Dissolving metals carbonyl compounds

The metal-NHs reductions of carbonyl groups are exceedingly fast reactions for the reaction of acetone with an ammoniated electron the rate is 9 x 10 M" s". Although many, particularly older, published experimental procedures for the metal-NHs reduction of ketones employ prolonged reaction times with excess metal, these conditions are unnecessarily harsh. The reactions of carbonyl compounds with metals in NH3 are effectively instantaneous and by using short reaction times it appears that reduction of terminal alkenes and disubstituted alkynes can be avoided.In addition to the functional groups mentioned above, alcohols, amines and ethers, other than epoxides, are usually stable to reductions of aldehydes and ketones by dissolving metals. " ... 

On a related front, the reactions of carbonyl compounds with metallated derivatives of 2-methylthia-zoline furnish adducts (85). Although the initial nucleophilic addition occurs smoothly with a wide variety of aldehydes and ketones, the intermediate 3-hydroxythiazolines (85) suffer thermal reversion upon attempted purification by distillation. Moreover, attempted cleavage of the corresponding 3-hydroxythia-zolidines, which are readily produced from (85) upon dissolving metal reduction (Al-Hg), leads to the formation of 3-hydroxy aldehydes only in simple systems numerous complications arising from dimerization, dehydration and retroaldol processes of the products usually intervene. Consequently it is necessary to protect the initial 1,2-adducts (85 R2 = H) as the corresponding O-methoxymethyl ether derivatives (86 R2 = MOM), which can then be easily transformed into protected 3-hydroxy aldehydes by sequential reduction and hydrolysis (Scheme 32).55... 

The sonochemistry of solutes dissolved in organic Hquids also remains largely unexplored. The sonochemistry of metal carbonyl compounds is an exception (57). Detailed studies of these systems led to important mechanistic understandings of the nature of sonochemistry. A variety of unusual reactivity patterns have been observed during ultrasonic irradiation, including multiple ligand dissociation, novel metal cluster formation, and the initiation of homogeneous catalysis at low ambient temperature (57). 

Phosphate groups can also be removed by dissolving-metal reduction. Reductive removal of vinyl phosphate groups is one method for conversion of a carbonyl compound to an alkene.224 (See Section 5.7.2 for other methods.) The required vinyl phosphate esters are obtained by phosphorylation of the enolate with diethyl phospho-rochloridate or /V A /V -tetramethyldiamidophosphorochloridate.225... 

The third method is the simplest to do, but has the most complicated mechanism. The Clemmensen reduction is also rather violent, and really reasonable only for compounds with just the one functional group. It uses zinc metal dissolvmg in hydrochloric acid. As the metal dissolves, it gives up two electrons—in the absence of something else to do, these electrons would reduce the H+ in the acid to H2, and give ZnCl2 and H2. But in the presence of a carbonyl compound, the electrons go to reduce the C=0 bond. 

A good example of the carbonylation process is the reaction of the tetracarbonyl ferra dianion [Fe(CO)4 ] with alkyl halides. This reagent is made by dissolving metal reduction of the l electron Fe(0) compound Fe(CO)5. Addition of two electrons would give an unstable 20-electrc species but the loss of one of the ligands with its two electrons restores the stable 18-electron stru ture. 

The contemporary view of these reductions recognizes that the reactions of carbonyl compounds with dissolving metals follow one of two general reaction paths. One of these prevails in reductions carried out in the absence of proton donors, the other in reductions in the presence of an alcohol or other proton source, frequently NH4CI. Two recent reviews present rather different mechanistic explanations for these reactions, particularly those in liquid NH3 in the absence of added proton donors. ... 

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. - ... 

It must be emphasized that reduction of carbonyl compounds by dissolving metals, either in the presence or absence of an added proton donor is a kinetically controlled process. This was tacitly stated in 1972," and has been repeated or implied in more recent reviews of this topic. A recent study of the reduction of several bicyclo[2.2.1]heptanones using alkali metal-NH3-NH4Cl systems emphasizes that these reductions are kinetically controlled. ... 

The vast majority of the dissolving metal reductions of carbonyl compounds which have been carried out synthetically have used either alcohols or liquid NH3 as the solvent. " However, a variety of other solvents have been employed, frequently in connection with studies of the mechanism of the reductions or in exploratory synthetic studies. 

Aromatic ketones represent a rather special case in dissolving metal reductions. Under many conditions pinacol formation is the predominent reaction path (see Volume 3, Chapter 2.6). Also, the reduction potentials of aromatic carbonyl compounds are approximately 1 V less negative than their aliphatic counterparts. The reductions of aromatic ketones by metals in ammonia are further complicated by the fact that hydrogenolysis of the carbon-oxygen bond can take place (Chapter 1.13, this volume) and Birch reduction may intervene (Chapter 3.4, this volume). 

In general, the reduction of aromatic carbonyl compounds to the corresponding alcohols by dissolving metals is not a particularly valuable synthetic procedure. Better yields and chemoselectivity are usually obtained using complex metal hydrides. 

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