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  5.06  hiiranes and hiirenes D C DITTMER  yracuse University 5.06.1  INTRODUCTION 132 5.06.2  STRUCTURE 132 5.06.2.1 Bond Lengths and Bond Angles  1325 06.2.2 Stereochemistry and NMR Spectra  1335 06.2.2.1 Stereochemistry  133 5.06.2.2.2 Chemical shifts and coupling constants  134 5.06.2.3 Mass Spectra and Photoelectron Spectra  1355 06.2.4 Electronic Spectra, Optical Rotatory Dispersion-Circular Dichroism  136 5.06.2.5 Infrared, Raman and Microwave Spectra  1385 06.2.6 Thermodynamic Properties  1385 06.2.7 Miscellaneous  Properties:  Dipole Moments; Magnetic Properties  139 5.06.3  REACTIVITY 1395 06.3.1 Introduction  139 5.06.3.2 Thermal and Photochemical Reactions  1405 06.3.2.1 Extrusion of sulfur, sulfur monoxide or sulfur dioxide  140 5.06.3.2.2 Rearrangement and isomerization  1425 06.3.2.3 Polymerization and oligomerization  144 5.06.3.3 Electrophilic Attack on Sulfur  145 5.06.3.3.1 Introduction  145 5.06.3.3.2 Protonation and basicity  145 5.06.3.3.3 Lewis acids  146 5.06.3.3.4 Alky I halides, oxonium salts and related compounds  147 5.06.3.3.5 Acyl halides and related compounds  147 5.06.3.3.6 Halogens  148 5.06.3.3.7 Electrophilic sulfur, nitrogen, phosphorus and arsenic  149 5.06.3.3.8 Per acids and other sources of electrophilic oxygen  150 5.06.3.3.9 Carbenes  151 5.06.3.4 Nucleophilic Attack on Sulfur  1515 06.3.4.1 Oxygen nucleophiles  151 5.06.3.4.2 Nitrogen nucleophiles  153 5.06.3.4.3 Sulfur nucleophiles  153 5.06.3.4.4 Phosphorus nucleophiles  1535 06.3.4.5 Halide ions  154 5.06.3.4.6 Carbanions  1545 06.3.4.7 ir-Systems  155 5.06.3.4.8 Hydride  155 5.06.3.4.9 Low-valent metals  156 5.06.3.5 Nucleophilic Attack on Carbon  1565 06.  j 5.1 Introduction  156 5.06.3.5.2 Oxygen nucleophiles  157 5.06.3.5.3 Nitrogen nucleophiles  158 5.06.3.5.4 Sulfur, selenium and phosphorus nucleophiles  160 5.06.3.5.5 Halide ions  161 5.06.3.5.6 Carbon nucleophiles  164 5.06.3.5.7 ir-Systems  165 5.06.3.5.8 Hydride and equivalents  165 5.06.3.5.9 Others  166 5.06.3.6 Nucleophilic Attack on Hydrogen {Proton Abstraction 166 5.06.3.7 Reactions with Radicals  166 5.06.3.7.1 Radical attack on sulfur  166 5.06.3.7.2 Radical reactions involving ring substituents  167 5.06.3.7.3 Electrochemical reactions  167 131  132  Thiiranes and Thiirenes 5.06.3.8 Reactions Involving Cyclic  Transition  States  167 5.06.3.8.1 Dipolar additions to  thiirene  1-oxides and 1,1-dioxides  167 5.06.3.9 Reactions of  Substituents  on  Thiirane  Rings  170 5.06.3.9.1 Reactions on carbon  1705. 06.3.9.2 Reactions on oxygen  1715.06.4 SYNTHESIS 171 5.06.4.1 From  Non heterocyclic  Sulfur  Compounds  1715. 06.4.1.1 By formation of a C—S bond 1115.06.4.1.2 By  formation  of a  C—  C bond  175 5.06.4.2  Formation  by Making Two Bonds  176 5.06.4.3  Formation  from Oxiranes  178 5.06.4.4  Formation  from  Four membered Heterocycles  179 5.06.4.5  Formation  from  Five membered Heterocycles  1795. 06.4.6  Formation  from  Six membered Heterocycles  1815. 06.4.7  Miscellaneous  Methods  1825.06.5 APPLICATIONS AND IMPORTANT COMPOUNDS 182 5.06.5.1 Naturally  Occurring  Thiiranes  1825. 06.5.2 Polymers  182 5.06.5.3 Drugs  1825. 06.5.4 Toxicity  1835. 06.5.5 Insecticides and  Herbicides  183 5.06.5.6 Miscellaneous  184 5.06.1 INTRODUCTION Three-membered rings containing one sulfur atom are named as thiiranes (1) or thiirenes (2),  ring numbering starting at the sulfur atom. In more complex systems such as (3), thesubstitution method of nomenclature is used; the position of the sulfur atom which replacesa carbon atom in the parent hydrocarbon is indicated by number and the prefix 'thia'. Thus(3) is 7-thiabicyclo[4.1.0]heptane. Other systems of nomenclature which have been usedare (1) name of alkene + sulfide; (2) name of alkene + episulfide; (3) episulfide of 'name ofalkene'; (4) epithioalkane with position of the functional group being given by numbers.By these systems compound (3) may be called cyclohexene sulfide, cyclohexene episulfide,the episulfide of cyclohexene or 1,2-epithiocyclohexane. The designation episulfide orepithio has been used for larger rings and their use may be ambiguous unless the ring sizeis made clear. 