Compr. Heterocyclic Chem. III Vol. 6 Other Five-membered Rings with Three or more Heteroatoms

749 Pages·2008·10.137 MB·English

by  Katritzky A.R., et al. (eds.)

6.01
1,2-Oxa/thia-3-azoles
O.A.Rakitin
ZelinskiInstituteofOrganicChemistry,Moscow,Russia
ª2008ElsevierLtd.Allrightsreserved.
6.01.1 Introduction 2
6.01.2 TheoreticalMethods 2
6.01.3 ExperimentalStructuralMethods 4
6.01.3.1 X-RayDiffraction 4
6.01.3.2 1HNMRSpectroscopy 6
6.01.3.3 13CNMRSpectroscopy 7
6.01.3.4 UVSpectroscopy 8
6.01.3.5 ESRSpectroscopy 9
6.01.3.6 CyclicVoltammetry 9
6.01.3.7 IRSpectroscopy 10
6.01.3.8 MassSpectrometry 10
6.01.4 ThermodynamicAspects 11
6.01.5 ReactivityofFullyConjugatedRings 11
6.01.5.1 UnimolecularThermalReactions 11
6.01.5.2 ElectrophilicAttackatRingAtoms 12
6.01.5.3 NucleophilicAttackatRingSulfur 12
6.01.5.4 NucleophilicAttackatRingCarbon 14
6.01.5.5 ReactionsInvolvingRadicals,Electron-DeficientSpecies,ReducingAgents,
andatSurfaces 19
6.01.6 ReactivityofNonconjugatedRings 20
6.01.6.1 ReactionsofHydrogenatedDerivativesof1,2,3-OxathiazoleS-Oxides 20
6.01.7 ReactivityofSubstituentsAttachedtoRingCarbonAtoms 22
6.01.8 ReactivityofSubstituentsAttachedtoRingHeteroatoms 24
6.01.9 RingSynthesesfromAcyclicCompoundsClassifiedbyNumberofRingAtoms
ContributedbyEachComponent 25
6.01.9.1 FormationofOneBondAdjacenttoaHeteroatom 25
6.01.9.2 FormationofTwoBonds:Four-AtomFragmentandSulfur 26
6.01.9.3 FormationofTwoBonds:[3þ2]AtomFragmentbyCycloaddition 27
6.01.9.4 FormationofTwoBonds:[3þ2]AtomFragmentbyOtherProcesses 28
6.01.10 RingSynthesesbyTransformationofAnotherRing 30
6.01.11 SynthesisofParticularClassesofCompoundsandCriticalComparison
oftheVariousRoutesAvailable 31
6.01.12 ImportantCompoundsandApplication 31
6.01.13 FurtherDevelopments 33
References 33
1
2 1,2-Oxa/thia-3-azoles
6.01.1 Introduction
Inthischapter,thechemistryof1,2,3-dithiazolesand1,2,3-oxathiazolesiscoveredfortheperiodfrom1996to2005
inclusive.1,2,5-Oxathiazoleshavenotbeeninvestigatedduringthisperiod.
The synthesis, theoretical and extensive spectral study of new families of neutral heterocyclic 1,2,3-dithiazolyl
radicals have been described by the Oakley group, and these compounds have been proposed as conductive
molecular materials. A new application of Burgess reagent (methoxycarbonylsulfamoyltetraethylammonium
hydroxide, inner salt) in the synthesis of enantiopure 1,2,3-oxathiazole di-S-oxides has been realized by Nicolaou
andco-authorsintheyearsreviewed.Anabundanceofnewchemistryhasbeenrevealedfor4-chloro-1,2,3-dithiazolo-
5-imines,5-one,5-thione,and5-ylideneseasilyobtainedfromAppelsalt(4,5-dichloro-1,2,3-dithiazoliumchloride).
The reactions of cyclic sulfamidates giving enantiopure aminoacids, (cid:2)-aminoalcohols, and sugars with functional
groupshavebeenintensivelyinvestigated.
1,2,3-Dithiazoles investigated are given in Figure 1(a): these are 1,2,3-dithiazolium cations 1, 1,2,3-dithiazolyl
radicals2,1,2,3-dithiazole-3-ones3andrelatedcompounds4,their2-oxides5.1,2,3-Oxathiazoleshavebeenobtained
andinvestigatedintheformoftheirS-oxides(Figure1(b)andnamedascyclicsulfamidates7and8andsulfimidates
6,9,and10.
