A Detailed Kinetico-Mechanistic Investigation on the Palladium C–H Bond Activation in Azobenzenes and Their Monopalladated Derivatives

Abstract

Palladium C-H bond activation in azobenzenes with R1 and R2 at para-positions of the phenyl rings (R1=NMe2, R2=H (L1); R1=NMe2, R2=Cl (L2); R1=NMe2, R2=I (L3); R1=NMe2, R2=NO2 (L4); R1=H, R2=H (L5)) and their monopalladated derivatives, using cis-[PdCl2(DMF)2], has been studied in detail by in situ 1 H NMR spectroscopy in N,N-dimethylformamide-d7 (DMF-d7) at room temperature; the same processes have been monitored in parallel via time-resolved UV-Vis spectroscopy in DMF at different temperatures and pressures. The final goal was to achieve, from a kinetico-mechanistic perspective, a complete insight on previously reported reactivity results. The results suggest the operation of an electrophilic concerted metallation and deprotonation mechanism for both the mono-and dipalladation reactions, occurring from the coordination compound and the monopalladated intermediates, respectively. The process involves deprotonation of the C-H bond assisted by the presence of a coordinated DMF molecule, that acts as a base. For the first time, NMR monitoring provides a direct evidence of all the intermediate stages, that is: i) coordination of the azo ligand to Pd II center, ii) formation of the monopalladated species, iii) coordination of the monopalladated species to another Pd II unit, which finally result in the iv) formation of the dipalladated product. All of these species have been identified as intermediates in the dipalladation of azobenzenes, evidenced also by UV-Vis spectroscopy time-resolved monitoring. The data also confirms that the cyclopalladation of asymmetrically substituted azobenzenes occurs by two concurrent reaction paths. In order to identify the species observed by NMR and by UV-Vis spectroscopies, the final products, intermediates and the Pd II precursor have been prepared and characterized by X-ray diffraction, IR and NMR spectroscopies. DFT calculations have also been used in order to explain the isomerism observed for the isolated complexes, as well to assign their NMR and IR spectra.

Mechanochemical halogenation of unsymmetrically substituted azobenzenes

“…Unlike bromination, iodination of L6–8 with NIS resulted in a mixture of the mono- and diiodinated products at the ortho positions of one or both phenyl rings ( LnI-IV and LnI-V , Scheme 2 and Table 2 , entries 10–12), as confirmed by NMR spectroscopy (Figures S104–S130 in Supporting Information File 1 ). Compared to L1 [ 51 ], iodination of its para -halogenated derivatives resulted in lower yields of diiodinated products ( Table 2 , entries 9–12) since the activation/halogenation of the C–H bond occurs preferentially at the unsubstituted azobenzene phenyl ring [ 57 ].…”

“…Compared to the bromination of substrates L3-5 containing amino substituents, the analogous reaction of L2 is slower because the methoxy substituent has weaker donor strength than amino substituents [57].…”

Piano-Stool Ruthenium(II) Complexes with Delayed Cytotoxic Activity: Origin of the Lag Time

“…It is believed that the first step consists of the substitution of an halido ligand by a DMSO molecule ( k 1 ), converting 2 CI 2 iPr or 2 I 2 iPr into intermediate A . This intermediate is readily converted into 3 Cl iPr or 3 I iPr ( k 2 ; Figure ), through a cyclometalation reaction with DMSO acting as a Lewis base, in the so-called concerted metalation–deprotonation (CMD) mechanism. , It is indeed proposed that coordinated DMSO makes an intramolecular hydrogen bond with the pyrenyl group, hence fostering the cyclometalation (Figure S11). 3 Cl iPr and 3 I iPr then undergo substitution of the second halido ligand by a DMSO molecule, producing the cationic cyclometalated compound 3 dmso iPr ( k 3 ), which is finally aquated (i.e.…”

“…This intermediate is readily converted into 3 Cl iPr or 3 I iPr (k 2 ; Figure 8), through a cyclometalation reaction with DMSO acting as a Lewis base, in the so-called concerted metalation−deprotonation (CMD) mechanism. 60,61 It is indeed proposed that coordinated DMSO makes an intramolecular hydrogen bond with the pyrenyl group, hence fostering the cyclometalation (Figure S11). 3).…”

Mechanistic study of the mechanochemical PdII-catalyzed bromination of aromatic C–H bonds by experimental and computational methods

“…The observed changes were attributed to the formation of the intermediate adduct, in which the intramolecular C-H bond activation takes place. 21 LAG of Pd(OAc)2 with L and TsOH, using 25 µL of MeCN as an additive, in a molar ratio of 1:1:1 (L:Pd(OAc)2:TsOH) or 1:1:2 (L:Pd(OAc)2:TsOH) also gave the monopalladated product I1 (S33). Monitoring of these reactions by Raman spectroscopy corroborated the analogous reaction course as for the reaction with Pd(OTs)2(MeCN)2 (Fig.…”