of activated multiple bonds. The reactivity, however, can be dramatically enhanced by the addition of catalytic amount of palladium, thus allowing rapid and facile reduction of nonactivated acyclic and cyclic alkenes.194... 

Besides heterogeneous and homogeneous catalytic hydrogenations, chemical reductions can also transform alkynes to cis alkenes. Interestingly, activated zinc in the presence of a proton donor (alcohol), although a dissolving-metal reagent, reduces disubstituted alkynes to cis alkenes 199... 

This reaction has been used to make crystalline triphenylmethyl derivatives that have been characterized by X-ray structure determinations,22 and a technique has been developed for the reduction of hydrocarbons by liquid cesium activated by ultrasound irradiation in the presence of ethers such as diglyme.23 The blue solutions obtained when cesium metal is dissolved in THF in the presence of [18]-crown-6 have been used to metallate a series of 1,4- or 1,5-hexadienes. The organocesium compounds have not been isolated but they have been identified by derivatization by carbonation and trimethylsilylation.24 Substituted cyclopentadienyl derivatives of sodium have also been synthesized by this method.25... 

Trialkyl ferrates(II) or the related manganese(II) and cobalt(II) compounds are excellent catalyst for reduction of organic compounds by dissolved metals such as magnesium. The protocol provides for tuning of catalytic activity by way of metal and ligand.304... 

Chemical reduction of pyridines can be achieved with hydride, dithionite, dissolving metal reagents, or hydrogenation. The pyridine nucleus can be activated to reduction by conversion to a pyridinium species <1984CHEC(2)165, 1996CHEC-II(5)80>. 

In some cases, the electrode material is the limiting factor of the electrochemical stability window. In a metal salt solution, underpotential deposition (UPD) may occur. In some examples, such as gold or platinum electrodes in the presence of lithium ions, the UPD appears at potentials that are substantially higher than the bulk metal deposition [4-6], In addition, some metals may possess catalytic activity for specific reduction or oxidation processes [7-12], Many nonactive metals (distinguished from the noble metals), including Ni, Cu, and Ag, which are commonly used as electrode materials, may dissolve at certain potentials that are much lower than the oxidation potentials of the solvent or the salt. In addition, some electrode materials may be catalytic to certain oxidation or reduction processes of the solution components, and thus we can see differences in the stability limits of nonaqueous systems depending on the type of electrode used. 

In terms of synthetic utility, the reduction of carbonyl compounds by a dissolving metal in liquid NH3 in the presence of an alcohol, water or NH4CI is far more common and usually far more efficient than reduction in the absence of a proton donor. Historically these reductions were carried out using active metals, usually Na, in alcohols and the experimental results are similar in both systems. - ... 

Reductions of cyclic ketones by dissolving metals are frequently highly stereoselective and these reductions have been used to obtain secondary alcohols which are difficult or impossible to prepare by metal hydride reduction. In terms of yield, the best results are usually obtained either by reductions with alkali metals (commonly Li) in liquid NH3 in the presence of proton donors or with active metals in an alcohol. Although a number of explanations have been advanced for the stereoselectivity of these reductions, they are all rationalizations with dubious predictive value." There are, however, a number of empirical generalizations which are based on a considerable body of experimental data, specifically ... 

The reduction of imines to amines (equation 21) by dissolving metals is usually carried out using active metals in a protic solvent, typically Na-alcohol, Zn-NaOH and A1 or Mg in alcohols. - ""- Although the mechanism of these reductions has not been investigated in detail it is almost certainly analogous to that of the reduction of ketones (Section 1.4.2). It has been established that radical anions are intermediates in these reductions and in the absence of a proton donor reductive dimerization is the principal reaction path. ... 

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