Figure1 Known1,2-Oxa/thia-3-azoles.
6.01.2 Theoretical Methods
Charge distribution and p-bond orders in condensed 1,2,3-dithiazoles was the main subject for calculation in the
1980s<1996CHEC-II(4)409>.Heteropentalenescontaining1,2,3-dithiazoleand1,2,3-oxathiazolemoietywerealso
ofsignificantinterest.
Aseriesofdensityfunctionaltheory(DFT)calculationsattheB3LYP/6-31G** levelonthenoveltetrathiadiaza-
fulvalene 11 hasshown thatthesemolecules exhibitclosed shell11a ratherthan biradical 11bground states, as a
resultofstronginterannularinteractionacrossthebridgeheadC–Cbond(Equation1)<1999JA6657>.
ð1Þ
Electron correlation calculations using the DFT method on the benzo-bis(1,2,3-dithiazole) 12 have been
performed. The asymmetry of the pairwise combination of two singly occupied molecular orbitals (SOMOs)
leads, upon mixing with benzene, to a much greater splitting of the two frontier orbitals a and b , and a quinoid
u g
singlet ground state 12a. The diradical singlet 12b is predicted to lie 0.72 eV above the singlet ground state of
12a <1997JA12136>.
1,2-Oxa/thia-3-azoles 3
Calculationofthefrontiermolecularorbital(FMO)interactionshowedthattheeffectofthehighelectronegativity
oftheN–Nbridgeontheelectronicdistributioninbis(1,2,3-dithiazole)13isofparticularimportance;itstabilizesthe
singletstaterelativetothediradicaltripletbyloweringofhighestoccupiedmolecularorbital(HOMO)andpolarizes
thetwooccupiedp-levelsontotheazinebridge.Fromthechargedistribution,itisapparentthatadiazine-bridged
bis(1,2,3-dithiazole) is best described using covalent formula 13a rather than by means of polar structure 13b
(Equation2)<2001IC2709>.
ð2Þ
Dispersion of the valence and conduction bands in the bis(1,2,3-dithiazolyl) radical 14 has been determined by
extended Hu¨ckel band structure calculations which suggest that the band gap (ca. 0.4eV) has a value which is
considerablysmallerthanthatfoundinotherradicaldimerstructuresandarisesfromtheweaknessoftheintradimer
interaction<1999JA969>.
The distribution of the total spin density in radicals 14 and 15 was calculated with UB3LYP/6-31G* methods
<2001PCA7615>.Asignificantspindelocalizationwasfoundinthebenzoannulated1,2,3-dithiazolylderivative15.
Inadditiontopolarresonanceforms,therearealsononpolarallylic-typeresonanceformsallowingforspindelocaliza-
tionontotheadjacentbenzenering.Thedisproportionationenergiesforradicals14and15,calculatedbythesame
method, were compared with the available conductivity data for radicals showing a qualitative correlation. Low
disproportionation energies and hence high conductivity were obtained for large p-systems, such as dithiazole 14
containingan–N–S–S–arrayofheteroatoms<2001PCA7626>.
AseriesofextendedHu¨ckeltheory(EHT)bandcalculationsoncrystalstructures16and17havebeenperformed
<2003JA14394,2004CM1564>.Theyshowthatthedispersioncurvesplottedalongthestackingdirectionarisefrom
theSOMOsoftheradicalsinthecellunit,thatis,theputativehalf-filledconductionbandofthemolecularmetal.
Clearly,noneofthematerialsaremetallic,butthedispersioncurvesnonethelessprovideinsightintotheextentof
theintermolecularinteractionalongandperpendiculartotheslippedp-stacks.
4 1,2-Oxa/thia-3-azoles
To explain the reactivity observed in the methylation of spirosugar 18, an ab initio theoretical study
using the UB3LYP/6-31G* basis set was carried out <1999T12187>. The total electronic energy values
show that amine 18a is energetically the most stable tautomer (Scheme 1). The geometry calculations showed
that the nitrogen of the amino group is in the same plane as the spiro oxathiazolidine ring. The electronic
density values on the nitrogen of amine 18a and imines 18b and 18c are high; consequently, these compounds
should be good nucleophiles. According to the electronic parameters for the amine 18a, the highest value of
electronic density is located on the endo-nitrogen whereas for the imines 18b and 18c the highest electronic
density is located on the exo-nitrogen. Thus, in a charge-controlled reaction, the methylation of amine 18a would
take place of the endo-nitrogen and on the exo-nitrogen in the imine 18b and 18c. These calculations are in
agreement with the experimental results of the methylation of compound 18. The ab initio calculations were
carried out on simplified structure 18 without taking into consideration the rest of the sugar structure of these
molecules.
