Benzo[1,2,3]Dithiazole Compounds

reactions Review Review Benzo[1,2,3]dithiazole Compounds: A History of Synthesis and Their RenewedRenewed ApplicabilityApplicability inin MaterialsMaterials andand SyntheticSynthetic Chemistry, OriginatingOriginating fromfrom thethe HerzHerz ReactionReaction AlexanderAlexander J. Nicholls * andand Ian Ian R.R. Baxendale Baxendale Department of Chemistry, University of Durham, SouthSouth Road,Road, DurhamDurham DH1DH1 3LE,3LE, UK;UK [email protected]@durham.ac.uk * Correspondence:Correspondence: [email protected]@durham.ac.uk Abstract: The benzo[1,2,3]dithiazole is a unique heteroaromatic functionality whose conjugatedconjugated profileprofile instils some fascinating electronic properties. This has been historically recognized in the design and manufacture of organic dyes early last century. Although, with the benefitbenefit of increased diagnostic techniques and improved understanding, these structures are attracting greater attention inin additional research settings,settings, including applicationsapplications as organicorganic radicalsradicals andand semiconductors.semiconductors. In addition, the benzodithiazolebenzodithiazole functionalityfunctionality has has been been shown shown to to be be a valuablea valuable synthetic synthetic intermediate intermediate in inthe the preparation preparation of aof variety a variety of other of other privileged privileged aromatic aromatic and heteroaromatic and heteroaromatic targets, targets, many ofmany which of whichare important are important APIs. InAPIs. this In review, this review, the authors the authors aim to criticallyaim to critically analyse analyse the potential the potential applicability applica- of bilitythese compoundsof these compounds to the fields to the of notfields only of small-scalenot only small laboratory-scale laboratory synthetic andsynthetic medicinal and chemistrymedicinal chemistrybut also commercial-scale but also commercial processes-scale andprocess increasinglyes and increasingly materials chemistry. materials chemistry. Nicholls, A.J.; Baxendale, Citation: Nicholls, A.J.; Baxendale, Keywords: thiazole;thiazole; disulfurdisulfur dichloride;dichloride; Herz Herz reaction; reaction; heterocycle heterocycle formation; formation; organic organic radical radical anion; an- I.R. Benzo[1,2,3]dithiazole ion; semiconductor; dye Compounds: A History of Synthesis semiconductor; dye and Their Renewed Applicability inin Materials and Synthetic Chemistry, Originating from the Herz Reaction. Reactions 2021,, 22,, 175–208.x. https://doi.org/ https:// 1. Introduction to Benz Benzo[1,2,3]dithiazoleo[1,2,3]dithiazole Compounds 10.3390/xxxxx doi.org/10.3390/reactions2030013 The benzo[1,2,3]dithiazole is a rare and enigmatic heterocycle (Figure(Figure 11),), whichwhich atat firstfirst glance has has similarity similarity to to the the more more common common benzothiazole benzothiazole structure, structure, with with the thecarbon car- Academic Editors: Silvana Pedatella Academic Editors: Silvana Pedatella bonat the at the2-position 2-position exchanged exchanged for forsulphur sulphur.. In Inaddition addition,, there there also also exists exists the the isomeric and Dmitry Yu. Murzin and Dmitry Yu. Murzin benzo[1,3,2]dithiazole,benzo[1,3,2]dithiazole, which which,, although also also displaying some some similar similar properties properties and and ap- ap- plications [1 [1––4]4],, will will not not be be covered covered in inthis this review review owing owing largely largely to the to differences the differences in syn- in Received: 303 May May 2021 2021 synthesisthesis [5] [(and5] (and from from here here forward forward for for simplicity, simplicity, ‘benzodithiazole’ ‘benzodithiazole’ will will refer refer only only to to the Accepted: 24 June 2021 [1,2,3]dithiazole derivatives). Published: 2928 June 2021 [1,2,3]dithiazole derivatives). Publisher’s Note: MDPI stays neu- 4 4 4 Publisher’s Note: H MDPI stays neutral 5 3 5 3 3 tral with regard to jurisdictional N S 5 N with regard to jurisdictional claims in 2 2 claims in published maps and institu- S NH published maps and institutional affil- 6 6 2 6 tional affiliations. S S S iations. 7 1 7 1 7 1 Benzo[1,2,3]dithiazole Benzo[1,3,2]dithiazole Benzo[1,3]thiazole Figure 1. 