Aminolysis of iridium thiophene compounds

.


Scheme 1
Thus in order to expand our knowledge concerning the behavior of compound 6, we decided to perform a thorough investigation of the chemical properties of this compound in the presence of secondary amines.

Scheme 2
The reaction was so well behaved for cyclic amines, it gave the iminium salts in good yields; meanwhile diethylamine, which is an acyclic derivative, showed that there was a competition between the formation of compound [Cp*Ir{SC(Me)CHCH(Me)C=N(Et) 2 }]BF 4 (16) and its hydrolysis to afford the acylthiolate compound [Cp*Ir{SC(Me)CHCH(Me)C=O}] (7), as well as its corresponding quaternary salt [NH 2 (Et) 2 ]BF 4 .Diisopropylamine and dicyclohexylamine showed the formation of compound 7 along with the corresponding quaternary salt [NH 2 (R) 2 ]BF 4 [R = CHMe 2 ; C 6 H 11 )] without evidence of the iminium salt.Finally, the chiral methylbenzylamine lead to the exclusive formation of compound [Cp*Ir{SC(Me)CHCH(Me)C=N(Me)(Bz)}]BF 4 (17); however, several attempts to isolate it were unsuccessful and there was only 1 H and 13 C NMR spectroscopic evidence of its presence (Tables 1 and 2).
There was no reaction between compound 6 and diphenylamine, both of which were recovered, and there was no evidence of the iminium salt under the conditions described in the experimental section (vide infra) in the case of ephedrine, pyridine and aniline.Particularly, ephedrine was not a selective reaction.When a stoichiometric ratio of reactants was used, the reaction, monitored qualitatively by 1 H NMR, showed that no considerable conversion to products took place.Hence, it is clear that an excess of ligand is always necessary.The role of the basicity of the amine is not quite clear because piperidine and morpholine afford quite similar results.Apparently, steric effects may also prevent the nucleophilic addition reactions, as observed from the diphenylamine.The iminum carbon is demonstrably electrophilic, because it is susceptible to base hydrolysis attack, as observed for ruthenium analogue species. 1 Partial recyclization of the thiophene ligand proceeds in the thermodynamic compound 12 in presence of triflic acid.It is known that the kinetic isomers of the ruthenium analogues are rapidly transformed after treatment with protic sources. 1 The iridium compound 13 showed faster interconversion between kinetic and thermodynamic products, which did not allow to compare the reactivity of the kinetic isomer with the ruthenium analogue.
Attempts to remove the Cp*Ir fragment from compound 13 were carried out in presence of (n-Bu 4 )NCl or MeOTf unsuccessfully, which suggest the high stability of the η 3 -allyl-Ir bond.Addition of D 2 O to compound 12 did not show any change even after a week; meanwhile an equimolar water solution of compound 13 and H 2 O/KOH (0.03M) afforded quantitatively the acylthiolate 7. Compounds 12-14 melted, 13 and 14 without decomposition, and crystallized from nitromethane/diethyl ether as single crystals, except 13 which crystallized from CHCl 3 at room temperature.The crystallographic studies were carried out at room temperature and the assignment of these species as an iminium derivative is supported by the bond distances and angles at C4.The C=N distances of 12, 13 and 14 are 1.313(6), 1.306(12) and 1.308(10) Å, respectively, which is appropiate for a double bond, whereas bonds between nitrogen and the carbon atoms of the corresponding cyclic amines are longer N-C7 1.491(6), N-C11 1.494(7) 12; N-C7 1.464(12), N-C10 1.441(16) 13; and N-C7 1.479(11), N-C10 1.477(10) 14 Å.The molecular structures are presented in Figures 1-3, respectively.Table 3 includes the corresponding crystal data and Tables 4 and 5 selected bond lengths and bond angles, respectively.
The structure of these iminium salts in the solid state revealed a sandwich structure wherein the thiophene ligand is cleaved.These iminium centers are conjugated via a π-bonding network with the rest of the allylthiolate, where the highest delocalization was observed for compound 12, and similar behavior has been reported for the aminolysis of dicationic ruthenium thiophene compounds. 1he distances of Ir(1)-S(1) and S(1)-C(1) are decreasing in the following order: 12 > 14 > 13 and 14 > 12 > 13, respectively.The piperidine and morpholine fragments showed a chair conformation and the pyrrolidine, an envelope one.The IR spectra of 12-16 shows a strong band, assigned to the ν C=N , at around (cm -1 ):1578, 12; 1594, 13; 1571, 14; 1574, 15 and 1590, 16.