Scheme1
A theoretical study was carried out to shed light on the different reactivities of sulfamides 19a and 19b with
fluorideanioninS 2andE2reactions <2004CC980>.Allgroundstateandtransitionstate(TS)geometrieswere
N
locatedusinghybriddensityfunctionaltheory(B3LYP).Forsulfamide19a,theTSobtainedfortheE2reactionwas
slightly lower in energy (0.60kcalmol(cid:2)1) than that resulting from the nucleophilic attack of the fluoride anion. In
contrast,withsulfamide19b,thedifferenceinenergybetweenTSinE2andTSinS 2isconsiderablyhigherwith
N
theS 2routebeingfavoredbyca.2kcalmol(cid:2)1.Thissituationisinqualitativeagreementwiththechemoselectivity
N
experimentallyobserved.
6.01.3 Experimental Structural Methods
2,5-Diaza-1,6-dioxa-6a-thiapentalene has been extensively studied by various diffraction methods: X-ray, neutron
andelectronspectroscopy<1996CHEC-II(4)409>.Therehasbeensubstantialprogressinthespectralinvestigation
ofdisubstituted1,2,3-oxathiazolidine2-oxides.Infrared(IR)andmassspectrawerelessemployedfordithiazolesand
oxathiazolesthanothermethods.
6.01.3.1 X-Ray Diffraction
TheX-rayanalysisof4,5-dichloro-1,2,3-dithiazoliumchloride20,oftenreferredtoastheAppelsalt,revealsapattern
ofbondingwithinthedithiazoleringthatindicatesdelocalizationthatextendsaroundtheringfromonesulfuratom
1,2-Oxa/thia-3-azoles 5
totheother,andindeedthereisalsoevidenceforashorteningoftheS–Sbondlength(2.034(2)A˚)comparedwith
that normally associated with the S–S single bond <2002J(P1)1535>. The distance seen here is only marginally
longerthanthatreportedforthecorrespondingseparation(2.023A˚)inafullydelocalizeddithiolering.IntheAppel
salt 20, there is a short triangular and almost in-plane approach of the chloride anion to the sulfur centers of the
dithiazolering.Inaddition,insalt20,thereisalsoanear-linearside-onapproachofthechlorideaniontotheS–S
bond:thechlorideionisalsopositionedoverthecenterofthedithiazoleringofanadjacentmoleculeatadistance
compatiblewithanelectrostaticinteraction.
A single crystal structure analysis shows bis(1,2,3-dithiazole) 21 to have a planar structure, all of the atoms
being required to lie within a crystallographic mirror plane <2002J(P1)1535>. The two dithiazole rings have
essentially identical geometries, but a pattern of bonding that differs markedly from that seen in salt 20. Here
with the exception of the CTN linkages which have pronounced double bond character, all the bonds are
noticeably lengthened compared to their values in salt 20, and are more consistent with conventional single
bonds.
The crystal and molecular structures of bis(1,2,3-dithiazoles) 13 and 22 have been determined by single-crystal
X-ray diffraction <2001IC2709>. Molecules of dithiazole 13 lie on a crystallographic center of inversion and are
planartowithin0.03A˚.Theintramolecularbondlengthsofcompound13aresimilartothoseseeninbis(dithiazole)
11(seebelow)and,alongwiththelongN–Nbond,areconsistentwiththeazineresonancestructure.Moleculesof
benzene-bis(dithiazole)22arealsocrystallographicallycentrosymmetricbutarefarfromplanar.Toavoidpotential
stericcongestionbetweenS-1andtheC-3proton,the1,2,3-dithiazolering(whichisplanartowithin0.07A˚)isrotated
abouttheN(1)–C(4)bondtoformadihedralangleof137.30(7)(cid:3)withtheplaneofthebenzenering.Asaresultofthis
torsionalmotion,conjugationwiththebenzenebridgeisrestricted.Thepackingofmoleculesinbis(dithiazole)22
consistsofamoreconventionalslippedstackstructurealong(cid:3),withtheregistryofadjacentstacksbeingsomewhat
ruffledbythetwistinthephenylenebridge.
