Durene, alkylation

Fluoroalkjiations are frequentiy performed indirectly using tandem reactions. Arenes react with sodium borohydride in trifluoroacetic acid to afford otherwise difficult to obtain l,l,l-trifluoro-2,2-diarylethanes. Presumably sodium borohydride reacts initially with the trifluoroacetic acid to produce the trifluoroacetaldehyde or its equivalent, which rapidly undergoes Friedel-Crafts-type condensation to give an intermediate carbinol. The carbinol further alkylates ben2ene under the reaction conditions giving the observed product. The reaction with stericaHy crowded arenes such as mesitylene and durene... 

Koch Chemical Company is the only U.S. suppHer of all PMBs (except hexamethylbenzene). Its process has the flexibility of producing isodurene, prehnitene, and pentamethylbenzene, should a market develop. Koch s primary process (20) is based on isomerization, alkylation, and disproportionation conducted in the presence of a Friedel-Crafts catalyst. For the synthesis of mesitylene and hemimellitene, pseudocumene is isomerized. If durene, isodurene, or prehnitene and pentamethylbenzene are desired, pseudocumene is alkylated with methyl chloride (see Alkylation Friedel-CRAFTSreactions). 

There are many other examples of this type of rearrangement particularly in the reactions of poly-alkylated benzenes163 for example, the sulphonation of durene (CXXII) gives prehnitenesulphonic acid (CXXIII) and octahydrophenanthrene (CXXIV) similarly yields the octahydroanthracenesulphonic acid (CXXV), viz. 

Cross-alkylations have been reported on a number of occasions. Thus, ethylbenzene when treated with aluminium bromide and hydrogen bromide at 0 °C forms some benzene and diethylbenzene168, and in the sulphonation of durene some trimethyl- and pentamethyl-benzenesulphonic acids are formed as well as the tetra-methyl compound. It has been suggested169 that these transfer reactions involve an SN2 type process... 

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

There have been only a few reports of direct hydroxylation362 by an electrophilic process (see, however, 2-26 and 4-5).363 In general, poor results are obtained, partly because the introduction of an OH group activates the ring to further attack. Quinone formation is common. However, alkyl-substituted benzenes such as mesitylene or durene can be hy-droxylated in good yield with trifluoroperacetic acid and boron trifluoride.364 In the case of mesitylene, the product is not subject to further attack ... 

The method has been applied to the preparation of substituted benzene hydrocarbons containing as many as four alkyl-groups (durene. Ct H (CH3)4(1.2,4,5) and isodurene, CfiHffCHslaf 1,2.3,5 ) 1. 

Kotsuki et al.203 have reported high yields of alkylated products formed in the presence of triflic acid using cyclohexyl methanesulfonate [Eq. (5.79)]. Similar performance was found for mesitylene, durene, and naphthalene, as well as for other secondary alcohol methanesulfonates. 

In the MTG process, durene is mostly formed by alkylation of lower molecular weight aromatics with methanol and/or ether. Low methanol partial pressures and high reaction temperatures tend to reduce the durene level, presumably by reducing the concentration overlap of methanol/ether with the aromatics formed. This overlap tends to increase with larger catalyst particles. As a consequence the early fixed-bed development work was conducted using 1 mm diameter extrudates. 

Scheme 12. Octa-alkylation, -allylation, or -benzylation of durene in high yields by a series of eight deprotonation/alkylation (or allylation or benzylation) sequences induced by the 12-electron activating group CpFe+ in a one-pot reaction under mild conditions.

The bisacetato ruthenium complex 28, on heating in 2-propanol, leads to the bridged hydrido dinuclear complexes 73 and 74. The bistrifluoroacetato complex 28 also leads to complex 73. The Tj2-acetato complex 39 was transformed in hot 2-propanol to another bridged hydrido derivative (75, arene = durene, mesitylene, p-cymene, hexamethylbenzene 60-70%). The introduction of alkyl substituents on the benzene ring is reflected by a shift of the p FI resonance toward high field (14,53). 

Many di- and poly-alkylated benzenes have been prepared by the Friedel-Crafts reaction. Alkyl groups on the nucleus do not exert a strong directive influence upon the orientation, nor do they greatly affect the rate of further alkylation. The composition of the alkylated product varies widely, depending upon the conditions of the reaction. Appreciable quantities of m-dialkylated and sywz-trialkylated products are obtained under vigorous conditions. The composition of many products is in doubt, as has been shown by later, more accurate analyses. Methylation of xylene gives 1,2,4,5-tetramethylbenzene (durene), pentamethylbenzene, and hexamethylbenzene. ... 

