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

Despite the ubiquity of singlet carbenes in chemistry, their utility as true monomeric building blocks for the synthesis of functional organic polymers has been underexplored. In this work, we exploit the capability of purposely designed mono- and bis-acyclic amino(aryl)carbenes to selectively dimerize as a general strategy to access diaminoalkenes and hitherto unknown amino-containing poly(p-phenylene vinylene)s (N-PPV’s). The unique selectivity of the dimerization of singlet amino(aryl)carbenes, relative to putative C-H insertion pathways, is rationalized by DFT calculations. Of particular interest, unlike classical PPV’s, the presence of amino groups in α-position of C=C double bonds in N-PPV’s allows their physico-chemical properties to be manipulated in different ways by a simple protonation reaction. Hence, depending on the nature of the amino group (iPr2N vs. piperidine), either a complete loss of conjugation or a blue-shift of the maximum of absorption is observed, as a result of the protonation at different sites (nitrogen vs. carbon). Overall, this study highlights that singlet bis-amino(aryl)carbenes hold great promise to access functional polymeric materials with switchable properties, through a proper selection of their substitution pattern.


Materials.
THF and Et2O were dried over sodium/benzophenone and distilled prior to use. Dichloromethane, chloroform and pentane were dried over CaH2 and distilled prior to use. Acetonitrile solvent was dried using a MBraun Solvent Purification System (model MB-SPS 800) equipped with alumina drying columns. Benzaldehyde (Alfa Aesar, 98%) was dried over CaH2, distilled and stored under argon. n-Butyllithium solution 11M in hexanes, Lithium bis(trimethylsilyl)amide, Phosphazene (P4-tBu) 0.8M in hexane and terephthalaldehyde were purchased from Sigma-Aldrich and used without further purification. Diisopropylformamide (Alfa Aesar) and piperidinoformamide (Sigma Aldrich) were stored under argon over activated molecular sieves 4 Å. Trifluoromethanesulfonic acid (TCI), Trifluoromethanesulfonic anhydride (ABCR) and benzaldehyde (Alfa Aesar) were distilled prior to use. Phenyllithium 1.9M in di-n-butyl ether was purchased from Alfa Aesar and used without purification.

Instrumentation.
NMR spectra were recorded on a Bruker Avance 400 (1H, 13C, 19F, 400.2, 100.6 and 376.53 MHz respectively) in appropriate deuterated solvents. Molar masses were determined by size exclusion chromatography (SEC) in THF (1 mL/min) with trichlorobenzene as flow marker, using both refractometric (RI) and UV detectors. Analyses were performed using a three-column TSK gel TOSOH (G4000, G3000, G2000) calibrated with polystyrene standards. HRMS were recorded on various spectrometer: a Waters Q-TOF 2 spectrometer and a GCT premier waters mass spectrometer in the chemical ionization (CI) mode. Melting point were determined by using a Stuart Scientific SMP3 apparatus. Ultraviolet visible spectra were collected on a Thermostated UV/Vis Spectrometer (Agilent Carry 4000). Fluorescence spectra were obtained via Spectrofluorimeter (Jasco FP-8500ST). XRay structures were done on a Rotating Anode Rigaku FRX 3kW with microfocus (Hybrid Dectris Pilatus 200 K pixel detector).

Precursors synthesis:
Benzylidenediisopropyliminium triflate 1a: Alder's route: To a stirred solution of diisopropylformamide (1.38 mL, 9.5 mmol) in dry ether (50 mL) cooled at -78°C was added phenyllithium (1.9M, 7.9 mL) dropwise and the resulting mixture was stirred at this temperature for 30 min then at room temperature for 1h. Then, to the reaction mixture cooled at -78°C was added trifluoromethanesulfonic anhydride and the mixture was stirred for 1h at this temperature then for 2h at room temperature. The precipitated solid was filtered under argon and washed with dry ether (3x5 mL) and THF (3x5mL) and dried under vacuum to obtain the compound 1a, as white crystals (2.1 g, 70%).
Schroth's route: To a stirred solution of benzaldehyde (1 mL, 10 mmol) and 1-(trimethylsilyl) diisopropylamine (1.45 g, 10 mmol) in dry ether (50 mL), TMSOTf (1.8 mL, 10 mmol) was added dropwise at room temperature and the resulting mixture was stirred at the same temperature for 6h under inert atmosphere. The precipitated solid was filtered under inert atmosphere and dried under vacuum to obtain the compound 1a as white crystals (2.32 g, 70%).