1.The structureThe structure and parent and numbering parent system numbering for benzo[1, system2,3]dithiazole, for benzo[1,2,3]dithiazole, benzo[1,3,2]dithi- Copyright: © 2021 by the authors. Li- benzo[1,3,2]dithiazoleazole and benzothiazole. and benzothiazole. Copyright:censee MDPI,© 2021Basel, by Switzerland. the authors. LicenseeThis article MDPI, is an Basel,open access Switzerland. article While all but neglectedneglected as aa syntheticsynthetic intermediateintermediate inin APIAPI andand bulk/finebulk/fine chemicals, Thisdistributed article under is an the open terms access and article con- the valuable electronic properties of the dithiazoledithiazole structures havehave engenderedengendered renewedrenewed distributedditions of the underCreative the Commons terms At- and interest in recent years. As a result, the general benzo[1,2,3]dithiazole structure appears conditionstribution (CC of BY) the license Creative (http://crea- Commons interest in recent years. As a result, the general benzo[1,2,3]dithiazole structure appears roughly equally in both the scientific and patent literature over the previous ten years Attributiontivecommons.org/licenses/by/4.0/). (CC BY) license (https:// roughly equally in both the scientific and patent literature over the previous ten years as creativecommons.org/licenses/by/ asit did it did in the in thenear near 100 100years years of its of preceding its preceding existence existence [6]. Despite [6]. Despite a renewed a renewed interest interest in the 4.0/). in the compounds recently, much of the synthetic methods and understanding remain Reactions 2021, 2, Firstpage–Lastpage. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/reactions Reactions 2021, 2, 175–208. https://doi.org/10.3390/reactions2030013 https://www.mdpi.com/journal/reactions Reactions 2021, 2, FOR PEER REVIEW 2 Reactions 2021, 2 176 compounds recently, much of the synthetic methods and understanding remain mostly transcribedmostly transcribed from the from original the original literature literature (early (early-to-mid-to-mid last century). last century). This This review review aims aims to highlightto highlight their their interesting interesting synthetic synthetic chemistry chemistry and andsignpost signpost some some of the of most the most popular popular end- useend-use applications. applications. 1.11.1.. Categorisation Categorisation of Benzo Benzo[1,2,3]dithiazole[1,2,3]dithiazole Compounds The benzo[1,2,3]dithiazole structure structure exists exists most commonly in three different formsforms with vastly different characteristic properties: the benzodithiazolium cation, the neutral compound andand the the benzodithiazoyl benzodithiazoyl radical. radical. All threeAll three are reversibly are reversibly interconverted interconverted through throughsimple chemical simple chemical transformations transformations [4,7]. The [4,7] benzodithiazolium. The benzodithiazolium cation cation is a planar, is a planar, 10-π 10aromatic-π aromatic system system and isand the is most the most facile facile benzo[1,2,3]dithaizole benzo[1,2,3]dithaizole to generate to generate experimentally. experimentally.Supported Supported by a by counterion a counterion (most (most commonly commonly chloride), chloride), it is itgenerically is generically referred referred to to as as a ‘Herza ‘Herz salt,’ salt, and’ and these these compounds compounds are are generally generally amorphous amorphous andand areare sparinglysparingly solublesoluble in most organic solventssolvents andand aqueousaqueous media media unless unless there there are are large large substituents substituents or or counterions counteri- onspresent. present. In suchIn such circumstances, circumstances crystalline, crystalline polymorphs polymorphs have have been been isolated,isolated, allowing analysis of of the the solid solid structure structure by by X X-ray-ray diffraction diffraction (XRD) (XRD) analysis analysis [8] [8. ].The The second second form form of benzo[1,2,3]dithiazoles,of benzo[1,2,3]dithiazoles, the the neutral neutral compounds, compounds, can can normally normally only only form form when when specific specific substituents areare locatedlocated atat the the 2 2 or or 6 6 positions; positions; otherwise, otherwise the, the molecule’s molecule’s lack lack of conjugationof conjuga- tionmakes makes the structure the structure too high too in high energy. in energy. The most The common most common examples examples being the being 2-sulfoxides the 2- sulfoxidesand 6-malononitrile and 6-malononitrile derivatives. derivatives. These compounds These compounds generally generally replicate replicate the Herz the salts Herz in saltstheir in physical their physical properties properties (high melting (high melting solid, poorsolid, solubility), poor solubility), although although they canthey have can haveextremely extremely different different absorption absorption properties, properties, notably notably the intense-purplethe intense-purple appearance appearance of the of theS-sulfoxides. S-sulfoxides. These These materials materials tend tend to beto bench-stable,be bench-stable whereas, whereas the the Herz Herz salts salts are are found found to togradually gradually hydrolyse hydrolyse to theto the 2-sulfoxides

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