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A Dynamic Nuclear Magnetic Resonance Study of the Mercury Keto-Enol Tautomerization in Bis( 1,1,1,2,2,3,3hep tafluoro-7,7-dime thyl-4,6-oc tanedion-5-yl) mercuryla Richard H. Fishlb
Contribution from the Western Regional Research Laboratory, Agricultural Research Seroice, U.S. Department of Agriculture, Berkeley, California 94710. Received February 6, 1974 Abstract: The reaction of 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione (HFOD) with mercuric acetate (4). The gave, in quantitative yield, bis(1,1,1,2,2,3,3-heptafluor0-7,7-dimethyl-4,6-octanedion-5-yl)mercury mercury atom in 4, which is initially bonded to both central carbon atoms of the two FOD nuclei, undergoes a unique and stereospecific intramolecular keto-enol tautomerization from a central carbon atom to a carbonyl oxygen in one of the FOD nuclei (4 e 5 ) in acetone-& as studied by dynamic nuclear magnetic resonance spectroscopy (dnmr). The tautomers 4 and 5 were assigned their respective structures on the basis of their J l 9 ~ geminal couplings, which was 257 Hz for 4 (C-Hg-C) and 387 Hz for 5 (C-Hg-0) at -75" in acetone-&. A total line-shape computer analysis from +40 to -60' gave the following thermodynamic parameters for the process 4 e 5 . An Arrhenius plot of log k us. l / T gave an activation energy, E,* = 9.90 * 0.4 kcal;"ol, and a frequency factor, log A = 9.82 + 1.6 sec-I, while from an Eyring plot of log kfT us. 1f T , we obtained AH* = 9.35 + 0.4 kcal/mol and AS* = -3.45 =t1.6 eu. The free energy of activation at 25 and -60" was calculated to be AG* = 10.35 i 0.4 and 10.09 i 0.4 kcal/mol, respectively. The factors affecting this interesting tautomerization will be discussed.
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ne of the most interesting findings that Musso and coworkers 2a--c discovered in their study of the structure of bis(dipivaloylmethy1)mercury (1) was that the
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lowed us to further study (dnmr) the mercury tautomerWe found, using 95 % ization (eq 1) in 95 % ethanol& ethanol-ds, nmr signals for 1 (-5Oz) and 2 (-5Oz~ but could not substantiate their assignments because of experimental problems with this solvent. Unfortunately, 95 % ethanol-d, masked the Ig9Hgsatellites of 1 and 2 and masked the signal of 3. Thus, the diagnostic J ~ o ~ Hcouplings3a ~-H which we utilized to identify bonding situations, such as HC-Hg-CH and HC-Hg-0, could not be ascertained in this solvent. In view of these tenuous results on the mercury tautomerization in compound 1 (eq I), we wish to present evidence for the first substantiated mercury keto-enol tautomerization occurring in bis(l,l,l,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedion-5-yl)mercury(4) as studied by dnmr in acetone-d6.
mercury atom undergoes a keto-enol tautomerization Results (eq 1) as studied in chloroform-d solution utilizing Mercury Keto-Enol Tautomerization (4 e 5). Since nuclear magnetic resonance spectroscopy (nmr). Howour results with compound 1 were tentative with regard ever, this keto-enol tautomerization only occurs to a to the question of mercury keto-enol tautomerization very small extent in 1 and was not unequivocally verified (see Experimental Section for details), we decided t o by these workers.2a-c Our recent work on the mechanistic aspects of P-diketone-mercury c o m p o ~ n d s ~prepare ~ ~ ~ compound 4 with the hope that it would be a better model system for studying this interesting prompted us t o reexamine this mercury tautomerization. tautomerization. In our present dynamic nuclear magnetic resonance We found that compound 4 in the solid state has the spectroscopy (dnmr) study, we found that, indeed, mercury atom bonded to the central carbon atoms of tautomers 1 (>98 %) and 3 (
