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Huw M. L. Davies - One of the best experts on this subject based on the ideXlab platform.
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d2 symmetric dirhodium catalyst derived from a 1 2 2 triarylcyclopropanecarboxylate ligand design synthesis and application
Journal of the American Chemical Society, 2011Co-Authors: Changming Qin, Vyacheslav Boyarskikh, Jorn H Hansen, Kenneth I Hardcastle, Djamaladdin G Musaev, Huw M. L. DaviesAbstract:Dirhodium tetrakis-(R)-(1-(4-bromophenyl)-2,2-diphenylcyclopropanecarboxylate) (Rh2(R-BTPCP)4) was found to be an effective chiral catalyst for enantioselective reactions of aryl- and styryldiazoacetates. Highly enantioselective cyclopropanations, tandem cyclopropanation/Cope rearrangements and a combined C–H functionalization/Cope rearrangement were achieved using Rh2(R-BTPCP)4 as catalyst. The advantages of Rh2(R-BTPCP)4 include its ease of synthesis, its tolerance to the size of the ester group in the styryldiazoacetates, and its compatibility with dichloromethane as solvent. Computational studies suggest that the catalyst adopts a D2-symmetric arrangement, but when the Carbenoid binds to the catalyst, two of the p-bromophenyl groups on the ligands rotate outward to make room for the Carbenoid and the approach of the substrate to the Carbenoid.
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Organic Reactions - Intermolecular C–H Insertions of Carbenoids
Organic Reactions, 2011Co-Authors: Huw M. L. Davies, Phillip M. PelphreyAbstract:The metal-catalyzed reactions of diazo compounds have broad utility in organic synthesis. The resulting high-energy metal Carbenoid intermediates are capable of a range of useful transformations, including cyclopropanation, ylide formation, and C-H insertion. The intermolecular C-H insertion by metal Carbenoids is the most versatile reaction to date for stereoselective C-H functionalization. This chapter covers the historical background of C-H insertions and describes how the utilization of new catalysts and more stabilized Carbenoids has resulted in major advances in the field. Now that highly diastereoselective and enantioselective C-H functionalization can be achieved, the method can be effectively applied to the synthesis of pharmaceutical agents and natural products. This chapter focuses exclusively on intermolecular C-H insertions of metal Carbenoids in sp3-hybridized C-H bonds. The Carbenoids can be classified into three major classes: 1, acceptor-substituted Carbenoids; 2, acceptor/acceptor-substituted Carbenoids; and 3, donor/acceptor-substituted Carbenoids Keywords: Intermolecular C-H insertions; Carbenoids; Hydrocarbons; Activated C-H bonds; Site selectivity; Donors; Acceptors; Aryldiazoacetates; Metal catalysts; Experimental procedures; Mechanisms
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silver triflate catalyzed cyclopropenation of internal alkynes with donor acceptor substituted diazo compounds
Organic Letters, 2011Co-Authors: John F Briones, Huw M. L. DaviesAbstract:Silver triflate was found to be an efficient catalyst for the cyclopropenation of internal alkynes using donor-/acceptor-substituted diazo compounds as Carbenoid precursors. Highly substituted cyclopropenes, which cannot be synthesized directly via rhodium(II)-catalyzed Carbenoid chemistry, can now be readily accessed.
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Rhodium Carbenoid Approach for Introduction of 4-Substituted (Z)-Pent-2-enoates into Sterically Encumbered Pyrroles and Indoles
Organic letters, 2010Co-Authors: Yajing Lian, Huw M. L. DaviesAbstract:An unusual rhodium Carbenoid approach for introduction of 4-substituted (Z)-pent-2-enoates into sterically encumbered pyrroles and indoles is described. These studies show that (Z)-vinylCarbenoids have a greater tendency than (E)-vinylCarbenoids to react at the vinylogous position of the Carbenoid rather than at the Carbenoid center.
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Functionalization of carbon-hydrogen bonds through transition metal Carbenoid insertion.
