bilization of a superoxide formulation Rum02-. A prime goal of the British Columbia group now is isolation of the ruthenium dioxygen complex. That goal has been elusive so far, since during workup oxidation to ruthenium(III) is enhanced. D
New synthesis route for arene oxides developed A group of Pennsylvania State University chemists at University Park has devised a simple, convenient, and inexpensive method for making large amounts of various arene oxides. The arènes belong to a family of fused polycyclic aromatic compounds, some of which form potent cancer-causing agents in the body upon oxidation. Having an abundant supply of these oxides will make it easier to study their role in causing tumors. And the new chemical mechanism might offer insight into how oxidation occurs in living cells. The new oxidation method involves use of a phase-transfer catalyst, such as tetrabutyl ammonium hydrogen sulfate, requires hypochlorite as an oxidizing agent, and probably takes the arènes through free radical intermediates [J. Amer. Chem. Soc, 99, 8121 (1977)]. u We think we understand what's happening," says Dr. Gordon A. Hamilton, leader of the Penn State group that includes Dr. Subramanian Krishnan, Dr. David G. Kuhn, and Hossein E. Fonouni. "We are working with simple alkenes to 'nail down' the
Chemists propose free radical sequence for arene oxidations Chain initiation CIO" + HOCI Ζ Cl20
cr + cioChain propagation
—H
L P + C I - + CIO·
Oxidation of arènes, such as phenanthrene here, probably involves free radical intermediates and is critically dependent on a phase-transfer catalyst.
mechanism," Hamilton continues. "But evidence so far strongly suggests that the reaction proceeds by a free radical mechanism." The phase-transfer catalyst plays a "critical" role in the oxidation reaction sequence, according to Hamilton. For one, it brings together the arene, which is in the organic phase, and the oxidizing agent, which is a hypochlorite ion. Hypochlorite ions have been used to oxidize alcohols, but this is the first time they've been used to oxidize aromatic compounds, Hamilton says. Another contribution of the phase-transfer catalyst is to stabilize the chloride ion that forms when the hypochlorite and the arene intermediate react. The chloride probably shifts back into the aqueous phase in exchange for another hypochlorite ion. Also important to the success of this oxidation scheme is regeneration of the chlorine monoxide radical, the species that keeps the reaction cascade going. Keeping the pH below 9 builds up enough hypochlorous acid to drive the initial reaction. However, permitting the pH to drop below 7 is not advantageous because undesirable side products form. The next reaction, decomposition of dichlorine monoxide, goes spontaneously at room temperature. It probably occurs in the organic phase, as dichlorine monoxide is not very soluble in water. The chlorine monoxide radical that forms in the decomposition reaction probably attacks the arene directly, forming a short-lived intermediate that decomposes when attacked by a hypochlorite ion. Thus, a chlorine monoxide radical is regenerated. Several observations make it plain that free radicals must be involved. For instance, oxygen and other free radical "traps" inhibit the overall reaction, but light accelerates it. Also, aza arènes (nitrogen-containing arènes) form epoxides under these conditions, and not the Noxides that might be expected if oxidation involved an electrophilic agent. Hamilton compares the new chemical mechanism to what probably occurs in living cells when arènes are oxidized en route to becoming active tumor-causing agents. In cells, oxidations are catalyzed by enzymes. Specifically, the cytochromes, which are heme-iron containing proteins, catalyze epoxidations. The oxidizing agent within the protein is believed to be the free radical, Fe(IV)-0\ This species probably attacks aromatic compounds to give an adduct similar to the one formed in the Penn State reaction. The Penn State chemists' phasetransfer reaction sequence, which probably goes through this kind of free radical intermediate, might be "one of the best models for reactions catalyzed by the cytochrome enzymes," Hamilton says. If so, the chemical reaction might provide insights into the biochemical apparatus. No matter how good a model the new sequence for arene oxidation proves to be, however, it offers a workable strategy for oxidizing arènes simply, under mild conditions, and in high yields. D
IT POURS AT-30°F. ITBOI1SAT600°F. IT'S NEW FROM GULF:
DXE (dixylylethane) CH 3 CH3 CH 3
I c.
m®Z
Mol. Wt. Sp. Gr. Pour Pt. Boil Pt. Flash Pt.
238 0.978 -30°F 600°F 325°F
Gulfs new DXE is a versatile molecule which offers many unique possibilities, such as: • A heat transfer fluid • Electrical insulation fluid • Synthetic fluid additive • Chemical intermediate • High boiling solvent For information, write C. E. Turtle, Gulf Specialty Chemicals, P.O. Box 3766, Houston Texas 77001. Or call (713) 750-3233.
Gulf Specialty Chemicals Gulf Oil Chemicals Company A division of Gulf Oil Corporation
Jan. 16, 1978 C&EN
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