Chlorine atoms complexes with arenes

The symmetric series provides functional cyclohexadienes, whereas the non-symmetric one serves to build deuterated and/or functional arenes and tentacled compounds. In both series, several oxidation states can be used as precursors and provide different types of activation. The complexes bearing a number of valence, electrons over 18 react primarily by electron-transfer (ET). The ability of the sandwich structure to stabilize several oxidation states [21] also allows us to use them as ET reagents in stoichiometric and catalytic ET processes [18, 21, 22]. The last well-developed type of reactions is the nucleophilic substitution of one or two chlorine atoms in the FeCp+ complexes of mono- and o-dichlorobenzene. This chemistry is at least as rich as with the Cr(CO)3 activating group and more facile since FeCp+ activator is stronger than Cr(CO) 3. 

The selectivity of chlorination reactions carried on in solution is increased markedly in the presence of benzene or alkyl-substituted benzenes because benzene and other arenes form loose complexes with chlorine atoms. This substantially cuts down chlorine-atom reactivity, thereby making the chlorine atoms behave more like bromine atoms. 

A crystalline 2 1 complex (Cl) of antimony trichloride and naphthalene was obtained from a hot petroleum ether solution on cooling. An X-ray analysis (275) revealed an interaction between the antimony atom and the arene n system. The structure consists of layers of antimony trichloride molecules alternating with layers of naphthalene molecules. The coordination sphere around the antimony atom is portrayed in Fig. 16. Two antimony-chlorine distances are equal, while the third is significantly longer. The antimony atom is 3.2 A away from the plane of the naphthalene molecule, thus indicating a weak n interaction. 

Catalytic asymmetric induction of planar chirality in an (arene)chromium complex has been reported in the cross-coupling of tricarbonyl (o-dichlorobenzene)chromium 30 with vinylic metals, where one of the meso chlorine atoms undergoes the coupling to give the monovinylation product 31 with up to 44% ee (Scheme 2-19) [38]. 

A mechanism for the final step is shown below. Overall, this step constitutes two subsequent electrophUic aromatic substitution reactions, resulting in the formation of two new B-C bonds. Addition of aluminum trichloride, a Lewis acid, serves to enhance the electrophilicity of the boron by forming a complex with one of the chlorine atoms. The boron is then attacked by one of the pendant arenes, with subsequent loss of AlCLt and formation of a resonance-stabilized sigma complex (several, but not all, of the many resonance structures are shown). Deprotonation with AlCl4 restores aromaticity, giving a neutral intermediate. An identical sequence of mechanistic steps (complexation with AICI3, attack of the arene, loss of AlCLt and deprotonation) results in fte formation of the second B-C bond, thus producing compound 3. 

Cyclometalation occurs with other kinds of C-H bonds besides arene C-H bonds. The 8-methylquinoline ligand and its derivatives provide many such examples of metalation at sp -carbon atoms. Treatment of 8-methylquinoline with Li2PdCLt, followed by addition of PEt3 to split the chlorine bridge, provides the cyclometalated complex (equation 69). 

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