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A cyclic alkylamidocarbene synthesis study and utility as a desulfurization reagent

A cyclic alkylamidocarbene synthesis study and utility as a desulfurization reagent
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  This journal is©The Royal Society of Chemistry 2016  Chem. Commun. Citethis: DOI:10.1039/c6cc01376g A cyclic (alkyl)(amido)carbene: synthesis, studyand utility as a desulfurization reagent † Zachary R. McCarty, a Dominika N. Lastovickova ab and Christopher W. Bielawski* bc The synthesis and study of a cyclic (alkyl)(amido)carbene is described.ThecarbenewasfoundtoundergoC–Hinsertionatlowtemperatures,formed cyclopropenes upon exposure to alkynes, and facilitateddesulfurization reactions. Spectroscopic studies revealed that thecarbene is strongly  p -accepting but retains a complimentary degreeof  r -donating properties.  While the chemistry of the  N  -heterocyclic carbenes (NHCs)continues to expand, 1 relatively less efforts have been directedtowards the study of carbenes which are structurally, electro-nically and chemically distinct from the diaminocarbene arche-type. 2 For example, our group 3–7 and others 8–17 have shown that the incorporation of carbonyl groups into the heterocyclicscaffolds of various NHCs strongly influences the electronicproperties of the resultant compounds. In particular, whencompared to their nucleophilic NHC analogues ( e.g. ,  I , Fig. 1),the  N  ,  N  0 -diamidocarbenes (DACs;  e.g. ,  II ) were found to bemore electrophilic and engaged in reactions more commonly associated with transient, electrophilic carbenes, including inter- and intramolecular C–H insertions, 7,18 the reversiblefixation of carbon monoxide, 3,4 and a range of cycloadditionreactions. 19,20  As disclosed by Bertrand and co-workers, the cyclic (alkyl)-(amino)carbenes (CAACs;  e.g. ,  III ) also display pronouncedelectrophilic characteristics, which is believed to stem fromthe relatively small singlet–triplet gap of the corresponding carbene nucleus; a consequence of the amino group bondedto the carbene nucleus. 21–25 Building on this knowledge, wepostulated that a carbene linked to an amide moiety and a s -donating alkyl group should display an unsurpassed degreeof electrophilicity, ‡  and thus potentially expand the rangeof carbene-facilitated transformations. Herein, we report thesynthesis and study of a cyclic (alkyl)(amido)carbene ( IV  ). As summarized in Scheme 1, condensation of 2,2,4,4-tetra-methylglutaryl dichloride 26 and 2,6-diisopropylaniline in thepresence of triethylamine/CH 2 Cl 2  afforded the tetramethyl-piperidine-2,6-dione  1  in 90% yield after purification. Subsequent treatment of   1  with 1.5 equivalents of diisobutylaluminumhydride (DIBAL) afforded the mono-reduced alcohol  2  in 48% yield after purification. Finally, the desired precursor  3  wasobtained in high yield  via  the treatment of   2  with thionylchloride (SOCl 2 ) following purification. Single crystals suitablefor a single crystal X-ray diffraction (XRD) analysis were obtainedby cooling a hexanes solution saturated with  3  to   30  1 C (Fig. 2). Fig. 1  Structures of various carbenes. DIPP = 2,6-diisopropylphenyl,Mes = 2,4,6-trimethylphenyl. Scheme 1  Synthesis of the cyclic (alkyl)(amido)carbene precursor  3 . a  Department of Chemistry, The University of Texas at Austin, Austin,Texas 78712, USA b Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science(IBS), Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea. E-mail:  c  Department of Chemistry and Department of Energy Engineering, Ulsan National  Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea †  Electronic supplementary information (ESI) available: Detailed experimentalprocedures, X-ray crystallographic data for  3 ,  5 ,  7 , and  9 , and NMR spectra. CCDC1024535 ( 3 ), 1024536 ( 5 ), 1024537 ( 7 ), and 1024538 ( 9 ). For ESI and crystallo-graphic data in CIF or other electronic format see DOI: 10.1039/c6cc01376g  Received 14th February 2016,Accepted 14th March 2016DOI: 10.1039/c6cc01376g ChemComm COMMUNICATION    P  u   b   l   i  s   h  e   d  o  n   2   4   M  a  r  c   h   2   0   1   6 .   