PMB

Six protective groups for alcohols, which may be removed successively and selectively, have been listed by E.J. Corey (1972B). A hypothetical hexahydroxy compound with hydroxy groups 1 to 6 protected as (1) acetate, (2) 2,2,2-trichloroethyl carbonate, (3) benzyl ether, (4) dimethyl-t-butylsilyl ether, (5) 2-tetrahydropyranyl ether, and (6) methyl ether may be unmasked in that order by the reagents (1) KjCO, or NH, in CHjOH, (2) Zn in CHjOH or AcOH, (3) over Pd, (4) F", (5) wet acetic acid, and (6) BBrj. The groups may also be exposed to the same reagents in the order A 5, 2, 1, 3, 6. The (4-methoxyphenyl)methyl group (=MPM = p-methoxybenzyl, PMB) can be oxidized to a benzaldehyde derivative and thereby be removed at room temperature under neutral conditions (Y- Oikawa, 1982 R. Johansson, 1984 T. Fukuyama, 1985). 

We can monitor the progress of this reaction by coupling it to a second reaction in which 12-MPA is reduced to form heteropolyphosphomolybdenum blue, PMB,... 

Polymethylbenzenes (PMBs) are aromatic compounds that contain a benzene ring and three to sis methyl group substituents (for the lower homologues see Benzene Toluene Xylenes and ethylbenzene). Included are the trimethylbenzenes, (mesit iene (1), pseudocumene (2), and hernimeUitene (3)),... 

PMBs are primarily basic building blocks for more complex chemical intermediates. 

The structures of the eight PMBs ate shown here and their physical and thermodynamic properties ate given in Table 1. 

The PMBs, when treated with electrophilic reagents, show much higher reaction rates than the five lower molecular weight homologues (benzene, toluene, (9-, m- and -xylene), because the benzene nucleus is highly activated by the attached methyl groups (Table 2). The PMBs have reaction rates for electrophilic substitution ranging from 7.6 times faster (sulfonylation of durene) to ca 607,000 times faster (nuclear chlorination of durene) than benzene. With rare exception, the PMBs react faster than toluene and the three isomeric dimethylbenzenes (xylenes). 

PMB Sulfon ation Acylation Chloromethylation Ring chlorination... 

The relative basicity of PMBs is summarized in Table 4, which shows that basicity increases with increa sing number of methyl groups, and that methyl groups ia the 1,3-position to one another gteatiy influence basicity. 

Mesitylene can be synthesized from acetone by catalytic dehydrocyclization (17). Similarly, cyclotrimerization of acetylenes has produced PMBs such as hexamethylbenzene (18). Durene has been recovered from Methanex s methanol-to-gasoline (MTG) plant in New Zealand (19). 

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). 

The thermodynamic equilibria are illustrated in Figures 1 and 2. Figure 1 shows the resulting composition after pure pseudocumene or a recycle mixture of C PMBs is disproportionated with a strong Friedel-Crafts catalyst. At 127°C (400 K), the reactor effluent contains approximately 3% toluene, 21% xylenes, 44% C PMBs, 29% C q PMBs, and 3% pentamethylbenzene. The equihbrium composition of the 44% C PMB isomers is shown in Figure 2. Based on the values at 127°C, the distribution is 29.5% mesitylene, 66.0% pseudocumene, and 4.5% hemimellitene (Fig. 2). After separating mesitylene and hemimellitene by fractionation, toluene, xylenes, pseudocumene (recycle plus fresh), C q PMBs, and pentamethylbenzene are recycled to extinction. 

The only significant U.S. producer of PMBs is Koch Chemical Company. The only PMBs produced ia any sizable quantity ate pseudocumene, durene, mesitylene, and hemimellitene. Koch s production capabiUty for pseudocumene is ca 35,000 t/yr the combiaed production rate of the other PMBs is less than 5,000 t/yr. 

Pseudocumene is shipped ia barges, tank cars, tank tmcks, isocontainers, and dmms. Mesitylene is shipped ia tank tmcks, isocontainers, and dmms, whereas durene is shipped molten ia heated tank tmcks, isocontainers, and occasionally as a cast soHd in dmms. Mesitylene, pseudocumene, and hemimellitene are classified as flammable Hquids the higher homologues are classified as combustible. The higher melting PMBs requite additional precautions when handled in the molten state to avoid thermal bums. Detailed shipping and handling procedures are described in manufacturers material... 

The typical properties of commercial PMBs are shown in Table 6. 

The PMBs, as higher homologues of toluene and xylenes, ate handled in a similar manner, even though their flash points are higher (see Table 1). Containers are tightly closed and use areas should be ventilated. Breathing vapors and contact with the skin should be avoided. Toxicity and primary... 

All the PMBs are Hsted on the U.S. EPA s Toxic Substances Control Act NonConfidential Chemical Substances Inventory (Table 8). In the early to mid-1980s, pseudocumene, mesitylene, hemimellitene, and trimethylbenzene were coveted by TSCA Section 8(a) Preliminary Assessment Information Rule (PAIR) reporting requirements (22) and by TSCA Section 8(d) for health and safety data (23). Mesitylene is the subject of a test rule subacute oral toxicity and subchtonic oral toxicity in tats were underway in 1994 (24). The Safe Drinking Water Act (SDWA) allows monitoring for pseudocumene and mesitylene at the discretion of the State (25). Of the PMBs, only pseudocumene is subject to SARA Tide III section 313 annual release reporting (26). 

PMB CAS Registry Number TSCA" Hsted EINECS" number Canadian b status... 

All eight PMBs are Hsted on the U.S. EPA s TSCA Inventory (May 1, 1994 ed.) and on the European Communities EINECS inventory (June 15, 1990). All but pentamethylbenzene are Hsted on the Canadian Domestic Substances List (DSL) (Apr. 6, 1994). Pentamethylbenzene is Hsted on the Canadian Nondomestic Substances List (NDSL) (Apt. 6, 1994) and as of July 1, 1994 was subject to New Substance Notification if manufactured in or imported into Canada. 

Similar regiospecific syntheses employing PMBs and diaLkylnaphthalene have been used to prepare 2,6-dimethyl naphthalene [581-42-0] which is oxidized to the corresponding 2,6-naphthalenedicarboxyHc acid [1141-38-4] also used as a monomer for Hquid crystal polymers. 

Important derivatives are obtaiaed from PMBs by oxidation (see Phthalic acid and otherbenzenecarboxylic acids). For example, hemimellitene is oxidized to hemimellitic acid dihydrate [36362-97-7] or hemimellitic acid [569-51-7] which can be dehydrated to hemimellitic anhydride [3786-39-8]. 

In[107] = Clear[PmB,PmA,Am,VI,VII,CIBo,CIDo,plI,pill,plIII plIV,t ]... 

---