Indole protonation

In the case of purification of triamidophosphites [51] the production of compound 3d requires a considerable amount of time the yields of the target compounds are increased by 10-18%. In this case, probably, the reaction takes place under the influence of the weakly acidic properties of indole. Protonation of the ambident system >P - N< of amidophosphite by the proton of indole clearly takes place initially with the formation of an intermediate, the dissociation of which leads to the formation of the target compound and amine. 

Smith and Utley hydrogenated indoles and 1,2,3,4-tetrahydrocarbazoles in about 1 1 ethanol-aqueous fluoroboric acid at room temperature and atmospheric pressure using platinum oxide as catalyst. The yields of the corresponding indolines and hexahy-drocarbazoles were very high or quantitative as analyzed by GC (Table 12.3).16 It has been suggested that the species that was subjected to hydrogenation might be the indoles protonated at the 3 position (8) with the C=N double bond in the indolenine system. 

Fig. 6.4.4. N spectra obtained at 20.3 MHz of labeled gramicidin A from different sample preparations in oriented lipid bilayers. (A) deuterium exchange of the indole proton in Trpy] gramicidin A results in the observed dipolar triplet centred on the observed (B)...

Indole protonation is a functional step in the cleavage of tryptophan by tryptophan indole-lyase, based on kinetic isotope effect measurements [26]. 

Based on these results, the thiourea catalyst 95 is believed to direct the approach of indole to acyl phosphonate 93 by forming weak hydrogen bonding with the indolic proton, while the electrophile is activated and positioned for the nucleophilic attack from the Re-face by the thiourea motif (Scheme 6.37). 

O2. As the rate of proton abstraction increases with increasing pH, the D2O isotope effect decreases from 4.4 at pH 7.0 to 1.2 at pH 8.5, indicating that abstraction of the indole proton is partially rate-determining. The inverse isotope effect increases by the same pH change = 0.96 at pH 7.0 and 0.92 at pH 8.5) and it is proposed that the... 

Ricci et al studied a series of thiourea catalysts for the Friedel-Crafts alkylation of aromatic and heteroaromatic compounds with nitroalkenes. They have succeeded in developing the Friedel-Crafts alkylation of indoles with nitroalkenes for the first time by means of a novel thiourea catalyst (13) (Scheme 2.48) [101]. The authors proposed the bifunctional nature of the thiourea catalyst, where thiourea activates the nitro group and at the same time the free alcoholic function interacts with the indole proton through a weak hydrogen bond, directing the attack of the incoming nucleophile on the Si face of the nitroalkene (Figure 2.18). 

---