Communication Synthesis of a Stable Primary-Alkyl-Substituted Selenenyl Iodide and Its Hydrolytic Conversion to the Corresponding Selenenic Acid

Shohei Sase, Ryo Kakimoto, Ryutaro Kimura and Kei Goto *

Received: 1 November 2015 ; Accepted: 25 November 2015 ; Published: 2 December 2015 Academic Editor: Thomas G. Back

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan; [email protected] (S.S.); [email protected] (R.K.); [email protected] (R.K.) * Correspondence: [email protected]; Tel./Fax: +81-3-5734-3543

Abstract: A primary-alkyl-substituted selenenyl iodide was successfully synthesized through oxidative iodination of a selenol with N-iodosuccinimide by taking advantage of a cavity-shaped steric protection group. The selenenyl iodide exhibited high thermal stability and remained ˝ unchanged upon heating at 100 C for 3 h in [D8]toluene. The selenenyl iodide was reduced to the corresponding selenol by treatment with dithiothreitol. Hydrolysis of the selenenyl iodide under alkaline conditions afforded the corresponding selenenic acid almost quantitatively, corroborating the chemical validity of the recent proposal that hydrolysis of a selenenyl iodide to a selenenic acid is potentially involved in the catalytic mechanism of an .

Keywords: selenenyl iodide; selenenic acid; hydrolysis; kinetic stabilization; iodothyronine deiodinase

1. Introduction Selenenyl iodides (RSeI) have been attracting increasing attention as important intermediates of hormone deiodination by iodothyronine deiodinases, -dependent comprising three isoforms (ID-1, ID-2, and ID-3) that catalyze regioselective deiodination of iodothyronines [1,2]. However, model studies on the chemical processes involving the selenenyl iodide intermediates have often been hampered by the notorious instability of the species; selenenyl iodides usually undergo facile disproportionation to the corresponding diselenides and [3,4] (Scheme1). For the synthesis of stable selenenyl iodides, kinetic stabilization using bulky substituents or thermodynamic stabilization due to intramolecular coordination have been employed [5,6]. We previously succeeded in the isolation of a stable selenenyl iodide bearing a bulky aromatic group and applied it to the demonstration of the chemical transformations included in the catalytic cycle of ID-1 [7–10]. While all the isolable selenenyl iodides reported so far are aromatic derivatives or tertiary-alkyl derivatives, a primary-alkyl derivative is most relevant as a model compound of naturally occurring selenenyl iodides derived from selenocysteine. It is a common idea, however, that the steric demands of primary-alkyl groups are too small to protect such reactive species, and there has been no example of the synthesis of a primary-alkyl-substituted selenenyl iodide. In the course of our studies on biologically relevant, highly reactive species derived from thiols and selenols, we recently developed a primary-alkyl steric protection group, a BpqCH2 group (Figure1), with a cavity-shaped framework [11]. Despite being a primary-alkyl group, this substituent was found to be effective for the stabilization of reactive species such as a sulfenic acid (X = SOH) [12], a sulfenyl iodide (X = SI) [11], and a selenenic acid (X = SeOH) [13]. Here we report the synthesis of a stable primary-alkyl-substituted selenenyl iodide by utilizing this molecular cavity and its hydrolytic

Molecules 2015, 20, 21415–21420; doi:10.3390/molecules201219773 www.mdpi.com/journal/molecules Molecules 2015, 20, page–page has been proposed to be potentially involved in the second half-reaction of ID-3 [14], but no chemical evidenceMolecules has been2015, 20available, 21415–21420 for this reaction process.

Molecules 2015, 20, page–page Molecules 2015, 20, page–page conversion to the corresponding selenenic acid; hydrolysis of a selenenyl iodide to a selenenic acid has been proposed to be potentially involved in the second half-reaction of ID-3 [14], but no chemical hashas beenbeen proposedproposed toto bebe potentiallypotentially involvedinvolved inin thethe secondsecond half-reactionhalf-reaction ofof ID-3ID-3 [14],[14], butbut nono chemicalchemical evidence has been availableScheme 1. for Facile this reaction disproportionation process. of a selenenyl iodide. evidenceevidence hashas beenbeen availableavailable forfor thisthis reactionreaction process.process.