The molecules of condensed bis(1,2,3-dithiazoles) 23 and 24 lie on crystallographic inversion centers and are
essentially planar to within 0.02 A˚ <1997JA12136>. These molecules adopt slipped p-stack structure; adjacent
moleculesapproachoneanotherinaside-onmanner.
Simple ionic packing arrangement of radical cation 26 (that is, alternating cations and anions) affords few
interannularcation/cationcontacts;theshortestintermolecularS(cid:4)(cid:4)(cid:4)Sapproach(3.740A˚)iswelloutsidethevander
Waals distance (3.6A˚) for two sulfurs <1999CM164>. A comparison of the intramolecular distance in neutral
bis(dithiazole) 25and its radical cation 26 reveals the expected differences; thatis, the S–S,S–N, andS–C bonds
areshortened,andtheN–Cbondsarelengthenedbyone-electronoxidation.
trans-Dichlorotetrathiadiazafulvalene11anditsradical-cationsalts27wereinvestigatedbyX-raycrystallography.
For compounds 11, the intramolecular S–S, S–N, and S–C distances are all slightly longer than those observed in
1,2,3-dithiazoliumsalts<1998CC1039>,andthisisintheagreementwithvaluespredictedbyabinitiocalculations
<1999JA6657>. The crystal structures of radical cations 27 are interspersed with the corresponding anions.
Molecules in adjacent layers are linked, up and down, by intermolecular S2(cid:4)(cid:4)(cid:4)S39 and S2(cid:4)(cid:4)(cid:4)S49 contacts, which
againarenearthevanderWaalslimit.
6 1,2-Oxa/thia-3-azoles
The radicals 14, which are planar to within 0.07 A˚, are loosely associated into centrosymmetric or head-to-tail
dimerswiththeclosestintradimerS(cid:4)(cid:4)(cid:4)Scontactbeing3.233A˚.Interannularorbitalinteractionsatthisrangeareweak
butsufficienttoquenchparamagnetism,thatis,togenerateaweak‘chemicalbond’betweentworadicals.Withinthe
individual halves of the dimer, the bond lengths show changes that reflect the long-range nature of the electron
delocalizationintheradical.TheshorteningoftheN(1)–C(2)bondheraldsconsiderabledoublebondcharacter.This
change, coupledwiththe shorteningofN(2)–C(3) andN(5)–C(4),andthe lengtheningofN(2)–C(2), suggeststhe
collectiveinvolvementoftheseriesofvalencebondresonancestructures14showninScheme2<1999JA969>.
Scheme2
The crystals of dithiazolyl radical 28 consist of planar (to within 0.03 A˚) undimerized radicals aligned in a slipped
p-stack arrangement running parallel to the (cid:3)-axis <2002CC1872>. There are no S(cid:4)(cid:4)(cid:4)S intermolecular interactions
betweentheradicalsthatareinsidethenormalvanderWaalscontactof3.6A˚.TheclosestS(cid:4)(cid:4)(cid:4)Sinteractionsoutsidethis
rangearethehead-to-headcontact(3.843A˚),thehead-to-tailcontact(3.626A˚),andthep-stackingcontact(3.707A˚).
Crystalsofcompound16consistofslippedp-stacksofundimerizedradicalsrunningparalleltoZ(b:R1¼Et,Pr;
R2¼Cl)and(cid:3)(a:R1¼Me;R2¼Cl)<2002JA9498>.Assuch,theyrepresentthefirstexampleofundimerized1,2,3-
dithiazolyl radicals. In all examples, there are numerous close intermolecular S(cid:4)(cid:4)(cid:4)S contacts between neighboring
dithiazolerings.MostofthesecontactsareinsidethevanderWaalscontactforsulfur(3.6A˚),andsomeofthosein
structure16aareamongtheshortestnonbondedS(cid:4)(cid:4)(cid:4)Scontactsthathavebeenobservedinundimerizedheterocyclic
sulfur–nitrogenradicals.
