Full proton transfer

It should be emphasized that these two schemes are principally different. The second mechanism corresponds to a situation in which the formation of M- H bonds precedes the full proton transfer, while dihydrogen bonds can be formed only via direct protonation of the hydridic hydrogen. In addition, the protonation of the metal center can be slow at very fast transformation to the dihydrogen complex, masking detection of the dihydride molecule. Therefore, first, it is necessary to show what hydride center is protonated faster. 

Consequently, the processes most relevant to the topic of this chapter, that is, hydrogen bonds in organocatalytic transition states, are (i) transition state stabilization by pure hydrogen bonding (without full proton transfer), and (ii) general Bronsted-acid/Bronsted-based catalyzed reactions which are initiated by hydrogen bonding but move further to distinct proton transfer. 

Proton-transfer reactions and hydrogen-bonding interactions may occur within one molecule or between two molecules (intra- and intermolecular proton transfer, respectively). Photoacids such as the phenols or the naphthols readily undergo intermolecular proton transfer reactions in aqueous solutions . When the proton is transferred to a solvent molecule the reaction is sometime called a proton-transfer-to-solvent reaction (PTTS reaction) . In non-aqueous solutions, hydroxyarenes form moderately strong hydrogen bonds of the type O—H+ O which usually do not lead to a full proton-transfer reaction either in the ground or the excited state of the photoacid. 

Within the phenol family there are connections between hydrogen-bond acidities and full proton transfer acidity. Abraham and colleagues fonnd two good correlations between the log LT scale and a parameter characteristic of proton transfer, the pATa value in water. Equations 14 and 15 might be valnable in the conversion of pK into log... 

The TT bond of an olefin can act as a nucleophile (see the red center in the EPS of ethylene and propene shown in Appendix 2) to remove a proton from hydronium ion (Scheme 10.1), and full proton transfer results in a carbenium ion. This highly electrophilic species reacts with water, and deprotonation gives the product, a simple alcohol. Note that every reaction in this mechanism involves the negative region of one molecule interacting with a positive region of the other molecule. 

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