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An Unusually Persistent Triplet Carbene molecules Article Diphenylcarbene Protected by Four ortho-Iodine Groups: An Unusually Persistent Triplet Carbene Katsuyuki Hirai 1,*, Kana Bessho 2, Kosaku Tsujita 2 and Toshikazu Kitagawa 2,* 1 Community-University Research Cooperation Center, Mie University, Tsu, Mie 514-8507, Japan 2 Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie 514-8507, Japan; [email protected] (K.B.); [email protected] (K.T.) * Correspondences: [email protected] (K.H.); [email protected] (T.K.); Tel.: +81-59-231-9417 (K.H.); +81-59-231-9416 (T.K.) Academic Editor: Shunichi Fukuzumi Received: 20 October 2016; Accepted: 11 November 2016; Published: 15 November 2016 Abstract: Diphenyldiazomethane with four iodine groups at the ortho positions and two tert-butyl groups at the para positions, i.e., bis(4-tert-butyl-2,6-diiodophenyl)diazomethane (1a-N2), was synthesized as a sterically hindered triplet carbene precursor. Irradiation of 1a-N2 in solution effectively generated the corresponding triplet diphenylcarbene 31a, which was characterized by UV-vis spectroscopy at low temperature, along with laser flash photolysis techniques at 3 room temperature. The UV-vis spectrum of 1a was obtained by irradiating 1a-N2 in a 2-methyltetrahydrofuran matrix at 77 K. The ESR spectrum showed no triplet carbene signals, while a radical species was observed at the anticipated temperature of the decomposition of triplet carbene 31a. Transient absorption bands ascribable to 31a were observed by laser flash photolysis of 1a-N2 in a degassed benzene solution and decayed very slowly with a second-order rate −3 −1 constant (2k/"l) of 5.5 × 10 ·s . Steady-state irradiation of 1a-N2 in degassed benzene afforded 9,10-diarylphenanthrene derivative 2a in a 31% yield. Triplet carbene 31a was also trapped by 5 −1 −1 −1 −1 either oxygen (kO2 = 6.5 × 10 M ·s ) or 1,4-cyclohexadiene (kCHD = 1.5 M ·s ) to afford the corresponding ketone 1a-O or the diarylmethane 1a-H2. The carbene was shown to be much less reactive than the triplet diphenylcarbene that is protected by two ortho-iodo and two ortho-bromo groups, 31b. Keywords: triplet carbene; diazo compound; photolysis; steric hindrance; substituent effect 1. Introduction Although stable N-heterocyclic carbenes (NHCs) have been frequently used as effective ligands for transition-metal catalysts [1,2], the instability of their triplet counterparts has hampered their use as materials. Thus, the stabilization and isolation of a triplet carbene has emerged as the next challenging target. Recent growing interest in triplet carbenes as potential organic ferromagnets [3–5] has added a more practical aspect to the project. Steric protection is an ideal method to achieve sufficient stabilization of a triplet carbene with its electronic integrity (one centered diradical) intact. Based on this concept, we generated many diphenylcarbenes that have various substituents at the ortho positions [6–9]. The strategy, however, encounters limitations when alkyl or aryl groups are employed as protecting groups. The central carbon atom of carbenes abstracts hydrogen even from very poor sources of electrons such as C-H bonds. Thus, the tert-butyl group, which has been successfully used to protect many reactive centers, is almost powerless to stabilize a triplet carbene center [10]. On the other hand, the van der Waals radius of bromine atom (185 pm) is larger than that of methyl group (175 pm) [11], and furthermore, the C-Br bond length (181 pm) of bromobenzene is longer than that of the C-CH3 (177 pm) in toluene [11]. Thus, the bromine groups at ortho positions on Molecules 2016, 21, 1545; doi:10.3390/molecules21111545 www.mdpi.com/journal/molecules Molecules 2016, 21, 1545 2 of 15 Molecules 2016, 21, 1545 2 of 15 diphenylcarbene overhang the carbene carbon more effectively. Moreover, halides involving bromine bromineatom are atom not reactive are not withreactive the with triplet the state triplet [12 stat–14e]. [12–14]. For example, For example, we previously we previously showed showed that triplet that tripletbis(2,6-dibromo-4- bis(2,6-dibromo-4-tert-butylphenyl)carbenetert-butylphenyl)carbene (31c) had (31c a) half-lifehad a half-life of 16 s of in 16 a benzenes in a benzene solution solution at room at roomtemperature temperature [15,16 ].