Topics in current chemistry, 2009Co-Authors: Huw M. L. Davies, Allison R. DickAbstract:The functionalization of carbon–hydrogen bonds through transition metal Carbenoid insertion is becoming a powerful method for the construction of new carbon–carbon bonds in organic synthesis. This chapter will highlight recent developments in this field, while placing it within its historical context. Intramolecular Carbenoid C–H insertion will be covered first, focusing on formation of three- and six-membered rings, as well as the use of nontraditional substrates. Additionally, the most recent progress in asymmetric catalysis will be discussed. The bulk of the chapter will concentrate on intermolecular transformations, emphasizing both the effect of substrate structure and the influence of carbene substituent electronics on the regioselectivity of the reactions. Vinyldiazoacetates will be covered as a distinct class of Carbenoid precursor, as they have been shown to initiate a variety of unique transformations, such as the combined C–H activation/Cope rearrangement. Finally, the synthetic utility of Carbenoid C–H insertion reactions, both intra- and intermolecular, will be displayed through their use in the total syntheses of a number of natural products and pharmaceuticals.
David Lee Phillips - One of the best experts on this subject based on the ideXlab platform.
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On the Mechanism and Stereochemistry of Chiral Lithium‐Carbenoid‐Promoted Cyclopropanation Reactions
Chemistry (Weinheim an der Bergstrasse Germany), 2007Co-Authors: Yubing Zhou, Cunyuan Zhao, Hui Gao, David Lee PhillipsAbstract:An investigation into the mechanism and stereochemistry of chiral lithium-Carbenoid-promoted cyclopropanation reactions by using density functional theory (DFT) methods is reported. Previous work suggested that this type of cyclopropanation reaction may proceed by competition between a methylene-transfer mechanism and a carbometalation mechanism. In this paper, it is demonstrated that the intramolecular cyclopropanation reactions promoted by chiral Carbenoids 1 and 2 proceed by the methylene-transfer mechanism. The carbometalation mechanism was found to have a much higher reaction barrier and does not appear to compete with the methylene-transfer mechanism. The Lewis base group does not enhance the carbometalation pathway enough to compete with the methylene-transfer pathway. The present computational results are consistent with experimental observations for these cyclopropanation reactions. The factors governing the stereochemistry of the intramolecular cyclopropanation reaction by the methylene-transfer mechanism were examined to help elucidate the origin of the stereoselectivity observed in experiments. Both the directing group and the configuration at the C 1 centre were found to play a key role in the stereochemistry. Carbenoid 1 has a chiral C 1 centre of R configuration. The Lewis base group directs the cyclization of Carbenoid 1 to form a single product. In contrast, the Lewis base group cannot direct the cyclization of Carbenoid 2 to furnish a stereoselective product due to the S configuration of the chiral C 1 centre in Carbenoid 2. This relationship of the stereochemistry to the chiral character of the Carbenoid has implications for the design of new efficient Carbenoid reagents for stereoselective cyclopropanation.
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A Density Functional Theory Study of Aluminum Carbenoid (CH3)2AlCH2X (X = Cl, Br, I) Promoted Cyclopropanation Reactions Compared to IMCH2I (M = Li, Sm, Zn) Carbenoids
Organometallics, 2006Co-Authors: Cunyuan Zhao, Zhi-yuan Geng, Yong-cheng Wang, David Lee PhillipsAbstract:Density functional theory calculations are reported for the cyclopropanation reactions of selected aluminum Carbenoids with ethylene for two reaction channels: methylene transfer and carbometalation. The aluminum Carbenoids react with ethylene via an asynchronous attack on one CH2 group of ethylene with a relatively high barrier (11−15 kcal/mol). In contrast, the reaction barriers for cyclopropanation via the carbometalation are much higher (about 30 kcal/mol). These computational results are in good agreement with experimental results, and this suggests that the methylene transfer process is favored and the competition from the carbometalation pathway is negligible. The (CH3)2AlCH2Cl Carbenoid (reaction barrier of 11.3 kcal/mol) is found to be the most reactive Carbenoid in the (CH3)2AlCH2X (X = Cl, Br, I) series of Carbenoids, and the (CH3)2AlCH2I Carbenoid is the least reactive one. The present computational results are briefly compared with previously reported results for related lithium, samarium, a...