Thecrystalsofradical29,whicharediamagneticbyelectronspinresonance(ESR),consistofcofacialdimerswith
four dimers per unit cell linked by two long, albeit unequal (3.053 and 3.309A˚), interannular S–S bond
<2005IC1837>. This mode of association has not hitherto been observed for 1,2,3-dithiazolyls. Dimers of radical
29donotformp-stackedarrays;instead,theyadopttheclosedpackedherringbonearrangementwhichallowsfora
clusteringoftheradicalheadssoastomaximizeS(cid:4)(cid:4)(cid:4)SandS(cid:4)(cid:4)(cid:4)Ncontacts.
6.01.3.2 1H NMR Spectroscopy
1H nuclear magnetic resonance (NMR) spectroscopy was used for the study of stereoisomerism in 5-alkylidene-4-
chloro-5H-1,2,3-dithiazoles<1999T9651>.Thesignalsofthemethylgroupsincompounds30and31,whicharesyn
1,2-Oxa/thia-3-azoles 7
to S1 of the 1,2,3-dithiazole moiety, appear upfield compared with those having the opposite stereochemistry
(compounds32and33,respectively).
Me
F C FC Me O
3 3
O O O O
Me O F3C F3C
Cl Me Cl Cl Cl
O O O O
S N S N S N S N
S S S S
30 31 32 33
2.39ppm 1.31ppm 2.73ppm 1.39ppm
Withthe helpof1HNMR, itwasshownthatthemixtureofstereisomers 34aand34bexistsinanequilibrium
betweentwoisomers,whichisslowlyachievedin24h(ratio53:47atroomtemperature)(Equation3).NMRdataof
thecarbethoxygroupsuggestthat34ahavingthegroupantitoS-1isthemajorcompound<1999T9651>.
ð3Þ
Theconformationsoftrans-fusedcyclohexathiazolidines35aand35bweredeterminedby1HNMRspectroscopy
<1996ACS1036>.FromtheanisotropyeffectoftheSTObonditisobviousthat,ofthetwotransconformations,35a
((cid:4) 2.60and(cid:4) 4.57ppm)istheonewithpseudoaxialsulfinyloxygen.Similarconsiderationhasbeenmadefor
H-3a H-7a
correspondingcis-isomers.
The absolute configuration at C-3 for the spiro derivative 18a was unequivocally determined by nuclear
Overhausereffect(NOE)experiments.Thus,irradiationoftheNH groupcausedenhancementsofthesignalsfor
2
H-2andH-5ofthetetrahydrofuran(THF)ring.ThesecorrelationsindicatedthattheNH groupisontheupperside
2
ofthefuranoseringandareonlycompatiblewithariboconfigurationofthefuranosemoiety<1999T12187>.
6.01.3.3 13C NMR Spectroscopy
The13Cspectrumofdithiazole36whosestereochemistrywasclearlydeterminedbyX-raycrystallographyexhibited
signalscorrespondingtotheCF COandPhCOcarbonylcarbonsat173.3and191.1ppm<1999T9651>.Basedon
3
thespectrumofylidene36,thesignalat172.4ppmexhibitedbyylidene37canbeassignedtothecarbonylcarbonsyn
toS-1ofthedithiazoleringandtheotherpeakthatappeareddownfield(182.2ppm)canbeassignedtothecarbonyl
carbonantitoS-1ofdithiazole37.Similarlythe13CNMRsignalat171.9and186.2ppmexhibitedbyamixtureof
compounds30and32wereassignedtotheCF COcarbonylgroupsyn32andanti30toS-1,respectively.Thesame
3
propensityofthe13CNMRchemicalshiftswasobservedforthecarbonylcarbonsoftheacetylgroupsinylidenes30
and 32. That is, compound 30 having an acetyl group syn to S-1 exhibited the signal of the carbonyl carbon at
188.0ppm,whereasdithiazole32havingthecorrespondingabsorptionat197.8ppm.
8 1,2-Oxa/thia-3-azoles
Theoxathiazoles38canbeidentified,eveniftheyarenotstableenoughtosurvivepreparativechromatography,
by the characteristic low-field resonances ((cid:4)(cid:5)97–98ppm) for the C-5 ring atom indicating a neighboring oxygen
<1996J(P1)1629>.
