Mercury carbon—transition—metal

Although the tin hydride + alkyl halide method is probably the most important way of making alkyl radicals, we should mention some other methods that are useful. We said at the beginning of the chapter that carbon-metal bonds, particularly carbon-transition metal bonds, are weak and can homolyse to form radicals. Alkyl mercuries are useful sources of alkyl radicals for this reason. They can be made by a number of routes, for example, from Grignard reagents by transmetailation. 

In certain instances (Ir, Pt, Au), the carbon-transition-metal-mercury complexes are actually isolable. They also decompose in the manner indicated. When organomercury halides are employed in such reactions, similar oxidation-reduction processes occur, but evidence suggests that the transition metal inserts primarily into the mercury-halogen bond followed by loss of mercury " ... 

The chemistry of cyclopropanol [7] has long been studied in the context of electrophilic reactions, and these investigations have resulted in the preparation of some 3-mercurio ketones. As such mercury compounds are quite unreactive, they have failed to attract great interest in homoenolate chemistry. Only recent studies to exploit siloxycyclopropanes as precursors to homoenolates have led to the use of 3-mercurio ketones for the transition metal-catalyzed formation of new carbon-carbon bonds [8] (vide infra). 

Transition metal compounds with covalent carbon-metal bonds include organo-zinc, organo-cadmium, and organo-mercury compounds. Carbon-13 shifts of the methyl derivatives (Table 4.71) indicate a heavy atom deshielding. Diphenylmercury displays carbon shifts similar to those of phenyllithium and phenylmagnesium bromide (Table 4.53). 

CARBIDES. A binary solid compound of carbon and another element. The most familiar carbides are those of calcium, tungsten, silicon, boron, and iron (cemcntitc) Two factors have an important bearing on the properties of carbides (1) the difference in electronegativity between carbon and the second elemenl. and (2) whether the second element is a transition metal. Saltlike carbides of alkali metals are obtained by reaction with acetylene. Those ohlained from silver, copper, and mercury sails are explosive. See also Carbon and Iron Metals, Alloys, and Steels. 

These organometallic nucleophiles show most of the typical reactions with carbon electrophiles associated with benzenoid Grignard reagents and aryllithiums They also allow the introduction of other metals, and nonmetals, on to the ring, such as mercury, boron, phosphorus, tin, and arsenic (Scheme 104) (see also Section 3.2.3.10.2.5), some of which are of great use as partners in transition metal-catalyzed processes. 

The carbon-iodine bond in perfluoroalkyl iodides is usually susceptible to homolytic fission this was exploited in early work on the synthesis of mercurials and in later work relating to group IVB and transition-metal derivatives (Figure 10.2). 

This approach is especially useful for the synthesis of organopalladium and -platinum compounds. These oxidation-reduction reactions proceed via initial insertion of the transition metal into the carbon-mercury bond and subsequent loss of mercury ... 

Generally, the concentrations of the interfering transition metal ions in the brine are decreased sufficiently by simple adsorption onto the colloidal precipitates of calcium carbonate, magnesium carbonate, and barium sulfate produced by conventional brine pretreatment. The brine is then acidified with hydrochloric acid to about pH 4. This helps to maintain a high hydrogen overvoltage on mercury in order to minimize the formation of hydrogen in the... 

Due to this lack of acidity, the internal carbon often does not activate upon metalation with many transition metals. This results in a novel form of porphyrin metal binding where the internal C-H bond(s) remain intact but lie close to the bound metal. Several metals exhibit this type of bonding, including manganese (25-27), rhodium (25), iron (29), cobalt (30 zinc (57), palladiiun (52), silver (55), cadmium, and mercury (34), The C-H bond can reside very close to the metal ion, often within 2.5 A. As a result of this proximity, this interaction has been referred to as agostic, however, the key measurement that would define the agostic interaction, the C- H coupling constant, has yet to be measured. 

There have been several reports of transition metal-carborane complexes with covalent two-electron bonds. An early report indicates that carbon-mercury bonds were formed by the reaction of C-lithiocarboranes with mercuric halides (6, 7). 

Sulphur Dioxide. The observed products and high rates of insertion of sulphur dioxide into tin-carbon bonds in organotin allenes and alkynes have been interpreted in terms of an electrophilic cleavage mechanism. Such a mechanism is also proposed for sulphur dioxide insertion into mercury-carbon and into lead-carbon bonds. The mechanism of sulphur dioxide insertion into transition metal (Mo, W, or Mn)-carbon bonds is again similar. ... 

Wang XF, Andrew L, Riedel S, Kaupp M (2007) Mercury is a transition metal The first experimental evidence for HgF4. Angew Chem Int Ed 46 8371-8375 Li X, Wang L-S, Boldyrev Al, Simons J (1999) Tetracoordinated planar carbon in the Al4C anion. A combined photoelectron spectroscopy and ab initio study. J Am Chem Soc 121 6033-6038... 

Based on the previous analysis, 3-methyl-l-hexene reacts with mercuric acetate to give secondary carbocation intermediate 72. Is this carbocation unusual relative to previously discussed carbocations, such as 30 in Section 10.3 Note that mercury is a transition metal with d-orbitals that can donate electron density to the carbocation, stabilizing that positive center via what is known as back-donation (similar to what is observed with the halogens in the formation of halonium ions). Despite the manner in which is it drawn, this carbocation is not a three-membered ring, but rather a secondary carbocation that is stabilized by back-donation from the mercury atom. The dashed line (--) indicates significant coordination between the carbon and mercury. 

The organometallic compounds prepared from alkali metal derivatives of metal carbonyls include compounds in which a transition metal of a metal carbonyl residue is cr-bonded to another atom such as a carbon atom in an alkyl, acyl, aryl, or perfluoroalkyl group, to the heavier congeners of carbon —silicon, germanium, tin, and lead—to mercury and gold which form unusually stable compounds, and even to other transition metals. The preparation and properties of all of these classes of compounds will be considered and, for the sake of completeness, a few related compounds prepared by routes other than those involving anionic metal carbonyl derivatives will be discussed. 

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