Intermolecular reactions hydrogenation

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. 

With a radical-scavenging compound present in the reaction mixture, an alkyl radical species like 5 can be trapped, thus suggesting a fast conversion of the alkoxy radical 3 by intramolecular hydrogen abstraction, followed by a slow intermolecular reaction with nitrous oxide. 

The ene reaction as a reaction principle has been first recognized and systematically investigated by Alder It is a thermal addition reaction of a double bond species 2—the enophile—and an alkene 1—the ene—that has at least one allylic hydrogen. The intramolecular variant is of greater synthetic importance than is the intermolecular reaction. 

One of the most dramatic recent developments in metal carbene chemistry catalyzed by dirhodium(II) has been demonstration of the feasibility and usefulness of intermolecular carbon-hydrogen insertion reactions [38, 91]. These were made possible by recognition of the unusual reactivity and selectivity of aryl- and vinyldiazoacetates [12] and the high level of electronic control that is possible in their reactions. Some of the products that have been formed in these reactions, and their selectivities with catalysis by Rh2(S-DOSP)4, are reported in Scheme 10. 

The poor stability of anthocyanins with simple stractures can be overcome by intermolecular copigmentation reaction, that is, an association with different compounds, especially polyphenolic ones, stabilized by n-n intermolecular and hydrogen bonds. However, this complex dissociates at high temperatures. ... 

The most common rearrangement reaction of alkyl carbenes is the shift of hydrogen, generating an alkene. This mode of stabilization predominates to the exclusion of most intermolecular reactions of aliphatic carbenes and often competes with intramolecular insertion reactions. For example, the carbene generated by decomposition of the tosylhydrazone of 2-methylcyclohexanone gives mainly 1- and 3-methylcyclohexene rather than the intramolecular insertion product. 

Krische and co-workers have revealed efficient reductive generation of rhodium enolates under hydrogenation conditions.403 40311-403 Both inter- and intramolecular reductive aldol reactions proceed smoothly and stereoselec-tively, although intermolecular reactions generally show low diastereoselectivity (Equations (52) and (53)). 

Undesirable intermolecular reactions can be avoided during certain synthetic conversions. Thus it is often useful to carry out C-alkylation and C-acylation of compounds that form enolate anions, for example, esters with a-hydrogens. Such reactions are often complicated by self-condensation since the enolate anion can attack the carbonyl group of a second ester molecule. Attachment of the enolizable ester to a polymer support at low loading levels allows the alkylation and acylation reactions (Eq. 9-79) to be performed under... 

Thermal decomposition in three different ways, i.e. homolytic, polar and radical induced decomposition, as well as intermolecular reaction of sulfonyl peroxides are the main reactions displayed by sulfonyl peroxides. When bis(arylsulfonyl) peroxides are allowed to decompose at 25-40 °C in chloroform, homolytic 0—0 bond fission followed by hydrogen abstraction from the solvent results in the formation of the corresponding arylsnlfonic acids. Mixed acyl sulfonyl peroxides undergo complicated thermal decomposition in solution, and have been used commercially as polymerization initiators, since they provide a source of free radicals at a relatively low temperature . 

Many of the limitations of C—C bond formation by C —H insertion outlined for intermolecular reactions (Section 1.2.1.) can be overcome by making the reaction intramolecular. Thus, hydrogen atom abstraction followed by intramolecular radical-radical coupling or radical addition to an alkene are increasingly popular processes. Two-electron carbene insertions, either thermal or transition metal catalyzed, have also been used extensively. In either case, ring construction involves net C—C bond formation at a previously unactivated C-H site. 

The photolysis of n-octyl nitrite in n-heptane provides a favorable situation for comparing the tendency of an alkoxyl radical to undergo the Barton-type intramolecular reaction or, alternatively, the intermolecular reaction as indicated in eqs. 1 and 2. Calculations23 reveal that both intramolecular and intermolecular hydrogen abstraction involving a second-... 

In Equation 16, the first step is the activation of a cyclohexene molecule by electron impact, and the second step is the reaction of the activated cyclohexene molecule with a normal cyclohexene molecule resulting in intermolecular allylic hydrogen transfer. In Equation 17 the first step is ionization of a cyclohexene molecule by electron impact, and... 

Catalytic hydrogenation of l,2,3,4-tetrahydro-ll//-pyrido[2,l-6]quina-zolin-ll-one and its 6-, 7-, 8-, and 9-methyl derivatives over Pd/C catalyst in ethanol gave 1,2,3,4,6,7,8,9-octahydro-ll //- derivatives (87JMC1543). A solution of a 1 1 mixture of 9-methyl-l,2,3,4-tetrahydro-ll/f-pyrido-[2,l-6]quinazolin-11 -one (73) and 2-benzyl-9-methyl-l,2,3,4-tetrahydro-ll//-dipyrido[l,2-a 4,3-catalyst yielded a complex reaction mixture containing 15% of 9-methyl-l,2,3,4,6,7,8,9-octahydro-ll//-pyrido[2,l-/)]quinanolizin-Tl-one and other products (87T1157). The double bonds of the pyridine of compound 73 was also saturated by intermolecular catalytic hydrogen transfer from 2-(4-cyclohexenyl)-l,2,3,4,5,6,7,8-octahydroquinazolin-4-one in the presence of Pd/C catalyst [85H(23)3095]. 

Intramolecular azomethine ylide cycloaddition to the C—O double bond of an aldehyde was reported in 197369 and cycloaddition to the C—C double bond was first reported in 1975.70 Competition between 1,1- and 1,3-cycloaddition is observed in intramolecular reactions, although intermolecular reactions give only 1,3-cycloaddition. Photolysis of 2//-azirines is one generation method of nitrile ylides applicable to intramolecular cycloaddition.70 Another method involves the base-catalyzed 1,3-elimination of hydrogen halide from alkenyl imidoyl halides. Still other procedures involve thermolytic and photolytic cycloreversions of oxazolinones and dihydrooxazaphospholes. 

This intermolecular reaction is reversible, and occurs with certain other nitro compds. [Thermal extraction of the hydroxyl-hydrogen of hydro-quinone by TNT is cited in the section on The Action of Heat (Ref 96)]... 

In the absence of oxygen, effects of pH on the polymer are complicated by possible effects of pH on yields of hydrogen atoms from the radiolysis of water. However, it is difficult to see how the increase in hydrogen atom yield at low pH could lead to a decrease in yield of D-glutamate and glycine. If these were formed by intermolecular reactions—e.g., Reaction 22 for forming D-glutamate—the explanation put forward by Sokol et al. to account for the effects in O2 could also explain the observations on yields in the absence of 02. 

Thus, it is likely, that after imine formation a second catalyst precoordination occurs and an intramolecular imine hydrogenation takes place. Such an intramolecular process should be kinetically favored compared to a corresponding intermolecular reaction pathway. Hence, the catalyst-directing o-DPPB group may be acting within one sequential transformation in... 

Some intermolecular reactions involving the coordinated ligands (e.g., hydrogen or proton transfer) may be fast enough to compete with the excited state decay. However, except for a few cases [e.g., Ru(bpy)2(CN)2 34 and Ru(bpy)2 (bpy-4,4 -(COOH)2 )2 + 35)] reactions of this kind have not yet been well documented for transition metal complexes, although they are very common for organic molecules19, 36,37. ... 

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