D  o  w  n   l  o  a   d  e   d   b  y   F   L   O   R   I   D   A   A   T   L   A   N   T   I   C   U   N   I   V   E   R   S   I   T   Y  o  n   2   4   /   0   3   /   2   0   1   6   1   2  :   1   5  :   1   1 . View Article Online View Journal  Chem. Commun.  This journal is©The Royal Society of Chemistry 2016 The C(1)–Cl(1) bond distance of   3  (1.850(3) Å) was found to berelatively elongated when compared to a typical sp 3 C–Cl bond(1.76 Å), 27 and similar tothe analogous bondlengthmeasured inthe solid state structure of the HCl precursor of   II  (1.888(2) Å). 4  With  3  in hand, subsequent efforts were directed toward thegeneration of the corresponding cyclic (alkyl)(amido)carbene(Scheme 2). Treating   3  with 1.2 equivalents of sodium hexa-methyldisilazide (NaHMDS) at ambient temperatureimmediately resulted in the formation of the C–H insertion product   5 , asdetermined by   1 H NMR spectroscopy (C 6 D 6 ). In particular, theappearance of a septet (3.58 ppm) and a singlet (3.42 ppm) wereconsistent with the methine groups of the corresponding bicyclic product. Similar results were obtained when the depro-tonation reaction was performed at 0  1 C, as determined using  1 H NMR spectroscopy. The structure of   5  was unambiguously confirmed using a single crystal XRD analysis of colorlesscrystals of   5 , which were grown by slowly evaporating a diethylether/pentane (1:1 v/v) solution saturated with the compound(Fig. 3). The propensity of   4  to readily undergo C–H insertionis unique when compared to the intra- and intermolecularC–H insertion reactions previously reported for the DACs, assuch transformations typically require elevated temperatures(50–120  1 C) as well as relatively long periods of time. 3,18 Despite its high affinity to undergo intramolecular C–Hinsertion, the generation of the free carbene  4  was corroborated via  a series of trapping experiments. For example, generating the carbene  4  in the presence of S 8  afforded the corresponding thione  6 . Notably, the  13 C NMR resonance assigned to the C Q Sbond (219.02 ppm, CDCl 3 ; 220.08 ppm, THF- d  8 ) of   6  was foundto be significantly downfield when compared to those recordedfor the analogous five- (177.96 ppm; CDCl 3 ) 12 and six- (177.4 ppm;CDCl 3 ) 10 membered DAC-sulfur adducts, but similar to the sulfuradduct of   III  (213.6 ppm; THF- d  8 ). 28 Moreover, the treatment of  in situ  generated  4  with 1-phenyl-1-butyne at 0  1 C afforded thecyclopropene adduct   7 in54%yield. The structure of   7  wasfurthersupported by a single crystal XRD analysis of single crystals of   7 grown by slowly evaporating a saturated pentane/diethyl ethersolution (1:1 v/v) (Fig. 4). While other electron deficient carbenes,such as  III , have been shown to activate hydrogen, 23 deprotonat-ing   3  under an atmosphere of H 2  afforded  5  as the main product,even at elevated temperatures. As it is well known that NHCs formstable zwitterionicadducts with electrophilic heteroallenes, such as carbon disulfide (CS 2 ), we sought to utilize this benchmark reaction to probe theintrinsic nucleophilicity of   4 . 