pubs.acs.org/JACS Article Two Copper-Carbenes from One Diazo Compound María Á lvarez, Maria Besora, Francisco Molina, Feliu Maseras,* Tomás R. Belderrain,* and Pedro J. Pérez* Cite This: J. Am. Chem. Soc. 2021, 143, 4837−4843 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Many transition-metal complexes MLn decompose diazo compounds N2CR1R2 generating metal-carbenes LnM CR1R2 which transfer the carbene group to other substrates, constituting an important tool in organic synthesis. All previous reports have shown that the CR1R2 fragment at the metal-carbene remains intact from the parent diazo compound. Herein we report the detection and isolation of a monosubstituted copper carbene where the CR1R2 ligand has undergone a modification from the initial diazo reagent. When TpMsCu(THF) (TpMs = hydrotris(3- mesityl)pyrazolylborate ligand) was reacted with N,N-diethyl diazoacetamide [N2C(H)(CONEt2)], the stable copper carbene TpMsCuC(H)(NEt2) was isolated, resulting from a decarbon- ylation process, with carbon monoxide being trapped as TpMsCu- (CO). The simultaneous observation of products derived from the intramolecular carbene insertion reaction into C−H bonds demonstrates that the expected TpMsCuC(H)(CONEt2) complex is also formed. Experimental data, DFT calculations, and microkinetic models allow us to propose that the latter undergoes CO loss en route to the former. ■ INTRODUCTION More than a century after Buchner postulated the existence of are diazocarbonyl compounds,4 with a CO group directly bonded to the diazo functionality: the acceptor nature of the carbene CR2 groups during the thermal decomposition of ethyl −COR group favors the transfer of the carbene group toward diazoacetate,1 the catalytic transfer of such moiety from diazo the nucleophile. compounds yet constitutes an area of continuous growth.2,3 Herein we report the observation of the unprecedented Such process consists of the metal-induced decomposition of modification of the carbene unit during the course of a copper- the diazo reagent in a process in which molecular N2 is catalyzed transformation. The use of a diazoacetamide extruded and a metallocarbene intermediate MC is formed compound (Scheme 1c) bearing a CONR2 substituent leads (Scheme 1a).4 This species is electrophilic5 in nature and to the formation of the expected, undetected, hot, highly reacts with available nucleophiles transferring the carbene reactive copper carbene MCE (Scheme 1c) which promotes group, thus liberating the metal to continue the catalytic cycle. the intramolecular C−H bond functionalization of the ethyl This strategy has been successfully employed in the addition of groups of the amide fragment. In a parallel manner, MCE carbene groups to unsaturated bonds or in its insertion into undergoes the loss of CO en route to the formation of the C−H or other C−X bonds, both intra- and intermolecu- unexpected, stable and isolable, cold, Fischer carbene complex larly.6−8 MCU (Scheme 1c), which has been structurally characterized. With the appropriate tuning of the carbene substituents and the metal precursor, several metallocarbene complexes have been detected, some of them being isolated and structurally ■ RESULTS AND DISCUSSION Reaction of TpMsCu(THF) and N2C(H)(CONEt2). Our characterized. All metals from groups 8 to 11 are known to group has been involved in the area of carbene transfer from catalyze the carbene transfer from diazo compounds (Scheme diazo compounds, with emphasis on its application to the 1b), and at least one detected/isolated example of a metal- carbene intermediate formed from a diazo compound has been reported for each of them.8−18 In all cases, the CR1R2 moiety Received: February 10, 2021 in the initial diazo compound appears unmodified in the Published: March 18, 2021 subsequent metal-carbene intermediate (Scheme 1b), from where it is further transferred to the nucleophile. From here, it is assumed that the carbene ligand is always transferred without modification. One of the most popular diazo reagents © 2021 American Chemical Society https://doi.org/10.1021/jacs.1c01483 4837 J. Am. Chem. Soc. 2021, 143, 4837−4843 Journal of the American Chemical Society pubs.acs.org/JACS Article Scheme 1. Metal-Catalyzed Carbene Transfer from Diazo Scheme 2. (a) Copper-Catalyzed Intramolecular C−H Bond Compounds and the Formation of Metalocarbene Functionalization of N−N-Diethyldiazoacetamide; (b) 13 Intermediates C{1H} NMR Spectrum of the Reaction of Copper Complex 1 and 13C-Labeled Diazoacetamide (2*) enriched in 13C at the 13CN2 site, in toluene-d8 (Scheme 