Experimental Section
General Procedures.All reactions were carried out with exclusion of air using Schlenk-tube techniques.Solvents were dried by the usual procedures and distilled under nitrogen prior to use.The starting material Cp*Ir(2,5-dimethyl-η 5 -thiophene)][BF 4 ] 2 (6) was prepared by the published method. 5n the 1 H and 13 C, 11 B and 19 F NMR spectra, chemical shifts are expressed in ppm downfield from Me 4 Si, BF 3 •OEt 2 and CF 3 COOH, respectively.Coupling constants, J, are given in Hertz.The addition of an excess of the amine (2 equiv) in ∼1-2 mL of CH 3 NO 2 to a solution of compound 6 (150 mg, 0.245 mmol) in CH 3 NO 2 (30 mL) was stirred at room temperature for 18 h.Immediately after the addition of the amine, a strong change in color was observed (vide infra).The solvent was evaporated and the product was extracted with ∼30 mL of CH 2 Cl 2 , the volume was reduced to ∼10 mL and precipitated with Et 2 O (∼60 mL).The corresponding solids of 12 and 14 were recrystallized from CH 3 NO 2 /Et 2 O, at -10 °C, to afford powders or single crystals.Compound 13 was recrystallized from CHCl 3 at room temperature.See Table 1 and 2 for data related to melting point, elemental analyses, yield, mass spectra and NMR spectroscopy.The different compounds were: R 2 = piperidyl (12).The orange solution afforded orange crystals.R 2 = pyrrolidyl(13).The orange solution afforded red crystals.R 2 = morpholyl (14).The orange-dark solution afforded cherry-red crystals.R 2 = piperazyl (15).The orange-red solution afforded an orange-red solid of 15 along with an unknown orange compound in a 6:4 ratio, respectively.R = diethyl (16).The yellow-brown solution afforded a brown-orange solid, which after recrystallization with Et 2 O gave an insoluble white powder corresponding to the quaternary salt [NH 2 (Et) 2 ]BF 4 in 47.8% yield (related to the amine), 6 [ 1 H NMR (CDCl 3 ) δ: 6.8(s,br); 3.14(m); 1.35(m). 13C NMR (CDCl 3 ) δ: 44.0 (CH 2 ) and 11.2 (Me)]; meanwhile in the soluble fraction of the remaining solid, the acylthiolate iridium compound 7 was isolated in 44.9% yield (related to compound 6). 3,4A chromatographic column of alumina was carried out for separation of both cationic compounds in CH 2 Cl 2 ; the quaternary salt stayed at the top of the column and compound 6 was eluted with CH 2 Cl 2 /acetone to afford an orange oil which after recrystallization in CHCl 3 /Et 2 O gave an orange solid.R = methyl, benzyl (17).In our hands, it was not possible to isolate this product.However, according with 1 H and 13 C NMR spectroscopy this is the only product formed during the reaction.Attempts to isolate 17 led only to unidentified decomposition products.R = diisopropyl The yellow-green solution afforded a yellow solid which after purification afforded the quaternary salt of [NH 2 (CHMe) 2 ]BF 4 in 52.3% yield (related to the amine)

Table 2 .
13C NMR, a mass spectra and elemental analysis of compounds 12-17 The presence of only one isomer for derivatives 12-17 was established by 1 H and 13 C NMR which showed only one set of Cp* signals.However, the reaction was monitored through 1 H NMR for compound 6 in presence of pyrrolidine, in a 1:2 ratio.This showed the formation of the kinetic product as the anti conformer 11 [ 1 H NMR (CD 3 NO 2 ) δ: 5.62 (d, 4.7 Hz, CH A ); 3.60 (br, CH B ); 2.34 (s, Me) 2.00 (s, Cp*)] which easily went to the corresponding syn thermodynamic one 13 [ 1 H NMR (CD 3 NO 2 ) δ: 6.06 (d, 6.2 Hz, CH A ); 2.47 (d, 6.2 Hz, CH B ); 2.33 (s, Me) 1.92 (s, Cp*), The addition and C-S cleavage processes occur regiospecifically at the (Me)S-C linkage.

Table 1 ]
(Scheme 2).Additional to the 1 H and 13 C NMR spectra of 12-17, HETCOR and APT experiments were performed to support the corresponding assignment.The 11 B and 19 F NMR confirmed the presence of the BF 4 counterion in all cases.

Table 3 .
X-ray data collection parameters for compounds 12-14