Formation of ji Complexes

Studies of the relative rates of the zinc chloride-catalysed bromination of alkyl-and halogeno-benzenes in nitromethane at 25 °C have lead to the suggestion that the rate-determining step of the reaction is formation of Ji-complex, since low substrate selectivity was found to be coupled with high (i.e. normal) positional selectivity323. Under some conditions (column 1 in Table 75) the low selectivity... 

Table 3.56). An explanation was put forward by postulated formation of Ji-complex of cuprous chelate which coordinates to aryl halide could be affected by nitro group drawing electron density from the benzene moiety.

Photoamination of 6 proceeds via the nucleophilic addition to the radical ion pairs between 6 and m-DCB followed by one-electron reduction of the aminated radicals with m-DCB" and protonation to give 26 according to Scheme 6.8. However, the highly reactive localized cation radicals of 6, in general, tends to cause side reactions involving dimerization, deprotonation, and isomerization, resulting in the amination in lower yields. It is well known that aromatic hydrocarbons (ArH) work as tr-donor that can interact with a cation radical [49]. The additive effect of 1,3,5-TPB or m-TP appears to come from the formation of Ji-complex with 6". The Jt-complex formation with ArH would lower the reactivity of the cation radicals and suppress the side reactions, resulting in the effective photoamination of 6. 

It is interesting to note, as was pointed out to me some years ago by J. L. Hoard, that these considerations lead to an explanation of the difference in stability of cobalt (II) and cobalt (I JI) as compared with iron (II) and iron (III) in covalent octahedral complexes. The formation of covalent complexes does not change the equilibrium between bipositive and tripositive iron very much, as is seen from the values of the oxida-... 

For examples of P-carbon elimination in late transition metal systems, Bergman et al. identified P-methyl transfer with four-membered ruthenacycles, which is driven by the formation of Ji-allyl and Ji-oxallyl complexes. Warming the solution of oxaruthenacycle 58 to 45°C led to formation of methane and cyclic enolate complex 60 [76]. ji-Oxallyl complex 59 initially arises from P-methyl... 

The most stable oxidation state for all lanthanide elements is the +3 state. This primarily arises as a result of the lack of covalent overlap, which stabilizes low and high oxidation states in the d-block metals by the formation of Ji bonds. While some zero-valent complexes are known, only the +2 and -1-4 oxidation states have an extensive chemistry and even this is restricted to a few of the elements. The reasons for the existence of compounds in the -1-4 and -j-2 oxidations states can be found in an analysis of the thermodynamics of their formation and decomposition reactions. For example, while the formation of all LnF4 and LnX2 is favorable with respect to the elements, there are favorable decomposition routes to Ln for the majority of them. As a result, relatively few are known as stable compounds. Thus L11X4 decomposition to L11X3 and X2 is generally favorable, while most UnX2 are unstable with respect to disproportionation to LnXs and Ln. 

Scheme 1). Introduction of a jt bond into the molecular structure of 1 furnishes homoallylic amine 2 and satisfies the structural prerequisite for an aza-Prins transform.4 Thus, disconnection of the bond between C-2 and C-3 affords intermediate 3 as a viable precursor. In the forward sense, a cation ji-type cyclization, or aza-Prins reaction, could achieve the formation of the C2-C3 bond and complete the assembly of the complex pentacyclic skeleton of the target molecule (1). Reduction of the residual n bond in 2, hydro-genolysis of the benzyl ether, and adjustment of the oxidation state at the side-chain terminus would then complete the synthesis of 1. 

The vacant orbital in 16e -zirconocene(IV) complexes allows a Ji-interaction with an incoming alkene or aUcyne. However no metal— alkene/alkyne backbonding is possible with the d°-Zr-metal center. As a consequence, the metal-olefin interaction is not stabilized, and formation of the thermodynamically favored o-bound organozirconocene complex (>10 kcal/mol) is then observed [36]. The product is the result of an overall cis addition of the zirconocene metal fragment and the hydrogen across the carbon-carbon multiple bonds. 

Lithium complex bearing a r 1 1 -[ 1,2,4]diazaphospholide ligand, [(ri1 ri1-dp)-(ji-Li) (DME)]2 (DME= 1,2-dimethoxyethane) was first reported by Gudat and coworkers [43], Lithium [l,2,4]diazaphospholide further binds to two M(CO)5 fragments by coordination via the lone pair of the P and one N atom. Formation of mononuclear P-coordinated complexes as intermediates is supported by indication of more efficient M—>L back donation for P- than for IV-bound fragment by X-ray and spectroscopic studies. 

The alkyl complexes mentioned above are very electron rich, and thus their susceptibility to reaction with O2 is not a great surprise, though the formation of stable organometallic oxidation products may be. Somewhat more unusual is the reaction of metal(III) hahdes with O2. Relatively recent results in this area begin with the report by Morse et al. of the oxidative addition of O2 to [Cp CrBr( ji- Br)]2, see Eq. 9 [23]. 

Reaction with carboxylic acids leads to formation of /i-amido-ji-carboxylato complexes, and complexes with structure X have been isolated for X"- = CH3COO- and HCOO-. For equilibrium Eq. (65), a value of K = 53 M l (50°C, 2 M LiC104) has been determined from kinetic data (386). 

The formation of compound 175 could be rationalized in terms of an unprecedented domino allene amidation/intramolecular Heck-type reaction. Compound 176 must be the nonisolable intermediate. A likely mechanism for 176 should involve a (ji-allyl)palladium intermediate. The allene-palladium complex 177 is formed initially and suffers a nucleophilic attack by the bromide to produce a cr-allylpalladium intermediate, which rapidly equilibrates to the corresponding (ji-allyl)palladium intermediate 178. Then, an intramolecular amidation reaction on the (ji-allyl)palladium complex must account for intermediate 176 formation. Compound 176 evolves to tricycle 175 via a Heck-type-coupling reaction. The alkenylpalladium intermediate 179, generated in the 7-exo-dig cyclization of bro-moenyne 176, was trapped by the bromide anion to yield the fused tricycle 175 (Scheme 62). Thus, the same catalytic system is able to promote two different, but sequential catalytic cycles. 

The Ji-complexes formed between chromium(O), vanadium(O) or other transition metals, and mono- or poly-fluorobenzene show extreme sensitivity to heat and are explosive [1,2], Hexafluorobenzenenickel(O) exploded at 70°C [3], and presence of two or more fluorine substituents leads to unstable, very explosive chromium(O) complexes [1]. Apparently, the aryl fluorine atoms are quite labile, and on decomposition M—F bonds are formed very exothermically. Laboratory workers should be wary of such behaviour in any haloarenemetal Ji-complex of this type [1]. However, in later work, no indications of explosivity, or indeed of any complex formation, were seen [4]. Individually indexed compounds are ... 

---