Benzylidenepiperidineiminium triflate 1b:
To a stirred solution of benzaldehyde (1.0 mL, 10 mmol) and 1-(trimethylsilyl)piperidine (1.57 g, 10 mmol) in dry ether (50 mL), TMSOTf (1.8 mL, 10 mmol) was added dropwise at room temperature and the resulting mixture was stirred at the same temperature for 6h under inert atmosphere. The precipitated solid was filtered under inert atmosphere and dried under vacuum to obtain the compound 1b as yellow crystals (2.62 g, 80%). To a stirred solution of fluorene-aldehyde (1.81 g, 5 mmol) and 1-(trimethylsilyl)piperidine (0.79 g, 5 mmol) in dry ether (25 mL), TMSOTf (0.9 mL, 5 mmol) was added dropwise at room temperature and the resulting mixture was stirred at the same temperature for 14h under inert atmosphere. After the solvent was removed under reduced pressure, the resulting mixture was washed with dry cyclohexane (3x20 mL). The resulting solid was filtered under inert atmosphere to obtain the desired compound 1c as a purple solid (2.25 g, 78%).

1,4-dilithiobenzene:
To a stirred solution of dibromobenzene (2.62 g, 11 mmol) in dry ether (100 mL), n-BuLi (4 mL, 44 mmol) was added dropwise at -78°C and the resulting mixture was stirred at room temperature overnight. The precipitated solid was then filtered, washed with dry ether (3x10 mL) under inert atmosphere and dried under vacuum to obtain the title compound as a light yellow solid (0.890 g, 80%).

Benzylidenebis(diisopropyliminium) bis-triflate 7a:
To a stirred solution of 1,4-dilithiobenzene (0.456 g, 5 mmol) in Et2O at -78°C, diisopropyl formamide (1.32 mL, 9.1 mmol) was added dropwise. The resulting mixture was stirred at -78°C for 30 minutes, followed by 1 hour at room temperature. The mixture was then cooled to -78°C before an ethereal solution of triflic anhydride (ca. 1.8 equivalents) was added dropwise with vigorous stirring and let to room temperature overnight. The precipitated solid was filtered, washed with dry ether (3x10 mL), DCM and THF under inert atmosphere and dried under vacuum to obtain the compound 7a as a light brown solid (1g, 33%). Single crystals were grown from the slow evaporation of a solution of ether in an acetonitrile solution of 7a. 1

Benzylidenebis(piperidineiminium) bis-triflate 7b:
To a stirred solution of terephthalaldehyde (0.27 g, 2 mmol) and 1-(trimethylsilyl) piperidine (0.63 g, 4 mmol) in dry ether (30 mL), TMSOTf (0.72 mL, 4 mmol) was dropwise added at room temperature and the resulting mixture was stirred at the same temperature overnight under inert atmosphere. The precipitated solid was filtered, washed with dry ether (2x25 mL) under inert atmosphere and dried under vacuum to obtain the compound 7b as a white solid (0.98 g, 86%). To a stirred solution of 9,9'-dihexyl-9H-fluorene-2,7-dicarbaldehyde 1 (1.17 g, 3 mmol) and 1-(trimethylsilyl) piperidine (1.04 g, 6.6 mmol) in dry ether (25 mL), TMSOTf (1.2 mL, 6.6 mmol) was dropwise added at room temperature and the resulting mixture was stirred at the same temperature overnight under inert atmosphere. The precipitated solid was filtered under inert atmosphere, washed with dry ether (2x25 mL) and dried under vacuum to obtain the compound 7c as a yellow solid (1.97 g, 80%). Although not mentioned in the main text, other iminiums, such as the thiophene derivative 7d, the morpholino-7e and diethylamino-iminiums 7f have also been prepared with the aim to investigate the scope of the deprotonation reaction as a mean to access original diaminoalkenes.