2-Methylthiirane is propylene sulfide or 1,2-epithiopropane. Thiirane  1-oxides and  1,1-dioxides  are sometimes called episulfoxides and episulfones, and the simplestexamples are often referred to as ethylene sulfoxide and ethylene sulfone. The thiirane,thiirene or substitution system of nomenclature is preferred, although the episulfide ter-minology is often useful in general discussions of complex systems. Thioethylene oxide isa poor name for thiirane. l s Z\ (1) (2) 5.06.2 STRUCTURE5.06.2.1 Bond Lengths and Bond Angles Bond lengths and bond angles for some thiiranes and thiirenes are given in Table 1. Thedata for other thiirane derivatives are similar although the C—C and C—S bond lengthsare as high as 1.60 and 1.92 A respectively in (4) (78CC555), presumably because of stericcrowding, and as low as 1.375 and 1.729 A respectively in 2-triphenylsilylthiirane becauseof thermal motion of the ring during X-ray analysis (79JOM(172)285>. The C-3 —S bonds ina-thiolactone (5) (77AG(E)722), thiiranimine (6) (80AG(E)276> and 2-methylenethiirane(1.849 A) (78JA7436) are also unusually long, no doubt because of the large CCS angleassociated with the sp 2 -hybridized carbon atoms. The exocyclic C—H bonds in thiiranesare comparable in length (1.08 A) to those in ethylene.  Thiiranes and Thiirenes  133 : NTs(5) (6) The increase in C—C bond length and decrease in C—S bond length noted in Table 1on going from thiirane to thiirane  1,1-dioxide  is explained by a bonding scheme whichconsiders these compounds as complexes of ethylene with sulfur or sulfur dioxide respec-tively (73JA7644). The dominant interaction is electron donation from the SO 2  fragment tothe antibonding 7r*-orbital of the ethylene fragment. This weakens the C—C bond andlengthens it. Participation of sulfur 3^-orbitals, which is more important in the sulfone,depletes bonding electron density from the ethylene fragment, further weakening it. TheC—S and, for the sulfoxide-sulfone, S—O bond shortening and strengthening also isexplained by invoking 3d-orbitals which when mixed with available 3s- and 3p-orbitalsprovide better overlap with atoms, carbon or oxygen, bound to sulfur. This model forbonding predicts that substitution of 7r-electron donors (e.g. OR, NR 2 , hal) on thiirane orthiirane  1,1-dioxide  will decrease the C—C bond length and that substitution of 7r-electronacceptors (e.g. CN, COR, NO2) will increase it, this being due principally to raising orlowering the energy of the antibonding 7r*-orbital.  Ab initio  SCF-MO calculations withincorporation of rf-orbitals give good agreement for the C  —C,  C  —S  and S —O bond lengthswith the experimentally observed values (75JA2025). Table 1  Bond Lengths and Angles for some Thiiranes and Thiirenes Compound SS: V s IX X S: / \Ph PhO O ^ã# S/ \—-\Ph Ph Me v  BF Bu' Bu 1 C-C 1.4841.5041.590f  1.288 <1.251 b U.27 1.3051.3541.277 Bond length  (A) C-S 1.8151.8221.731 1.790 1.978 1.81 1.784 1.716,  1.7031.820 S-O— 1.4831.439 —— 1.467 1.444,  1.453 1.802 c Bond angle  (°) C-S-C48.348.854.7 —— 42.946.741.1 o-s-o— 133.6 a 121.4 —— 126.0116.1 — Ref. 74MI5060076AX(B)217176AX(B)217176AX(B)217180JA250780PAC162376AX(B)217176AX(B)217179MI50600 a  Twice the angle between the S —O bond and the ring. b  Calculated value. c S-Me. 5.06.2.2 Stereochemistry and NMR Spectra 5.06.2.2.1 Stereochemistry The configurations of chiral thiiranes and thiirane  1-oxides  can be determined by *HNMR experiments in chiral solvents such as (i?)-(-)-l-phenyl-2,2,2-trifluoroethanol. The  134  Thiiranes and Thiirenes protons  a  to sulfur usually are shifted more downfield for the (5)-configuration than forthe  (R) -configuration, all shifts being relative to the shifts in carbon tetrachloride (77T999).The  cis  and  trans  isomers of 2,3-diphenylthiirane can be distinguished easily by thedifference in *H NMR shifts caused by shielding of the protons in the  trans  isomer by aphenyl group; other  cis-trans  isomers may be differentiated by the fact that the  cis  couplingconstant is generally larger than the  trans. In thiirane  1-oxides  and 1-substituted thiiranium salts the stereochemistry around thesulfur atom is pyramidal. In the sulfoxides the oxygen