[15,16]. The iodine iodine atom atom has has been been regarded regarded as asa better a better kine kinetictic protector protector of carbene of carbene because because its van its der van Waals der Waalsradius radius(198 pm) (198 is much pm) islarger much than larger that thanof the that bromine of the atom bromine (185 pm) atom [11]. (185 In pm) addition, [11]. according In addition, to theoreticalaccording tocalculations theoretical [17,18], calculations the length [17,18 of], the the C-I length bond of (210 the C-Ipm) bondof iodobenzene (210 pm) of is iodobenzenegreater than thatis greater of the thanC-Br bond that of(189 the pm) C-Br in bromobenzene. bond (189 pm) Recently, in bromobenzene. we generated Recently, and characterized we generated a triplet and diphenylcarbenecharacterized a triplet bearing diphenylcarbene two iodine and bearingtwo bromine two iodine groups and at the two ortho bromine positions, groups (2,6-dibromo- at the ortho positions,4-tert-butylphenyl)(4- (2,6-dibromo-4-tert-butyl-2,6-diiodophenyl)carbenetert-butylphenyl)(4-tert-butyl-2,6-diiodophenyl)carbene 31b [19] (Scheme 1). This3 1bspecies[19] (Schemewas found1). Thisto bespecies a persistent was found triplet to carbene, be a persistent but contrary triplet to carbene, our expectations, but contrary its to half-life our expectations, (t1/2 = 24 s) its was half-life only 3 3 3 (1.5t1/2 times= 24 longer s) was than only that 1.5 times of 1c longer. The bimolecular than that of rate1c. constants The bimolecular of the decay rate constants of 1b by oftriplet the decaycarbene of quenchers31b by triplet were carbene also only quenchers moderately were smaller also only than moderately those of 31c smaller. Although than thosean iodine of 31c group. Although shielded an theiodine carbene group center shielded more the effectivel carbene centery than morea bromine effectively group, than the a brominehalf-occupation group, theof an half-occupation iodine atom ofat thean iodinefour ortho atom positions at the four of aortho diphenylcarbenepositions of a was diphenylcarbene insufficient to was protect insufficient the carbene to protect center. the One carbene may expectcenter. Onethat mayan extreme expect thatstabilization an extreme would stabilization be achieved would by be thorough achieved bysubstitution thorough substitution of the four ofortho the positionsfour ortho positionswith iodo with groups. iodo groups.In the Inpresent the present study, study, we were we were successful successful in inpreparing preparing a aprecursor precursor diphenyldiazomethane with four ortho-iodo groups, bis(4- tert-butyl-2,6-diiodophenyl)diazomethane 3 (1a-N2).). Here Here we we report that triplet diphenylcarbene 1a1a generated by the photolysis of 1a-N2,, is an unusually persistent triplet diphenylcarbene with an extended half-life of 18.4 min.min. I N2 I I I hν –N2 I I I I 1a-N2 1a Br N2 I Br I hν –N2 I I Br Br 1b-N2 1b Br N2 Br Br Br hν Br –N2 Br Br Br 1c-N 1c 2 Scheme 1. Generation of diarylcarbenes 1. 2. Results Results and and Discussion Discussion 2.1. Preparation of Precursor Diazomethane Most diphenyldiazomethanes that that bear four kinetic protectors at the ortho positions have been prepared from the corresponding diphenylmethanolsdiphenylmethanols via via the carbamate derivatives derivatives [10,15,16,19]. [10,15,16,19]. For the synthesis of 1a-N2 (Scheme(Scheme 22),), thethe startingstarting alcoholalcohol waswas expectedexpected toto bebe obtainedobtained byby aa couplingcoupling reaction of of a a 2,6-diiodobenzaldehyde 2,6-diiodobenzaldehyde with with a 2,6-d a 2,6-diiodophenyllithium.iiodophenyllithium. According According to the to synthesis the synthesis plan, plan,we attempted we attempted the lithiation the lithiation of 5- oftert 5-tert-butyl-1,2,3-triiodobenzene-butyl-1,2,3-triiodobenzene (3), ( 3of), ofwhich which the the terttert-butyl-butyl group affords a para-substituent in 1a.A. A para-tert-butyl group is known to serve serve as as a a good good protecting protecting group in diphenylcarbenesdiphenylcarbenes [15,16]. [15,16]. Fortun Fortunately,ately, the the lithiation proceeded proceeded exclusively exclusively at at the 2-position of triiodobenzene to selectively afford the corresponding diphenylmethanol 4 by the subsequent reaction with 4-tert-butyl-2,6-diiodobenzaldehyde [19]. The success of the synthesis of alcohol 4 opened the way to diazomethane 1a-N2. Alcohol 4 was chlorinated, substituted with ethyl carbamate, nitrosated, Molecules 2016, 21, 1545 3 of 15 triiodobenzene to selectively afford the corresponding diphenylmethanol 4 by the subsequent reaction withMolecules 4-tert 2016-butyl-2,6-diiodobenzaldehyde, 21, 1545 [19]. The success of the synthesis of alcohol 4 opened the3 wayof 15 to diazomethane 1a-N2. Alcohol 4 was chlorinated, substituted with ethyl carbamate, nitrosated, and andtreated treated with potassiumwith potassiumtert-butoxide tert-butoxide to give theto give crude the diazomethane. crude diazomethane. The purification The purification by preparative by preparativethin layer chromatography thin layer chromatography (alumina, hexane (alumina, as eluent) hexane afforded as eluent) pure afforded1a-N2 as pure yellow 1a-N crystals2 as yellow in a crystals33% yield. in Diazoa 33% compoundyield. Diazo1a -Ncompound2 was very 1a stable-N2 was for very a diazomethane stable for a anddiazomethane did not decompose and did innot a decomposerefrigerator in (at a− refrigerator20 ◦C) over (at an − extended20 °C) over period an extended of time. period of time. I I I OH I Cl I I I I I2 1.
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