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Samarium(III) Carbenoid as a competing reactive species in samarium-promoted cyclopropanation reactions.
Journal of Organic Chemistry, 2004Co-Authors: Dongqi Wang, Cunyuan Zhao, David Lee PhillipsAbstract:The trivalent samarium Carbenoid I2SmCH2I-promoted cyclopropanation reactions with ethylene have been investigated and are predicted to be highly reactive, similarly to the divalent samarium Carbenoid ISmCH2I. The methylene transfer and carbometalation pathways were explored and compared with and without coordination of THF solvent molecules to the Carbenoid. The methylene transfer was found to be favored, with the barrier to reaction going from 12.9 to 9.2 kcal/mol compared to barriers of 15.4−17.5 kcal/mol for the carbometalation pathway upon the addition of one THF molecule.
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DENSITY FUNCTIONAL STUDY OF SELECTED MONO-ZINC AND GEM-DIZINC RADICAL Carbenoid CYCLOPROPANATION REACTIONS: OBSERVATION OF AN EFFICIENT RADICAL ZINC Carbenoid CYCLOPROPANATION REACTION AND THE INFLUENCE OF THE LEAVING GROUP ON RING CLOSURE
Journal of Theoretical and Computational Chemistry, 2003Co-Authors: Cunyuan Zhao, Dongqi Wang, David Lee PhillipsAbstract:We report a theoretical study of the cyclopropanation reactions of EtZnCHI, (EtZn)2CH EtZnCHZnI, and EtZnCIZnI radicals with ethylene. The mono-zinc and gem-dizinc radical Carbenoids can undergo cyclopropanation reactions with ethylene via a two-step reaction mechanism similar to that previously reported for the CH2I and IZnCH2 radicals. The barrier for the second reaction step (ring closure) was found to be highly dependent on the leaving group of the cyclopropanation reaction. In some cases, the (di)zinc Carbenoid radical undergoes cyclopropanation via a low barrier of about 5–7 kcal/mol on the second reaction step and this is lower than the CH2I radical reaction which has a barrier of about 13.5 kcal/mol for the second reaction step. Our results suggest that in some cases, zinc radical Carbenoid species have cyclopropanation reaction barriers that can be competitive with their related molecular Simmons-Smith Carbenoid species reactions and produce somewhat different cyclopropanated products and leaving groups.
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Density functional theory investigation of the remarkable reactivity of geminal dizinc Carbenoids (RZn)2CHI (R = Et or I) as cyclopropanation reagents with olefins compared to mono zinc Carbenoids RZnCHI2, etchiznr (R = Et or I)
Journal of the American Chemical Society, 2002Co-Authors: Cunyuan Zhao, Dongqi Wang, David Lee PhillipsAbstract:Density functional theory calculations for the cyclopropanation reactions of several mono zinc Carbenoids and their corresponding gem-dizinc Carbenoids with ethylene are reported. The mono zinc Carbenoids react with ethylene via an asynchronous attack on one CH2 group of ethylene with a relatively high barrier to reaction in the 20−25 kcal/mol range similar to other Simmons−Smith type Carbenoids previously studied. In contrast, the gem-dizinc Carbenoids react with ethylene via a synchronous attack on both CH2 groups of ethylene and substantially lower barriers to reaction (about 15 kcal/mol) compared to their corresponding mono zinc Carbenoid. Both mono zinc and gem-dizinc Carbenoid reactions can be accelerated by the addition of ZnI2 groups as a Lewis acid, and this lowers the barrier by another 1.0−5.1 kcal/mol and 0.0−5.5 kcal/mol, respectively, for addition of one ZnI2 group. Our results indicate that gem-dizinc Carbenoids react with CC bonds with significantly lower barriers to reaction and in a noti...