6,19 functionalization of C−H bonds of unmodified alkanes, for 2b). Monitoring of the reaction by 13C{1H} NMR spectros- which the design of very active catalysts precluded the copy showed the appearance of a resonance centered at 236.6 observation of intermediates. However, we recently detected ppm. This is within the typical region of CuC moieties copper-carbene species20 in solution upon reacting TpMsCu- reported by Hofmann25 for the complexes [tBu2P(NSiMe3)2- (THF) (1) and ethyl phenyldiazoacetate (PheDA), which κ2N]CuC(Ar)C(O)R) (235.8−219.0 ppm), by Warren13 were stable at temperatures below 10 °C. After those findings, for [β-diketiminate]CuCPh2 (253.1 ppm), or by our group we aimed to detect copper-carbene intermediates with for complexes TpxCuC(Ph)(CO2Et) (Tpx = TpiPr2, Tp*, monosubstituted diazo compounds, which yet remains a TpMs; 233.9, 236.8, and 248.5 ppm, respectively).20 Under challenge, particularly with the most popular catalysts within these conditions, the resonances for 3, 4, and 5 were also this field, i.e., rhodium and copper, for which only observed, the latter being the major product. The reaction was disubstituted carbene species have been detected or immediate, and no free diazo compound 2* was observed after isolated.11,13,21 Toward that end we chose TpMsCu(THF)22 5 min. Successive registration of 13C{1H} NMR spectra for as the copper source, since the TpMs ligand provides a high hours showed no change in the composition of the mixture. degree of steric protection to the metal center and, when In view of the apparent stability of the species bearing the formed, to the carbene ligand. Regarding the diazo reagent, we CuC unit, we repeated the reaction at a bench scale (see selected N,N-diethyl diazoacetamide (2), in view of the Supporting Information (SI) for optimization conditions). previously reported23 intramolecular carbene insertion into When 0.45 g of 1 were reacted with 5 equiv of diazo 2 in the C−H bonds of the ethyl N-substituents in the presence of toluene at room temperature for 1 h, a yellowish solution was 1 as the catalyst (Scheme 2a), leading to mixtures of lactams 3 formed from which, after workup, the new compound 7 was and 4. Olefin 5 was also observed from the catalytic reaction of isolated as crystalline material in 35% yield. The 1H NMR two molecules of 2.24 Since heating at 70 °C was needed to spectrum of 7 showed three equivalent pyrazolyl rings, two reach efficient ratios, we reasoned that perhaps the copper- inequivalent ethyl groups, and a singlet at 8.02 ppm (Scheme carbene intermediate expected from the reaction of 1 and 2, 2c), which corresponds to one proton that correlates in the TpMsCuC(H)(CONEt2) (6), could be stable enough at 2D-HSQC experiment with the aforementioned signal at 236.6 room temperature to be detected. Based on this idea, we ppm (Scheme 2d). In the sample derived from the 13C-labeled started this study carrying out the reaction of TpMsCu(THF) diazo compound, the singlet at 8.02 in the 1H NMR spectrum (1) with 5 equiv of N,N-diethyl diazoacetamide (2*), 100% split into a doublet with JC−H = 115 Hz (Scheme 2c). 4838 https://doi.org/10.1021/jacs.1c01483 J. Am. Chem. Soc. 2021, 143, 4837−4843 Journal of the American Chemical Society pubs.acs.org/JACS Article Albeit the above data could support the assignment of Scheme 3. Reaction of TpMsCu(THF) with Doubly complex 7 as the pursued TpMsCuC(H)(CONEt2), some Isotopically Labeled 2** and Region of the 13C{1H} NMR other experimental data were not in agreement with that Spectrum Showing the Labeled Carbene and Carbonyl proposal: neither the FT-IR spectrum showed the expected Groups of 7* and 8* absorption for the CO group nor the 13C{1H} NMR spectrum displayed any resonance within the carbonyl region. We suspected that a decarbonylation process could have occurred, an idea that was confirmed when single crystals of this complex were grown, and the molecular structure determined by X-ray studies showed the formulation TpMsCuC(H)(NEt2) (7).26 As shown in Figure 1, this complex contains the TpMs ligand Figure 1. Molecular structure of the molecules for complex 7. Hydrogens have been omitted for clarity. bonded to copper in a κ3-fashion, and a carbene ligand with two substituents: a hydrogen and a diethylamido group, the latter resulting from the loss of the CO present in the initial diazo compound. The distance Cu1−C1, 1858(5) Å, is similar to that reported by Warren for [β-diketiminate]CuCPh2 (1.834(3) Å).13 Complex 7 constitutes the first example of a metal-carbene complex formed from a diazo compound