Lead—carbon bonds hydrogen halides

The usual method for the preparation of organolead halides is by the action of hydrogen halides upon tetra-organoleads. Initial reaction involves fission of one carbon-lead bond, with loss of a further alkyl/aryl group under more vigorous conditions. 

Dehydrohalogenations at two adjacent carbon atoms lead to compounds with C = CorCsC bonds and arc an important route to fluorinated alkencs and alkynes. Some dehydrohalogenations occur spontaneously, others require elevated temperatures, and the majority occur in a basic medium which takes up the eliminated hydrogen halide.1 4 In addition to aqueous or alcoholic alkalis, organic bases such as triethylamine are often used for this purpose. 

Halogen halides add across carbon-carbon double bonds. These additions follow Markovnikov s rule, which states that the positive part of a reagent (a hydrogen atom, for example) adds to the carbon of the double bond that already has more hydrogen atoms attached to it. The negative part adds to the other carbon of the double bond. Such an arrangement leads to the formation of the more stable carbocation over other less-stable intermediates. 

Due to their close relationship with carbonic acid halides, cyanuric halides can undergo Friedel-Crafts-type reactions which lead to the formation of C-C bonds between the triazine ring and an aromatic compound. By analogy, the conversion is catalysed by Friedel-Crafts catalysts such as aluminum trihalides, heavy-metal salts, and protonic acids. Generally, the hydrogen halide formed is evaporated or trapped by a base96,91,113-124... 

The 1,2-insertion of alkenes into transition metal-carbon o-bond leads to C-C bond formation under mild conditions, as described in Chapter 6. This reaction is considered to be a crucial step in the coordination polymerization and carbometalation of alkenes catalyzed by transition metal complexes. A common and important carbometalation is the Heck-type arylation or vinylation of alkene catalyzed by Pd complexes [118], The arylation of alkene, most typically, involves the formation of arylpalladium species and insertion of alkene into the Pd-aryl bond as shown in Scheme 5.20. The arylpalladium species is formed by the oxidative addition of aryl halides to Pd(0) complexes or the transmetalation of aryl compounds of main group metals with Pd(II) complexes. Insertion of alkene into the Pd-aryl bond produces 2-arylalkylpalladium species whose y6-hydrogen elimination leads to the arylalkene. Oxidative chlorination of the 2-arylalkylpalladium intermediate forms chloroalkanes as the product. 

With some metals the process of olefin coordination and insertion may continue, leading to polymerization, but with palladium the metal is expelled from the molecule by a P-hydride elimination reaction and the product is an alkene, plus a Pd(ll) complex. For the whole process to be catalytic, this Pd(ll) product of P-hydride elimination must be converted to a Pd(0). This occurs in the presence of base, which removes HX from the palladium(ll) species. This is another example of reductive elimination one that forms a hydrogen halide rather than a carbon-carbon or carbon-hydrogen bond, as you saw earlier. 

The reaction of an alcohol with a hydrogen halide is a substitution. A halogen, usually chlorine or bromine, replaces a hydroxyl group as a substituent on carbon. Calling the reaction a substitution tells us the relationship between the organic reactant and product but does not reveal the mechanism. The mechanism is the step-by-step pathway of bond cleavage and bond formation that leads from reactants to products. In developing a mechanistic picture for a particular reaction, we combine some basic principles of chemical reactivity with experimental observations to deduce the most likely sequence of steps. 

Examples of the formylation of aryl halides with synthesis gas catalyzed by palladium complexes are summarized in Equation 19.90. These reactions relied upon the development of ligands with particular steric and electronic properties. The dia-damantyl-n-butyl phosphine shown in the equation, in combination with palladium acetate, leads to the formation of aromatic aldehydes in high yields from electron-rich and electron-poor aryl bromides. Reactions of nitroarenes and 2-bromopyridine provided the aldehydes in low yield, but other examples occurred in satisfactor) yield with only 0.1-0.75 mol % catalyst. The identity of the base is important in this process, and TMEDA was the most effective base. The mechanism of this process was not proposed in the initial work, but is likely to occur by oxidative addition of the aryl halide, insertion of the carbon monoxide into the palladium-aryl bond, and a combination of hydrogenolysis of the acyl intermediate and elimination of hydrogen halide to regenerate palladium(O). The base would then be involved in the hydrogenol5 sis and consumption of hydrogen halide. 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Reactions leading to the formation of the catalytically active nickel hydride species from organonickel precursors (Section III) can be regarded as model reactions for olefin oligomerization reactions. The reactions described by Eq. (8) and Scheme 3 (Section III) show that RNiX compounds (R = methyl orallyl, X = halide or acetylacetonate) activated by Lewis acids add to double bonds under mild reaction conditions (-40° or 0°C). It follows further from these reactions that under conditions leading to olefin dimerization a rapid nickel hydride /3-hydrogen elimination reaction occurs. The fact that products resulting from olefin insertion into the nickel-carbon bond are only observed when /3-hydride... 

---