Carbon and Metal-Halogen Bonds

Heterocumulenes undergo insertion reactions with numerous substrates. In general, carbodiimides react faster than isocyanates and isothiocyanates, in that order. Insertions of carbodiimides into metal-hydrogen, metal-halogen, metal-mitrogen, metal-oxygen and metal-sulfur bonds are reported. Also insertions of carbodiimides into carbon-hydrogen bonds are known. 

Pesticides are almost always xenobiotics. If you look at the chemical structures in organic chemistry and biochemistry textbooks, the difference is very striking (Table 10.1). The synthetic compounds have many so-called xeno-biotic bonds, that is, bonds between carbons and halogens, direct bonds between carbons and phosphorus, and between carbons and metals or silicium. 

The analysis of Table 1 shows that the modifiers used in the epoxyanhydride compositions have such chemical bonds as metal-carbon (Table 1, NN 1), metal-oxygen (Table 1, NN 2-5, 7, 8, 11-16, 18-21, 24, 26, 28-36), metal-nitrogen (Table 1, NN 22, 24-28, 31, 34, 37), metal-sulfur (Table 1, N 17), and metal-halogen (Table 1, NN 6, 9,10, 23, 27). These chemical bonds can be both covalent (if the metal salts are used) and coordination bonds (if the metal complexes are used). 

The classical Reformatsky reaction consists of the treatment of an a-halo ester 1 with zinc metal and subsequent reaction with an aldehyde or ketone 3. Nowadays the name is used generally for reactions that involve insertion of a metal into a carbon-halogen bond and subsequent reaction with an electrophile. Formally the Reformatsky reaction is similar to the Grignard reaction. 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

The activated nickel powder is easily prepared by stirring a 1 2.3 mixture of NiL and lithium metal under argon with a catalytic amount of naphthalene (1(7 mole % based on nickel halide) at room temperature for 12 h in DME. The resulting black slurry slowly settles after stirring is stopped and the solvent can be removed via cannula if desired. Washing with fresh DME will remove the naphthalene as well as most of the lithium salts. For most of the nickel chemistry described below, these substances did not affect the reactions and hence they were not removed. The activated nickel slurries were found to undergo oxidative addition with a wide variety of aryl, vinyl, and many alkyl carbon halogen bonds. 

In marked contrast to the majority of activated metals prepared by the reduction process, cobalt showed limited reactivity toward oxidative addition with carbon halogen bonds. Iodopentafluorobenzene reacted with 2 to give the solvated oxidative addition products CoL and Co(C,F5)2 or Co(C F )L The compound CoiOJF 2PEt, was isolated in 54% yield by addition of triethylphosphine to tne solvated materials. This compound was also prepared in comparable yield from 1 by a similar procedure. This compound had previously been prepared by the reaction of cobalt atom vapor with C6F5I(81). 

These reactions are examples of 1,2-addition. In addition to insertion in the M-H bond as shown earlier, this type of reaction also occurs with metal-halogen and metal-carbon bonds. The following equation shows an example of a reaction in which insertion occurs between a metal and a halogen ... 

---