in which the CR1R2 moiety in the latter is different from that in the former. Additionally, it is also the first example of a monosubstituted copper carbene complex.27 Further investigation of the 1H NMR spectrum of the experiment carried out with a 1:5 ratio (excess of diazo is employed since C−H bond insertion also occurs in a catalytic manner to some extent) of 1 and 2* showed that, in addition to 7, another TpMsCu-containing species was formed, both accounting for all detectable TpMsCu cores. Such compound has been identified as TpMsCu(CO) (8) (Scheme 3), as the Figure 2. (a) Plot of the yields into complex 7 vs temperature (°C). result of the trapping of carbon monoxide by 1.22 To gain (b) Variation of the yield of complex 7 with time at different further information, we have prepared the doubly isotopically temperatures. enriched N2=13C(H)(13CONEt2) (2**) diazo compound and monitored its reaction with complex 1, observing the Supporting Information). We have also monitored the resonances of carbene and carbonyl ligands of 7 and 8, concentration of complex 7 with time at three different respectively (see Supporting Information also). Thus, this temperatures (Figure 2b) and have found that at 0 or 25 °C observation unambiguously demonstrates the existence of a such concentration remains constant after 24 h. However, at decarbonylation process. 70 °C, a process involving the disappearance of 7 takes place The yield in complex 7 is dramatically affected by with time in such a way that after 12 h it cannot be detected in temperature, as shown in Figure 2a. The reaction of 1 and 2 solution. (1:5 ratio) has been performed within the −30 to +70 °C The reactivity of isolated copper carbene 7 has also been range, showing an increase in the reaction yield from 2% to investigated. On one hand, no reaction was observed with 78%, respectively, in 1 h time experiments. The high yield styrene, tert-butanol, benzene, or cyclohexane from room preparation and isolation of 7 is better performed at 70 °C (see temperature to 70 °C. It seems clear that the loss of the CO 4839 https://doi.org/10.1021/jacs.1c01483 J. Am. Chem. Soc. 2021, 143, 4837−4843 Journal of the American Chemical Society pubs.acs.org/JACS Article Scheme 4. General Picture of the Reactivity Derived from the Interaction of TpMsCu(THF) (1) and N2C(H)CONEt2 (2) group modifies the electronic behavior of the carbene Scheme 5. (a) Wolff Rearrangement of Diazo Compounds; C(H)NEt2 ligand decreasing the electrophilicity from (b) Reversible Formation of Ketenes from Carbene and CO TpMsCuC(H)(CONEt2) (6) to TpMsCuC(H)NEt2 (7). Ligands Described by Grotjahn; (c) Plausible Intermediate Interestingly, heating toluene solutions of isolated 7 at 70 °C for the Conversion of 6 into 7 for hours/days did not lead to any transformation, whereas the same experiment under a CO atmosphere (4 bar) for 6 h induced a clean conversion of 7 into TpMsCu(CO) (8). NMR studies have also shown that the olefin (Et2N)(H)C C(H)(NEt2) (9) derived from the coupling of two of the carbene units in 7 is formed.This result agrees with the lack of observation of copper-carbene 7 in the reaction of 1 and 2 at 70 °C for several hours, because of the presence of CO from the decarbonylation process. The reactivity observed from the initial mixture of 1 and 2 (1:5 ratio) is rationalized in Scheme 4. The TpMsCu core is generated upon decoordination of THF and reacts with the diazo compound 2 to generate the transient copper carbene 6. This species undergoes either the intermolecular reaction with a second molecule of 2 or the intramolecular insertion of the carbene group into the secondary or primary C−H bonds of the ethyl groups of the NEt2 fragment, thus yielding lactams 3 and 4, these catalytic cycles taking place at room temperature. CO ligand (Scheme 5b).29 Based on these precedents, it could Intermediate 6 can alternatively undergo a decarbonylation be possible that copper-carbene 6 undergoes a related process which generates the isolable copper carbene 7, a rearrangement in the coordination sphere of copper, followed process which occurs, at least, within the −30 to +70 °C by CO extrusion (Scheme 5c). As mentioned above, the interval. Evolved CO can be trapped by TpMsCu cores leading reverse reaction is not observed in our case, at least apparently, to the formation of TpMsCu(CO) (8). At 70 °C, copper- as inferred from the experiment carried out with 7 and CO. carbene 7 originates olefin 9 and carbonyl 8 in a process DFT Studies and Microkinetic Model. Given the requiring carbon monoxide. This proposal agrees with the unexpected formation of 7, we further explored the process initial observation of the need for heating at 70 °C to induce with DFT studies (B3LYP-D3 optimizations in solution; see catalysis by 1,23 since the copper−carbonyl 8 under the details in the Supporting Information) and microkinetic reaction conditions in the presence of 2 also generates TpMsCu simulations. Computational results are summarized in Figure cores to catalyze the C−H bond functionalization reaction. 