Scheme 1. Facile disproportionation of a selenenyl iodide. SchemeScheme 1. 1.FacileFacile disproportionation disproportionation ofof a a selenenyl selenenyl iodide. iodide.

Figure 1. Reactive species stabilized by a cavity-shaped primary-alkyl steric protection group.

2. Results and Discussion Figure 1. Reactive species stabilized by a cavity-shaped primary-alkyl steric protection group. FigureFigure 1. Reactive 1. Reactive species species stabilized stabilized by by a a cavity-sha cavity-shapedped primary-alkylprimary-alkyl steric steric protection protection group. group. Reaction of selenol 1 [13] bearing the BpqCH2 group with 1.2 equivalent of N-iodosuccinimide afforded selenenyl iodide 2 in 98% yield (Scheme 2). Selenenyl iodide 2 was isolated as air-stable 2.2. ResultsResults2. Results andand andDiscussionDiscussion Discussion purple crystals and characterized by NMR (1H-, 13C-, and 77Se-) and UV-vis spectroscopies, mass Reaction of selenol 1 [13] bearing the BpqCH2 group with 1.2 equivalent of N-iodosuccinimide spectrometry,ReactionReaction and of elementalselenol of selenol 1 [13]1 analysis.[13 bearing] bearing The the the BpqCHFD-MS BpqCH2 spectrumgroup withwith of 1.2 1.2 2 equivalent equivalentshowed ofthe N-of iodosuccinimidemolecularN-iodosuccinimide ion peak at affordedaffordedafforded selenenylselenenyl selenenyl iodideiodide iodide 22 in2inin 98%98% 98% yieldyield yield (Scheme(Scheme 2 ).2).2). Selenenyl SelenenylSelenenyl iodide iodideiodide2 was22 waswas isolated isolatedisolated as air-stable asas air-stableair-stable m/z 1090 with a characteristic isotopic pattern1 1of the1313 selenium7777 atom. In the UV-vis spectrum of 2 in purplepurplepurple crystalscrystals crystals andand and characterizedcharacterized characterized byby by NMRNMR NMR ( (1H-,H-, 13C-,C-, andandand 77Se-)Se-)Se-) and andand UV-vis UV-visUV-vis spectroscopies, spectroscopies,spectroscopies, mass massmass CHCl3, aspectrometry, weak diagnostic and elemental band due analysis. to the The Se–I FD-MS functionality spectrum of 2 wasshowed observed the molecular at 511 ion nm. peak The at m/zabsorption spectrometry,spectrometry, andand elementalelemental analysis.analysis. TheThe FD-MSFD-MS spectrumspectrum ofof 22 showedshowed thethe molecularmolecular ionion peakpeak atat band of 10902 showed with a characteristica blue shift isotopic compared pattern to of that the seleniumof the aryl-substituted atom. In the UV-vis selenenyl spectrum of iodide2 in CHCl without, the m/zm/z 10901090 withwith aa characteristiccharacteristic isotopicisotopic patternpattern ofof thethe seleniumselenium atom.atom. InIn thethe UV-visUV-vis spectrumspectrum ofof3 22 inin a weak diagnostic band due to the Se–I functionality was observed at 51177 nm. The absorption band of methyleneCHCl3, moietya weak diagnostic(BpqSeI ( 3band), λmax due = to553 the nm, Se–I Figure functionality 2) [7]. wasIn the observed Se-NMR at 511 spectrum nm. The absorption of 2, a signal CHCl3, a weak diagnostic band due to the Se–I functionality was observed at 511 nm. The absorption was bandobserved2 showedof 2 showedat aδ blue420, a shift whichblue compared shift is comparable compared to that of to the to that aryl-substituted that of ofthe 3 aryl-substituted(δ 465) selenenyl [7]. The iodide selenenylmethylene without iodide the protons methylene without adjacent the to band of 2 showed a blueλ shift compared to that of