Aromaticity metal aromatic species reactivity

Methoxy-substituted aromatic compound 4 is lithiated metalation with Buli in THF, a step in which it proves useful to include lithium chloride. Because of the greater basicity of /t-butyllithium relative to 4. direct metallation is in fact possible thermodynamically, but /i-butyllithium is generally present in solution as a tetra-mer, and this reduces its reactivity. Addition of lithium chloride destroys these aggregates, and that eliminates the kinetic inhibition. Lithiated aromatic species 18 is further stabilized through chelate formation between lithium and the orr/icr-methoxy groups (ortho effect).8... 

The inteimolecular hydroamination of allenes is readily catalyzed by early transition metal complexes to yield imines. An addition of aromatic and ahphatic amines to aUene requires high reaction temperatures (90-135°C) and long reaction times (1-6 days) when mediated by zirconocene- [41] and tantalum-imido [178] catalysts. The more efficient titanium half-sandwich imido-amide complex 42 operates under significantly milder reaction conditions (27) [179], Because the metal-imido species are prone to dimerization, sterically more hindered aliphatic and aromatic amines are more reactive. Simple, sterically unencumbered aliphatic amines add to aUenes in the presence of the bis(amidate) titanium complex 43 (28), although higher reaction temperatures are required [180]. 

The reactive species that iaitiate free-radical oxidatioa are preseat ia trace amouats. Exteasive studies (11) of the autoxidatioa mechanism have clearly estabUshed that the most reactive materials are thiols and disulfides, heterocycHc nitrogen compounds, diolefins, furans, and certain aromatic-olefin compounds. Because free-radical formation is accelerated by metal ions of copper, cobalt, and even iron (12), the presence of metals further compHcates the control of oxidation. It is difficult to avoid some metals, particularly iron, ia fuel systems. 

The highly reactive species methylene inserts into C—H bonds,both aliphatic and aromatic,though with aromatic compounds ring expansion is also possible (see 15-62). This version of the reaction is useless for synthetic purposes because of its nonselectivity (see p. 248). This contrasts with the metal carbene insertion reaction, which can be highly selective, and is very useful in synthesis. Alkylcarbenes usually rearrange rather than give insertion (p. 249), but, when this is impossible. 

In the presence of low reactive aromatic substrates such as o-tolyl chloride, it has been shown that production of the aromatic zinc occurs only at the potential at which the zinc(II) bipyridine is reduced, i.e. at about —1.4 V/SCE. Furthermore, the nickel(II) complex is also reduced at this potential value. In this case, the formation of either a bi-metallic species, or a cluster (18), intermediary combining Ni(0) and Zn(0) via the bipyridine ligand is suggested. This complex would react with the aromatic chloride to produce the corresponding ArZnCl along with the regeneration of the nickel catalyst. 

Neither C5- nor C6-cyclization involve carbonium-ion intermediates over platinum metal. The rates of the -propylbenzene - indan reaction (where the new bond is formed between a primary carbon atom and the aromatic ring) and the n-butylbenzene- 1-methylindan reaction (which involves a secondary carbon atom) are quite similar (13). Furthermore, comparison of the C6-cyclization rates of -butylbenzene and n-pentylbenzene (forming naphthalene and methylnaphthalene, respectively) over platinum-on-silica catalyst shows that in this reaction a primary carbon has higher reactivity than a secondary carbon (Table IV) (29). Lester postulated that platinum acts as a weak Lewis acid for adsorbed cyclopentenes, creating electron-deficient species that can rearrange like carbonium ions (55). The relative cyclization rates discussed above strongly contradict Lester s cyclization mechanism for platinum metal. 

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