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Oxiranes and Oxirenes: Monocyclic 1.03 Oxiranes and Oxirenes: Monocyclic IHSAN ERDEN San Francisco State University, CA, USA 1.03.1 INTRODUCTION TO OXIRANES AND OXIRENES 98 1.03.2 OXIRANES: STRUCTURE AND PROPERTIES, INCLUDING SPECTRA 98 1.03.2.1 Molecular Geometry and Energetics 98 1.03.2.2 NMR Spectra 98 1.03.2.3 Mass Spectra 99 1.03.2.4 UV Spectra 99 1.03.2.5 IR Spectra 99 1.03.3 OXIRANES: REACTIVITY 100 1.03.3.1 Thermal Reactions 100 1.03.3.2 Photochemical Reactions 101 1.03.3.3 Electrophilic Ring Opening 101 1.03.3.4 Reactions with Carbonyl Compounds 105 1.03.3.5 Nucleophilic Attack on Ring Carbon 105 1.03.3.5.1 Introduction+ and mechanistic aspects 105 1.03.3.5.2 H - or Lewis acid-assisted ring opening 105 1.03.3.6 Reactions with Halogens 108 1.03.3.7 Ring Opening with Neutral or Basic Nucleophiles 108 1.03.3.7.1 Halides 108 1.03.3.7.2 N-, P-, O-, S-, and Se-based nucleophiles 109 1.03.3.7.3 Intramolecular nucleophilic attack 110 1.03.3.7.4 Organometallic reagents 111 1.03.3.7.5 Carbanions 114 1.03.3.7.6 Enzyme-catalyzed reactions 117 1.03.3.8 Free Radical Reactions 118 1.03.3.9 Base-catalyzed Isomerizations 119 1.03.3.10 Reductions 121 1.03.3.11 Deoxygenations 124 1.03.3.12 Cycloaddition Reactions 124 1.03.3.13 Palladium-mediated Reactions 126 1.03.4 OXIRANES: SYNTHESIS 127 1.03.4.1 General Survey of Synthesis 127 1.03.4.2 Oxiranes by Intramolecular Substitution 128 1.03.4.3 Oxiranesfrom Carbonyl Compounds with CH^equivalents (CH N , LiCH X, S, Se, and As Ylides) 129 2 2 2 1.03.4.4 Oxirane Synthesis from [2 +1] Fragments 130 1.03.4.4.1 Peroxy acid epoxidation 130 1.03.4.4.2 Oxaziridine epoxidations 131 1.03.4.4.3 Epoxidations with tertiary amines-oxides 131 1.03.4.5 Metal-mediated Epoxidations 132 1.03.4.5.1 t-Butylhydroperoxide (tbhp) epoxidations catalyzed by titanium tartrate systems (Sharpless epoxidation) 132 1.03.4.5.2 Metal-catalyzed epoxidations of alkenes 132 1.03.4.6 Epoxidations with Dioxiranes 134 1.03.4.7 Epoxidations with Molecular Oxygen 135 97 98 Oxiranes and Oxirenes: Monocyclic 1.03.4.8 Nucleophilic Epoxidations 135 1.03.4.9 Epoxidations with a-Azohydroperoxides 136 1.03.4.10 Enzyme-catalyzed Epoxidations 136 1.03.4.11 Miscellaneous Methods 137 1.03.5 ALLENE MONO- AND BISOXIRANES 138 1.03.6 OXIRANES: BIOLOGICAL ASPECTS, OCCURRENCE 140 1.03.6.1 Biological Aspects 140 1.03.6.2 Occurrence (Natural Products) 141 1.03.7 OXIRENES 142 1.03.7.1 Background and Theoretical Studies 142 1.03.7.2 Syn the tic Approaches to Oxirenes 142 1.03.7.3 Conclusions 144 1.03.1 INTRODUCTION TO OXIRANES AND OXIRENES Oxiranes are among the 1most intensely studied group of compounds. Owing to the considerable ring strain (~27 kcal mol" ), as well as the polarization of the C—O bonds in the three-membered ring system, oxiranes exhibit such varied modes of reactions that it is impossible to cover all of the work reported in literature in this area since 1982. The synthesis of oxiranes can be accomplished from a very large number of substrates, using a plethora of reagents and reagent systems by direct or indirect oxygenation methodologies. This area, in particular the field of enantioselective epoxidations, has burgeoned in the past decade to the extent that the discussion of every method here is beyond the scope of this chapter. The nomenclature of oxiranes is discussed in <B-79MI 103-01, 84CHEC-I(7)95>. The generic name of the three-membered oxygen heterocycle is oxirane, according to the Hantzch-Widman system <B-74MI 103-01, 83PAC409). A search of current literature in this area reveals that the names oxirane and 1,2-epoxide are used interchangeably, as well as additive nomenclature (e.g., ethylene oxide). The most systematic method of naming heterocyclic compounds, including oxiranes, is the replacement nomenclature <B-79MI 103-01), according to which an oxygen-containing three-membered ring is named oxacyclopropane. However, this nomenclature system is more frequently used for hetero- cycles containing unusual heteroatoms, as well as bridged and spiro systems. There have been a large number of excellent review articles on oxiranes published since 1982; these are mentioned in each of the sections below. 1.03.2 OXIRANES: STRUCTURE AND PROPERTIES, INCLUDING SPECTRA 1.03.2.1 Molecular Geometry and Energetics The microwave structure of oxirane has been determined by Hirose <74BCJl3ll). Molecular geometries of oxirane have been determined by ab initio calculations at various levels with remark- able agreement with the experimental values (Figure 11) <85JA3800, 89JA6957, 89JPC3025). The con- ventional ring strain energy of oxirane is 27.2 kcal mol"" <B-74MI 103-02). 