Burgess reagent

Abstract

5224 the pH range 6 < pH < 9, the difference in the water nO line widths of the two solutions is ~4 Hz, indicating that there is, at most, only a weak second coordination sphere interaction between H20 and Ni-EDTA. Below pH 6 there is little change (<10%) in the width of the 170 resonance of the Mg-EDTA solution but the 170 line width of the Ni-EDTA solution increases to 350 Hz at pH 2. If the pH dependence of the 170 relaxation is treated in terms of the formation of lb, then a pK value of 3.05 is obtained for reaction l.16 Although the ,3C shift data are not inconsistent with structure lc, the 170 relaxation rates in the pH range 6-9 rule out a strong first coordination sphere interaction between Ni(II) and H20. The protonation of la is also accompanied by changes in the electronic absorption spectrum. Bhat and Krish-namurthy7 used dilute solutions (|Ni2+] = 1CF3-10~2 M and ionic strength µ = 1) and obtained pK values of 2.73 and 3.12 for the protonation of Ni-EDTA from changes in the electronic spectrum at 980 and 380 nm, respectively. In our analysis of the pH dependence of the electronic spectra of 0.2 (µ ^ 1) and 0.5 (µ = 2.5) Ni-EDTA solutions, we obtain pK = 3.1 ± 0.05 for both solutions at 985 and 785 nm. In contrast to the solution behavior of the Ni-EDTA complex, 13C and 170 nmr studies in progress indicate that the FeHI-EDTA complex is seven-coordinate in solution (has structure lc) and that, if the Con-EDTA complex has structure la, it is stereochemically nonrigid. Acknowledgments. We are indebted to Dr. B. B. Mclnteer and Mr. R. M. Potter of the Los Alamos Scientific Laboratory (LASL) for supplying the enriched 1SC and to Dr. D. G. Ott and Mr. Arthur Mur-ray, also of LASL, for preparing the 13C-enriched EDTA. (15) No 170 contact shifts were observed for these solutions at 32° in the pH range 2-6, indicating that the rate of 17OH: exchange from the Ni(H) coordination sphere of lb is much smaller than the 170 shift. At temperatures above 90°, a large nO shift is observable, consistent with the entry of the nO relaxation rate into the region of rapid H2O exchange. Similar observations have been made for the relaxation and chemical shift of 17OH2 by the (> 4 ion.16 (16) T. We have found a synthetically useful and facile method for the mild dehydration of secondary and tertiary alcohols to the corresponding olefins which employs an alcohol-derived new leaving group, tri-alkylammonium or sodium A-carboalkoxysulfamates (la,b). Despite the charge initially associated with this leaving group, such derivatives appear to ionize at low temperatures in nonpolar solvents to provide tight ion pairs which undergo fast stereospecific proton transfer to give high yields of olefins and the corresponding salt of ZV-carboalkoxysulfamic acid (2). Similar studies with primary alcohols indicate that thermolysis of such derivatives provides urethanes in high yield by an Sn2 (or Sní) pathway.1 The triethylammonium TV-carbomethoxysulfamates employed in this investigation were prepared by the interaction of the alcohol and the readily available methyl(carboxysulfamoyl)triethylammonium hydroxide inner salt (3) neat or in hydrocarbon solution at 30° or below. The electrophilic species responsible for the observed exothermic reaction even in the case of hindered tertiary alcohols may be methyl A-sulfonyl-carbamate (4).2 The tert-alkyl sulfamate esters and their salts are sufficiently liable at room temperature as to preclude isolation and characterization. In many cases it was found to be operationally convenient to exchange the triethylammonium for a sodium cation in the more stable primary or secondary sulfamate ester salts and this could be readily accomplished by treatment of la with sodium hydride in THF at 30°. Table I summarizes the results of our product investigation on the thermal decomposition of these sec-and terl-N-carbo-methoxysulfamate salts. A kinetic study of the solvolytic elimination reaction of l^-diphenylethyl-A-carbomethoxysulfamate triethyl-ammonium salt in ethanol to give only Zrarcs-stilbene provided a first-order rate constant at 35° of 2.66 ± 0.03 X 10-6 sec-1 with AH = 21.7 keal/mol and AS =-3.3 eu.3 As a probe into the stereochemical requirements we examined the behavior of the corresponding erythro-and ?A/ieo-2-deuterio-l,2-diphenyl-ethyl-iV-carbomethoxysulfamate salts4 (5a,b) in ben-zene at 50°. The former provided only zrans-stilbene containing 97% deuterium while the latter gave only protio-Zrans-stilbene as determined by mass spectral analysis and this result remains invariant in substituting dimethylformamide as the solvent. A small ß-hydrogen isotope effect was kinetically observed with kH/kD = 1.05 ± 0.02 and 1.08 ± 0.03 for the erythro and threo isomers, respectively, in ethanol at 35°. These kinetic and stereochemical results are consistent with an initial rate-limiting formation of an ion pair followed by a fast cis ß proton transfer5 to the departing anion at a rate greater than (1) An elegant method for the conversion of alcohols that yields moderately stable carbonium ions to amines via the Sní rearrangement of N.iV-dialkylsulfamate esters has been published: E. H. White and C. A. Elliger, /. Amer. Chem. Soc., 87, 5261 (1965). (2) G. M. Atkins, Jr., and E. M. Burgess, ibid., 90, 4744 (1968), (3) Product development was followed spectrophotometrically at 295 nm and good first-order kinetics were displayed to 60% reaction. (4) The precursor deuteriated alcohols were prepared by the procedure of D. Y. Curtin and D. B. Kellom, J. Amer. Chem. Soc., 75, 6011 (1953).