atom is bent about 67° out of theplane of the ring. The anisotropy of the S —O bond is useful in distinguishing alkyl groups syn  or  anti  to the oxygen atom by *H NMR, although the increased thermal (roomtemperature) instability of thiirane  1-oxides  with alkyl groups  cis  to oxygen presentsdifficulties. Relative to the episulflde, hydrogens on alkyl groups  syn  to the oxygen aredeshielded (downfield shifts) and those  anti  are shielded (upfield shifts) (71T4821). Hydrogensdirectly bonded to the ring of the sulf oxide did not show this behavior to a significant extentalthough the ring hydrogens in fra«5-2,3-di-f-butylthiirane  1-oxide  are well differentiated,5 = 3.11  (syn),  1.87p.p.m.  (anti),  7 = 12 Hz. Assignments were confirmed by benzenesolvent assisted shifts (relative to CCU) which were more upfield for protons  anti  to oxygen.Both  cis-  and £ratts-2,3-dimethylthiirane  1,1-dioxides  are differentiated by the chemicalshifts for the ring hydrogens which absorb at  S  3.36 and 2.78 p.p.m. respectively (70S393).The *H NMR spectrum of 2-methylthiirane in FSO 3 H-SbF 5 -SO 2  shows two doublets forthe methyl group for  syn  and  anti  configurations in the protonated thiirane (7UOC1121).The barrier to inversion at sulfur in  S  -protonated thiirane is calculated to be high(326.8 kJ moF 1 ) (75JCS(P2)1722). The calculated barrier for pyramidal inversion for 1-methyl-2,3-di-r-butylthiiranium tetrafluoroborate is similar (313.8 kJmor 1 ) (79MI50600).Barriers calculated for 2-methylthiirene  1-oxide  and the 1,2-dimethylthiirenium ion aregreater by about 6 kJ moF 1  than those for 2-methylenethiirane  1-oxide  and the l-methyl-2-methylenethiiranium ion, the range of barriers being from 355.6 to 272.4kJmor 1 (71IJS(A)(1)66). 5.06.2.2.2 Chemical shifts and coupling constants Chemical shifts ( H, C) and coupling constants for some thiiranes and thiirenes aregiven in Table 2. The shifts of substituted derivatives may be calculated by the use ofadditivity relationships found in textbooks on NMR.In the series of tetralin 2,3-thiiranes, oxiranes and aziridines, the deshielding effects ofthe heteroatoms are in the sequence S > O > NH (80JST(63)73) which differs from the orderfound in the parent three-membered heterocycles (O>S>NH) (B-73NMR138, 366,425). Thedifference in shielding in both systems between sulfur and oxygen is not great. Thiiranehydrogens in steroidal a-episulfides are at higher field than the hydrogens of the /2-isomers.Vicinal hydrogen-hydrogen coupling constants  (/H_H)  are larger and geminal hydrogen-hydrogen coupling constants  (JH-H)  are smaller for thiiranes than for oxiranes, but careshould be taken in comparing  JH-H  values because of solvent influences  (JH-H  is morenegative in more polar solvents). Three-bond carbon-hydrogen and hydrogen-hydrogencoupling constants  ( 3 / C -H>  3 ^H-H)  are larger in thiiranes than in aziridines or oxiranes, thevalues paralleling the length of the ring C—C bond (78JOC4696). The  JC-H  values for  cis Me and H ring substituents are larger than for  trans  substituents, which is an aid in theassignment of stereochemistry.The *H NMR spectrum of thiirane  1-oxide  is complex (AA'BB'); at 60 MHz 24 linesare observed consisting of two sets of 12 centered about a midpoint. The *H NMR chemicalshift in thiirane  1,1-dioxide  is fairly sensitive to solvent variations partly because of thehigh dipole moment (4.4 D) of the sulfone. The benzene-induced shift,  AS  (C 6 D 6 -CCl4),is large (—1.04 p.p.m.), as expected from the presence of a sulfone group. Oxygen-17chemical shifts for thiirane  1-oxide  and thiirane  1,1-oxide  are -71 and +111 p.p.m.respectively, relative to H 2 O.In  S -protonated 2,3-di-f-butylthiiranes (7)-(9) the more crowded S—H proton in (9)appears at higher field  (S  2.68 p.p.m.) than the S—H protons of (7;  S  3.01 p.p.m.) and(8,5 3.54 p.p.m.) in the solvent FSO 3 H. The methine hydrogens of the ring are coupledprincipally to the S —H proton  ( 3 / H -H  =  6-8 Hz). Proton and carbon chemical shift differen-ces for C —H in  cis  and  trans  derivatives (7)-(9) are very small, although  VC-H  for (7; 167 Hz)
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