Hydrogenation polycyclic arenes

Apart from the construction of phenanthrenes, carbene complexes have also been used for the synthesis of more extended polycyclic arenes. An unusual dimerization of chromium coordinated ortbo-ethynyl aryl carbenes results in the formation of chrysenes (Scheme 37) [81]. This unusual reaction course is presumably due to the rigid C2 bridge that links the carbene and alkyne moieties, and thus prevents a subsequent intramolecular alkyne insertion into the metal-carbene bond. Instead, a double intermolecular alkyne insertion favored by the weak chromium-alkyne bond is believed to occur forming a central ten-membered ring that may then rearrange to the fused arene system. For example, under typical benzannulation conditions, carbene complex 97 affords an equimolar mixture of chrysene 98a and its monochromium complex 98b. The peri-interactions between the former alkyne substituent (in the 5- and 11-positions) and the aryl hydrogen induce helicity in the chrysene skeleton. 

With nickel(II) 2-ethylhexanoate and triethylaluminum, tetralin (59) is obtained by hydrogenation of naphthalene (55). Polycyclic aromatics, such as anthracene (57 equation 8), 9-methylanthracene and 9-trifluoroacetylanthracene, are partially hydrogenated to 1,2,3,4-tetrahydroanthracene derivatives by use of [Rh(DPPE)(arene)]+ in methanol and by ruthenium hydride complexes having triphenylphosphine ligands... 

Isopropyl alcohol under UV irradiation converts bromobenzene to benzene in 72% yield (Table 4). Similar replacement of bromine by hydrogen is accomplished by treatment of aryl bromides dissolved in dichloromethane with a mixture of ethanethiol and anhydrous aluminum chloride. This hard acid-soft base combination reacts with polycyclic aromatic halides and halogenated phenols by an addition-elimination mechanism, leading to an aryl ethyl sulfide through a radical anion intermediate. This is converted by another molecule of ethanethiol to the debrominated arene and diethyl disulfide. 1-Bro-monaphthalene is thus transformed into naphthalene (equation 59), 2,4,6-tribromophenol into phenol (equation 60), and bromochlorophenols into chlorophenols in 61-91% yields. ... 

Raney nickel and platinum, palladium, and rhodium catalysts have been used to accomplish the hydrogenation of polycyclic aromatics. Hydrogenation of fused polycyclic arenes leads to the cis- or fran -substituted cyclohexane derivatives. The cis product is usually obtained again this can be understood in terms of the mechanism proposed for aromatic hydrogenation (vide supra). 

The coordination chemistry of this anionic, cyclometallated precatalyst and its derivatives, along with its stoichiometric reactions with anthracene, have been reported by Halpem. On the basis of these studies, a plausible mechanism can be proposed for the hydrogenation of polycyclic arenes by this catalyst. Although kinetic studies of the complete catalytic cycle have not been reported, Halpem s investigation of the stoichiometric reactions of this system provides strong evidence that the reaction occurs by a series of steps involving soluble complexes. 

Polycyclic arenes are also hydrogenated in the presence of the monohydride catalyst, HCo(CO) (Equahon 15.101). The regioselectivity of this reduction is different from that of the hydrogenation of anthracene catalyzed by the ruthenium complex of Grey and Pez. This difference in regioselectivity results from a different mechanism. Halpem showed that the hydrogenation of polycyclic arenes catalyzed by HCo(CO) occurs by hydrogen atom transfer. ... 

An efficient catalyst for the hydrogenation of polycyclic arenes, such as anthracene, is derived from an anionic orthometallated hydride ruthen-ate complex that, according to equation (24), reacts with hydrogen to jdeld /ac-[RuH3(PPh3)3], followed by the reaction with anthracene to yield... 

Intramolecular HAS induced by reducing reagents has been intensively applied to obtain a large number of compounds, where either alkyl, vinyl, aryl, or heteroaryl radicals successfully add onto arenes [31b, 40]. Different synthetic strategies, such as ring expansion [41], ipso substitution [42], 1,5-hydrogen translocation, and tandem cyclization [43], among others, have been applied to afford important cyclic compounds such as phenanthridines [44], polycyclic arenes [45], 6//-benzo[c] chromen-6-ones [13a], and strained helicenes [46]. 

The trichloromethyl radical is known to attack the rings of polycyclic aromatics (7). In order to correct for this, it was assumed that any enhanced reactivity of the methylarene relative to the parent arene reflected only hydrogen abstractions. A competition between these two species can thus be used to determine (allowing for statistical correction) the amounts of hydrogen abstraction and ring substitution for each arylmethane. Such an approach presumes that the methyl substituent does not electronically influence attack in the aromatic unit. At the time, however, both experimental (24) and theoretical (25)... 

A review has been published regarding computational studies of arenes, linear polycyclic aromatics, and their reactivities in electrophilic and nucleophilic processes. There have also been reviews of theoretical studies of electron delocalization and its relevance to electrophilic substitution, and of mechanisms of activation of carbon-hydrogen bonds to reactions with electrophiles. ... 

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