29 To our surprise, treatment of   4  with excess CS 2  (3 equivalents) did not afford the expectedzwitterionic adduct; rather, the two major products isolated wereidentified as the thione  6  and the thiol  8  (Scheme 3). Due to thepresence of substantial insoluble precipitates observed inthe reaction mixture, we believe that   4  desulfurized CS 2  andproduced highly reactive carbon monosulfide which is known 30 to spontaneously polymerize upon formation. As part of a seriesof control reactions, no reaction was observed between  3  andCS 2  in the absence of base. Likewise, performing a similar Fig. 2  POV-ray representation of  3  with the thermal ellipsoids set at 50%probability and most of the H-atoms omitted for clarity. Selected bondlengths (Å) and angles ( 1 ): C1–Cl1, 1.850(3); C1–N1, 1.442(4); C2–O1,1.223(4); C2–N1–C1, 124.0(2); N1–C1–Cl1, 109.95(19), O1–C2–N1, 119.2(3). Scheme 2  Generation of and reactivity displayed by  4 . Fig. 3  POV-ray representation of  5  with the thermal ellipsoids set at 50%probability and most of the H-atoms omitted for clarity. Selected bondlengths (Å) and angles ( 1 ): C1–N1, 1.4964(17); C1–C16, 1.5666(19); C2–N1,1.3775(18); C2–O1, 1.2222(17); N1–C1–C16, 101.95(10); C1–N1–C2,123.82(11); O1–C2–N1, 121.16(12). Fig. 4  POV-ray representation of  7  with thermal ellipsoids set at 50%probability and the H-atoms omitted for clarity. Selected bond lengths (Å)and angles ( 1 ): C1–N1, 1.491(2); C1–C24, 1.497(2); C1–C25, 1.501(2);C24–C25, 1.302(2); C24–C1–C25, 51.48(11); C24–C25–C1, 64.10(12);C25–C24–C1, 64.43(12). Communication ChemComm    P  u   b   l   i  s   h  e   d  o  n   2   4   M  a  r  c   h   2   0   1   6 .   D  o  w  n   l  o  a   d  e   d   b  y   F   L   O   R   I   D   A   A   T   L   A   N   T   I   C   U   N   I   V   E   R   S   I   T   Y  o  n   2   4   /   0   3   /   2   0   1   6   1   2  :   1   5  :   1   1 . View Article Online  This journal is©The Royal Society of Chemistry 2016  Chem. Commun. reaction with 0.5 equivalents of NaHMDS with respect to  3 resulted in the same molar ratio of products  6  and  8 . Collectively,these observations indicated that the free carbene  4  facilitatedthe desulfurization process.Building on aforementioned results, we explored the ability of   4  to desulfurize carbonyl sulfide (COS), a reagent which hasalso been shown to form zwitterionic adducts with NHCs. 31  When  4  was generated under an atmosphere of COS, the thiol  8  was isolated in 57% yield (Scheme 3). The reaction with COSlikely favors the release of the more stable CO gas over CS, asproducts with increased oxygen content ( e.g. ,  1  or  2 ) were not observed. Although similar transformations were reported by Berry and co-workers 32 using a Zr complex, to the best of ourknowledge, these are the first examples of carbene-facilitateddesulfurizations and reveal the potential of   4  to serve as anorganic alternative to metal-mediated processes.To gain additional insight into the unique reactivity displayedby   4 , its  s -donor and  p -acceptor abilities were quantified by synthesizing and analyzing various organometallic adducts(Fig. 5). Treating the  in situ  generated  4  with [Ir(COD)Cl] 2  (COD =1,5-cyclooctadiene) afforded the Ir complex [( 4 )Ir(COD)Cl] ( 9 ).The  13 C NMR resonance of the carbene nucleus in  9  was recordedat 287.6 ppm (CDCl 3 ). To the best of our knowledge, this is themost deshielded signal to be reported amongst the analogouscarbene-supported Rh or Ir carbene complexes, 33 including that of the Rh complex of an acyclic variant of   III  (279.5 ppm,CDCl 3 ). 