Thiophenylidenepiperidine iminium triflate 7d:
To a stirred solution of thiophenecarboxaldehyde (0,47 mL g, 5 mmol) and 1-(trimethylsilyl)piperidine (0.79 g, 5 mmol) in dry ether (25 mL), TMSOTf (0.9 mL, 5 mmol) was added dropwise at room temperature and the resulting mixture was stirred at the same temperature for 14h under inert atmosphere. After the solvent was removed under reduced pressure, the resulting mixture was washed with dry cyclohexane (3x20 mL). The resulting solid was filtered under inert atmosphere to obtain the desired compound 36 as a violet solid (1.2 g, 73%).

Benzylidenemorpholine iminium triflate 7e:
To a stirred solution of benzaldehyde (0,51 mL g, 5 mmol) and 1-(trimethylsilyl)morpholine (0.79 g, 5 mmol) in dry ether (25 mL), TMSOTf (0.9 mL, 5 mmol) was added dropwise at room temperature and the resulting mixture was stirred at the same temperature for 14h under inert atmosphere. After the solvent was removed under reduced pressure, the resulting mixture was washed with dry cyclohexane (3x20 mL). The resulting solid was filtered under inert atmosphere to obtain the desired compound 36 as a white solid (1.14 g, 70%). To a stirred solution of benzaldehyde (0,51 mL g, 5 mmol) and 1-(trimethylsilyl)diéthylamine (0.73 g, 5 mmol) in dry ether (25 mL), TMSOTf (0.9 mL, 5 mmol) was added dropwise at room temperature and the resulting mixture was stirred at the same temperature for 14h under inert atmosphere. After the solvent was removed under reduced pressure, the resulting mixture was washed with dry cyclohexane

1,2-bis(diisopropylamino)stilbene 3a:
With P4-t Bu base: To a stirred solution of 1a (0.1 g, 0.3 mmol) in THF (6 mL) cooled at -78ºC, P4-t Bu base (0.38 mL, 0.3 mmol, 0.8M in hexanes) was added and the resulting mixture was stirred at this temperature for 1h followed by 24h at room temperature. The solvent was evaporated under reduced pressure and the reaction mixture was diluted with pentane (20 mL) and passed through a short column of basic alumina. The final compound 3a (0.034 g, 60%) was obtained as a yellow solid. Single crystals were grown from the slow evaporation of a pentane solution of 3a at -20°C.
With LiHMDS base: 1a (0.1 g, 0.3 mmol) and LiHMDS (50 mg, 0.3 mmol) were dissolved in cold THF (6 mL, -78°C). The resulting mixture was stirred at this temperature for 1h followed by 24h at room temperature. The solvent was evaporated under reduced pressure and the reaction mixture was diluted with pentane (20 mL) and passed through a short column of basic alumina. The final compound 3a (0.034 g, 60%) was obtained as a yellow solid. Single crystals were grown from the slow evaporation of a pentane solution of 3a at -20°C.

1,2-bis(piperidine)stilbene 3b:
To a stirred solution of 1b (0.32 g, 1 mmol) in THF (3 mL) cooled at -78ºC, P4-t Bu base (1.38 mL, 1.1 mmol, 0.8M in hexanes) was added and the resulting mixture was stirred at this temperature for 1h followed by 24h at room temperature. The solvent was evaporated under reduced pressure and the reaction mixture was diluted with pentane (20 mL) and passed through a short column of basic alumina. The final compound 3b (0.122 g, 70%) was obtained as a yellow solid. Single crystals were grown from the slow evaporation of a pentane solution of 3b at -20°C. 1,2-bis(9,9'-dihexyl-9H-fluorene)-1,2bis(piperidine)ethene 3c: To a stirred solution of 1c (1.16 g, 2 mmol) in THF (10 mL) cooled at -78ºC, P4-t Bu base (3 mL, 2.4 mmol, 0.8M in hexanes) was added and the resulting mixture was stirred at this temperature for 1h followed by 24h at room temperature. The solvent was evaporated under reduced pressure and the reaction mixture was diluted with pentane (20 mL) and passed through a short column of basic alumina. The final compound 3c (0.48 g, 56%) was obtained as an orange solid. Single crystals were grown from the slow diffusion of a pentane solution of 3c at -20°C.

