Double hydrogenation

A loss of 55 is possibly the loss of C4H7 from esters (double hydrogen rearrangement). The loss suggests a butyl or isobutyl group, especially when m/z 56 is also present. 

Benzoates (by double hydrogen rearrangement) (The ester must be ethyl or higher.)... 

Kelly and colleagues91 explored the use of bisphenylenediol 103 as a catalyst in Diels-Alder reactions of a,/i-unsaturated carbonyl compounds. Activation of the dieno-phile occurred through double hydrogen bonding of the two hydroxyl functions on 103 to the carbonyl group on the dienophile. The reaction of cyclopentadiene with methyl vinyl ketone (equation 31) at ambient temperature showed, after a reaction time of 10 minutes, 3% of product formation in the absence of 103 against 90% of product formation in the presence of 0.4 equivalents of 103. 

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. 

McLafferty Rearrangement with Double Hydrogen Transfer... 

Alkene loss via McLafferty rearrangement at the alkoxy group of aliphatic and aromatic carboxylic acid esters competes with yet another reaction path, where two hydrogens instead of one as in the normal McLafferty product are transferred to the charge site. This second pathway leading to alkenyl loss has early been noticed [94] and became known as McLafferty rearrangement with double hydrogen transfer (r2H) ... 

Studying the competition of McLafferty rearrangement either with charge retention or charge migration and double hydrogen transfer has revealed that ion-neutral complex intermediates (Chap. 6.12) can also play a role for the latter two processes. [102]... 

Benoit, F.M. Harrison, A.G. Hydrogen Migrations in Mass Spectrometry. II. Single and Double Hydrogen Migrations in the Electron Impact Fragmentation of Propyl Benzoate. Org. Mass Spectrom. 1976, 1056-1062. 

Tajima, S. Azami, T. Shizuka, H. Tsu-chiya, T. An Investigation of the Mechanism of Single and Double Hydrogen Atom Transfer Reactions in Alkyl Benzoates by the Ortho Effect. Org. Mass Spectrom. 1979,14,499-502. 

The other two hydroxyls in each POi, group of the chain link it with water or some other proton acceptor molecules in the solution. In a diluted PA there are less direct P0i,-H-P0h connections, with more water molecules inserted between POit groups, which means that dilution hinders the formation of long PA chains. In a more concentrated PA double hydrogen bonds between POi, groups appear (17) leaving less free hydroxyls for the side linkages of PA chain with other molecules in the solution. 

Tetrahydrofolate (THF, 6) is a coenzyme that can transfer Cj residues in different oxidation states. THF arises from the vitamin folic acid (see p. 366) by double hydrogenation of the heterocyclic pterin ring. The Ci units being transferred are bound to N-5, N-10, or both nitrogen atoms. The most important derivatives are ... 

Fig. 2 Double hydrogen bonding enamine catalysts developed by Gong and Singh...

Activation of carbonyl compounds by double hydrogen bonding an emerging tool in asymmetric catalysis (P. M. Pihko, 2004) [lb]. 

For organizational reasons, several catalytic systems possessing multiple hydrogen bond donating functionalities have been included in this section which may not be classified as BBAs. For example, the bistrifylamide Mikami catalyst could perhaps be classified as a double-hydrogen bond donor catalyst akin to thiourea catalysis. 

Scheme 6.1 Active sites of enzymes employing a double hydrogen-bonding motif for substrate coordination and activation in various biochemical transformations Haloalcohol dehalogenase (1), formate dehydrogenase (2), and serine protease (3).

Figure 6.1 Chronological order of milestone achievements toward catalytically active (thio)urea organocatalysts utilizing explicit double hydrogen-bonding interactions for substrate activation.

---