6.01.3.4 UV Spectroscopy
Structurally similar naphthodithiazolones 39 and 40 are both formally 14p heteroaromatic systems with analogous
possibilities for electronic delocalization <1998T223>. The striking difference in color and electronic spectra of
dithiazole39((cid:5) 461nm,log"4.03)anddithiazole40((cid:5) 602nm,log"3.63)ispresumablyassociatedwiththe
max max
angular tricyclic structure of dithiazole 39 compared with the higher-energy linear structure of dithiazole 40
(Equations 4 and 5), as found in similar carbocyclic systems like phenanthrene and anthracene and their aza
derivatives.
ð4Þ
ð5Þ
Theintensep–p-transitioninbis(dithiazole)24(622nm)<1998CC1939>isduetothelongerwavelengthofthe
correspondingabsorptionmaximainbis(dithiazoles)23(522nm)and11(565nm)<1998CC1039>.
4-Chloro-1,2,3-dithiazoles41and42showstrongultraviolet(UV)absorptionat(cid:5) 423–431nm(log"3.8–4.0)and
max
aweakerabsorptionat(cid:5) 330nm(log"3.2–3.3)<1998J(P1)2505>.
max
The UV spectra of condensed dithiazole 43 ((cid:5) 412nm, "¼23928) contrasted the UV spectrum of the
max
chloro derivative 44 ((cid:5) 546nm,"¼2618) <2005EJO5055>. Apparently, the morpholino group in dithiazole 43
max
disrupts the charge-transfer band between the two rings by conjugation between the amine and cyano groups.
The UV spectrum of cycloheptadithiazole 45 showed a broad spectral absorption in the near-IR region ((cid:5)
max
700nm,"¼864).
1,2-Oxa/thia-3-azoles 9
The5-5-6fuseddithiazole46andangular5-6-6-647and48systemsrevealstrong(log"¼3.9–4.2)absorptionat
ca.570–635nm,whereasthelinear5-6-6-6system49absorbsatshorterwavelength,533nm(log"¼4.7),withonlya
weakdependenceofthecorrespondingp!p*transitionsinextendedpolyheteroatomp-systemuponsubstitutionof
fluorine for hydrogen. Additionally, the 5-6-6-6 systems 47–49 demonstrate strong fluorescence in the range of
ca.660–680nm<2005EJI4099>.
6.01.3.5 ESR Spectroscopy
Asolutionofradicalcationof23inliquidsulfurdioxideexhibitsanextremelystrongandpersistentESRsignalasa
1:2:3:2:1 quintet, confirming that spin density is fully delocalized over both nitrogen atoms <1997JA12136>. The
effectsofspin-orbitcoupling(atsulfur)ontheg-valuearemorepronouncedinthedithiazolylcationradicalof23than
in simple monofunctionalized dithiazolyls, and, consistently, the observed g-value of 2.0114 is larger than in, for
example, benzo-1,2,3-dithiazolyl 15 (g¼2.008).Similarly, the more extensive delocalization ofspin densityin the
cation radical of 23 relative to simple 1,2,3-dithiazolyls leads to a smaller hyperfine coupling constant at nitrogen
(a ¼0.201mT).Additionalcouplingtotwopairsofhydrogenswitha ¼0.079and0.048mTincationradicalof50
N N
is also observed <1998CC1939>. The ESR spectrum of compound 14 is considerably more complex but spectral
simulationsrevealtheeffectsofhyperfinecouplingtoallfivenitrogennuclei.Spindensityismovedawayfromthe
5-positionandisredistributednotonlytothenitrogenattachedtothe4-positionofthe1,2,3-dithiazoleringbutalso
totheothernitrogensofthethiadiazolopyrazineligand.Asaresultofthisreorganizationofspindensity,theradical
14resistsassociationviaC–Cbondformation<1999JA969>.
Inadditiontotheexpectedtriplet(a ¼0.498mT)arisingfromhyperfinecouplingtothedithiazolenitrogenof
N
radical28,thespectrumalsodisplaysappreciable(a ¼0.135mT)couplingtotheisothiazolylnitrogen,aswellas
N
smaller coupling to two of the three chlorines present in the molecule. This is indicative of substantial spin
delocalizationawayfromthedithiazolylring<2002CC1872>.
Thea constantsandg-valuesofknown1,2,3-dithiazolylradicalsarelistedinTable1.
N
6.01.3.6 Cyclic Voltammetry
Electrochemicalpropertieswereexaminedtogainmorequantitativeinsightintotheredoxpropertiesofthissystem.
