Quinoline reductive dimerization

On treatment with zinc in acetic anhydride, quinoline is readily transformed to the product 46 showing a pentacyclic quinobenzazepine skeleton. This formal reductive dimerization process is likely to be initiated by SET (single electron transfer) of the A-acetylchinolinium intermediate 45 followed by radical dimerization via 4-position [94]. 

Grignon-Dubois and coworkers have shown that reduction of a quinoline using zinc and acetic acid in THF gives the dimeric compound 3-177 via intermediate 3-176 (Scheme 3.47) [70J. Usually, a mixture of the syn- and anti-products is formed the substituent has some influence on the regioselectivity of the dimerization and cydization step. With R = H and R = 6-Me, only the benzazepine 3-177 were produced, by a head-to-head dimerization. 

Reduction of quinolines in acid solution at a lead cathode or by dissolving zinc leads to attack on the heterocyclic ring with the formation of 4,4-coupled products, together with the tetrahydroquinoline [82,83]. In the case of 2- and 4-methyl substituted quinolines, dimeric products are obtained in 10 90 % yields. In these processes, dimerization of the one-electron addition product is in competition with further reduction to give the 1,4-dihydroquinoline, The latter is an enamine and it... 

Air stable materials of formula MPc (Pc = phthalocyanine) have been obtained from MCI5, phthalonitrile and quinoline at 220 °C.694 Nb11 alanates [LiNb2(AlH4)5] and [LiNb(AlH4)3] were obtained from the low temperature reaction of NbCl5 with excess LiAlH. 2 Reduction of (44) yielded a singly bonded Nb—Nb dimer (equation 79).568... 

Cuprous acetate monomer, complexed with the quinoline solvent, is in rapid equilibrium with dimer. The equilibrium constant is such that dimer formation is incomplete. Activation of the hydrogen occurs by a slow reaction between dimer complex and dissolved molecular hydrogen. Following activation of the hydrogen, the substrate quickly reacts with the hydrogen. Reaction (11) is believed rate controlling. Weller and Mills (5) attempted to establish whether the reaction went through a two-step oxidation and reduction of the Cu1 catalyst however, the conclusion was that the reaction depicted above best fits the observed facts. 

The reduction of 2, either catalytically or with zinc in acetic acid, leads to 3-amino-4-hydroxy-2-quinolone 20 [72TH000], These amino compounds are rather unstable they dimerize with loss of ammonia to "bis-amines", which in turn are readily oxidized to dyes similar to those obtained from ninhydrin and primary amines [68M1205] [68M1543], The amino derivatives 20 are therefore conveniently converted into 0,N-diacetyl derivatives, the N-acetyl derivative 21, or its dehydrated form, the oxazolo derivative 22 [95MI000], The variety of biological activity of oxazolo-quinolines of type 22 has been detected only in recent years [94JHC1647],... 

A benzene solution of the cyclic dimer 61 when refluxed with a saturated solution of t-BuOK in BuOH gave a red solution of the bisdehydro[20]annulene 63 [62]. The cyclic trimer 62 was rearranged similarly to tridehydro[30]annulene 64. Both species show characteristic UV-vis spectra [62]. The bisdehydro[20]annulene was partially hydrogenated in benzene solution over a Lindlar catalyst (quinoline-poisoned palladium calcium carbonate catalyst). This procedure led to the [20]an-nulene 65. Similarly, the reduction of tridehydro[30]annulene 64 with Lindlar catalyst yielded the [30]annulene 66 [62]. 

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