0 r 147.0 pm <C-O-C 61.7° cc rr 143.4 pm <H-C-H 116.3 co H H CH 108.5 pm Figure 1 1.03.2.2 NMR Spectra Proton and carbon-13 NMR chemical shifts, geminal, and vicinal proton-proton coupling con- stants for oxirane and derivatives have been discussed in the first edition of Comprehensive Het- erocyclic Chemistry <84CHEC-I(7)95>. The stereochemical assignment of several epoxy alcohols has been achieved by a combination of H—H and C—H coupling constants and nuclear Overhauser effect (NOE) data <92JOC6025>. The NMR arguments have been supported by molecular modeling (MMX force field) and semiempirical quantum mechanical (AMI) calculations. A report in 1986 13 17 Oxiranes and Oxirenes: Monocyclic 99 on C NMR and O chemical shifts of a large number of mono- and disubstituted oxiranes has been used to determine the direc 13 t additivity parameters for calculating chemical shifts of oxiranes <86MRC15>. A comparison of C experimental shifts and calculated values for di- and trisubstituted oxirane17 s indicates good agreement in most cases. Discrepancies between experimenta 17 l and calculated O shift values fall in the range 0 + 14 ppm. A striking feature of the O NMR shift data is the possibilit17 y of distinguishing between different molecular configuration for isomeric compounds by O NMR. Oxygen-17 NMR data of 17 variously substituted oxirane 17s have been reported by the same authors <83OMR(2i)403>. Table 1 17depicts some characteristic O NMR shifts for selected oxiranes. An excellent discussion of O NMR spectroscopy of epoxides can be found in <B-91MI 103-01). Table 1 Oxygen-17 NMR shift values of some oxiranes. OOO O O /A ZA . ZA /A (1) (2) (3) (4) (5) 17 <5( O)(ppm) -49 -16 -18 -9.5 -8 13 The C NMR data of 42 ring-halogenated oxiranes containing F, Cl, Br (and I in one case) as substituents have been reported and discussed with respect to the influence of the halogen and other substituents on the chemical shifts of the ring carbons <85MRC524>. For monocyclic mono- and dichlorooxiranes, increments have been determine 13d which allow the calculation of the chemical shifts of the ring carbon atoms. Comparison of the C NMR data of substituted 1,2-dihaloethylenes (C(l)—C(2) ca. 111-158 ppm) with those of the corresponding oxiranes (C(l)—C(2) ca. 60-90 ppm) shows that the signals of the ring carbons of the halogenated oxiranes invariably appear at considerably higher field than the vinylic carbons in the alkene precursors. 1.03.2.3 Mass Spectra The mass spectra of oxiranes are discussed in (B-71MI 103-01 >. Ionized oxiranes undergo uni- molecular decomposition in the mass spectrometer; the fragment ions observed are in general due to rearrangements, transannular hydrogen transfer, and a- and /^-cleavage <(89MI 103-01). Mass spectroscopic studies on trans-chalcone epoxides reveal that the most stable fragments are formed by bond cleavage to the oxirane ring carbons (onium cleavage and aryl fragmentation) <89JPR37>. 1.03.2.4 UV Spectra Oxiranes do not have an absorption in the UV spectrum above 200 nm to be of diagnostic value for structural characterization. The A values of substituted oxiranes are surveyed in <64CHE17). Unsubstituted oxirane has an absorptiomaxn at 171 nm (gas phase, e-5600) <63PMH(2)i>. 1.03.2.5 IR Spectra The routine employment of high-resolution FT-IR spectroscopy in organic chemistry has allowed the assignment of IR signals of oxiranes to the corresponding vibrational modes with greater confidence. The controversy around the assignment o1f the B (asymmetric) ring deformation in oxirane has1 been resolved by high-resolution (0.04 cm" ) FT-IRx techniques <86MI 103-01 >. The peak at 897 cm" in the vapor-phase FT-IR spectrum of oxirane has now been assigned as the Q-branch of the expected type-A band, and results from the B ring deformation. The IR group frequencies complementing the existing data <B-75MI 103-01) havex been reported in an attempt to confirm and expand previous IR spectra-structure correlations of oxiranes <86MI 103-02). In1 all cases studied, the symmetric ring deformation has the highest IR band intensity (830-877 cm" ). The absorbance ratio for ring-breathing (1248-1268 cm" ^/symmetric ring deformation varies between 0.22 and 0.43, and the absorbance ratio for antisymmetric in-plane deformation (883-932 cm" ^/symmetric in-plane ring deformation varies between 0.009 and 0.96. The band intensity ratios are of diagnostic 100 Oxiranes and Oxirenes: Monocyclic value in specific spectra-structur1 e identification1 s of oxiranes.
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