3, showing the free energy profile for the conversion from the The transformation of 6 into 7 somehow resembles the starting TpMsCu(THF) complex (1) to compound 7. The Wolff rearrangement of diazo compounds (Scheme 5a), which initial steps follow the usual mechanism up to the formation of originate short-lived ketenes4,28 which further react with metal-carbene 6, with the highest barrier corresponding to nucleophiles, in a process triggered by light, heating, or nitrogen extrusion (17.3 kcal/mol−1). Species such as metal- silver(I) salts. Grotjahn and co-workers described a ketene carbene 6 have been previously shown to be very reactive iridium complex which undergoes the reversible conversion of toward homocoupling or C−H activation.19,23,24 The novelty the coordinated ketene into a metallocarbene which contains a in this system is the availability of an alternative low energy 4840 https://doi.org/10.1021/jacs.1c01483 J. Am. Chem. Soc. 2021, 143, 4837−4843 Journal of the American Chemical Society pubs.acs.org/JACS Article Figure 3. (a) Simplified reaction free energy profile from DFT calculations for the generation of copper carbenes 6 and 7. (b) Simplified reaction mechanisms for the formation of olefin 9. path via decarbonylation. The latter proceeds through a rather This value is significantly lower than the 13.8 kcal/mol stable ketene intermediate INTDCO2, with a linear arrange- reported above for decarbonylation, which from a simplistic ment between the CO and the former carbene carbon (see view would indicate that decarbonylation should not take Supporting Information for more details). The barrier for the place. However, for convoluted mechanisms such as the one decarbonylation process is low at 298.15 K (13.8 kcal/mol) present here, it is better to translate the computed free energy while the reverse barrier is 28.0 kcal·mol−1, too high to take barriers to rate constants and use those to estimate reaction place significantly at ambient temperature. At 343.15 K the times through microkinetic modeling.31 We performed such profile is similar, with a barrier of 13.9 kcal/mol for microkinetic modeling with additional energy adjustment32 decarbonylation and a barrier of 30.3 kcal/mol for the reverse (see Supporting Information for details), and we obtained the process. The lack of observation of the reverse pathway when results shown in Figure 4. The agreement between calculations reacting complex 7 and CO at 70 °C is due to the more and experimental data (displayed in Figure 2) is certainly favorable path of formation of olefin 9 and carbonyl adduct 8 remarkable, and this strongly supports the validity of our (see below). Further calculations were carried out for the case mechanistic proposal. where the toluene solvent is replaced by dichloromethane, also reproducing the experimental observation of pyrrolidinone (3) and azetidinone (4). The detailed computational results for ■ CONCLUSION We herein report the first example of the modification of a these cases are given in the Supporting Information. carbene group during its metal-catalyzed transfer from a diazo The formation of 9 has also been investigated. Several paths functionality, a discovery that should result in further previously reported for such carbene coupling have been consideration from the community involved in this area. discarded based on their high energy barriers: (a) direct From now on, the belief that the carbene moiety remains interaction of two molecules of 7;24 (b) carbene C(H)NEt2 undisturbed in this type of processes must be reconsidered dissociation and reaction with 7;24 and (c) coupling of two when analyzing reaction outcomes, particularly in those dissociated carbene units30 (see Supporting Information). At catalytic systems which either seem deactivated or require variance with that, we have found an accessible route which anomalous heating. Additionally, the chemistry of these involves two molecules of 7 and one of CO. The simplified isolated metal-carbenes is yet to be developed and could find mechanism for the formation of olefin 9 is depicted in