4085isomer, (-)-(2S,8&)-2 (entries 7 and 13). Asymmetric enolate oxidation of 2-methyltetralone (6) with (+)-(2R,8aS)-1 gave much lower stereoselectivities (entries [15][16][17][18][19]. Changing the base or solvent failed to increase the asymmetric induction. Asymmetric oxidations using (-)-(S,S)-3 also reduced the asymmetric induction for 4a,b (entries 8 and 14) but nearly doubled it from 16% ee to 23.5% ee for ketone 6 (compare entries 17 with 20).The results summarized in Table I are best understood in terms of an open transition state, where nonbonded steric interactions are principally responsible for the chiral recognition.10 Dreiding models and an X-ray crystal structure of the conformationally locked oxaziridine (+)-(2R,8aS)-1, suggest that in the vicinity of the active site oxygen the most sterically demanding region is the bridgehead 5-4a bond.12 We consider, by analogy with recent studies by Heathcock and co-workers,13 that the largest enolate group is the "OM" (M = Li, Na) solvent aggregate complex. The two extreme geometries for the oxidation of the si-faces of the metal (2)-enolates of 4a,b by (+)-(2R,8aS)-l are depicted in Scheme I. From a consideration of the nonbonded interactions, the planar transition-state geometry is favored over the spiro form.14 The lower enantioselectivities noted for 2-methyltetralone (6) are consistent with this hypothesis because all rotational conformations of this enolate are sterically demanding. It is interesting to note that planar transitionstate geometry is also preferred for asymmetric epoxidations of alkenes by chiral oxaziridineslg and certain conformationally restricted peracids.lg If our assumptions concerning the factors that control the transition state geometries are correct, then it follows that "NaO" can also be considered as a large group. The higher enantioselectivities associated with sodium vs. lithium enolates of 4a,b probably reflect the lower temperature of oxidation for the former counterion (Table I).2o We speculate that the generally lower stereoselectivities seen in the presence of HMPA (entries 2,4,10,12, 16) and for the potassium enolates (entries 5, 13, 18) are the result of a smaller effective size for "OM" group. K+ is a poorer chelating metal than either Li+ or Na+, and HMPA is (10) No change in the 'H or 13C NMR spectra of oxaziridine (+)-(2R,BaS)-A and the shift reagent Pr(fod), at 1:l molar ratios could be detected. (11) Davis, F. A.; Towson, J. T., manuscript in preparation. (12) Details of the X-ray structure of (+)-(2R,BaS)-l will be published elsewhere." (13) (a) Heathcock, C. H.; Henderson, M. A.; Oare, D. A.; Sanner, M. A. J. Org. Chem. 1985,50,3019. (b) Heathcock, C. H.; Oare, D. A. J. Org. Chem. 1985,50, 3022. (14) In analogy with other studies,lB the enolate and oxaziridine are considered to approach in a perpendicular fashion. Recent ab initio calculations by Houk and Paddon-Row suggest that the transition state for reaction of MeF with the acetaldehyde enolate is product-like; Le., MeF approaches the enolate at an angle o...

640 1 unilateral electron flow should result, establishing one bond between the reactants, Le., generation of a zwitterionic intermedi~te.~ Sulfur and carbon possess the same electronegativity on the Pauling scale. Absence of the more electronegative 0 or N atoms brings thiocarbonyl ylides6 close to the high M O energies of the allyl anion, electronic prototype of 1,3-dipoles. An aliphatic thiocarbonyl ylide and an ethylene derivative bearing four electron-attracting substituents offer a reactant pair with extremely different M O energies.

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Dedicated to Siegfried Hünig on the occasion of his 80th birthday Thiobenzophenone and diazomethane afford thiadiazoline 1 at À 788. By elimination of N 2 from 1 at À 458 (t 1/2 ca. 1 h), (diphenylmethylidenesulfonio)methanide (2), which cannot be isolated but is interceptible by dipolarophiles, is set free. The nucleophilic 1,3-dipole 2 undergoes cycloadditions with electrophilic C,C multiple bonds; the structures of 16 cycloadducts were elucidated. One-step and two-step cycloaddition pathways are discussed in the light of the steric course observed for (E)/(Z)-isomeric ethylene derivatives. Competition experiments with pairs of dipolarophiles at À 458 and HPLC analysis of the adducts provided relative rate constants of 26 dipolarophiles, involving 2 CC, 13 CC, 9 CS, and 2 NN bonds. In accordance with Sustmanns reactivity model of concerted cycloadditions, 2 shows the highest selectivity of all known 1,3-dipoles, i.e., the largest spread of rate constants (k rel 1 for methyl propiolate and 33 Â 10 6 for TCNE). As a consequence of low LU energies, thiones are very active dipolarophiles, and fluorene-9-thione (k rel 79 Â 10 6 ) stands at the top.1. Introduction. ± Thiobenzophenone (3) and diazomethane furnish 4,4,5,5-tetraphenyl-1,3-dithiolane (4) as a 2 : 1 product at 08, as observed by two groups in 1930 and 1931 [2] [3]. Schönberg and his school extended the reaction to many pairs of thiones and diazoalkanes, but the mechanistic pathway remained unclear [4]. The key to the mechanism was offered by the observation that 3 and CH 2 N 2 react by a 1 : 1 stoichiometry at À 788 to produce 2,5-dihydro-2,2-diphenyl-1,3,4-thiadiazole (1) [5] [6]. The latter loses N 2 at À 458 with a half-life of 56 min in THF and affords (diphenylmethylidenesulfonio)methanide (2) in a 1,3-dipolar cycloreversion. The methanide 2 is not isolable, but is easily intercepted in situ by suitable dipolarophiles; with a second equiv. of 3 in that role, the 1,3-dithiolane 4 is formed in 95% yield (Scheme 1).The first-order rate constant k 1 for the conversion 1 3 2 increases faster with rising temperature than k 2 , which refers to the cycloaddition of CH 2 N 2 with 3. At À 788, k 2 b k 1 is found, but the ratio switches to k 2 ( k 1 at 08, i.e., the initial cycloaddition becomes rate-determining, and thiadiazoline 1 assumes the role of a transient intermediate. Thus, the Schönberg reaction consists of two 1,3-dipolar cycloadditions linked by a 1,3-dipolar cycloreversion (Scheme 1) [6].

Abstract