Photochirogenesis, the control of chirality in photoreactions, is one of the most challenging problems in stereocontrolled photochemistry, in which the stereodifferentiation has to be imprinted within the short lifetime of the electronically excited state. Singlet oxygen (1O2), an electronically excited molecule that is known to be sensitive to vibrational deactivation, has been selected as a model case for testing stereoselective control by vibrational deactivation. The stereoselectivity in the reaction of 1O2 with E/Z enecarbamates 1, equipped with the oxazolidinone chiral auxiliary, has been examined for the mode selectivity ([2 + 2]-cycloaddition versus ene-reaction) and the stereoselectivity in the oxidative cleavage of the alkenyl functionality to the methyldesoxybenzoin (MDB) product. Through the appropriate choice of substituents in the enecarbamate, the mode selectivity (ene versus [2 + 2]), which depends on the alkene geometry (E or Z), the steric bulk of the oxazolidinone substituent at the C-4 position, and the C-3' configuration on the side chain, may be manipulated. Phenethyl substitution gives exclusively the [2 + 2]-cycloaddition product, irrespective of the alkene geometry. The stereoselection in the resulting methyldesoxybenzoin (MDB) product is examined in a variety of solvents as a function of temperature by using chiral GC analysis. The extent (% ee) as well as the sense (R versus S) of the stereoselectivity in the MDB formation for the E isomer depends significantly on solvent and temperature, whereas the corresponding Z isomers are not affected by such variations. The complex temperature and solvent effects are scrutinized in terms of the differential activation parameters (DeltaDeltaS++, DeltaDeltaH++) for the photooxygenation of E/Z-enecarbamates in various solvents at different temperatures. The enthalpy-entropy compensations provide a mechanistic understanding of the temperature dependence of the ee values for the MDB product and the difference in the behavior between the Z and E enecarbamates. The E enecarbamates show a relatively high contribution from the entropy term and an appreciable contribution from the enthalpy term; both terms possess the same sign. In contrast, the corresponding relative insensitivity of Z enecarbamates to temperature and solvent variation is convincingly explained by the near-zero DeltaDelta S++ and DeltaDelta H++. Such effects, associated with temperature- and solvent-dependent conformational factors, are most likely dictated by the stereogenic center at the C-3' phenethyl substituent. The high stereocontrol during the photooxygenation of the chiral enecarbamates is shown to be independent of the steric demand of the oxazolidinone substituent at the C-4 position. In view of the reduced stereocontrol on deuteration of the oxazolidinone substituent at the C-4 position, we propose that the unusual stereoselective vibrational quenching of the attacking singlet oxygen (excited-state reactivity), a novel mechanistic concept, works in concert with the usual steric...

Chiral auxiliaries have been successfully employed in controlling the diastereoselectivity of numerous reactions. 1 A notoriously difficult problem is the manipulation of the stereochemical course of singlet oxygen (1 O 2), the smallest possible cyclophile. In the past few years, we have shown that through the appropriate choice of the chiral inductor it is possible to control the stereoselectivity of the [4 + 2] cycloaddition of 1 O 2. 2 For this purpose, optically active 2,2-dimethyloxazolidines proved to be highly effective chiral auxiliaries when attached to sorbic acid in the form of an amide linkage. Recently we have also discovered the diastereoselective ene reaction of 1 O 2 with optically active oxazolidines, which have been equipped with an urea functionality. 3 The diastereoselective control was achieved in this ene reaction through remote hydrogen bonding between the urea NH bond and 1 O 2 , whereas steric factors were held responsible in the [4 + 2] cycloaddition. Similar efficacious hydrogen-bonding effects have been documented 4 for the [2 + 2] cycloadditions of 1 O 2 with chiral allylic alcohols. To date, no cases appear to be known in which steric interactions efficiently guide the π-facial attack of 1 O 2 in the dioxetane formation; however, the stereoselective dioxetane formation in the ozonolysis of vinylsilanes has been reported. 5 Herein we report the first example of a chiral-auxiliary-induced [2 + 2] cycloaddition between 1 O 2 and oxazolidinone-functionalized enecarbamates, which proceeds with complete diastereoselectivity as a result of steric repulsions. Evans' chiral auxiliary 6 was introduced into the enecarbamate substrate 2 by condensing the 4-alkyl-substituted oxazolidinones with the aldehyde 1, which was prepared in three steps from methyl phenylacetate (Scheme 1). The 1-phenylethyl substituent at the C3 position of the double bond was chosen to minimize the ene reaction since the required coplanar alignment of the only allylic hydrogen atom is encumbered. 7 The optically active enecarbamates 2 were photooxygenated at-35 °C with 5,10,15,20-tetrakis(pentafluo-rophenyl)porphine (TPFPP) as sensitizer and a 800-W sodium lamp as light source. The dioxetanes 3 (Table 1) were obtained exclusively, but they readily decomposed at room temperature (ca. 20 °C) to the expected carbonyl products because of their thermally labile nature. The [2 + 2] cycloaddition between the unsubstituted enecar-bamate 2a and 1 O 2 displayed no diastereoselectivity (Table 1, entry 1), whereas the methyl derivative 2b(3S) (Table 1, entry 2) and the isopropyl derivative 2c (entries 3 and 4) afforded the diaste-reomerically pure dioxetanes [1S,2S]-3c(3R) and [1S,2S]-3c(3S). Within the experimental error of 5%, the other possible diastereomer could not be observed in the 600-MHz 1 H NMR spectra of the photooxygenate. When the configuration of the oxazolidinone stereogenic center was changed from R to S, the inverse configuration at the new stereogenic centers was obtained (entries 4 and 5). Also, in the polar solv...

The direct molybdenum-catalyzed sulfuration of a variety of isonitriles with elemental sulfur or propene sulfide as sulfur donors affords the corresponding isothiocyanates in good yields and under mild reaction conditions. A catalytic cycle is suggested, in which the molybdenum oxo disulfur complex operates as the active sulfur-transferring species. A novel adduct between the isonitrile and the molybdenum complex has been characterized by X-ray analysis and its association constant determined by UV-vis spectroscopy, but this adduct appears not to be involved in the sulfur-transfer process.

The stereochemical course of the oxidation of chiral oxazolidinone-substituted enecarbamates has been studied for singlet oxygen ((1)O(2)), dimethyldioxirane (DMD), and m-chloroperbenzoic acid (mCPBA) by examining of the special structural and stereoelectronic features of the enecarbamates. Valuable mechanistic insight into these selective oxidations is gained. Whereas the R(1) substituent on the chiral auxiliary is responsible for the steric shielding of the double bond and determines the sense of the pi-facial diastereoselectivity, structural characteristic such as the Z/E configuration and the nature of the R(2) group on the double bond are responsible for the extent of the diastereoselectivity. Stereoelectronic steering by the vinylic nitrogen functionality controls the mode selectivity (ene reaction vs [2+2] cycloaddition) in the case of (1)O(2).