the77 aryl-substituted selenenyl iodide without the moiety (BpqSeI (3), max = 553 nm, Figure2)[7]. In the Se-NMR spectrum77 of 2, a signal was observed the Se–Imethylene moiety moiety in 2 showed (BpqSeI a signal(3), λmax at = δ 553 4.73, nm, which Figure is substantially2) [7]. In the 77 shifSe-NMRted downfield spectrum compared of 2, a signal to that methyleneat δ 420, moiety which (BpqSeI is comparable (3), λmax to that= 553 of nm,3 (δ Figure465) [7 ].2) The [7]. methyleneIn the Se-NMR protons spectrum adjacent to of the 2, Se–Ia signal was observed at δ 420, which is comparable to that of 3 (δ 465) [7]. The methylene13 protons adjacent to of selenolwas observedmoiety 1 (δ 4.03). in 2atshowed δ A420, downfield which a signal is atcomparable shiftδ 4.73, was which also to isthat observed substantially of 3 (δ 465) for shifted [7].the The α downfield-carbon methylene in compared protonsC-NMR to adjacent that spectroscopy of to (δ 28.2thethe Se–IforSe–Iselenol 2 moietymoiety and1 (δ δ in4.03).in 15.3 22 showedshowed A for downfield 1 a).a signalsignalThese shift atat downfield δ wasδ 4.73,4.73, also whichwhich observed shifts isis substantiallysubstantially forobserved the α-carbon shif shiffortedted 2 in downfieldwoulddownfield13C-NMR be compared spectroscopycomparedattributable toto thatthat to the 13 electron-withdrawingofof selenolselenol(δ 28.2 11 for ((δδ 4.03).24.03).and character AδA 15.3 downfielddownfield for 1of). the Theseshiftshift iodine waswas downfield alsoalso atom. observedobserved shifts Seleneyl observed forfor iodide thethe for αα-carbon-carbon 22would exhibited inin be 13C-NMRC-NMR attributable high thermal spectroscopyspectroscopy to the stability. ((δδ 28.228.2electron-withdrawing forfor 22 andand δδ 15.315.3 for characterfor 11).). TheseThese of the downfielddownfield iodine atom. shiftsshifts Seleneyl observedobserved iodide 2forforexhibited 22 wouldwould high bebe thermal attributableattributable stability. toto thethe In the solid state, 2 was decomposed at 181–183 °C.˝ In solution, 2 remained unchanged upon heating electron-withdrawingelectron-withdrawingIn the solid state, 2 wascharactercharacter decomposed ofof thethe iodine atiodine 181–183 atom.atom.C. SeleneylSeleneyl In solution, iodideiodide2 remained 22 exhibitedexhibited unchanged highhigh thermal uponthermal heating stability.stability. at 60 °C for 16˝ h and even at 100 °C ˝for 3 h in [D8]toluene. The selenenyl iodide 2 was reduced to the In theat solid 60 C state, for 16 2 h was and evendecomposed at 100 C forat 181–183 3 h in [D 8°C.]toluene. In solution, The selenenyl 2 remained iodide unchanged2 was reduced upon to theheating correspondingIn the solid state,selenol 2 was 1 by decomposed dithiothreitol at 181–183(Scheme °C. 3), In as solution, was in the2 remained case with unchanged the aromatic upon counterpart, heating at 60corresponding °C for 16 h and selenol even1 byat dithiothreitol100 °C for 3 h (Scheme in [D8]toluene.3), as was inThe the selenenyl case with theiodide aromatic 2 was counterpart, reduced to the at 60 °C for 16 h and even at 100 °C for 3 h in [D8]toluene. The selenenyl iodide 2 was reduced to the BpqSeI (BpqSeI3) [7]. (3)[7]. correspondingcorresponding selenolselenol 11 byby dithiothreitoldithiothreitol (Scheme(Scheme 3),3), asas waswas inin thethe casecase withwith thethe aromaticaromatic counterpart,counterpart,

BpqSeIBpqSeI ((33)) [7].[7].