Reinhard W. Hoffmann - One of the best experts on this subject based on the ideXlab platform.
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Formation of Olefins by Eliminative Dimerization and Eliminative Cross-Coupling of Carbenoids: A Stereochemical Exercise
Angewandte Chemie (International ed. in English), 2017Co-Authors: Paul R. Blakemore, Reinhard W. HoffmannAbstract:Two Carbenoids combine to generate an olefin by a mechanism involving formation of an ate complex, 1,2-metalate rearrangement, and β-elimination. As each stage of this eliminative coupling is stereospecific, the overall stereochemical outcome can be understood and, in principle fully controlled, providing that the absolute stereochemical configurations of the reacting Carbenoid species are defined. In contrast to traditional alkene syntheses, the eliminative cross-coupling of Carbenoids offers a connective approach to olefins capable of precisely targeting a given isomer regardless of the nature of the features distinguishing the isomers. The formation of olefins by the eliminative dimerization and eliminative cross-coupling of Carbenoids is reviewed with a range of illustrative examples, including the reactions of α-lithiated haloalkanes, epoxides, and carbamates. An emphasis is placed on stereochemical analysis and methods to generate sp3 -hybridized Carbenoids in stereodefined form are surveyed.
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Carbenoid Homologation Reactions of Grignard Reagents: A Closer Look
Organometallics, 2004Co-Authors: Oliver Knopff, Hans Christian Stiasny, Reinhard W. HoffmannAbstract:The Carbenoid homologation reaction of α-haloalkyl Grignard reagents 3 with isopropylmagnesium chloride gives rise not only to the expected secondary Grignard reagents 5 but also to substantial amounts of the tertiary Grignard reagent 7. The latter is postulated to arise by a Carbenoid C−H insertion reaction within the mixed halide-bridged aggregate 14.
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Chiral Organometallic Reagents XXI. Stereoselective Carbenoid Cyclization Reactions
Chemische Berichte, 1997Co-Authors: Hans Christian Stiasny, Volker P. W. Böhm, Reinhard W. HoffmannAbstract:The dichotomy between concerted cyclopropanation and carbolithiation pathways on intramolecular Carbenoid cyclopropanation reactions has been studied. These studies have been extended to the intramolecular Carbenoid/aldehyde addition reaction.
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diastereoselective bromine lithium exchange applied to the synthesis of a c 1 c 9 segment of the bryostatins
Tetrahedron Letters, 1995Co-Authors: Reinhard W. Hoffmann, Hans Christian StiasnyAbstract:Abstract The α-bromo-alkyllithium compounds 5 are generated by diastereoselective bromine/lithium exchange on the dibromo-compound 4. The minor Carbenoid 5b generated cyclizes spontaneously at −110°C to the bicyclo[3.1.0]hexane 6, thus, leaving the major Carbenoid 5a in diastereomerically pure form. Application of the boronate extension reaction to this Carbenoid 5a ed to 1,3- or 1,6-diol derivatives, viz. 9, 13, 6, with complete stereocontrol. This technique was applied to the synthesis of racemic 22, corresponding to the segment C-1/C-9 of the bryostatins.
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The Stereochemistry of Carbenoid Cyclopropanation Reactions
Chemistry - A European Journal, 1995Co-Authors: Hans Christian Stiasny, Reinhard W. HoffmannAbstract:The stereochemical course of the intramolecular Carbenoid cyclopropanation reaction has been studied for the epimeric Carbenoids 12a and 12b. In these reactions the tert-butyldimethylsilyloxy substituent serves as an internal stereochemical reference point. It was found that 12b cyclizes rapidly at −110°C in a complexation-assisted concerted process to give the bicyclo[3.1.0]hexane 16. The diastereomer 12a cyclizes more slowly at −100°C to give both 16 and 17; the former is probably formed by a complexation-assisted carbolithiation pathway.
Cunyuan Zhao - One of the best experts on this subject based on the ideXlab platform.
