Unactivated C-C bond selectively cleaved Chemists in Israel have demonstrated a way to selectively cleave a carbon-carbon bond between an aromatic ring and a methyl group under relatively mild conditions in solution. Their approach—if extended generally to other hydrocarbons— could eventually change the way petroleum is converted to fuels and other useful chemicals. "The petroleum industry takes large hydrocarbon molecules and cleaves them into smaller pieces by the addition, at high temperature, of hydrogen in the presence of a heterogeneous catalyst," explains chemistry professor William D. Jones of the University of Rochester in New York. The transition-metal catalysts currently used in this process usually aren't fussy about which C-C bond they break, so a diverse mixture of products is obtained. The holy grail being sought by many chemists is the capability to break C-C bonds selectively in unreactive, saturated hydrocarbons—preferably under mild, easily controlled conditions. The Israeli group, led by professor David Milstein of Weizmann Institute of Science, Rehovot, has shown how to do this in one specific, functionalized molecule. Milstein hopes the method, described last week in Nature
[364,699 (1993)], can be applied generally, even to simple hydrocarbons. Milstein's discovery is "an important step" toward the ultimate goal of selectively cleaving totally unactivated C-C bonds, says chemistry professor Robert G. Bergman of the University of California, Berkeley. It now looks even more likely, he says, that one day someone will cleave the C-C bond in ethane. The Israeli scientists performed their bond-breaking legerdemain on an aromatic ring with three methyl and two phosphine groups attached. When this molecule interacts with a rhodium complex in solution, the phosphines chelate to the metal center so that it preferentially targets the closest ring methyl. In this first step, the rhodium undergoes a wellknown reaction—insertion into the methyl C-H bond—to yield a stable molecule with an aryl-CH 2 -Rh linkage. When this molecule is heated at 90 °C under pressurized hydrogen, that C-C bond is cleaved quantitatively, yielding an aryl-Rh complex and methane. This finding is the first reported example of a metal inserting into a fairly unactivated C-C bond in solution, Milstein tells C&EN. In earlier work at other labs,
researchers had managed to insert a metal into a C-C bond in solution in certain special circumstances, such as when the bond was part of a strained ring system or was adjacent to an activating group. Milstein has proposed a mechanism to explain the C-C cleavage. He suggests that the aryl-CH2-Rh species reversibly converts, in two steps, to an aryl-CH3 species in which the rhodium is held close to this C-C bond by being anchored to the phosphine groups on either side. The metal then inserts into this C-C bond to give an aryl-Rh-CH3 intermediate. Because the rhodium also has a hydrogen on it, it can eliminate methane to give the observed aryl-Rh complex. The irreversible removal of methane drives the C-C cleavage reaction to completion. Milstein says he has direct spectroscopic evidence for two of the proposed intermediates. In his paper, Milstein says it should be possible to find other systems in which "precoordination" of the metal atom enhances its insertion into a C-C bond. But, he tells C&EN, such precoordination may not be required since, under hydrogen, C-C bond cleavage can compete favorably with C-H bond cleavage (the more common reaction). And although his paper focuses on intramolecular activation of a C-C bond, intermolecular activation should also be possible.