SchemeScheme 2. Synthesis 2. Synthesis of the of the primary-alkyl- primary-alkyl-substitutedsubstituted selenenyl selenenyl iodide iodide2. 2. SchemeScheme 2.2. SynthesisSynthesis ofof thethe primary-alkyl-primary-alkyl-substitutedsubstituted selenenylselenenyl iodideiodide 22..

21416

FigureFigureFigure 2. 2.2. Aryl-substituted Aryl-substitutedAryl-substituted selenenylselenenyl iodideiodide iodide 33. . 3 .

22 Molecules 2015, 20, page–page has been proposed to be potentially involved in the second half-reaction of ID-3 [14], but no chemical evidence has been available for this reaction process.

Scheme 1. Facile disproportionation of a selenenyl iodide.

Figure 1. Reactive species stabilized by a cavity-shaped primary-alkyl steric protection group.

2. Results and Discussion

Reaction of selenol 1 [13] bearing the BpqCH2 group with 1.2 equivalent of N-iodosuccinimide afforded selenenyl iodide 2 in 98% yield (Scheme 2). Selenenyl iodide 2 was isolated as air-stable purple crystals and characterized by NMR (1H-, 13C-, and 77Se-) and UV-vis spectroscopies, mass spectrometry, and elemental analysis. The FD-MS spectrum of 2 showed the molecular ion peak at m/z 1090 with a characteristic isotopic pattern of the atom. In the UV-vis spectrum of 2 in CHCl3, a weak diagnostic band due to the Se–I functionality was observed at 511 nm. The absorption band of 2 showed a blue shift compared to that of the aryl-substituted selenenyl iodide without the methylene moiety (BpqSeI (3), λmax = 553 nm, Figure 2) [7]. In the 77Se-NMR spectrum of 2, a signal was observed at δ 420, which is comparable to that of 3 (δ 465) [7]. The methylene protons adjacent to the Se–I moiety in 2 showed a signal at δ 4.73, which is substantially shifted downfield compared to that of selenol 1 (δ 4.03). A downfield shift was also observed for the α-carbon in 13C-NMR spectroscopy (δ 28.2 for 2 and δ 15.3 for 1). These downfield shifts observed for 2 would be attributable to the electron-withdrawing character of the iodine atom. Seleneyl iodide 2 exhibited high thermal stability. In the solid state, 2 was decomposed at 181–183 °C. In solution, 2 remained unchanged upon heating at 60 °C for 16 h and even at 100 °C for 3 h in [D8]toluene. The selenenyl iodide 2 was reduced to the corresponding selenol 1 by dithiothreitol (Scheme 3), as was in the case with the aromatic counterpart, BpqSeI (3) [7].

Molecules 2015, 20, 21415–21420 Scheme 2. Synthesis of the primary-alkyl-substituted selenenyl iodide 2.