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On the Mechanism and Stereochemistry of Chiral Lithium‐Carbenoid‐Promoted Cyclopropanation Reactions
Chemistry (Weinheim an der Bergstrasse Germany), 2007Co-Authors: Yubing Zhou, Cunyuan Zhao, Hui Gao, David Lee PhillipsAbstract:An investigation into the mechanism and stereochemistry of chiral lithium-Carbenoid-promoted cyclopropanation reactions by using density functional theory (DFT) methods is reported. Previous work suggested that this type of cyclopropanation reaction may proceed by competition between a methylene-transfer mechanism and a carbometalation mechanism. In this paper, it is demonstrated that the intramolecular cyclopropanation reactions promoted by chiral Carbenoids 1 and 2 proceed by the methylene-transfer mechanism. The carbometalation mechanism was found to have a much higher reaction barrier and does not appear to compete with the methylene-transfer mechanism. The Lewis base group does not enhance the carbometalation pathway enough to compete with the methylene-transfer pathway. The present computational results are consistent with experimental observations for these cyclopropanation reactions. The factors governing the stereochemistry of the intramolecular cyclopropanation reaction by the methylene-transfer mechanism were examined to help elucidate the origin of the stereoselectivity observed in experiments. Both the directing group and the configuration at the C 1 centre were found to play a key role in the stereochemistry. Carbenoid 1 has a chiral C 1 centre of R configuration. The Lewis base group directs the cyclization of Carbenoid 1 to form a single product. In contrast, the Lewis base group cannot direct the cyclization of Carbenoid 2 to furnish a stereoselective product due to the S configuration of the chiral C 1 centre in Carbenoid 2. This relationship of the stereochemistry to the chiral character of the Carbenoid has implications for the design of new efficient Carbenoid reagents for stereoselective cyclopropanation.
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The influence of the leaving group X (X = F, Cl, Br, I) on the Carbenoid nature of the Carbenoids X2AlCH2X – A theoretical study
Journal of Molecular Structure-theochem, 2006Co-Authors: Zhi-yuan Geng, Cunyuan Zhao, Yong-cheng Wang, Le-yan LiuAbstract:Abstract The cyclopropanation reaction of ethene with aluminum Carbenoid has been studied by means of the B3LYP hybrid density functional method. The reaction goes through two pathways: methylene transfer and carbometalation. In methylene transfer pathway, a quantum-chemical investigation shows that the reactions of the Carbenoids X 2 AlCH 2 X 1-X, (X = F, Cl, Br, I) with ethene 2 to cyclopropane 3 + lX 3 profit from a weakening of the C–X bonds by the C–Al bonds in the Carbenoids 1-X and in the complex [1-X∗2]. The C–F bond is more affected than the C–I bond. Since in the transition states 3 [1-X∗2] ‡ AlHal is strongly decomplexed, the cleavage of the C–Hal bond is essential compensated by the formation of the Al–Hal bonds, which leads to almost equal transition state energy for the reactions of 1-X with 2. In contrast with methylene transfer, the cyclopropanation reaction of the carbometalation pathway profit from a weakening of the C–Al bonds by the C–X bonds.
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A Density Functional Theory Study of Aluminum Carbenoid (CH3)2AlCH2X (X = Cl, Br, I) Promoted Cyclopropanation Reactions Compared to IMCH2I (M = Li, Sm, Zn) Carbenoids
Organometallics, 2006Co-Authors: Cunyuan Zhao, Zhi-yuan Geng, Yong-cheng Wang, David Lee PhillipsAbstract:Density functional theory calculations are reported for the cyclopropanation reactions of selected aluminum Carbenoids with ethylene for two reaction channels: methylene transfer and carbometalation. The aluminum Carbenoids react with ethylene via an asynchronous attack on one CH2 group of ethylene with a relatively high barrier (11−15 kcal/mol). In contrast, the reaction barriers for cyclopropanation via the carbometalation are much higher (about 30 kcal/mol). These computational results are in good agreement with experimental results, and this suggests that the methylene transfer process is favored and the competition from the carbometalation pathway is negligible. The (CH3)2AlCH2Cl Carbenoid (reaction barrier of 11.3 kcal/mol) is found to be the most reactive Carbenoid in the (CH3)2AlCH2X (X = Cl, Br, I) series of Carbenoids, and the (CH3)2AlCH2I Carbenoid is the least reactive one. The present computational results are briefly compared with previously reported results for related lithium, samarium, a...
