Double proton donor

It is usually energetically unfavorable for a molecule to act as a double proton acceptor as BH would in Fig. 5.1. For similar reasons, cooperativity is typically negative also when a molecule acts as double proton donor. Of course, even in the case of negative cooperativity, formation of the second H-bond is usually energetically favorable when compared to the complete absence of a second H-bond. That is, even though the CH -BH interaction above is weaker than it would be in the absence of the other proton donor, AH, this interaction energy is still negative, and so wiU form spontaneously. In other words, two H-bonds are always better than one (or usually so). 

For the sake of consistency of terminology, triads of molecules in which the central unit acts simultaneously as both proton donor and acceptor will be termed sequential to distinguish such configurations from those in which the central molecule acts as double proton donor or double acceptor. A perhaps more quantitative expression of cooperativity is referred to in the literature as nonadditivity. The latter term is commonly taken as the difference between the total interaction energy of an aggregation of molecules on one hand and the sum of all the pairwise interactions on the other. 

Selective reduction of a benzene ring (W. Grimme, 1970) or a C C double bond (J.E. Cole, 1962) in the presence of protected carbonyl groups (acetals or enol ethers) has been achieved by Birch reduction. Selective reduction of the C—C double bond of an a,ft-unsaturated ketone in the presence of a benzene ring is also possible in aprotic solution, because the benzene ring is redueed only very slowly in the absence of a proton donor (D. Caine, 1976). 

With very electrophilic olefins, an alternative hydrogen fluoride addition process is often preferred This process, involving reaction of the olefin with fluoride ion in the presence of a proton donor, is applicable to certain perhalogen ated alkenes [/] and substrates with other electron attracting groups attached to the double bond [i5, 36] (equations 4 and 5)... 

Ionic hydrogenation reactions85 involve the use of a hydrogenating pair consisting of a proton donor and a hydride ion donor. The ionic hydrogenation is based on the principle that the carbenium ion formed by the protonation of the double bond abstracts a hydride ion from the hydride source. 

The two more stable structures jomo and ietero are characterized by a double hydrogen bond between Ej and Pj or P. The Ej molecule acts as proton donor towards the nitrogen of prolinol, and as acceptor towards the alcoholic proton of P . In the two less stable structure IIhomo and Utetero. the prolinol maintains an intramolecular H-bond between the alcoholic oxygen and nitrogen and, thus, only one hydrogen bond with the Ej molecule is possible, in which the oxygen of Pr/s accepts a proton. 

Birch reduction-alkylation of (2S)-2-methoxymethyl-l-(2-phenylbenzoyl)pyrrolidine (1) gives products 2 in high diastereoselectivities29. In contrast to the previous examples, only one double bond remains in the product (if one equivalent of rm-butyl alcohol is used as proton donor). Formally this procedure is a stereoselective cis addition, and is thus particularly useful. Thus, two stereogenic centers are created in the same reaction step with high diastereoselectivities. Subsequent hydrolysis furnishes acids, whereas reaction with methyllithium yields chiral ketones29. 

After the ESIPT the molecule exhibits a pronounced ringing in this mode. The 295 cm-1 mode is a symmetric in-plane stretching vibration (see Fig. 3). The corresponding contraction of the molecule reduces the donor acceptor distances in both chelate rings simultaneously and initiates the electronic configuration change of the concerted ESIPT. The concerted double proton transfer leads therefore to a ringing of the molecule in this second mode. 

Since proton addition is known (3, 4) to be a rate-determining step in the chemical reduction of aromatic and olefinic double bonds in the alkali-amine system, the effect of adding a proton donor (ferf-butyl alcohol) on current efficiency in the electrochemical reduction of 1-decene was investigated. 

The pepsin family is most active in the low pH range 1-5. All of the enzymes contain two especially reactive aspartate carboxyl groups.378 One of them (Asp 215 in pepsin) reacts with site-directed diazoni-um compounds and the other (Asp 32) with site-directed epoxides.379 It is attractive to think that one of these carboxyl groups might be the nucleophile in a double displacement mechanism. The second carboxyl could then be the proton donor to the cleaving group. 

Reactions studied include dehydrations of alcohols, double bond shifts in olefins, isomerization of hydrocarbons, racemization of optically active compounds, etc.. In the literature a rather rigid separation is made between a Brested acid, which is actually a proton donor, and a Lewis acid, which works as a hydride abstractor. We may illustrate this difference by using the double bond shift in olefins as the model reaction. 

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