Figure 2. Aryl-substituted selenenyl iodide 3.. Molecules 2015, 20, page–page

Molecules 2015, 20, page–page 2

Scheme 3. Reduction of selenenyl iodide 2. Scheme 3. Reduction of selenenyl iodide 2. Scheme 3. Reduction of selenenyl iodide 2. Sulfenic acids (RSOH) [15,16] and selenenic acids (RSeOH) [17,18] play important roles as reactive intermediatesSulfenic acidsin various (RSOH) biological [15,16] processes. and selenenic It is acidswell known (RSeOH) that [17 they,18] playare formed important through roles the as Sulfenic acids (RSOH) [15,16] and selenenic acids (RSeOH) [17,18] play important roles as reactive reactiveoxidation intermediates of cysteine or in selenocysteine various biological residues processes. by reactive It is welloxygen known species. that Meanwhile, they are formed in the through course intermediates in various biological processes. It is well known that they are formed through the theof our oxidation studies of concerning cysteine or sulfur-containing selenocysteine residues reactive by species, reactive we oxygen previously species. demonstrated Meanwhile, that in the a oxidation of cysteine or selenocysteine residues by reactive oxygen species. Meanwhile, in the course coursesulfenic of acid our studiescan be concerningobtained by sulfur-containing alkaline hydrolysis reactive of the species, corresponding we previously thionitrate demonstrated (RSNO2 that) or of our studies concerning sulfur-containing reactive species, we previously demonstrated that a asulfenyl sulfenic bromide acid can (RSBr) be obtained [19]. This by alkalinecorroborates hydrolysis the possible of the formation corresponding of sulfenic thionitrate acids (RSNO by similar2) or sulfenic acid can be obtained by alkaline hydrolysis of the corresponding thionitrate (RSNO2) or sulfenyltransformations bromide in (RSBr) biological [19]. systems This corroborates [20,21]. As the for possiblethe selenium formation counterpart, of sulfenic it has acids recently by similar been sulfenyl bromide (RSBr) [19]. This corroborates the possible formation of sulfenic acids by similar transformationsproposed that related in biological hydrolytic systems formation [20,21 of]. a As selenenic for the seleniumacid is potentially counterpart, involved it has in recently the catalytic been transformations in biological systems [20,21]. As for the selenium counterpart, it has recently been proposedmechanism that of relatedID-3; Steegborn hydrolytic et formational. suggested of athat selenenic a selenenyl acid is iodi potentiallyde intermediate involved generated in the catalytic in the proposed that related hydrolytic formation of a selenenic acid is potentially involved in the catalytic mechanismactive site of of the ID-3; Steegborn after etdeiodination al. suggested of thatan iodothyronine a selenenyl iodide substrate intermediate might be generated hydrolyzed in the to mechanism of ID-3; Steegborn et al. suggested that a selenenyl iodide intermediate generated in the activeafford a site selenenic of the enzymeacid, which after is deiodinationthen reduced ofto ana selenol iodothyronine via a selenenyl substrate sulfide might [14]. be However, hydrolyzed there to active site of the enzyme after deiodination of an iodothyronine substrate might be hydrolyzed to affordhas been a selenenic no chemical acid, evidence which is for then the reduced hydrolytic to aco selenolnversion via of a selenenyla selenenyl sulfide iodide [14 to]. a However, selenenic thereacid. afford a selenenic acid, which is then reduced to a selenol via a selenenyl sulfide [14]. However, there hasAs a been model no chemicalreaction of evidence this chem forical the process, hydrolytic the conversion hydrolysis ofof a2 selenenyl was examined. iodide The to a selenenictreatment acid.of 2 has been no chemical evidence for the hydrolytic conversion of a selenenyl iodide to a selenenic acid. Aswith a an model aqueous reaction solution of this of chemical NaOH at process, 0 °C for the 2 hydrolysish led to the of formation2 was examined. of selenenic The treatmentacid 4 in 93% of 2 As a model reaction of this chemical process, the hydrolysis of 2 was examined. The treatment of 2 withyield an(Scheme aqueous 4). solutionSelenenyl of iodides NaOH atand 0 ˝selenenicC for 2 h acids led to are the intrinsically formation ofunstable selenenic because acid 4 ofin their 93% with an aqueous solution of NaOH at 0 °C for 2 h led to the formation of selenenic acid 4 in 93% yieldpropensity (Scheme to undergo4). Selenenyl facile iodides decomposition and selenenic via bimolecular acids are intrinsicallypathways. The unstable cavity-shaped because ofBpqCH their 2 yield (Scheme 4). Selenenyl iodides and selenenic acids are intrinsically unstable because of their propensitygroup effectively to undergo stabilizes facile both decomposition reactive species, via bimolecular enabling the pathways. experimental The cavity-shaped demonstration BpqCH of this2 propensity to undergo facile decomposition via bimolecular pathways. The cavity-shaped BpqCH2 groupelusive effectivelyreaction process. stabilizes both reactive species, enabling the experimental demonstration of this group effectively stabilizes both reactive species, enabling the experimental demonstration of this elusive reaction process. elusive reaction process.

