No abstract

The dielectric properties of Ices II, III, V, and VI have been measured up to 300 kc/sec over a range of temperatures and pressures. All except Ice II exhibited well-defined dielectric dispersion and so are orientationally disordered under the experimental conditions. The dispersion loci were slightly broader than Debye curves, which may reflect the presence of nonequivalent crystal sites. As for Ice I, the static dielectric constants correspond to values of about 3 for the Kirkwood orientational correlation factor. This suggests that these forms of ice are four coordinated, in agreement with infrared and (for Ice III) x-ray evidence at low temperatures. The relaxation rates are considerably faster than for Ice I, and the activation energies and entropies somewhat lower. The volumes of activation are all about 4.6 cm' mole-i. The relaxation mechanism appears to be similar to that in Ice I, i.e., relaxation occurs by diffusion of orientational defects.X-ray and infrared studies have indicated that Ice II is rotationally ordered near liquid-nitrogen temperature. The absence of orientational polarization in Ice II at temperatures as high as -30 0 shows it to be ordered throughout its region of stability.

The effect of pressure up to about 2 kbar on the dielectric properties of Ice I at − 23.4° has been measured. The static dielectric constant increases with increasing pressure by about the amount predicted by the Kirkwood theory. The increase in relaxation time yields a volume of activation for the relaxation process of 2.9±0.2 cm3 mole−1 which supports current views that dielectric relaxation occurs by rotation at orientational defects. The volume of activation given by the rapid rise of dc conductivity with increasing pressure is − 11±∼3 cm3 mole−1 which is consistent with the proton-transfer mechanism.

The hoil~ogeneous thermal decomposition of Ni(C0)r between 35' C and 80" C and over a pressure range of 15 111111 to 80 mm of Hg has been distinguished from the heteroge~leous reaction occurring on the freshly deposited ~liclcel surface. The activation energy of the homogeneous reaction was found to be 10

Activated vinylaziridines (N-benzoyl, N-p-toluenesulfonyl, N-t-butoxycarbonyl and N-benzyloxycarbonyl) have been prepared from N-substituted 1,4-aminoalcohols using Mitsunobu conditions. Activated vinylaziridines containing an electron withdrawing group are versatile synthetic building blocks, because the N-substituent facilitates regiospecific aziridine opening by a nucleophile. 1,2 2-Vinylaziridines are particularly useful intermediates for organic synthesis and thus have been used in the synthesis of various natural products. 3 Hudlicky has reviewed the chemistry of vinyl aziridines, 4 and used a vinyl N-ptoluenesulfonyl (Ts) aziridine in the first asymmetric synthesis of the antitumor alkaloid pancratistatin. 5 Unfortunately, the lowest yielding step in the synthesis was the aziridination of enantiomerically pure 3-bromo-1,2-isopropylidenedioxycyclohexa-3,5-diene. 2 A widely employed activating group is the p-toluenesulfonyl group because of the accessible methods for the preparation of N-Ts aziridines. 6 However, removal of this protecting group frequently requires harsh conditions. 7 Synthetic efforts have been directed toward the development of direct methods of preparing aziridines containing other activating groups. 8There is a plethora of methods for the preparation of aziridines, and they have been recently reviewed by Sweeney 9a and Tanner. 9b An obvious method utilizes 1,2-aminoalcohols as precursors. 10 The reaction is readily achieved when the alcohol is converted to a nucleofuge. However, this method is limited to N-phosphonylated 11 and N-Ts aziridines; 6 when applied to the synthesis of N-acyl or N-carbamoyl aziridines, the nucleophilic attack by the carbonyl oxygen leads to the formation of oxazolines. 12 Conceptually, vinylaziridines should also be easily accessed from 1,4-aminoalcohols as shown in Scheme 1. Scheme 1We present in this communication a new method for the preparation of 2-vinylaziridines from 1,4-aminoalcohols. This method allows the preparation of functionalized aziridines while avoiding protectiondeprotection steps and permits the direct preparation of aziridines containing N-Ts, N-acyl and N-carbamoyl activating groups. Enantiomerically pure aziridines are prepared by enantiospecific synthesis from enantiomerically pure dienes.1,4-Aminoalcohols are prepared easily in a high yielding two-step sequence starting from a conjugated diene. Nitroso dienophiles are then generated in situ as transient intermediates by oxidation of the corresponding hydroxamic acid with periodate (NaIO 4 , MeOH-H 2 O). 13 Nitroso dienophiles generated in the presence of a conjugated double bond furnish 3,6-dihydro-1,2-oxazines. 14 This hetero Diels-Alder cycloaddition reaction occurs regio-and stereospecifically with chiral dienes. 15 The cis-1,4-aminoalcohols are obtained by reductive cleavage of the nitrogen-oxygen bond of the corresponding 1,2-oxazines using either Keck's aluminium amalgam procedure 16 or refluxing the oxazine in acetonitrile in the presence of molybdenum hexacarbonyl. 17 Cycli...