LIVING IN SYN In conventional alkene anti-dichlorination (top), Cl+ and Cl− attack opposite faces of the carbon-carbon double bond. In the new syn reaction (bottom), two Cl− ions attack the same face, giving a stereoisomeric product. Conventional approach: Cl2
Cl2
ALTERNATIVE ROUTE FOR ADDITION
R1
R2
R1
R1 Cl–
Cl
Cl+ R2
R2
R1 Cl
New syn reaction: Cl Cl
Cl Cl
CHEMICAL SYNTHESIS: New catalyst
expands stereochemical repertoire of alkene dichlorination
R2
PhSeCl3
R1
R2
Ph
Ph +
Se
Se
R1
R2 Cl–
R1 Cl–
Cl
PhSeCl
R2 Cl
R2
R1 Cl
R1, R2 = organic groups, Ph = phenyl
N A DEVELOPMENT that could revise organic chem-
I
istry textbooks, a new catalytic version of alkene dichlorination makes the reaction more versatile. Adding Cl2 to double-bonded carbons to give saturated dichlorinated products is a fundamental reaction taught early in the organic chemistry curriculum. It mostly proceeds in just one way: via an “anti” mechanism, in which Cl+ and Cl− ions attack opposite faces of the double bond. Now, researchers have devised the first catalytic alkene dichlorination that proceeds by the alternative “syn” route, in which two Cl− ions attack the same face of the double bond. This approach provides a direct route to stereoisomers of anti-dichlorination products. Chemists have reported alkene syn-dichlorinations before, using antimony and molybdenum chloride reagents, but these reactions were not catalytic and their applicability was severely restricted, primarily to nonsubstituted alkene substrates. The only other way to get alkene syn-dichlorination products has been to use multiple steps. Alexander J. Cresswell, Stanley T.-C. Eey, and synthetic organic chemist Scott E. Denmark at the University of Illinois, Urbana-Champaign, have now designed a selenium(IV) reagent that catalyzes alkene syn-dichlorinations in one step (Nat. Chem. 2015, DOI: 10.1038/nchem.2141). They report 27 examples of cyclic and acyclic dichlorinated products synthesized
using the strategy, including dichlorocyclohexane. In the reaction, Se4+ gets reduced to Se2+. According to synthetic chemist Ross Denton of the University of Nottingham, in England, the key to making the process catalytic was “identifying a suitable oxidant that would convert Se2+ back to Se4+ in the presence of the alkene and not interfere with the dichlorination process itself.” Denmark’s group did that, Denton says. In a classic anti-dichlorination, Cl+ forms a cyclic intermediate on one side of the alkene. The ring is opened when Cl− attacks the other side. In the synreaction, the Se4+ reagent forms a cyclic intermediate on one side of the alkene that is opened by Cl− on the opposite side. The Se4+ reagent then gets displaced by a second Cl− attacking on the same side as the first. “The magic comes about from a deep-seated appreciation of reaction mechanism that follows from analytical thinking about the individual steps that constitute the process,” says synthetic organic chemist Erick M. Carreira of ETH Zurich. “The simplicity and availability of catalyst and reagents ensure that the method—and more generally, the concepts—will be widely adopted.” Synthetic organic chemist Takehiko Yoshimitsu of Osaka University says the reaction “is a major breakthrough that could provide easier access to polychlorinated compounds, such as chlorosulfolipid natural products.”—STU BORMAN
CLIMATE CHANGE EPA seeks massive methane reductions from oil and gas sector The White House unveiled plans last week to reduce emissions of methane from the oil and gas industry by 40–45% through new EPA regulations. Methane emissions from this sector are projected to rise 25% by 2025, constraining White House international climate change policy objectives. EPA warns that increased emissions will be damaging because methane is a greenhouse gas 20 times as potent as carbon dioxide. The 40–45% reduction is targeted for 2025.
Methane accounted for 10% of greenhouse gas emissions in 2012, 30% of which came from the oil and gas sector. Under the Clean Air Act, rules for “new sources” of pollution would trigger regulation on existing sources, which underpins EPA’s proposals to reduce carbon emissions from coal-fired power plants. Environmental advocates say the plan is a positive step that aligns with White House actions on the electricity sector and vehicle fuel standards. Industry representatives, meanwhile,
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say the rules are onerous. Institute for Energy Research’s Thomas Pyle says it’s “yet another crushing regulation aimed at driving energy prices right back up again.” EPA is encouraging industry to pursue voluntary approaches and verify reductions, saying it could “reduce the need for future regulations.” Separately, the Obama Administration announced $25 million in new Department of Energy technology funding to detect methane loss and quantify emissions.—STEVEN K. GIBB