Cyclic voltammetry on bis(dithiazole) 23 in acetonitrile (with Pt electrodes and 0.1M n-Bu NPF as supporting
4 6
electrolyte) reveals a reversible oxidation wave with E (ox)¼0.93V and a second, irreversible oxidation process
1/2
10 1,2-Oxa/thia-3-azoles
Table1 ESRg-values,hyperfinecouplingconstants(a )of1,2,3-dithiazolylradicalsandcationradicals,andhalf-wave
N
potentialsof1,2,3-dithiazolyls
Structure g-Value a (mT) E (0/þ)(V) E (þ/2þ)(V) E (0/(cid:2))(V) Reference
N 1/2 1/2 pc
11 2.0117 0.096 0.80 1.25 (cid:2)0.95 2001IC2709
13 2.0102 0.236 1.36 1.60 (cid:2)0.91a 2001IC2709
14 2.009 0.514 1.14 0.15 1999JA969
16a 2.0083 0.310 0.005 1.415 (cid:2)0.835 2004CM1564
16d 2.0082 0.317 (cid:2)0.130 1.294 (cid:2)0.952a 2004CM1564
16e 2.0082 0.318 2003JA14394
16f 2.0082 0.310 2002JA9498
17a 2.0084 0.317 (cid:2)0.136 1.278 (cid:2)0.94 2004CM1564
17b 2.0086 0.320 (cid:2)0.104 1.305 (cid:2)0.956 2004CM1564
23 2.0114 0.201 0.93 1.5a (cid:2)0.95 1997JA12136
24 2.0106 0.235 0.41 0.66 (cid:2)1.06 1998CC1939
25 2.0117 0.161 0.61 1.10 (cid:2)0.98 1999CM164
28 2.00875 0.498 0.565 (cid:2)0.389a 2002CC1872
29 2.0081 0.748 0.207 (cid:2)0.91a 2005IC1837
50 2.0117 0.143 0.81 1.37 (cid:2)0.96 1999CM164
aIrreversible.
with an anodic peak potential E ¼1.5V <1997JA12136>. Attempts to suppress the irreversibility of the [23]þ/
pa
[23]2þ couple by varying the voltage sweep rate and substrate concentrations were unsuccessful. Reduction of
compound23occursviaasingle,broad,andstronglyirreversiblewavewithacathodicpeakpotentialE ¼(cid:2)0.95V.
pc
Alistingforknown1,2,3-dithiazolesofthehalf-wavepotentialsE (ox)ofthefirstandsecondoxidationaswellas
1/2
thecathodicpeakpotential(E )forthereductionprocessisgiveninTable1.
pc
6.01.3.7 IR Spectroscopy
The IR data recorded in KBr shows that the carbonyl stretching absorptions of the trifluoroacetyl group in 1,2,3-
dithiazoles 32, 33, and 36, which have the trifluoroacetyl carbonyl group syn to S-1, showed absorption at
1573–1586cm(cid:2)1, whereas compounds 31 and 37 having carbonyl group anti to S-1 showed absorptions at 1718–
1731cm(cid:2)1<1999T9651>.Thatis,thecarbonylgroupantitoS-1needsmoreenergythanthatofsynanalogstobea
vibrationallyexcitedsite.
TheIRspectrumof4-chloro-1,2,3-dithiazole-5-thione41showsstrongbandsat1041,1029,and1013cm(cid:2)1inthe
regionforsulfinesymmetricandasymmetricstretching<1998J(P1)2505>.
6.01.3.8 Mass Spectrometry
The presence of the 1,2,3-dithiazole ring in a mixture of isomers 51 was supported by the fragment ions m/z 137
(C ClNS )forthechlorinatedring,125(CClNS ),102(C NS )theringitself,93(CClNS)theCl–CTN–Sunit,70
2 2 2 2 2
(C NS) and 64 (S ) <1998J(P1)2505>. These assignments were supported by high-resolution mass spectrometry
2 2
(HRMS), which also identified the fragment ions m/z 120 (C Cl N) and 85 (C ClN) from substituents of the
3 2 3
dithiazolering.Thefragmentationofmolecule52occurredsoreadilythatastrongparention,m/z287,couldonly
beobtainedbyfastatombombardment(FAB)spectrometry<1998J(P1)2505>.