Cyclohexadienyl anion, protonation

The mechanism of the Birch reduction (shown next) is similar to the sodium/liquid ammonia reduction of alkynes to fnmy-alkencs (Section 9-9C). A solution of sodium in liquid ammonia contains solvated electrons that can add to benzene, forming a radical anion. The strongly basic radical anion abstracts a proton from the alcohol in the solvent, giving a cyclohexadienyl radical. The radical quickly adds another solvated electron to form a cyclohexadienyl anion. Protonation of this anion gives the reduced product. 

The first (reversible) electron transfer generates the ketyl anion-radical. The ketyl moiety then attacks the aryl group in the ortho position. The resulting cyclohexadienyl radical is reduced to a cyclohexadienyl anion by a second electron transfer, and the anion is finally protonated. HMPA as... 

The processes depend on the formation of the cyclohexadienyl anion intermediates in a favorable equilibrium (carbon nucleophiles from carbon acids with pKt > 22 or so), protonation (which can occur at low temperature with even weak acids, such as acetic acid) and hydrogen shifts in the proposed diene-chromium intermediates (25) and (26). Hydrogen shifts lead to an isomer (26), which allows elimination of HX and regeneration of an arene-chromium complex (27), now with the carbanion unit indirectly substituted for X (Scheme 9). 

The efficient trapping of the cyclohexadienyl anionic intermediates with protons raises the possibility of quenching with carbon electrophiles. The process is not as general as the proton quench early expert-... 

Initially, Hine (1966a) related PLM to the London-Eyring-Polanyi approach to three-center reactions. Since this method cannot easily be applied to most systems of interest, approximate methods have to be used. One of these minimizes bond motions and bond distortions. The geometric changes in pentadienyl or cyclohexadienyl anions brought about by protonation (Bates et al., 1967) in equation (192) are assumed to be roughly parallel to changes in bond order (BO). For deuteriation on the end carbons, 27(BO)2 = 2(2/3)2 + 2(l/3)2 = 10/9 for deuteriation... 

Thus, Hine (1966a) used PLNM successfully to rationalise the sites of attack on conjugated reactive intermediates (cations, radicals and anions). The data is puzzling since the thermodynamically less stable non-conjugated isomers predominate protonation of the cyclohexadienyl anion, for example, yields predominantly cyclohexa-1,4-diene. The PLNM rationalisation of this result is set out in Scheme 14 in terms of the resonance structures of the pentadienyl anion fragment. 

The efficient trapping of the cyclohexadienyl anionic intermediates with protons raises the possibility of qnenching with carbon electrophiles. The process is not as general as the proton quench. However, when the nucleophile adds essentially irreversibly, quenching with a limited set of carbon electrophiles is successful. For example, addition of 2-lithio-l,3-dithiane to benzene-Cr(CO)2T, followed by addition of ethyl iodide and then oxidation or addition of a donor ligand (CO, PhsP), produces a cyclohexa-l,3-diene substituted by both acetyl (Me + CO) and the nucleophile (Scheme 47).134,209 insertion of CO occurs, without... 

Transfer of another electron leads to the formation of a delocalized cyclohexadienyl anion, and protonation 1,4-Cyclohexadiene of this produces the 1,4-cyclohexadiene. 

Dehydrocbloiiaation. From tetrachlorocyclohexadienes HCl is eliminated by boding with potassium hydroxide in ethanol. Exclusively 1,2,4-trichlorobenzenes are formed as the reaction passes a tetrachlorocyclohexa-1,4-diene stage. The abstraction of a proton from tetrachlorocyclohexa-1,3-diene leads to the cyclohexadienyl anion (7) that is reprotonated in the 3 position - as in the Birch reduction - to tetrachlorocyclohexa-1,4-diene. From here the 1,4-elimination of HCl leads to 1,2,4-trichlorobenzene (eq 13). ... 

Obviously the structures and yields of Birch reduction products are determined at the two protonation stages. The ring positions at which both protonations occur are determined kinetically the first protonation or 7t-complex collapse is rate determining and irreversible, and the second protonation normally is irreversible under the reaction conditions. In theory, the radical-anion could protonate at any one of the six carbon atoms of the ring and each of the possible cyclohexadienyl carbanions formed subsequently could protonate at any one of three positions. Undoubtedly the steric and electronic factors discussed above determine the kinetically favored positions of protonation, but at present it is difficult to evaluate the importance of each factor in specific cases. A brief summary of some empirical and theoretical data regarding the favored positions of protonation follows. 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

Aromatic anion radicals can protonate on the ring to yield cyclohexadienyl radicals. This process has been observed with the electron adduct of benzene [reaction (81)] (Michael and Hart, 1970)... 

The bimetallic complex (CO)5Cr =C(OMe)CH2—C6H7-77 - Fe(CO)3 (68a) was synthesized by reacting the anionic carbene precursor with the cationic cyclohexadienyl complex [Fe(7j -cyclohexadienyl)(CO)3]+ (197). Complex 68a is chiral, and a structure determination showed that both enantiomers are present in the unit cell. Deprotonation of a second acidic proton from the position a to the carbene carbon of 68a by -BuLi and subsequent treatment with another equivalent of the iron substrate afforded the trimetallic cr,Tr,7r complex (CO)5Cr=C(OMe)CH C6H7-7 -Fe(CO)3 2 (68b). The complex 68b has three chiral centers and can as a... 

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