Intermolecular dehydrogenative

The first cross-dehydrogenative intermolecular arylation of a heteroarene with an arene was reported by Fagnou in 2007. N-acetyl-lH-indoles were coupled with simple arenes and selective C3-arylation was obtained (88 89) in the presence of Pd(TFA)2 as catalyst in combination with superstoichiometric Cu(OAc)2 as terminal oxidant (Scheme 40) (2007SCI1172). The N-acetyl group proved to be crucial as no reaction product was achieved with IH-indoles, furthermore N-methyl-lH-indoles gave only self-dimerized products. 

Photodehydrogenation reactions may lead to dimerization or cycli-zation. Intermolecular dehydrogenation with ultraviolet light has been recently illustrated in the formation of 2,2 - bipyridine, 6,6 -dimethyl-2,2 -bipyridine, and 2,2 -biquinoline on irradiation of pyridine, a-pico-line, and quinoline, respectively, under nitrogen in cooled quartz tubes.216... 

M-butane proceeds via an intermolecular mechanism with 2-butene involved intermediately.300-303 The role of the transition metal promoters such as Fe and Mn was shown to increase the surface concentration of the intermediate butene 304 The formation of butene is speculated to occur through an oxidative dehydrogenation on the metal site305 or by one-electron oxidation.306... 

Since the reaction conditions for the one-reactor synthesis of p-picoline from MGN are a compromise between hydrogenation and dehydrogenation, and since very reactive compounds are involved, several unwanted reactions were expected to take place, in addition to those indicated in Figure 1. For instance, intermolecular condensations may take place which yield heavy compounds. These compounds, depending on the reaction conditions, adsorb on the metal surface leading to deactivation. 

Compounds of type 106 are readily available by intramolecular dehydrogenation of appropriate triazenopyrazoles 105 (Scheme 61) <1987CB1375>. A general approach to the fused tetrazole system is provided via intermolecular [3 + 2] cycloadditions of the azide group with the activated nitrile functionality present in cyanamides, thiocyanates, and cyanates as illustrated in Schemes 6264 . 

Here hydrocarbon conversion reactions occur wholly or at least partly on the carbonaceous overlayer on the metal and oxide surfaces, as reported by others (13,15-20). Poly-condensed EDA complexes may behave as giant alkenes in which by reversible catalytic hydrogenation/dehydrogenation occurs. This mechanism is similar to the intermolecular hydrogen transfer mechanism proposed (IS) for hydrogenation of unsaturated hydrocarbons. 

Incorporated as a dienophile. These results strongly indicate that dehydrogenation at the isoprenyl portion of (48) followed by a [4 2]cycloaddition reaction with the a, p-double bond of another molecule of isoprenylchalcone leads to the formation of the Diels-Alder type metabolites. Furthermore, the Diels-Alder type metabolites from the precursory chalcones (47), (48), and (53) are all optically active, having the same stereochemistries as those of )cuwanon J (11) and chalcomoracin (21). This fact revealed the [4 + 2]cycloaddition step to be enzymatic. Administration of O-methylated precursory chalcone to the M. alba cell cultures has thus demonstrated that optically active mulberry Diels-Alder type adducts such as 11 and 21 are biosynthesized through an enzymatic intermolecular [4 + 2Jcycloaddition reaction. 

Compounds 1-4, 7-13, and 16 should be appropriate as monomeric precursors for modified PA-MPs, whereas the derivatives 17, 20, and 23 must be polymerized previously. The polymerization can be carried out either dehydrogenative (compounds 20 and 23) by the use of transition metal complex catalysts or by intermolecular hydrosilylation of Si-H to the -C=C- unit (compound 17) to yield crosslinked polymeric precursors for Si-alloyed PA-MPs. 

BD conversion on Co-Zn/porcelain catalyst includes dehydrogenation, dehydration, intra- and intermolecular condensation, C-C bond destruction processes. The reaction products - 2,3-DHF, THF, 4-hydroxybutanal (4-HB) (1-2%),... 

Intermolecular dehydrogenative oxidative homocouplings of (hetero)arenes turned out to be among the most important methods for the synthesis of symmetrically substituted biaryls [122]. A recent illustrative example is oxidative coupling reactions of 2-naphthols, which were accomplished in an asymmetric fashion employing an inexpensive iron catalyst (Scheme 9.47) [123]. 

Dehydration of alcohols over solid catalysts can yield alkenes by intramolecular dehydration, whereas ethers are the product of an intermolecular process. The catalysts used can be acidic or basic solids or bifunctional acid-base materials. Although selective synthesis of any desired product is possible, complications can arise as a result of side-reactions-dehydrogenation and decomposition of the starting alcohol, decomposition and consecutive transformations of intermediates and products (j9-cleavage of carbocations, oligomerization of alkenes). 

The mechanism of this dehydrogenation was investigated by studying the distribution of radioactivity using phenyP C-hydrazine, whereby it was shown that no intramolecular oxido-reduction takes place, but rather, that the enediamine form causes hydrogenation of the free phenyl-hydrazine present in the reaction mixture, to yield aniline and ammonia. This mechanism was criticized by Weygand when this work was presented, and the problem of intermolecular or intramolecular oxido-reduction still awaits settlement. 

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