Figure important applications as surrogates of well-known midtransi- 3b. The mechanism starts with one molecule of the metal- tion metal Fischer carbene complexes carbene 7 adding a molecule of CO to form the above- mentioned intermediate INTDCO2 which further reacts with a second molecule of 7. The formation of the olefin 9 must ■ EXPERIMENTAL SECTION Synthesis and Characterization of Complexes 7 and 7*. In a overcome a barrier of 23.1 kcal/mol. Thus, albeit olefin 9 is 250 mL Schlenk flask, TpMsCu(THF) (1, 0.450 g, 0.64 mmol) was more stable than metal-carbene 7, its formation involves a dissolved in 75 mL of toluene, and a solution of 2-diazo-N,N- higher barrier (see Supporting Information for further details). diethylacetamide (2, 0.451 g, 3.2 mmol) in the same solvent (37 mL) The third possible product from the interaction between the was added via canula. The mixture was stirred at 70 °C for 1 h before the volatiles were removed under reduced pressure. The residue was precursor 1 and N,N-diethyl diazoacetamide is olefin 5, which washed with dry acetone (5 mL) and cold Et2O (3 × 5 mL) and dried is also experimentally observed. We also computed the under vacuum, to afford 0.35 g of a yellow solid corresponding, mechanism of its formation from 6, which is detailed in the according to NMR studies, to a mixture of complexes 7 and 8 (yields Supporting Information. The key result is that the free energy of 47% and 10% referred to initial 1). Crystallization from a mixture barrier for CC bond formation leading to 5 is 8.9 kcal/mol. of toluene/hexane (4:4 mL) at −30 °C led to the isolation of complex 4841 https://doi.org/10.1021/jacs.1c01483 J. Am. Chem. Soc. 2021, 143, 4837−4843 Journal of the American Chemical Society pubs.acs.org/JACS Article Tomás R. Belderrain − Laboratorio de Catálisis Homogénea, Unidad Asociada al CSIC, CIQSO-Centro de Investigación en Química Sostenible and Departamento de Química, Universidad de Huelva, 21007 Huelva, Spain; Email: trodri@dqcm.uhu.es Feliu Maseras − Institute of Chemical Research of Catalonia, ICIQ, 43007 Tarragona, Spain; Departament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain; orcid.org/0000-0001-8806-2019; Email: fmaseras@iciq.es Authors María Á lvarez − Laboratorio de Catálisis Homogénea, Unidad Asociada al CSIC, CIQSO-Centro de Investigación en Química Sostenible and Departamento de Química, Universidad de Huelva, 21007 Huelva, Spain Maria Besora − Institute of Chemical Research of Catalonia, ICIQ, 43007 Tarragona, Spain; Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, 43007 Tarragona, Spain; orcid.org/0000-0002-6656-5827 Francisco Molina − Laboratorio de Catálisis Homogénea, Figure 4. (a) Computed yield (%) of metallocarbene 7 at 1 h of Unidad Asociada al CSIC, CIQSO-Centro de Investigación reaction for different reaction temperatures (K). (b) Evolution over en Química Sostenible and Departamento de Química, time (h) of the yield (%) of metallocarbene 7 at different Universidad de Huelva, 21007 Huelva, Spain temperatures (continuous line obtained from DFT and microkinetic simulations; dots obtained from experiment). Complete contact information is available at: https://pubs.acs.org/10.1021/jacs.1c01483 7 as yellow crystals in 35% yield (combined crops). Analytically calculated for C41H51BCuN7 (1): C, 68.75; H, 7.18; N, 13.69%. Notes Found: C, 69.41; H, 7.12; N, 13.66%. Complex 7* was prepared The authors declare no competing financial interest. ■ following the same procedure, with the corresponding labeled diazo compound (see SI) with an isolated yield of 28%. ACKNOWLEDGMENTS 1 H NMR (C6D6, 500 MHz): δ 0.28 (t, JHH = 7.3 Hz, 3H, NCH2CH3), 0.44 (t, JHH = 7.3 Hz, 3H, NCH2CH3), 2.12 (s, 27H, We dedicate this work to the memory of Professor Victor CH3), 2.23 (q, JHH = 7.3 Hz, 2H, NCH2CH3), 2.41 (q, JHH = 7.3 Hz, Riera, one of the pioneers in Organometallic Chemistry in 2H, NCH2CH3), 6.01 (d, JHH = 2.0 Hz, 3H, CH), 6.71 (s, 6H, CH), Spain. We acknowledge financial support from the Ministerio 7.80 (d, JHH = 2.0 Hz, 3H, CH), 8.02 (s, 1H, CuCH). 13C{1H} de Ciencia e Innovación (CTQ2017-82893-C2-1-R, NMR (C6D6, 125 MHz): δ 12.9 (CH3), 13.6 (CH3), 20.9 (CH3,Ms), CTQ2017-87792-R, P. O. Feder UHU-1260216 and P. O. 21.1 (CH3,Ms), 53.1 (NCH2), 54.9 (NCH2), 104.5 (CHpz), 127.8 Feder UHU-1254043, and Red Intecat CTQ2016-81923- (CHMs), 133.6 (Cq,Ms), 134.9 (CHpz), 136.2 (Cq,Ms), 138.3 (Cq,Ms), REDC). ■ 150.9 (Cq,pz), 236.6 (CuC). ■ * ASSOCIATED CONTENT sı Supporting Information REFERENCES (1) Buchner, E.; Curtius, T. 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