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Functionalised Oximes: Emergent Precursors for Carbon-, Nitrogen- and Oxygen-Centred Radicals

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Functionalised Oximes: Emergent Precursors for Carbon-, Nitrogen- and Oxygen-Centred Radicals molecules Review Functionalised Oximes: Emergent Precursors for Carbon-, Nitrogen- and Oxygen-Centred Radicals John C. Walton Received: 11 December 2015 ; Accepted: 30 December 2015 ; Published: 7 January 2016 Academic Editor: Roman Dembinski EaStCHEM School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, UK; [email protected]; Tel.: +44-01334-463-864; Fax: +44-01334-463-808 Abstract: Oxime derivatives are easily made, are non-hazardous and have long shelf lives. They contain weak N–O bonds that undergo homolytic scission, on appropriate thermal or photochemical stimulus, to initially release a pair of N- and O-centred radicals. This article reviews the use of these precursors for studying the structures, reactions and kinetics of the released radicals. Two classes have been exploited for radical generation; one comprises carbonyl oximes, principally oxime esters and amides, and the second comprises oxime ethers. Both classes release an iminyl radical together with an equal amount of a second oxygen-centred radical. The O-centred radicals derived from carbonyl oximes decarboxylate giving access to a variety of carbon-centred and nitrogen-centred species. Methods developed for homolytically dissociating the oxime derivatives include UV irradiation, conventional thermal and microwave heating. Photoredox catalytic methods succeed well with specially functionalised oximes and this aspect is also reviewed. Attention is also drawn to the key contributions made by EPR spectroscopy, aided by DFT computations, in elucidating the structures and dynamics of the transient intermediates. Keywords: oxime esters; oxime ethers; free radicals; organic synthesis; photochemical reactions; EPR spectroscopy; photoredox catalysis; N-heterocycles 1. Introduction A huge variety of compounds containing the carbonyl functional group is available from natural and commercial sources. Of these, aldehydes and ketones can readily and efficiently be converted to oximes R1R2C=NOH (1) by treatment with hydroxylamine hydrochloride and a base. Alternatively, oximes can be prepared from reaction of organic and inorganic nitrites with various compounds containing acidic C–H atoms. Consequently, oximes are accessible in great diversity for further functional group transformations. A few instances of oximes themselves being used directly for radical generation are known. However, their O–H bonds are comparatively weak, usually in the range of 76–85 kcal¨ mol´1 [1] and, therefore, any radicals X‚ generated in their presence abstract these H-atoms with production of iminoxyl radicals 2 (see Scheme1). Many iminoxyls have been generated and studied [2–4] and they resemble nitroxide (aminoxyl) radicals in a number of ways; especially in that most are persistent. Frequently, therefore, the presence of an oxime serves to impede further radical reactions. Oximes are easily derivatised so that oxime esters (O-alkanoyl and O-aroyl oximes) and oxime ethers (O-alkyl and O-aryl oximes) are straightforward to make. The N–O bonds in these compounds are usually comparatively weak; ~50 kcal¨ mol´1 in oximes [5] and only 33–37 kcal¨ mol´1 in O-phenyl oxime ethers [6]. Homolytic scission of these N–O bonds can be accomplished either by photochemical or by thermal means thus yielding for each type an N-centred radical accompanied by one equivalent of its O-centred counterpart. For this reason, oxime derivatives, especially carbonyl-oximes containing the Molecules 2016, 21, 63; doi:10.3390/molecules21010063 www.mdpi.com/journal/molecules Molecules 2016, 21, 63 2 of 23 >C=N–OC(=O)–Molecules 2016, 21 unit,, 63 are finding increasing use as selective sources for free radicals. They offer2 of tangible 23 advantages over traditional initiators, such as diacyl peroxides, azo-compounds or organotin hydrides; all oftangible which haveadvantages well-known over traditional troublesome initiators, features. such as Mostdiacyloxime peroxides, derivatives azo-compounds are easily or organotin handled; are non-toxic,hydrides; non-pyrophoric all of which andhave have well- longknown shelf troublesome lives. The fieldfeatures. has Most expanded oxime to derivatives encompass are a hugeeasily range handled; are non-toxic, non-pyrophoric and have long shelf lives. The field has expanded to encompass of structural elements resulting in diverse and varied possibilities for subsequent transformations a huge range of structural elements resulting in diverse and varied possibilities for subsequent of the radicals. These compound types are being subsumed into more environmentally friendly transformations of the radicals. These compound types are being subsumed