Scheme 4. Hydrolysis of 2 to afford selenenic acid 4. Scheme 4. Hydrolysis of 2 to afford selenenic acid 4. 3. Experimental Section Scheme 4. Hydrolysis of 2 to afford selenenic acid 4. 3. ExperimentalUnless otherwise Section stated, all operations were performed by using high-vacuum and standard 3. Experimental Section Schlenk techniques under an argon atmosphere. Anhydrous tetrahydrofuran (THF) was purchased Unless otherwise stated, all operations were performed by using high-vacuum and standard fromUnless Kanto otherwiseChemical and stated, passed all operations through a wereKayama performed Oxygen by solvent using purification high-vacuum system and standard prior to Schlenk techniques under an argon atmosphere. Anhydrous tetrahydrofuran (THF) was purchased Schlenkuse. CHCl techniques3 was washed under with an argonconc. atmosphere.H2SO4 and distilled Anhydrous over tetrahydrofuranCaH2. [D8]Toluene (THF) was was distilled purchased from from Kanto Chemical and passed through a Kayama Oxygen solvent purification system prior to frombenzophenone Kanto Chemical ketyl. Other and passedchemicals through were purchas a Kayamaed from Oxygen commercial solvent sources purification and used system as received. prior to use. CHCl3 was washed with conc. H2SO4 and distilled over CaH2. [D8]Toluene was distilled from use.Preparative CHCl thinwas layer washed chromatography with conc. H (PTLC)SO and was distilled performed over CaHusing. [DMerck]Toluene silica wasgel distilled60 PF254 benzophenone3 ketyl. Other chemicals were2 purchas4 ed from commercial 2sources8 and used as received. from(Darmstadt, benzophenone Germany). ketyl. The Other 1H-NMR chemicals spectra werewere purchasedrecorded on from a JEOL commercial ECX-500, sources a JEOL andECX-400, used or as Preparative thin layer chromatography (PTLC) was performed using Merck silica gel 60 PF254 received.a JEOL ECS-400 Preparative spectrometer thin layer (Tokyo, chromatography Japan), and (PTLC) the chemical was performed shifts of using1H are Merck referenced silica gelto the 60 (Darmstadt, Germany). The 1H-NMR spectra were recorded on a JEOL ECX-500, a JEOL ECX-400, or residual proton signal of CDCl3 (δ 7.25). The 13C-NMR spectra were recorded on a JEOL ECX-500 or a a JEOL ECS-400 spectrometer (Tokyo, Japan), and the chemical shifts of 1H are referenced to the JEOL ECX-400 spectrometer, and the chemical shifts of 13C are referenced to the signal of CDCl3 (δ 77.0). residual proton signal of CDCl3 (δ 7.25). The 13C-NMR21417 spectra were recorded on a JEOL ECX-500 or a All spectra were assigned with the aid of DEPT, COSY, HMQC, and HMBC NMR experiments. The JEOL ECX-400 spectrometer, and the chemical shifts of 13C are referenced to the signal of CDCl3 (δ 77.0). 77Se-NMR spectra were recorded on a JEOL ECX-500 or JEOL ECX-400 spectrometer, and the chemical All spectra were assigned with the aid of DEPT, COSY, HMQC, and HMBC NMR experiments. The shifts of 77Se are referenced to diphenyl diselenide (δ 480) as external standard. UV-vis spectra were 77Se-NMR spectra were recorded on a JEOL ECX-500 or JEOL ECX-400 spectrometer, and the chemical recorded on a JASCO V-650 UV-vis spectrometer (Tokyo, Japan). Mass spectra were measured on a shifts of 77Se are referenced to diphenyl diselenide (δ 480) as external standard. UV-vis spectra were JEOL JMS-T100GCv “AccuTOF GCv” spectrometer using a field desorption probe (Tokyo, Japan). recorded on a JASCO V-650 UV-vis spectrometer (Tokyo, Japan). Mass spectra were measured on a Melting points were measured with a Yanaco MP-S3 and are uncorrected (Kyoto, Japan). JEOL JMS-T100GCv “AccuTOF GCv” spectrometer using a field desorption probe (Tokyo, Japan).