Deprotonation of 7d, 7e, 7f:
To a stirred solution of 7d-f (0.1 g, 0.3 mmol) in THF (6 mL) cooled at -78ºC, P4-t Bu base (0.38 mL, 0.3 mmol, 0.8M in hexanes) was added and the resulting mixture was stirred at this temperature for 1h followed by 24h at room temperature. The solvent was evaporated under reduced pressure and the reaction mixture was extracted with pentane (2 mL). According to 1 H NMR in THF-d8, those reactions were not clean enough to be useful for the investigation of the dimerizing polymerization of related biscarbenes.

Poly(7,8-bisdiisopropylamino-1,4-phenylenevinylene) 9a:
To a stirred solution of 7a (0.300 g, 0.5 mmol) in THF (2 mL) cooled at -78ºC P4-tBu base (1.38 mL, 1.1 mmol, 0.8M in hexanes) was added and the resulting mixture was stirred at this temperature for 1h followed by 24h at room temperature. After the solvent was removed under reduced pressure, the mixture was washed with MeOH (10 mL) to remove the phosphazenium salts. The resulting mixture was dissolved in CHCl3 (15 mL) and filtered to remove the insoluble compounds. After evaporation of the organic solvent, the mixture was purified by precipitation using CHCl3 and MeOH at -10ºC to obtain the desired polymer 9a as a dark red solid (80 mg, 60%).

Poly(7,8-bispiperidine-1,4-phenylenevinylene) 9b:
To a stirred solution of 7b (0.28 g, 0.5 mmol) in THF (2 mL) cooled at -78ºC P4-t Bu base (1.38 mL, 1.1 mmol, 0.8M in hexanes) was added and the resulting mixture was stirred at this temperature for 1h followed by 24h at room temperature. After the solvent was removed under reduced pressure, the mixture was washed with MeOH (10 mL) to remove the phosphazenium salts. The resulting mixture was dissolved in CHCl3 (15 mL) and filtered to remove the insoluble compounds. After evaporation of the organic solvent, the mixture was purified by precipitation using CHCl3 and MeOH at -10ºC to obtain the desired polymer 9a as a wine red solid (80 mg, 60%). To a stirred solution of 7c (0.83 g, 1 mmol) in THF (15 mL) cooled at -78ºC P4-t Bu base (3.0 mL, 2.4 mmol, 0.8M in hexanes) was added and the resulting mixture was stirred at this temperature for 1h followed by 24h at room temperature. After the solvent was removed under reduced pressure, the mixture was washed with MeOH (10 mL) to remove the phosphazenium salts. The resulting mixture was dissolved in CHCl3 (15 mL) and filtered to remove the insoluble compounds. After evaporation of the organic solvent, the mixture was purified by precipitation using CHCl3 and MeOH at -10ºC to obtain the desired polymer 9c as a reddish-brown solid (425 mg, 81%).

Optimization of the experimental conditions:
We performed several experiments in order to access higher molecular weight N-PPVs; in particular, influence of the nature of the solvent, base and concentration over the molecular weights was investigated using mainly bis-iminium 7a as bis-carbene precursor (see Supplementary Table 1  For instance, NiPr2-PPV 9a was synthesized following this general procedure: Benzylidenebis(diisopropyliminium) triflate (0.30 g, 0.5 mmol) partially soluble in THF (50 mL) was deprotonated with a solution of tert-Butylimino-tri(pyrrolidino)phosphorane (0.32 mL, 1.05 mmol) in 2 mL THF to avoid freezing at -90 °C. NiPr2-PPV polymer was obtained as a dark red solid upon purification (105 mg, 70 %).