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Samarium(III) Carbenoid as a competing reactive species in samarium-promoted cyclopropanation reactions.
Journal of Organic Chemistry, 2004Co-Authors: Dongqi Wang, Cunyuan Zhao, David Lee PhillipsAbstract:The trivalent samarium Carbenoid I2SmCH2I-promoted cyclopropanation reactions with ethylene have been investigated and are predicted to be highly reactive, similarly to the divalent samarium Carbenoid ISmCH2I. The methylene transfer and carbometalation pathways were explored and compared with and without coordination of THF solvent molecules to the Carbenoid. The methylene transfer was found to be favored, with the barrier to reaction going from 12.9 to 9.2 kcal/mol compared to barriers of 15.4−17.5 kcal/mol for the carbometalation pathway upon the addition of one THF molecule.
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DENSITY FUNCTIONAL STUDY OF SELECTED MONO-ZINC AND GEM-DIZINC RADICAL Carbenoid CYCLOPROPANATION REACTIONS: OBSERVATION OF AN EFFICIENT RADICAL ZINC Carbenoid CYCLOPROPANATION REACTION AND THE INFLUENCE OF THE LEAVING GROUP ON RING CLOSURE
Journal of Theoretical and Computational Chemistry, 2003Co-Authors: Cunyuan Zhao, Dongqi Wang, David Lee PhillipsAbstract:We report a theoretical study of the cyclopropanation reactions of EtZnCHI, (EtZn)2CH EtZnCHZnI, and EtZnCIZnI radicals with ethylene. The mono-zinc and gem-dizinc radical Carbenoids can undergo cyclopropanation reactions with ethylene via a two-step reaction mechanism similar to that previously reported for the CH2I and IZnCH2 radicals. The barrier for the second reaction step (ring closure) was found to be highly dependent on the leaving group of the cyclopropanation reaction. In some cases, the (di)zinc Carbenoid radical undergoes cyclopropanation via a low barrier of about 5–7 kcal/mol on the second reaction step and this is lower than the CH2I radical reaction which has a barrier of about 13.5 kcal/mol for the second reaction step. Our results suggest that in some cases, zinc radical Carbenoid species have cyclopropanation reaction barriers that can be competitive with their related molecular Simmons-Smith Carbenoid species reactions and produce somewhat different cyclopropanated products and leaving groups.
Xing Wang - One of the best experts on this subject based on the ideXlab platform.
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solvent effects in a Carbenoid n h insertion route to triarylamines via 2 diazo 1 3 cyclohexanedione
Tetrahedron Letters, 2005Co-Authors: Peter Livant, Xing WangAbstract:The Rh-Carbenoid derived from 2-diazo-1,3-cyclohexanedione inserts into the N–H bond of arylalkylamines and diarylamines. A solvent for this reactive Carbenoid is suggested. The insertion products undergo a Pd-mediated aromatization to afford alkyldiarylamines and triarylamines.
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Solvent effects in a Carbenoid N–H insertion route to triarylamines via 2-diazo-1,3-cyclohexanedione
Tetrahedron Letters, 2005Co-Authors: Peter Livant, Yuanping Jie, Xing WangAbstract:The Rh-Carbenoid derived from 2-diazo-1,3-cyclohexanedione inserts into the N–H bond of arylalkylamines and diarylamines. A solvent for this reactive Carbenoid is suggested. The insertion products undergo a Pd-mediated aromatization to afford alkyldiarylamines and triarylamines.