34  A single crystal XRD analysis of orange crystals of   9 ,grown  via  the slow evaporation of a saturated heptanes solution,confirmed the structure of the complex (Fig. 6).The s -donor ability of   4  was determined from the Ir dicarbonylcomplex   10 , which was prepared by sparging a CH 2 Cl 2  solution of  9  with carbon monoxide. The corresponding product exhibiteda downfield  13 C NMR resonance at 280.6 ppm (CDCl 3 ), which wasassigned to the carbene nucleus, and supported its electrondeficient nature. The infrared spectrum of   10  exhibited twoprominent   n  CO  absorptions at 2068 and 1986 cm  1 (KBr,CH 2 Cl 2 ) which, using Nolan’s method, 33 translated into aTolman Electronic Parameter (TEP) of 2053 cm  1 , a valuesimilar to that calculated for SIMes  I  (2052 cm  1 ). 33 Based onthe reactivity observed for  4 , this result was surprising andindicated that the carbene is a relatively strong   s -donor whenused as a ligand. While the TEPvalue providesa useful correlationforassessing the donor ability of carbenes, several new methods have beenestablished to provide information on the corresponding  p -accepting abilities. 35 One valuable method entails an  77 Se NMR analysisofthe corresponding carbene–seleniumadductfollowedby comparison to other adducts. 36 Treatment of   4  (generated in situ ) with elemental selenium at ambient temperatureafforded  11  in 55% yield. In addition to the  13 C NMR resonanceassignedtothe carbene nucleus observedat 228.01 ppm (CDCl 3 ),a  77 Se NMR signal was recorded at 1179.71 ppm (acetone- d  6 ). When compared to the series of carbene–selenium adductsevaluated by Ganter and co-workers, the  77 Se NMR resonancedisplayed by   11  represents the most downfield signal reported todate and reflects the strong   p -accepting ability of   4 , surpassing even the five- and six-membered DACs ( 77 Se NMR: 856 and847 ppm, respectively). 36,37 In conclusion, we disclose the synthesis and study of a cyclic(alkyl)(amido)carbene. The corresponding precursor  3  wassynthesized in three steps and in good overall yield. Unliketraditional NHCs,  4  displayed a significant degree of electro-philicity as evidenced by an ability to undergo C–H insertion at low temperature and to cyclopropenate alkynes. The observedreactivity profile was supported by   77 Se NMR spectroscopy  which indicated that   4  is the most strongly   p -accepting carbenereported to date while its TEP value indicated that a relatively high degree of   s -donating ability is retained. The discovery of acarbene to function as a desulfurization reagent is expected tocatalyze further investigations into the unique transformationsfacilitated by electrophilic carbenes, particularly as metal-freealternatives in synthetic processes. Scheme 3  Carbene facilitated desulfurizations. Fig. 5  Structures of complexes used to evaluate the electronic propertiesof  4 . Fig. 6  POV-ray representation of  9  with thermal ellipsoids set at 50%probability and the H-atoms omitted for clarity. Selected bond lengths (Å)and angles ( 1 ): C1–Ir1, 1.944(5); C1–N1, 1.354(6); N1–C1–Ir1, 126.2(3). ChemComm Communication    P  u   b   l   i  s   h  e   d  o  n   2   4   M  a  r  c   h   2   0   1   6 .   D  o  w  n   l  o  a   d  e   d   b  y   F   L   O   R   I   D   A   A   T   L   A   N   T   I   C   U   N   I   V   E   R   S   I   T   Y  o  n   2   4   /   0   3   /   2   0   1   6   1   2  :   1   5  :   1   1 . View Article Online  Chem. Commun.  This journal is©The Royal Society of Chemistry 2016  We acknowledge the Institute for Basic Science (IBS-R019-D1), the National Science Foundation (CHE-1266323) and theBK21 Plus Program as funded by the Ministry of Education andthe National Research Foundation of Korea for their support. Notes and references ‡  Carbenes linked to amido and amino groups have been previously reported. 6,9,14–16,38–42 Such mixed (amido)(amino)carbenes typically exhibit reactivity profiles that are intermediate of prototypical NHCsand the DACs.1 For reviews on NHCs, see: F. E. Hahn and M. C. Jahnke,  Angew.Chem., Int. Ed. , 2008,  47 , 3122; T. 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