Protonation of 9b:
To a deep red solution of polymer 9b (0,134 g, 0.50 mmol) in CH3CN/CHCl3 (50/50) cooled at -40°C, TfOH (184.2 μL, 2.2 mmol) was added dropwise. The resulting mixture was warm to room temperature overnight, leading to a yellow solution. Finally, after evaporation of the solvents under vacuum 10b was isolated in 99% yield (0.245 g). According to UV/visible spectroscopy, the characteristic absorption (lmax = 476 nm) of 9b was lost and was replaced by an absorption (lmax = 254 nm) in the spectrum of 10b, suggesting a loss of the conjugation. Note that this reaction is reversible since addition of t-BuOK to CH3CN/CHCl3 (50/50) solution of 10b yielded back to 9b (9b'), where the lmax (462 nm for 9b') was restored. A similar procedure was followed for 9a and 9c; the same behavior was observed during the protonation of 9c into 10c.

Protonation of 9a:
A completely different scenario occurred during the protonation of 9a. In this case, the characteristic absorption (lmax = 462nm) of 9a was blue-shifted down to 352 nm in 10a, suggesting an alteration of the conjugation in 10a (the solution was orange-brownish).
To gain some insight into the different behavior of 9a and 9b,c, protonation of the corresponding molecular dimers 3a and 3b (as representative of piperidine-substituted dimers 3b,c) was carried out with 2 eq. of TfOH (see below). Hence, while both nitrogen atoms are protonated and the C=C bond is preserved in the case of 3a, protonation of 3b occurs at one nitrogen atom and at the enamine carbon, ultimately leading to a dicationic compound without C=C bond. By analogy, we may hypothesize that protonation of N-PPV's 9a and 9b led respectively to conjugated 10a and non-conjugated 10b, in agreement with respectively, the disappearance and a blue-shift of the lmax in the UV/vis spectrum of 9a and 9b upon protonation.

Protonation of 3a and 3b:
Procedure: 2 eq. of TfOH (0.26mL, 3 mmol) were added at -78°C to a stirred DCM solution (1mL) of 3-E/Z (1.5 mmol). The reaction was stirred at this temperature for 1h and the solution was slowly warmed to RT overnight. After evaporation of the solvent under vacuum, the remaining solid was washed with Et2O and dried under vacuum to afford a mixture of mono protonated Z-3-HOTf and di-protonated E-3-(HOTf)2 isomer. Further washing with THF (3x1mL) allowed to selectively removing mono-cationic Z-3-HOTf isomers. Recrystallization of E-3a-(HOTf)2 and E-3b-(HOTf)2 from THF/CH3CN mixture afforded single crystals suitable for diffraction analysis (see section Crystallographic data, section 8).

Addition reaction
Benzylidenediisopropyliminium-2-dithiocarboxylate 4a: To a stirred solution of 1a (0.146 g, 0.45 mmol) and S8 (1.16 g, 4.5 mmol) in THF (5 mL) cooled at -78ºC, LiHMDS (0.5 mL, 0.5 mmol, 1.0 M in THF) was added and the resulting mixture was stirred at this temperature for 1h then warmed to room temperature overnight. Evaporation of the solvent followed by silica-gel column chromatography (n-hexane/ethyl acetate) gave the compound 4a as a yellow powder (0.069 g, 70 %). Single crystals were grown from the slow diffusion of a solution of ether in a THF solution of 4a.

Film forming properties of N-PPV's:
The film forming properties of N-PPV's 9 were investigated with 9c as representative example. Gratifyingly, a film of 9c could be prepared from a CHCl3 diluted solution (15 mg/mL) and spin-coating In order to calculate the NMR DPn, the aldehyde peak was first integrated to a value of two, corresponding to the two aldehydes end-chain functions in the polymer chain. Then, the calculus was done as followed: addition of the protons corresponding of both green peaks, then, the result is divided by the number of protons corresponding of the green peaks in the repeating unit. For example for 9c, the values of both green peaks is equal to 366 protons divided by 8 protons in the repeating unit gave a DPn of 45.