Melting points were measured with a Yanaco MP-S3 and are uncorrected (Kyoto, Japan).

3 3 Molecules 2015, 20, 21415–21420

1 PF254 (Darmstadt, Germany). The H-NMR spectra were recorded on a JEOL ECX-500, a JEOL ECX-400, or a JEOL ECS-400 spectrometer (Tokyo, Japan), and the chemical shifts of 1H are referenced 13 to the residual proton signal of CDCl3 (δ 7.25). The C-NMR spectra were recorded on a JEOL ECX-500 or a JEOL ECX-400 spectrometer, and the chemical shifts of 13C are referenced to the signal of CDCl3 (δ 77.0). All spectra were assigned with the aid of DEPT, COSY, HMQC, and HMBC NMR experiments. The 77Se-NMR spectra were recorded on a JEOL ECX-500 or JEOL ECX-400 spectrometer, and the chemical shifts of 77Se are referenced to diphenyl diselenide (δ 480) as external standard. UV-vis spectra were recorded on a JASCO V-650 UV-vis spectrometer (Tokyo, Japan). Mass spectra were measured on a JEOL JMS-T100GCv “AccuTOF GCv” spectrometer using a field desorption probe (Tokyo, Japan). Melting points were measured with a Yanaco MP-S3 and are uncorrected (Kyoto, Japan).

3.1. Synthesis of BpqCH2SeI (2) A solution of selenol 1 (15.1 mg, 15.6 µmol) [13] in THF (0.6 mL) in a Schlenk flask with a J-young valve was degassed through three freeze-pump-thaw cycles, and the flask was flushed with argon. To the solution was added a suspension of N-iodosuccinimide (4.2 mg, 19 µmol) in CCl4 (0.6 mL) at room temperature, and the resulting reaction mixture was stirred for 2 h at room temperature. After evaporation of the solvent, the residue was washed with acetonitrile. Recrystallization from Et2O afforded 2 (16.7 mg, 15.3 µmol, 98%) as purple crystals.

˝ 1 2: purple crystals; m.p. 181.4–182.5 C (dec); H-NMR (500 MHz, CDCl3) δ 1.10 (d, J = 6.9 Hz, 24H), 1.16 (d, J = 6.9 Hz, 24H), 2.85 (sept, J = 6.9 Hz, 8H), 4.73 (s, 2H), 7.05 (t, J = 1.7 Hz, 2H), 7.19 (d, J =7.8 Hz, 8H), 7.20 (d, J = 1.7 Hz, 4H), 7.31-7.34 (m, 6H), 7.40 (t, J = 7.6 Hz, 1H); 13C-NMR (125 MHz, CDCl3) δ 24.25 (q), 24.29 (q), 28.2 (t), 30.5 (t), 122.5 (d), 127.5 (d), 127.9 (d), 128.7 (d), 129.9 77 (d), 130.0 (d), 132.9 (s), 138.8 (s), 140.1 (s), 140.6 (s), 143.3 (s), 146.8 (s); Se-NMR (95 MHz, CDCl3) + δ 419.7; UV-vis (CHCl3, 298 K) λmax 511 nm (ε =153); LRMS (FD-TOF) m/z 1090 (M ). Anal. Calcd 1 13 for C67H79ISe: C, 73.80; H, 7.30. Found: C, 73.82; H 7.23. For the H and C-NMR spectra, see Supplementary Materials.

