Rearrangement of Biaryl Monoaminocarbenes via Concerted Asynchronous Insertion into Aromatic C−H Bonds

Abstract

The biphenyl and binaphthyl diisopropylaminocarbenes were found to be only transient species that spontaneously and quantitatively rearrange into the corresponding aminofluorenes. DFT calculations confirm that these insertion reactions of aminocarbenes into proximal aromatic C-H bonds require only a moderate energy barrier and support a concerted, strongly asynchronous, mechanism dominated by C arom →C carbene proton transfer.Over the past 20 years, spectacular progress has been achieved in carbene chemistry. These divalent six-valence-electron fleeting intermediates have shifted into the realm of isolable compounds, 1 and the availability of a variety of stable singlet carbenes has allowed for considerable synthetic developments in metal-mediated 2 as well as organo-catalyzed transformations. 3 More insight has also been gained into typical carbene reactivity including cyclopropanation, 4 dimerization, 5 1,2-and 1,3-migration, 6 as well as O-H, N-H, 7 and Correspondence to: Karinne Miqueu, karine.miqueu@univ-pau.fr; Didier Bourissou, dbouriss@chimie.upstlse.fr; Guy Bertrand, gbertran@mail.ucr.edu. Supporting Information Available: Experimental procedures, spectroscopic characterizations for all new compounds, computational details, and Z-matrices for all optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org. C sp -and C sp 3-H 8,9 insertion reactions. In addition, unusual transformations such as radical fragmentation 10 and nucleophilic substitution at the carbene center 11 have been evidenced. NIH Public AccessIn this context, our interest for increasing further the structural variety of singlet carbenes prompted us to investigate biaryl monoaminocarbenes, and we recently reported the synthesis and original coordination behavior of the stable biphenyl and binaphthyl representatives I and II (Figure 1). 12 In continuation of this work, we became interested into the corresponding carbenes 1 and 2 free of methoxy groups on the biaryl moieties and report here on the spontaneous rearrangement of these transient carbenes via an unprecedented insertion into C sp 2-H bonds.A one-pot procedure 13 starting from commercially available 2-bromobiphenyl afforded the biphenyl iminium salt 3 in 58% overall yield (Scheme 1). The characteristic CH iminium signals were observed at 8.75 and 173.5 ppm in the respective 1 H and 13 C NMR spectra. In addition, the presence of only seven CH arom signals in the 13 C NMR spectrum indicated free rotation around the biphenyl axis on the NMR time scale. Monitoring the reaction of 3 with LiHMDS in THF by 1 H and 13 C NMR at low temperature indicated that deprotonation immediately and cleanly occurred at −80 °C. However, the characteristic low-field 13 C NMR signal expected for carbene 1 was not observed, and the resulting product was unambiguously identified as the diisopropylaminofluorene isomer 4. The 1 H NMR chemical shift for the central CH (4.96 ppm) is identical to that reported previously, 14 and the presence of only 4 CH a...

Reactivity Studies and Electronic Properties of an N-Arylated Acyclic Amino Carbene

“…The insertion product 3 was characterized by various NMR spectroscopic methods ( 1 H, 13 C, COSY, HMQC, and HMBC) and high-resolution mass spectrometry. Similar decomposition pathways of AACs were reported by Bertrand and co-workers. ,, …”

“…Similar decomposition pathways of AACs were reported by Bertrand and co-workers. 14,16,27 Since we were not able to isolate the free carbene, attempts were made to trap it. Thus, iminium salt 2 was treated with 2 equiv of KHMDS and subjected to crystallization from hexane at −30 °C.…”

Direct and selective access to amino-poly(phenylene vinylenes)s with switchable properties by dimerizing polymerization of aminoaryl carbenes

“…For that purpose, we kept in mind that methyl group at the nitrogen atom should be avoided to suppress any carbene to azomethine ylide isomerization 46 . The substitution pattern of the aryl moiety, especially at the ortho positions, has also been shown to dramatically influence the kinetic stability of the carbene, which eventually controls its deactivation pathways (dimerization versus C–H insertions) 11 , 46 , 47 .…”

“…Although the dimerization of (cyclic) diaminocarbenes has been theoretically investigated 13 , 15 , 17 , data concerning MACs, and particularly amino(aryl)carbenes, are lacking. Therefore, to gain some insight into the selective dimerization of carbenes 2a , relative to putative intramolecular C-H insertion reactions 47 , DFT calculations were carried out on the direct dimerization of 2a , as well as on the intramolecular C-H insertion of 2a into CHiPr and CH 3 iPr at the B3LYP/def2-SVP level of theory. The β-H elimination of propene from i Pr 2 N-substituted carbene 2a was also investigated in the Supplementary Information .…”

Lewis‐Acid‐Catalyzed Intramolecular Aza‐Friedel–Crafts Reaction of N‐tert‐Butanesulfinyl Imines: Efficient Synthesis of Optically Active 9‐Aminofluorene Derivatives

“…However, the synthetic approaches to access these valuable compounds have received considerably less attention to date; only examples dealing with racemic intramolecular Friedel–Crafts alkylation,3a,b addition of imines,3c benzylic arylation,3d,e intermolecular amination of the CH bond,3f,g or cross‐coupling of the CN bond3h,i have been described. To the best of our knowledge, despite the potential importance of their related optically active compounds (Scheme , compound C ), the stereoselective construction of 9‐aminofluorene derivatives is even more underexplored, with only one report that touched upon attempts at asymmetric synthesis 3k.…”