Investigation of the stability of 3a by variable temperature (VT) NMR analysis
To gain some insight into the stability of N-PPVs 9, we investigated the stability of the corresponding diaminoalkenes 3 towards their potential dissociation into the corresponding amino(aryl)carbenes. For this purpose, 3a was selected as representative example because i-Pr substituents give characteristic signals in 1 H NMR (CH gives a septet and CH3 gives a doublet); moreover, the chemical shift of CHi-Pr is very sensitive to the environment around the nitrogen atom (i.e. its hybridization). Thus, VT NMR was performed over 20-90 °C temperature range, using toluene-d8 as solvent. A superimposition of the different spectra is given in figure X. As can be seen, 3a remains perfectly stable over the whole 3c 9c range of temperature as no signals corresponding to the amino(aryl)carbene nor signals corresponding to rearrangement/decomposition products could be observed. Figure 8. VT 1 H NMR of dimer 3a in Toluene-d8 (full spectrum and partial spectrum with enlarged selected areas).

Size exclusion chromatography spectra
The calibration PS standard was used for the SEC experiments.

Crystallographic data
Crystal data and structure refinement for 3a:

Computational details
All calculations were performed using the Gaussian 09 package [1][2][3] and the B3LYP hybrid [4] functional on the real systems. The def2-SVP [5] basis set was employed for all atoms. All stationary points involved were fully optimized. Frequency calculations were undertaken to confirm the nature of the stationary points, yielding one imaginary frequency for transition states (TS), corresponding to the expected process, and all of them positive for minima. The connectivity of the transition states and their adjacent minima was confirmed by intrinsic reaction coordinate (IRC) [6,7] calculations. All the geometrical structures were plotted with Gaussview 5.0. [8] TD-DFT calculations have been performed at CAM-B3LYP/6-311++G** level of theory on the geometry optimized at B3LYP/def2-SVP in order to obtain the 1 st transition energy (lmax) for dimers 3ac, corresponding to HOMO®LUMO transition. In each isomer, the HOMO is mainly localized on the πC=C and nNorbitals while the LUMO is localized on the combination of the orbitals π*C-N and π*C=C on the phenyl ring. Moreover, the values obtained for the HOMO-LUMO gap have the same tendency found experimentally, with the highest gap found for 3a>3b>3c. These results are in agreement with the maximum of absorption observed with a λmax of 3c>3b>3a.
Supplementary Figure 23. Frontier orbitals for the Z isomer of the 3 dimers 3a-c (cutoff: 0.04). ). In the case of dimer 3c the C6H13 alkyl groups are replaced by CH3 groups.
In each isomers, the HOMO is mainly localized on the πC=C and nNorbitals while the LUMO is localized on the combination of the orbitals π*C-N and π*C=C on the phenyl ring. oscillator strengths (f) calculated at CAM-B3LYP/6-311++G** level of theory on the geometry optimized at B3LYP/def2-SVP. Associated gap in eV.
The trend in the maximum of absorption found by calculations is in agreement with the one obtained experimentally. Indeed, the maximum of absorption increases from 3a to 3c compounds. Supplementary Table 3. Relative stability of singlet (S) and triplet (T) states for carbene 2a computed at B3LYP/def2-SVP level of theory. Gibbs free energy and electronic energy into bracket, in kcal.mol -1 . Main geometrical parameters, bond lengths in Å and main bond angles in °.

∆GS-T (∆E S-T)
As expected, the singlet state is more stable than the triplet state around 18.5 kcal.mol -1 . The parameters calculated for the dimer 3a-E are in agreement with the ones observed by X-Ray analysis.

Direct dimerization
Supplementary Figure 24. Energy profiles computed for the dimerization of 2a, using B3LYP/def2-SVP level theory. Gibbs free energy in kcal.mol -1 .
The direct dimerization of the carbene into the alkene, with both the Z-and the E-configuration, was found strongly exergonic, with ∆G of -35 kcal.mol -1 for 3a-Z and -45 kcal.mol -1 for 3a-E. 3a-E is thermodynamically more stable by 10 kcal.mol -1 than 3a-Z, which could explain the E/Z ratio in favor of the E-isomer observed experimentally (thermodynamic control).
Moreover, the potential energy surface of the transformation 2a into 3a-E/Z was scrutinized. Two transition states connecting 2a and 3a-E, namely TS-3a-E, and 2a and 3a-Z, namely TS-3a-Z, could be localized, leading to close and moderate activation barriers of 18.5 and 17.2 kcal.mol -1 respectively.
For TS-3a-E, the C1 … C2 bond (2.428 Å) is strongly elongated compared to its corresponding dimer 3a-E (76 %). Moreover, the C1-N1 bond of 1.31 Å, the planar nitrogen and carbon atoms (ΣN1 = 360° and carbon ΣC1 = 360°, respectively) found in the carbene on the left part of TS-3a-E are close to the geometrical parameters of the starting carbene 2a. Similar geometrical parameters are observed for the carbene on the right part of TS-3a-E, with a C2-N2 bond distance of 1.33 Å, a ΣN1 of 359.9° and a ΣC1 of ~ 359.0°. Thus, TS-3a-E can be considered as an early transition state.