3.2. Heating of 2 in [D8]toluene

A solution of 2 (5.5 mg, 5.0 µmol) in [D8]toluene (0.5 mL) in a 5 mm o/d NMR tube with a J-young valve was heated at 60 ˝C for 16 h. In an independent experiment, a solution of 2 (5.5 mg, ˝ 5.0 µmol) in [D8]toluene (0.5 mL) was heated at 100 C for 3 h. In both cases, no change was observed in the 1H-NMR spectra.

3.3. Reduction of 2 with Dithiothreitol

To a solution of 2 (37.7 mg, 34.6 µmol) in CHCl3 (3 mL) in a Schlenk flask was added dithiothreitol (12.3 mg, 79.7 µmol) and triethylamine (11 µL, 0.11 mmol). The mixture was degassed through three freeze-pump-thaw cycles, and the flask was flushed with argon. The degassed solution was stirred ˝ at 40 C for 15 h and then quenched with 5% aq. NH4Cl. After extraction with CHCl3, the combined organic layer was dried over MgSO4, and the solvent was evaporated to give the crude mixture. Purification by PTLC (hexane:CHCl3 = 4:1) afforded 1 (22.2 mg, 23.0 µmol, 66.7%) as colorless solids.

3.4. Hydrolysis of 2 A solution of 2 (29.5 mg, 27.1 µmol) in THF (1 mL) in a Schlenk flask was degassed through three freeze-pump-thaw cycles, and the flask was flushed with argon. To the solution was added an aqueous solution of NaOH (1.4 M, 117 µL, 0.16 mmol) at 0 ˝C, and the reaction mixture was stirred at 0 ˝C for 1 h. Quenching and work-up were carefully performed under an argon atmosphere. Thus, the resulting white suspension in an ice bath was treated with phosphate buffer (pH 7, 0.4 mL) followed by degassed water (0.8 mL). After extraction with degassed CHCl3 (1 mL ˆ 3), the combined

21418 Molecules 2015, 20, 21415–21420

organic layer was dried over MgSO4. Evaporation of the solvent afforded selenenic acid 4 [13] as white solids (24.7 mg, 25.2 µmol, 93%).

4. Conclusions A primary-alkyl-substituted selenenyl iodide was successfully synthesized by taking advantage of a cavity-shaped steric protection group. Selenenyl iodides are usually unstable because of facile disproportionation to the corresponding diselenides and iodine. In contrast, the selenenyl iodide synthesized in this study showed remarkable thermal stability despite being a primary-alkyl derivative, demonstrating the effectiveness of the steric protection due to the cavity-shaped substituent. By utilizing the stable selenenyl iodide, a model reaction for a chemical process related to the function of an iodothyronine deiodinase was examined. Hydrolysis of the selenenyl iodide under alkaline conditions cleanly led to the formation of the corresponding selenenic acid, corroborating the chemical validity of the recent proposal that hydrolytic conversion of a selenenyl iodide to a selenenic acid is potentially involved in the catalytic mechanism of ID-3.

Supplementary Materials: Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/20/ 12/19773/s1. Acknowledgments: This work was partly supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Molecular Activation Directed toward Straightforward Synthesis” (No. 22105011), a Grant-in-Aid for Scientific Research (B) (No. 15H03776), a Grant-in-Aid for Scientific Research on Innovative Areas “Stimuli-responsive Chemical Species for the Creation of Functional Molecules” (No. 15H00930), and a Grant-in-Aid for Scientific Research (C) (No. 26410038) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Author Contributions: R. Kakimoto and R. Kimura performed the experimental works; S. Sase and K. Goto conceived and designed the project, and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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