Intramolecular dehydrogenative

Chiral benzo[t/ thiepine fused to pyrrole ring 163 was obtained via a tandem dehydrogenation/intramolecular arylation <2001TL573>. Treatment of ct-hydroxylactam 160 with trifluoroacetic acid (TFA) at 25 °C gave a separable mixture of 163 and 165 in 64% and 22% yields, respectively (Scheme 18). Formation of the minor product 165 was a consequence of deprotonation of /V-acyliminium ion 161, followed by isomerization of unstable enamide 164. Formation of benzo[t/ thiepine 163 can be explained by a 71-aromatic intramolecular arylation of... 

Pyrans. Nucleophilic substitution of chloroazulene 207 (Scheme 48) with cyanoacetate and subsequent reaction with DEX3 yields pen-fused pyran 208, presumably through dehydrogenation, intramolecular cyclization, and aromatization (97M11). 

In Section S.2.3.3, we discussed an example of the catalytic asymmetric construction of silicon stereogenic center-containing compounds by a Pd-catalyzed C—H bond functionalization reaction. Recently, Rh-catalyzed asymmetric C—H bond functionalization also found applications in this area. In 2013, Kuninobu, Takai, and co-workers demonstrated that Rh-catalyzed double dehydrogenative intramolecular cyclization of bis(biphenyl)silanes was an effective strategy for the asymmetric synthesis of spirosilabifluorene... 

Use of pivalic acid (PivOH) as a solvent with 10 mol % K2CO3 was highly effective for the acceleration of palladium-catalyzed dehydrogenative intramolecular coupling reaction [35]. For example, electron-rich diarylamines 71 underwent the cyclization smoothly to give 72 in much higher yields that those of the same reaction carried out in AcOH (Table 22.3). The examples illustrated in Table 22.3 are a synthesis of... 

In 2012, Takai and coworkers use a bis(biphenyl)phosphine oxide to synthesize a new phosphine oxide with a chiral phosphine center by dehydrogenative intramolecular cyclization via P-H and C-H bond activation under palladium... 

This chapter compares the reaction of gas-phase methylation of phenol with methanol in basic and in acid catalysis, with the aim of investigating how the transformations occurring on methanol affect the catalytic performance and the reaction mechanism. It is proposed that with the basic catalyst, Mg/Fe/0, the tme alkylating agent is formaldehyde, obtained by dehydrogenation of methanol. Formaldehyde reacts with phenol to yield salicyl alcohol, which rapidly dehydrogenates to salicyladehyde. The latter was isolated in tests made by feeding directly a formalin/phenol aqueous solution. Salicylaldehyde then transforms to o-cresol, the main product of the basic-catalyzed methylation of phenol, likely by means of an intramolecular H-transfer with formaldehyde. With an acid catalyst, H-mordenite, the main products were anisole and cresols moreover, methanol was transformed to alkylaromatics. 

A reasonable mechanism for their formation starts with the primary adduct 339, which is capable of ring-opening to the ketene 340 this can either be trapped by addition of water (337) or undergo intramolecular acylation followed by dehydrogenation (338). 

In 1979, Claesson et al. observed the formation of the dihydropyrrole 125 and the pyrrole 126 when trying to purify the amine 124 by GLC [85]. They suspected that an initial cycloisomerization first leads to 125 and a subsequent dehydrogenation then delivers 126. Guided by other intramolecular nucleophilic additions to alkynes that are catalyzed by AgBF4, they discovered that this catalyst efficiently allowed the transformation of 124 to 125 (Scheme 15.38). Reissig et al. found that with enantio-merically pure substrates of that kind a cyclization without racemization is possible with Ag(I) catalysts [86],... 

A key feature of our polyphenylene dendrimers is that they can be planarized and thus reduced in dimensionality by intramolecular dehydrogenation [29,35]. This results in large, fused polycyclic aromatic hydrocarbons (PAHs). PAHs serve as structurally distinct, two-dimensional subunits of graphite and show attractive properties such as high charge carrier mobility, liquid crystallinity, and a high thermal stability, which qualifies these materials as vectorial charge transport layers [81]. 

Propargylsulfinyl)tropones 69a-c, however, afford 3-acylthienotro-pones 70a-c, eventually accompanied by the 2,3-dihydro derivative (71b), which can be dehydrogenated by DDQ [91H(32)2099], In this case, crossover experiments demonstrate an intramolecular reaction. 

The second function, and the one pertinent to this section, is the decarboxylation of oxalosuccinic acid to 2-oxoglutaric acid. This is simply a biochemical example of the ready decarboxylation of a P-ketoacid, involving an intramolecular hydrogen-bonded system. This reaction could occur chemically without an enzyme, but it is known that isocitric acid, the product of the dehydrogenation, is still bound to the enzyme isocitrate dehydrogenase when decarboxylation occurs. 

CONNECTION BETWEEN INTRAMOLECULAR DIHYDROGEN BONDING AND DEHYDROGENATION REACTIONS... 

Intramolecular Mannich type reaction of the conjugated iminium salt 426 should lead to ellipticine (228) via an intermediate 427. Alternatively, the conjugated iminium salt 426 could hydrolyze to afford the 2-vinylsubstituted indole 428, which, on cyclization through an intermediate 429, would lead to guatambuine (233). This alkaloid, on demethylation and dehydrogenation, should afford olivacine (238a) (375) (Scheme 3.11). 

The reaction of n-hexene at 773 K and high dilution over H-ZSM5 produced almost exclusively cracked products propene. Under these conditions the formation of aromatics and paraffins were not observed. In contrast over Ga-HZSN-5 the main products were propene and benzene. The very rapid dehydrogenation of n-hexene over Ga-HZSM-5 into hexadiene and hexatriene which could easily form cyclic hydrocarbons by Intramolecular alkylation catalyzed by H will explain the different behaviour of H-ZSM-5 and Ga-HZSM-5 in the reaction of highly diluted n-hexene. These suggestions are consistent in view of the finding that Ga-HZSM-5 shows dehydrogenating properties. 

Benzofuranyl)butanoic acid readily forms the acid chloride, and this undergoes intramolecular Friedel-Crafts acylation on treatment with tin(IV) chloride in carbon disulfide at room temperature, providing 1,2,3,4-tetra-hydro-l-dibenzofuranone (54%). " This intermediate has been converted to dibenzofuran by lithium aluminum hydride reduction and subsequent dehydrogenation, to 1-methyldibenzofuran by Grignard reaction and dehydrogenation, and to 1-dibenzofuranol by reaction with iV-bromosuccinimide and subsequent dehydrobromination with pyridine. 

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