Side reaction (intramolecular C-H insertion)
Intramolecular C-H insertion reactions of aminoarylcarbene 2a into CHiPr or CH3iPr were predicted to be exergonic (∆G = -16 and -30.6 kcal.mol -1 respectively). Prohibitive activation barriers of 39.4 kcal.mol -1 and 45.6 kcal.mol -1 respectively, were calculated, in agreement with the high selectivity observed experimentally for the dimerization reaction.

TS-3a-Z a) C-H Insertion in β position
Insertion of carbene 2a into the CH3iPr was first investigated. A single transition state connecting 2a and the cyclobutane 6a, namely TS-6a, could be localized on the potential energy surface. Analysis of the geometrical parameters of TS-6a revealed that the disruptive H3-CiPr' bond was elongated by only 18 % compared to that of 2a (1.29 Å vs 1.09 Å) and that the forming C2a-H bond was also elongated by 18 % compared to the C2a-H bond of 6a (1.32 Å vs 1.11 Å). In parallel, the forming C2a-CiPr' bond was moderately elongated by 37 % compared to that of cyclobutane 6a (2.12 Å vs 1.54 Å). As a consequence of the C2a-CiPr' interaction, both C2a and N2a atoms are strongly pyramidalized (ΣN2a = 332° and ΣC2a = 333°, respectively), and the C2a-N2a bond length of 1.43 Å is characteristic of a C-N single bond (1.42 Å). Therefore, TS-6a can thus be considered as a late transition state. Overall, these data suggest that this insertion reaction is a concerted but asynchronous process. Figure 25. Energy profiles computed for the C-H insertion reactions of 2a at B3LYP/def2-SVP level of theory. Gibbs free energy in kcal.mol -1 and bond distance in Å.

b) C-H insertion in α position
The insertion of carbene 2a into CHiPr was next investigated. Insertion of carbene 2a into CHiPr involves a stepwise process, where the CHiPr proton of 2a is transferred to the carbene center (C2a) with a high activation barrier of 39.4 kcal.mol -1 (TS-11a) for the first step (2aà11a); the resulting azomethine ylide intermediate 11a then undergoes a ring closure in the second step (11aà5a) with a very low activation barrier of 5.3 kcal.mol -1 (TS-5a). Analysis of TS-11a allows some insight into the nature of this proton transfer step to be gained. The C-HiPr bond in TS-11a is strongly elongated by 48 % compared to that of the starting carbene 2a (1.62 Å vs 1.09 Å), while the forming C2a-H bond is only moderately elongated (21 %), compared to the C2a-H of compound 11a (1.30 Å vs 1.09 Å). In parallel, the C2a-CiPr bond distance remains long (2.26 Å) in comparison with that of the azomethine ylide 11a (2.56 Å). Besides, the short C2a-N2a bond distance of 1.30 Å, the sum of angles around C2a and N2a of 360° attest of the absence of any rehybridization of those atoms between 2a and 11a. From those data, it can be concluded that TS-11a is a late transition state.
The cyclopropane final compound is formed in the last step of this process by ring closure of the zwitterionic azomethine ylide 1a. This step proceeds via TS-5a, where the forming C2a-CiPr is strongly elongated by 40 % compared to that of the final compound 5a (2.12 Å vs 1.51 Å). The long C2a-N2a bond distance of 1.43 Å (characteristic of a single C-N bond) associated with the pyramidal N2a atom (ΣN2a = 339°) and the planar C2a atom (ΣC2a = 360°) suggest a zwitterionic structure for TS-5a, where both the negative and positive charges are localized at CiPr and C2a, respectively.