A brief review and reassessment of data on the photophysics of benzene has been presented by Pereira. Evidence for the l E2g valence state has been obtained by u.v. two-photon spectroscopy.Slow electron impact excites fluorescence in thin films of benzene at 77 K as well as emission from isomers." The fluorescence yields and quenching by chloroform of alkyl-benzenes and 1-methylnaphthalene after excitation into Si, Sz, and S3 states and after photoionization have been measured. The channel-three process has been reconsidered in terms of the effects of local modes and Morse oscillator potentials. Excited-state dipole moments of some monosubstituted benzenes have been estimated from solvent effects on electronic absorption spectra, Structural imperfections influence the photochemistry of durene in crystals at low temperatures. Relaxation time studies on excited oxido-substituted p-oligophenylenes have been made by fluorescence depolarization... 

However, the outcome of the oxidation of alkyl-substituted arenes is not easy to predict owing to the competition between nuclear dimerization and dimerization through an Qf-carbon. For instance, a major product from oxidation of mesitylene in CH2CI2 is bimesityl [Eq. (46)], and from durene in CH2CI2-CF3SO3H, the heptamethyldiphenyl-methane [Eq. (47)] [104,105] no diphenylmethane is formed from mesitylene nor biphenyl from durene. 

Nutaitis and Gribble have found that arenes react with NaBH4 in CF3CO2H to afford 1,1,1-trifluoro-2,2-diarylethanes, which were otherwise difficult to prepare. They proposed a Friedel-Crafts-type (Baeyer) condensation to give an intermediate carbinol (21 equation S2), which further alkylates benzene to give the observed products. The reaction with sterically congested arenes such as mesitylene and durene stops at the carbinol stage (equation 53). 

Although the oxidation affects only the alkyl group attached to the benzene nucleus, the presence of more than one methyl group accelerates the oxidation. Thus, in the liquid phase non-catalytic oxidation experiments of Stephens it was found that durene (sym.-tetra methylbenzene) was easily oxidized under conditions that scarcely affected toluene. However, in the presence of water the oxidation of m-xylene or mesitylene (1 3 4-trimethylbenzene) is almost entirely inhibited,128 a fact that Stephens interprets as showing the reversible elimination of water in one... 

Fig. 1. Hydrogenation of the ring of alkyl-substituted aromatic compounds with 5% Rh on AljOs powder as catalyst and 100 ml. glacial acetic acid as solvent. 1) 1 g. catalyst, 0.5 ml. benzene 2) 1 g. catalyst, 0.5 ml. toluene 3) 1 g. catalyst, 1 ml. p-xylene 4) 1 g. catalyst, 1 ml. mesitylene 5) 1 g. catalyst, 0.5 ml. butylbenzene 6) 1 g. catalyst, 500 mg. dibenzyl 7) 2 g. catalyst, 500 mg. durene.

The reaction was found particularly useful as a relatively selective and mild nitration method, for example allowing mononitration of durene and other highly alkylated benzenes, which with mixed acid usually undergo dinitration. (Table XIl). Methyl nitrate-boron trifluoride can also be used to achieve dinitration of tet-ramethylbenzenes by using two and three molar excess of methyl nitrate, respectively. Relative yields of mono- and dinitro product compositions are shown in Table XIII. Other Friedel-Crafts type catalysts can also be used, but boron trifluoride was found to be the most suitable. Aluminum trichloride and titanium (IV) chloride in the nitration of pentamethylbenzene caused formation of significant amounts of chlorinated derivatives, whereas sulfuric acid led to nitrodemethylation products. 

Mercuration of polyalkylbenzenes with mercuric trifluoroacetate [e.g. durene -< 2,3,5,6-tetramethyl-p-phenylenedimercury bis(trifluoroacetate)] has been noted during spectroscopic (u.v., n.m.r.) examination of solutions of alkylated aromatics in trifluoroacetic acid both in the presence and absence of its mercury(ii) salt. Mono- and di-mercuration of 1,2,4,5-tetrafluoro-benzene can be achieved by heating it with mercuric trifluoroacetate... 

If deprotonation of a polymethylbenzene complex is carried out in the presence of an excess base and compatible electrophile such as an alkyl iodide or allyl- or benzyl bromide, the deprotonation-alkylation sequences spontaneously follow one another in situ until steric inhibition is reached. This reaction has been exploited with arene = CgMeg for the synthesis of star-shaped molecules (examples below) and living polymers and with arene = durene or mesitylene for the synthesis of dendritic cores (see for instance Chap. 21.2.2). 

When dealing with polyalkylaromatic hydrocarbons, rearrangement of the substituents has been known to occur. Heilbrunn and Marechal have shown that when durene is reacted with MA in the presence of AICI3, a 75% yield of the indanone carboxylic acid 135 is obtained. Thus, both acylation as well as alkylation by the anhydride and olefin fragments of MA occur. 

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