into more environmentally preparativefriendlymethods preparative and methods for new and means for new of means access of to access ranges to ofranges aza-heterocycles. of aza-heterocycles. This This article article reviews modernreviews methods modern of releasingmethods of radicals releasing from radicals both thesefrom both precursor these types.precursor Ithighlights types. It highlights how their how use has enabledtheir the use structures, has enabled reactions the structures, and kinetics reactions of and sets kinetics of C-, Nof- sets and ofO C-centred-, N- and radicalsO-centred to radicals be elucidated to in greaterbe elucidated detail than in greater heretofore. detail than heretofore. SchemeScheme 1. Oximes, 1. Oximes, carbonyl carbonyl oximes oximesand and oximeoxime ethe ethers;rs; production production of ofiminoxyl iminoxyl radicals. radicals. In earlier synthesis-orientated research, Forrester and co-workers employed oximeacetic Inacids earlier MeCR(CH synthesis-orientated2)2CR1=NOCH2CO2H research, and their Forrestert-butyl peresters and co-workers [7,8]. Hasebe’s employed group developed oximeacetic 1 acidsphotochemical MeCR(CH2)2 CRalkylations=NOCH and2CO arylations2H and theirwith t-butyloxime perestersesters Ph2C=NOC(O)R [7,8]. Hasebe’s [9,10] group and developedZard photochemicalestablished ingenious alkylations preparative and arylations methodology with from oxime oxime esters benzoates Ph2 C=NOC(O)RPhRC=NOC(O)Ph [9, 10[11],] andfrom Zard establishedO-benzoyl ingenious hydroxamic preparative acid derivatives methodology and from from sulfenylimines oxime benzoates R2C=NSAr PhRC=NOC(O)Ph [12–14]. Since the [ 11turn], from of the century, research has zeroed in on two particular sets of oxime-derived compounds: (a) carbonyl O-benzoyl hydroxamic acid derivatives and from sulfenylimines R2C=NSAr [12–14]. Since the turn of 1 2 the century,oxime derivatives research has and zeroed (b) O-aryl in on oxime two ethers particular R R C=NOAr. sets of oxime-derived Recent advances compounds:, important insights (a) carbonyl and details of some surprising outcomes are described with particular emphasis on mechanistic and oxime derivatives and (b) O-aryl oxime ethers R1R2C=NOAr. Recent advances, important insights computational aspects. and details of some surprising outcomes are described with particular emphasis on mechanistic and computational2. Oxime Esters aspects. and Related Carbonyl Oximes 2. Oxime2.1. General Esters Features and Related of Carbonyl Carbonyl Oxime Reactions Oximes 2.1. GeneralGeneralised Features ofstructures Carbonyl of Oxime the principal Reactions types of carbonyl oximes so far investigated are displayed in Chart 1. These carbonyl oxime types all have strong absorption bands in the 250–350 nm range and Generalisedreadily dissociate structures on UV of irradiation the principal in solution types ofby carbonyl cleavage oximesof their soN–O far bonds. investigated Cleavage are of displayed the in ChartO–C(O)1. These bonds carbonyl does not oximecompete types in solution all have under strong the absorption normal conditions bands infor the organic 250–350 reactions. nm range and readilyOccasional dissociate observations on UVof iminoxyl irradiation radicals in solution(R1R2C=NO by•), cleavageand products of theirtherefrom N–O can bonds. most probably Cleavage of the O–C(O)be attributed bonds to does residual not competetrace impurities in solution of the under oximes the from normal which conditions the carbonyl for organicoximes were reactions. Occasionalprepared. observations Density Functional of iminoxyl Theory radicals (DFT) (R 1computationsR2C=NO‚), and [B3LYP/6-31+G(d)] products therefrom with canmodel most oxime probably esters 3 also indicated N–O cleavage was thermodynamically favoured over O–C(O) cleavage. It is be attributed to residual trace impurities of the oximes from which the carbonyl oximes were prepared. apparent from Chart 1 that, by appropriate choice amongst these carbonyl oxime precursors, selective Density Functional Theory (DFT) computations [B3LYP/6-31+G(d)] with model oxime esters 3 also generation of radicals centred on either C-, N-, or O-atoms, with differing substitution patterns, can indicatedbe achieved. N–O cleavage The associated was thermodynamically chemistry has been inve favouredstigated over spectroscopically O–C(O) cleavage. and, of Itcourse, is apparent by end from Chartproduct1 that, byanalyses. appropriate The most choice successful amongst and helpful these carbonyl method for oxime mechanistic precursors, and dynamic selective information generation of radicalsabout centred the intermediates on either C-, has N-, been or EPR O-atoms, spectrosco withpy differing coupled with substitution QM computations. patterns, canThe besynergy achieved. The associatedachievable by chemistry their mutual has deployment been investigated has been demonstrated
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