Tethered rhodium inserts into carbon-carbon bond C-H bond activation
C-C bond activation
Note: Structures in shaded area are part of proposed reaction mechanism, not identified intermediates
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NEWS OF THE WEEK Milstein's results suggest that C-C bonds in free alkylated aromatic hydrocarbons might also be vulnerable to attack, Jones says. That would be a boon not only to petroleum processing, but also to the effort to convert coal into liquid fuels. C-C bond activation, he thinks, could be the next frontier in hydrocarbon chemistry. Ron Dagani
Panel says no health risksfromfluoride A National Research Council (NRC) report released last week concludes that currently allowed levels of fluoride in drinking water do not cause cancer, bone disease, or kidney failure. After reviewing available data on fluoride toxicity, an NRC panel decided that the maximum level of fluoride in drinking water allowed by the Environmental Protection Agency—4 ppm or 4 mg per L—is "appropriate as an interim standard." EPA requested the study as part of its reevaluation of the fluoride standard in drinking water. Panel chairman Bernard M. Wagner, research professor of pathology at New York University's School of Medicine, says his group could find only one type of adverse effect from fluoride in drinking water: an increase in staining or pitting of tooth enamel called dental fluorosis. Currently, about 10% of the population has dental fluorosis. "More research is needed on patterns of fluoride exposure . . . as well as fluoride's effects on bone strength and tooth enamel," says Wagner. "The current standard should then be reviewed and changed, if necessary, based on the results of that research." Some data indicate a possible link between fluoridated drinking water and the risk of hip or other bone fractures. Today about 132 million Americans drink water with fluoride concentrations higher than 0.7 ppm. To prevent tooth decay, communities that add fluoride to their water supplies adjust the level to 0.7 to 1.2 ppm, depending on the average local temperature. Hundreds of U.S. communities have naturally fluoridated water with levels 0.7 ppm or higher; in some, the levels are as high as 4 ppm. The report says, "The weight of the evidence from more than 50 epidemiological studies does not support the hypothesis of 6
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an association between fluoride exposure and increased cancer risk in humans." In addition, fluoride at recommended levels will not cause kidney problems in humans, nor is it likely to have adverse effects on human reproduction. Fluoride "has produced mutations and chromosomal damage in several in-vitro tests with mammalian cells," but at levels that are at least 100 times higher than those found in human plasma. The panel notes that "the most severe forms of dental fluorosis might be more than a cosmetic defect if enough fluorotic enamel is lost to cause pain" or "compromise chewing efficiency." It adds that 10 to 30% of children who drink water with EPA's ceiling level of fluoride (4 ppm) develop moderate or severe dental fluorosis, and at about 1-ppm fluoride, close to 1% develop such fluorosis. The panel cites four ecological studies that make geographical comparisons
and find a weak positive association between hip fracture incidence and fluoridated water. However, the panel regards these studies as inconclusive. Robert Carton, a former EPA scientist and former president of the agency's professional union, says: 'If s obvious from the research they have reported on bone fractures and dental fluorosis that the fluoride standard should be lowered dramatically." In 1986, the professional union at EPA attempted to join the Natural Resources Defense Council in a lawsuit opposing EPA's fluoride standard. John R. Lee, physician and vice president of the Center for Health Action, an association of antifluoridation groups based in Springfield, Mass., echoes Carton's sentiments: "The NRC report shows me that even scientists with good credentials seem to be operating on an agenda that is not pure science." Bet te Hileman
Rhone-Poulenc explosion injures three seriously Three workers were gravely injured last Wednesday in an explosion and fire at Rhone-Poulenc's plant in Institute, W. Va., when a reactor cooler loop erupted in a thiodicarb insecticide production unit. Thousands of area residents were forced indoors for two hours as sirens and a computerized call-out system notified residents of the danger from the thick black smoke that engulfed the plant and adjacent neighborhoods. The next day, Occupational Safety & Health Administration (OSHA) and plant officials began investigating what triggered the explosion and ignited the fire, but at press time neither would speculate on the cause. An OSHA spokesman says it will take up to six months for a report on the mishap. A Rhone-Poulenc spokesman tells C&EN that operators were shutting down the insecticide unit to clear a blockage when an explosion ripped through a reactor cooler loop that contained chloroacetaldoxime (CAO), hydrochloric acid (HCl), and water. The explosion in the loop severed adjacent transfer lines containing methyl isobutyl ketone (MIBK) and methyl disulfide (MDS). CAO, HCl, MIBK and MDS can cause severe skin and respiratory irritation. In addition, CAO, MIBK, and MDS are flammable. Rhone-Poulenc's emergency squad put out the fire one hour and 45 minutes after it started, without needing to call in
help from local fire departments, which stood on alert outside the plant. The Kanawha County Sheriffs Office says two Rhone-Poulenc maintenance workers and one contract employee from Chempower, who were working near the explosion site, arrived at the hospital in serious condition with burns and difficulty in breathing. Ambulances responded to an additional 21 calls from area residents. And, police shut down traffic on state Highway 25 in front of the 460-acre plant, and on U.S. Highway 60 across the Kanawha River as smoke drifted over. Pam Nixon, a member of the Local Emergency Planning Committee for the Kanawha Valley, says the area was well prepared for the emergency. Sirens—and emergency broadcasts that interrupted transmission on area cable TV service— warned residents near the plant to remain indoors. One-way radio alerts effectively reached two facilities immediately outside plant gates: a rehabilitation center and West Virginia State College, where freshman orientation was under way. A new automatic telephone dialing system meant to alert area residents was not entirely effective, because too many people picked up their phones to call for information. However, the system worked more effectively to inform people that the indoor alert was lifted, because by then residents were calmer, says Nixon. Marc Reisch
