Pentadienyl anion, protonation

The structure of the products is determined by the site of protonation of the radical anion intermediate formed after the first electron transfer step. In general, ERG substituents favor protonation at the ortho position, whereas EWGs favor protonation at the para position.215 Addition of a second electron gives a pentadienyl anion, which is protonated at the center carbon. As a result, 2,5-dihydro products are formed with alkyl or alkoxy substituents and 1,4-products are formed from EWG substituents. The preference for protonation of the central carbon of the pentadienyl anion is believed to be the result of the greater 1,2 and 4,5 bond order and a higher concentration of negative charge at C(3).216 The reduction of methoxybenzenes is of importance in the synthesis of cyclohexenones via hydrolysis of the intermediate enol ethers. 

Electrocyclic closure of both pentadienyl cation and anion have been observed. Cations generated by protonation of dienones close in the predicted conrotatory manner as shown in Equation 12.55.99 The pentadienyl anion, a six-electron system, should close in the disrotatory sense a clear example is the rapid isomerization illustrated in Equation 12.56.100 Photochemical cyclization of pentadienyl cations has been observed Equation 12.57 shows an example in a cyclic system.101 The ready thermal reversion, which should be conrotatory and therefore difficult in the bicyclic system, may possibly occur by a stepwise path.102... 

Intramolecular hydroamination of cyclohexa-2,5-dienes has afforded the corresponding bicyclic allylic amines with high selectivity (Scheme 13).80 The reaction does not proceed through a direct hydroamination of one of the diastereotopic alkenes but more likely involves a diastereoselective protonation of a pentadienyl anion, followed by addition of a lithium amide across the double bond of the resulting 1,3-diene and a highly regioselective protonation of the final allylic anion. 

Protonation of pentadienyle anion gives rise to 1,4-pentadiene, instead of the expected conjugated 1,3-pentadiene. Is it really so surprising, when we look at the HOMO coefficients and the charge distribution (STO-3G calculations) ... 

In addition to the fact that hydrogen/deuterium exchange reactions can be helpful to probe ion structures as will be shown later, they can also reveal mechanistic details such as the site of reaction within ions. For example, the pentadienyl anion exchanges four protons rapidly, demonstrating, as shown in (12), that proton addition occurs more rapidly at the ends of the conjugated system than in the middle (Stewart et al., 1977 DePuy et al., 1978a). 

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. 

Sodium or lithium in liquid ammonia cleave benzylic ethers and esters in the presence of a proton source to provide a pentadienyl anion which expels an alkoxide or carboxylate leaving group [Scheme 1.15]. Aqueous acidic workup returns the carboxylic acid or alcohol and toluene. Many functional groups are unable to survive such powerful reducing conditions. 

The strong leaving group character of the 9-fluorenylmethyl (Fm) group, which derives from the aromaticity effect of a base-induced proton abstraction to form the dibenzocyclo-pentadienyl anion, is exploited in the 5-(9-fluorenylmethyl) groupf to obtain a cysteine derivative fully stable even to hydrogen fluoride, but cleavable under relatively weak basic conditions such as piperidine (10-50%) or DBU (2%) in DMF (Scheme 13) [154,155] qijjg S-protection is thus compatible with the Boc/Bzl chemistry and fully or-... 

Molecular orbital (MO) calculations indicate that the intermediate radical anions have more anionic character at the positions ortho or meta to a TT-donating substituent and para or ipso. ipso means at the substituent-containing carbon) to a 7r-accepting substituent. (See Birch, A. J. Hinde, A. L. Radom, L. J. Am. Chem. Soc. 1980, 102, 3370-3376). The pentadienyl anions formed after transfer of the second electron have more anionic character on the central carbon, and that is where the second protonation occurs. 

The function of the alcohol in the metal -NH3 reduction is to provide a proton source that is more acidic than ammonia to ensure efficient quenching of the radical anion and pentadienyl anion species. Furthermore, the presence of alcohol represses the formation of the amide ion NH2 , which is more basic than RO M and is capable of isomerizing the 1,4-cyclohexadiene product to the thermodynamically more stable conjugated 1,3-cyclohexadiene. 

When a benzoic acid derivative is reduced, the final protonation step does not occur, and a carboxylate enolate is obtained. The enolate can be protonated upon workup to give the usual product, or it can be alkylated by addition of an electrophile. The proton or the electrophile adds ipso to the carboxylate group exclusively. This position has the largest coefficient in the HOMO of the pentadienyl anion. 

In the bicydo[3.2.1]octadienyl anion, formulated as a bishomocyclo-pentadienyl anion (72, see Table 15), those protons which would be involved in the cyclopentadienyl unit have resonances ranging from t 4.61 to 7.16 215>. Even when corrected for charge (on the naive assumption of equal charge density at all five carbon atoms) this range (t 2.6 to 5.2) is too far upheld for protons attached to a delocalized 67U system in which a diamagnetic ring current is present, although the relative order of chemical shifts in the hve protons concerned is as expected from simple HMO calculations 212,215)... 

The regiochemistry of the final product is determined at the last protonation step—the anion itself is of course delocalized and could react at either end to give a conjugated diene, which would be more stable. Why then does it choose to pick up a proton in the middle and give a less stable isomer Kinetically controlled reactions of pentadienyl anions with electrophiles typically take place at this central carbon as a result of orbital interactions. For more information see the further reading section at the end of the chapter. 

The nonconjugated diene is produced on protonation of pentadienyl anion (7). The central carbon atom is harder. 

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