Pentachlorophenyl rings

In both structures the gold atoms are linearly coordinated to two pentachlorophenyl rings with similar Au-C distances and C-Au-C angles (see Table XVIII). A remarkable difference is observed in Au- -Tl distances, which range from 3.0331(6) to 3.1887(6) A in the butterfly-type compound while the dioxane derivative displays the shortest Au- -Tl distance described to date (2.8935(3) A),149 151-157 being even shorter than the sum of Au and Tl covalent radii (3.08158 or 2.92 A159). The first complex also presents an intramolecular Tl- -Tl interaction of 3.6027(6) A, similar to the distances observed in dimeric [Tl(S2CNEt)2]2 (3.60 and 3.62 A).160... 

Spin delocalization in the two pentachlorophenyl rings is high, as shown by the value of the esr linewidth (4.0 G) (which reflects multiple couplings with chlorines) about three times that in the radical PTM- (1.4 G) and similar to that of the radical PPF- (4.7 G)). 

Bis-(trifluoromethyl)-phenyl plays the role of migrating ring in the Smiles rearrangement in the Najera and co-workers [106,107] modification of the olefination reaction. Judka and Makosza (Judka and Makosza, 2006, personal commimication) have shown that pentachlorophenyl and some very simple phenyl derivatives may be conveniently used in the olefination reactions imder mild conditions. Indubitably, several other aromatic and hete-rocycHc systems will be estabhshed as the templates in the direct olefination reaction, allowing for convenient choice of experimental conditions and stereochemical bias. 

Thus, two types of active esters are of interest those formed from an acid and a substituted phenol (12-15) and those formed from an acid and a substituted hydroxylamine (16-19). Both types are reactive by virtue of the electron-withdrawing properties of the OR moiety in 2. The level of activation of the substituted phenyl esters varies directly with the electronic effect going from 4-nitrophenyl to 2,4,5-trichlorophenyl, pentachlorophenyl, and pentafluorophenyl, which corresponds with the increasing acidity of the phenols. A diminution in the rate of aminolysis is caused by the presence of a substituent in the ortho position of the ring.f l An additional phenomenon contributes to the reactivity of the esters formed from substituted hydroxylamines, namely anchimeric assistance. Since the anoinolysis of active esters is a bimolecular reaction, it is dependent on concentration and can be forced to completion by an excess of one of the reactants. Aminolysis is also characterized by a pronounced dependence on the polarity of the solvent in particular for the esters formed from substituted phenols, the half-life of a 2,4,5-trichlorophenyl ester in the presence of benzylamine being one hundred times less in dimethylformamide than in benzene. Furthermore, aminolysis is catalyzed by mild acid such as acetic acid. The rate of anoinolysis is slowed if the side chain of the active ester contains a P-methyl substituent. 

Figure 11 shows a space-filling model of the nonstrained, yet extremely overcrowded, radical PTM-, with its propeller-blade-like pentachlorophenyl groups. It may also be noted that the central trivalent carbon is completely shielded by the six o-chlorines and the three benzene rings. The normally reactive radical site, its central carbon atom where most of the electron spin density resides, is enveloped in a cage of carbons and chlorines, and is unable to approach within the chemical-bonding distance of other molecular species. Furthermore, no molecular distortion, no matter how extensive, would allow any additional medium-sized atom or group to be accommodated on the a-carbon. 

Perchloro(diphenylmethylen)cyclohexa-l, 4-diene (PTM—Cl). PTM— Cl is a reddish solid, the structure of which has been established by X-ray analysis (M. Ballester, J. Riera, J. Castanez, C. Carreras, J. J. Ubierma, C. Badia, C. Miravitlles and E. Molins, forthcoming publication). The three bond axes to the a-carbon lie in a plane. The two pentachlorophenyls form an angle of 58.5° with this plane, that with the cyclohexadiene ring being 27°. 

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