Transfer intramolecular hydrogen

Scheme 7.44 Cascade conjugate hydrogen transfer/intramolecular Michael reaction and application to the total synthesis of (+ )-ricciocarpin A.

Intramolecular hydrogen transfer Intramolecular hydrogen transfer is found in anils, benzylpyridines, triazoles, oxazoles, metal dithizonates, and perimidinespirohexadienones... 

Miscellaneous Reactions. Ahyl alcohol can be isomerized to propionaldehyde [123-38-6] in the presence of sohd acid catalyst at 200—300°C. When copper or alumina is used as the catalyst, only propionaldehyde is obtained, because of intramolecular hydrogen transfer. On the other hand, acrolein and hydrogen are produced by a zinc oxide catalyst. In this case, it is considered that propionaldehyde is obtained mainly by intermolecular hydrogen transfer between ahyl alcohol and acrolein (31). 

In addition, the frequency cooo, as well as the tunneUng distance can also be extracted from the same empirical data. Thus all the information needed to construct a PES is available. Of course, this PES is a rather crude approximation, since all the skeleton vibrations are replaced by a single mode with effective frequency cooo and coupling parameter C. From the experimental data it is known that the strong hydrogen bond (roo < 2.6 A) is usually typical of intramolecular hydrogen transfer. 

Wawzonek et al. first investigated the mechanism of the cyclization of A-haloamines and correctly proposed the free radical chain reaction pathway that was substantiated by experimental data. "" Subsequently, Corey and Hertler examined the stereochemistry, hydrogen isotope effect, initiation, catalysis, intermediates, and selectivity of hydrogen transfer. Their results pointed conclusively to a free radical chain mechanism involving intramolecular hydrogen transfer as one of the propagation steps. Accordingly, the... 

PE produced by a high-pressure polymerization process (pressure 1000-3000 atm) using a free radical initiator is a highly branched material that contains both LCBs and SCBs. The polymer so produced is a low-density material (density up to about 0.925 g/cc) and is known as high-pressure low-density PE (HP LDPE). The LCBs are formed via intermolecular hydrogen transfer [19], whereas SCBs are formed by intramolecular hydrogen abstraction [16]. 

A new process developed by Institut Francais du Petrole produces butene-1 (1-butene) by dimerizing ethylene.A homogeneous catalyst system based on a titanium complex is used. The reaction is a concerted coupling of two molecules on a titanium atom, affording a titanium (IV) cyclic compound, which then decomposes to butene-1 by an intramolecular (3-hydrogen transfer reaction. ... 

Another important family of elimination reactions has as its common mechanistic feature cyclic TSs in which an intramolecular hydrogen transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.20 depicts examples of these reaction types. These are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic TS dictates that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are often referred to as thermal syn eliminations. 

The success of such reactions depends on the intramolecular hydrogen transfer being faster than hydrogen atom abstraction from the stannane reagent. In the example shown, hydrogen transfer is favored by the thermodynamic driving force of radical stabilization, by the intramolecular nature of the hydrogen transfer, and by the steric effects of the central quaternary carbon. This substitution pattern often favors intramolecular reactions as a result of conformational effects. 

Intramolecular hydrogen transfer is also important in the photolysis of large-ring cycloalkanones such as shown below. The singlet state is thought to be the reactive species in these reactions(10a) ... 

Saponification of Ionized Esters by Intramolecular Hydrogen Transfer Cleavage of the Alkyl/Oxygen Bond... 

We have already reported that the use of cyclohexanol at 140°C in the hydrogenation of ergosterol over Cu/A1203 gives the 5(5 derivative with an 81% selectivity owing to an intramolecular hydrogen transfer reaction, whereas direct H2 addition over the same catalyst gives the 5a isomer with 89% selectivity (10) (Scheme 1). 

The Norrish-Yang reaction [20] is based on the photochemical excitation of ketones followed by an intramolecular hydrogen transfer with the formation of biradicals. Wessig and coworkers used this procedure to prepare functionalized cyclopropyl ketones as 5-75 from 5-72 (Scheme 5.15) [21]. The substrate employed con-... 

Rini M, Dreyer J, Nibbering ETJ et al (2003) Ultrafast vibrational relaxation processes induced by intramolecular excited state hydrogen transfer. Chem Phys Lett 374 13-19... 

In this chapter are summarized the photochemical reactions wherein the primary chemical event is inter- or intramolecular hydrogen transfer to the excited chromophor. In intermolecular reactions hydrogen abstraction usually implies reduction or hydrodimerization of the excited molecule intramolecular hydrogen abstraction is frequently followed by either ring closure of the diradical or fragmentation to afford unsaturated molecules. 

---