Pruebas del sistema propuesto

The mixed NHC/PPh3 and NHC/AsPh3 complexes were synthesized by adding a mixture containing triphenylphosphine (or triphenylarsine) and the silver carbene complex (5a-b or 5d) to a dichloromethane solution of the olefinic precursor of interest (Scheme 3.21). The reaction proceeds for about one hour and is characterized by the progressive precipitation of AgCl. After the filtration of the reaction mixture by a Millipore apparatus, the products were isolated by precipitation from a dichloromethane/diethylether mixture (16a, 17a, 18a, 20a, 21a and 22a) or obtained by the removal of the solvent (15a-b, 15d, 19a-b and 19d).

Scheme 3.21 Synthesis of the mixed NHC/PPh3 and NHC/AsPh3 complexes 15-22.

The obtained complexes were characterized by NMR and IR spectroscopy and the relevant responses are reported below.

All the ¹H-NMR spectra show the presence of the signals ascribable to the NCH3 (singlets at 3-4 ppm) and NCH2 groups (AB systems or singlets at 5-6 ppm) of the carbene moiety.

The same groups resonate in the ¹³C-NMR between 25 and 50 ppm, whereas the signal of the coordinated carbenic carbon is observed as a singlet for the mixed NHC/AsPh3 complexes and as a doublet, due to the coupling with phosphorus (JC-P ≈ 15 Hz), for the mixed NHC/PPh3 complexes, between 195 and 200 ppm.

Moreover, for complexes containing PPh3, it is possible to observe a single peak at 25-30 ppm in the

31P-NMR spectra (Fig. 3.53), shifted downfield of about 30-35 ppm with respect to the uncoordinated triphenylphosphine.

Fig. 3.53 ³¹P{¹H}-NMR spectrum of the complex 15d (T = 298K, CDCl3).

Complexes with tmetc

In the ¹H-NMR spectra of the complexes 15a-b and 15d (Fig. 3.54) it is possible to observe four signals ascribable to the OCH3 groups of the coordinated tmetc between 3.1 and 3.7 ppm. This differentiation is due to the presence of two different ancillary ligands and to the hindered rotation of the asymmetric carbene about the C-Pd bond.

Fig. 3.54 ¹H-NMR spectrum of the complex 15d (T = 298K, CDCl3).

Moreover, in the ¹³C{¹H}-NMR spectra (Fig. 3.55) of the complexes 15a-b and 15d it is possible to observe the signals ascribable to the OCH3 groups between 51 and 53 ppm and those related to the olefinic carbons between 60 and 65 ppm. These latter resonate as singlets in the case of NHC/AsPh3

complexes and as doublets (JC-P ≈ 38 Hz) in the case of the NHC/PPh3 derivatives. The highfield shift

of the olefinic carbons with respect to the uncoordinated olefin ( ≈ 80 ppm), is a direct consequence of its coordination on the palladium center and of the high degree of back-donation, particularly marked in the presence of strongly electron-withdrawing olefins.

Fig. 3.55 ¹³C{¹H}-NMR spectrum of the complex 15d (T = 298K, CDCl3).

Complexes with fumaronitrile

The ¹H-NMR spectra (Fig. 3.56) of complexes 16a and 20a are characterized by the following signals:

• one doublet of doublets ascribable to the olefin proton pseudo-trans to the phosphine (JH-H = 9.4 Hz and JH-P = 3.4 Hz) at about 2.9 ppm and one doublet ascribable to the olefin proton pseudo-trans to the carbene (JH-H = 9.4 Hz) at about 3 ppm, in the case of complex 16a.

• Two doublets related to the different olefin protons between 2.8 and 3.2 ppm, for complex 20a.

Fig. 3.56 ¹H-NMR spectrum of the complex 16a (T = 298K, CDCl3).

The ¹³C{¹H}-NMR spectra (Fig. 3.57) of complexes 16a and 20a are characterized by the following signals:

• One doublet ascribable to the olefin carbon pseudo-trans to the carbene (JC-P = 3 Hz) at about 22.5 ppm and a doublet ascribable to the olefin carbon pseudo-trans to the phosphine (JC-P = 38 Hz) at 22.9 ppm, for complex 16a.

• The signals of the coordinated olefin at about 38 ppm in the case of complex 20a.

• The signals of the CN carbons at about 123 ppm.

Fig. 3.57 ¹³C{¹H}-NMR spectrum of the complex 20a (T = 298K, CDCl3).

The IR spectra show the typical band of the coordinated fumaronitrile at 2191 cm^-1 and the signals of the carbonyl groups within 1665 and 1710 cm^-1.

Complexes with maleic anhydride

The ¹H-NMR spectra (Fig. 3.58) of complexes 17a and 21a are characterized by the following signals:

• One doublet of doublets ascribable to the olefin proton pseudo-trans to the carbene at about 4 ppm (JH-H = 3.9 Hz and JH-P = 2.9 Hz) and one doublet of doublets related to those pseudo-trans to the phosphine at about 4.2 ppm (JH-H = 3.9 Hz and JH-P = 9.8 Hz), for complex 17a.

• Two doublets related to the coordinated olefin protons between 3.5 and 4.6 ppm, in the case of complex 21a.

Fig. 3.58 ¹H-NMR spectrum of the complex 21a (T = 298K, CDCl3).

The ¹³C{¹H}-NMR spectra (Fig. 3.59) show the following signals:

• One singlet ascribable to the olefin carbon pseudo-trans to the carbene at 45.6 ppm and one doublet related to the olefin carbon pseudo-trans to the phosphine (JC-P = 27.8 Hz) at about 47.5 ppm, for complex 17a.

• The signals of the coordinated olefin carbons between 43 and 47 ppm, for complex 21a.

• A doublet ascribable to the carbonyl carbon of maleic anhydride pseudo-trans to the carbene (JC-P = 2 Hz) and a doublet ascribable to those pseudo-trans to the phosphine (JC-P = 6 Hz) at about 172 ppm, for complex 17a.

• The signals of the carbonyl carbons at about 173 ppm, for complex 21a.

Fig. 3.59 ¹³C{¹H}-NMR spectrum of the complex 17a (T = 298K, CDCl3).

Complexes with dimethylfumarate

The ¹H-NMR spectra (Fig. 3.60) of the complexes 18a and 22a show the following signals:

• One broad singlet and two singlets ascribable to the OCH3 groups of the coordinated dimethyl fumarate (3.2-3.6 ppm) for complexes 18a and 22a, respectively.

• One doublet of doublets ascribable to the olefin proton pseudo-trans to the carbene at about 4.1 ppm (JH-H = 9.5 Hz) and one doublet of doublets for those pseudo-trans to the phosphine at about 4 ppm (JH-H = 9.5 Hz and JH-P = 2.6 Hz), for complex 18a.

• Two singlets between 4 and 4.2 ppm related to the olefinic protons in the case of complex 22a.

Fig. 3.60 ¹H-NMR spectrum of the complex 18a (T = 298K, CDCl3).

The ¹³C{¹H}-NMR spectra show two peaks at about 38 ppm ascribable to the OCH3 groups and the signals of the olefinic carbons between 45 and 48 ppm.