The radical-chain addition of benzenethiol to the monoalkylallenes la-e and the 1,1-dialkylallenes 6 and 8 has been studied. Attack by the benzenethiyl radical occurs at C2 and C3 of lb-e in a ratio of 83:17. Increased attack (25%) at C3 occurs with tert-butylallene (la). The 1,1-dialkylallenes undergo attack only at C2. Deuterium-labeling studies indicate that the thiyl radical addition step is not detectably reversible under the reaction conditions. The internal alkene adducts 2,3,9, and 10 are the kinetically favored products. Rapid and reversible addition of the thiyl radical to the adducts results in complete stereochemical equilibration during the course of the reaction. The hydrogen atom abstraction step is reversible under the reaction conditions with la and 8 but not with 6 or the other monoalkylallenes. Treatment of the adducts derived from la and 8 with a catalytic

Second-year organic chemistry students analyze and compare the spectral data of three unknown ketones (2-heptanone, 3-heptanone, and 4-heptanone). Students acquire one-dimensional proton and carbon NMR data for each unknown and make signal assignments based on chemical shifts, integration values, and splitting patterns of signals. Difficulties are encountered during interpretation of NMR data for 2- and 3-heptanone. Acquiring and analyzing the two-dimensional NMR spectra of the heptanones resolve these problems.

The stereoselectivities, chemoselectivities, relative reactivities, and kinetic isotope effects have been determined in the cycloaddition reactions of substituted alienes with iV-phenylmaleimide. The comparison of these results with those derived from the studies of the cycloaddition of 1,1 -dichloro-2,2-difluoroethene and the radical-chain addition of benzenethiol to alienes strongly indicates that the cycloadditions with TV-phenylmaleimide occur via a two-step, diradical-intermediate process. The stereochemical features controlling the formation of the stereoisomeric diradical intermediates and their ring closures are discussed. In addition to the cycloaddition processes, competitive ene reactions occur to produce intermediate dienes, which react further to produce 1:2 adducts or nonreactive alkyne-containing 1:1 adducts. These ene reactions also appear to proceed via diradical intermediates.Considerable attention has been devoted to the study of cycloaddition reactions of alienes. Stereochemical studies have revealed that the stereochemistry about both the alkene and aliene portions is retained. The cycloaddition of 1,1-dimethylallene with dimethyl fumarate has been reported to produce two adducts in which >99% stereoselectivity is retained,1

The di-O-methylated cyclodextrin was first described by J. Staerk and H. Schlenk. These authors suggested that the unsubstituted hydroxyls are those at C-2 (J. Staerk and H. Schlenk, 149th National Melting of the American Chemical Society, Detroit, Spring, 1965, Abstract 11c). This assignment has since been shown to be incorrect. The methylation occurs at C-2 and C-6, leaving the C-3 hydroxyls unsubstituted.

The relative reactivities of several monoalkyl-and 1,1-dialkylallenes, one trialkylallene, and one tetraalkylallene toward the addition of phenylthiyl radical have been measured. Isotope effects have been measured for the addition of the phenylthiyl radical to 1,1-dimethylallene and its 3,3-dideuterio and l,l-bis(trideuteriomethyl) analogues and to ethylallene and its 3,3-dideuterio analogue. The results are interpreted in terms of a very early transition state in which very little rotation about one end of the aliene system has occurred. The relative reactivities and product distributions are determined by frontier molecular orbital interactions between the singly occupied MO of the phenylthiyl radical and the occupied w MO's of the aliene.