Trapping Highly Electrophilic Metalladiphosphanylcarbenes
2003; Wiley; Volume: 42; Issue: 39 Linguagem: Inglês
10.1002/anie.200219604
ISSN1521-3773
AutoresJavier Ruiz, Marta E. G. Mosquera, Gabriel Garcı́a, Elena Patrón, Vı́ctor Riera, Santiago García‐Granda, Juan F. Van der Maelen,
Tópico(s)Catalytic Cross-Coupling Reactions
ResumoA nonstandard method of carbene generation is provided by reaction of 1 with AgBF4 to give the transient metalladiphosphanylcarbene 2. This unique dicationic singlet carbene can be trapped with bases (pyridine (Py) for example, see scheme) showing its electrophilic character, which is also confirmed by DFT calculations. In recent years much attention has been paid to the study of nucleophilic carbenes of Arduengo type (N-heterocyclic carbenes)1 and related diaminocarbenes (R2N)2C:,2 which are stabilized by the presence of two π-donor nitrogen atoms bonded to the carbene carbon atom. The synthesis of the first push-pull phosphanylsilylcarbene of formula (R2P)(SiMe3)C: (R=NiPr2) by Bertrand and co-workers was also a milestone in carbene chemistry.3 Mixed-substituent aminophosphanylcarbenes have recently been prepared.4 Of great interest are also the analogous diphosphanylcarbenes (R2P)2C:, although they are only known as transient species,5 which have been investigated by ab initio calculations.6 Interestingly, their protonated phosphoniumphosphanylcarbene derivatives [(R2P)(R2PH)C:]+ ions are known to be stable.7 We report herein on the generation and trapping reactions of the transient highly electrophilic metalladiphosphanylcarbene [Ru(CNtBu)4(PPh2)2C:]2+ (2).8 The electrophilic character of 2 has been confirmed experimentally and by density functional theory (DFT) calculations.9 Transient carbene 2 is formed from the cationic complex [Ru(CNtBu)4{(PPh2)2CI}]+ (1)10 by iodide abstraction with silver(I) salts (AgClO4 or AgBF4) (Scheme 1), a nonstandard method of carbene generation. Thus the formally anionic diphosphanyliodomethanide ligand {(PPh2)2CI}− is converted into the neutral ligand (PPh2)2C:. Coordination of the diphosphanylcarbene (PPh2)2C: to the metal fragment through the phosphorus atoms avoids dimerization to form the corresponding 1λ5,3λ5-diphosphete derivative,11 or even 1,2-migration of a phenyl group to give the phosphaalkene,5b and enhances the electrophilicity of the carbene as the phosphorus lone pairs can no longer be used for π donation to the carbene carbon atom. Owing to its high electrophilicity (see calculated electron affinities (EA) values in Table 1) 2 is very unstable and instantaneously decomposes, unless a trapping reagent is added. Thus, reaction of 1 with AgBF4 in CH2Cl2 as solvent in the presence of pyridine (Py) gives complex 3, the pyridine adduct of carbene 2 (Scheme 2).12 Complex 3 cocrystallizes with the silver complex salt [Ag(Py)2]BF4 giving suitable crystals for X-ray analysis (Figure 1).13 In the new pyridinium diphosphanylylide ligand the C1N1 bond is slightly shortened with respect to single bond, and the pyridinium plane is coplanar with the P1-P2-C1 plane allowing a π interaction of the ylide carbon atom C1 with the π system in the ring. The P1C1 and P2C1 bonds are appreciable shorter than single bonds indicating that the negative charge at the ylide carbon atom is also shared with both coordinated phosphorus atoms, thus stabilizing the ylide structure. Another adduct of carbene 2 is formed by treatment of 1 with AgBF4 in the presence of tetrahydrothiophene (tht). The reaction occurs readily to give complex 4 (Scheme 2).14 The 1H NMR spectrum of 4 shows three resonance signals for the CH2 groups of tht (see Experimental Section), as the two protons of each α-CH2 unit are inequivalent owing to its different orientation respect to the lone pair of sulfur. The solid-state structure of 4 was solved by X-ray analysis (Figure 2), and clearly shows a tht solvent molecule stabilizing carbene 2 with a rather strong CS interaction (C1-S1 1.707(6) Å). A view of the structure of the cation 3 (thermal ellipsoids set at 20 % probability). Methyl groups in the CNtBu ligands are omitted for clarity. Selected bond lengths [Å] and angles [°]: Ru1-P1 2.355(2), Ru1-P2 2.359(2), P1-C1 1.761(9), P2-C1 1.756(10), N1-C1 1.428(10); P1-Ru1-P2 70.48(9), N1-C1-P2 128.9(7), N1-C1-P1 129.8(7), P2-C1-P1 101.3(4). A view of the structure of the cation 4 (thermal ellipsoids set at 20 % probability). Methyl groups in the CNtBu ligands are omitted for clarity. Selected bond lengths [Å] and angles [°]: P1-C1 1.774(6), P2-C1 1.771(6), S1-C1 1.707(6), S1-C2 1.820(7), S1-C5 1.812(7); P1-Ru1-P2 70.51(5), S1-C1-P2 124.5(3), S1-C1-P1 134.9(4), C1-S1-C2 114.2(3), C1-S1-C5 109.8(3). Formation of the transient metalladiphosphanylcarbene 2. R=tBu. Trapping reactions of carbene 2. [Ru]=[Ru(CNtBu)4], tht=tetrahydrothiophene. B3LYP6-31G*,LanL2DZ[a] B3LYP6-31G*,3-21G B3LYP6-31G**,3-21G* B3LYP6-311++G**,3-21+G* C-P [Å] 1.673 1.681 1.682 1.662 P-C-P [°] 134.3 129.2 129.3 137.9 ΔEST [kcal mol−1] −8.96 −9.86 −10.00 −10.80 EA [eV] 8.94 8.99 When carbene 2 is generated by treatment of 1 with an excess of AgClO4, the diphosphanylketone complex [Ru(CNtBu)4{(PPh2)2CO}]2+ (5) is obtained, after precipitation of AgI (Scheme 2). This result implies the transfer of an oxygen atom from the ClO4− group to the transient carbene 2, which probably proceeds through nucleophilic attack of the oxoanion to the carbene carbon atom. The IR spectrum of 5 reveals the presence of the ketone group (CO=1721 cm−1 (m)), as well as the 13C{1H} NMR spectrum which shows a triplet at δ=216.6 ppm (1J(P,C)=16 Hz) for the P2CO carbon atom.15 The solid structure of 5 has also been confirmed by X-ray analysis (not shown). The formation of carbene 2 by treatment of 1 with AgBF4 in the presence of H2O yields the cationic complex [Ru(CNtBu)4{(OPPh2)(PPh2)CH2}]2+ (6; Scheme 2), which contains a mono-oxidized methylenebis[(diphenyl)phosphane] (dppm) ligand arising from activation of both OH bonds in the water molecule by the carbene. The structure of 6 has been confirmed by 31P and 1H NMR spectroscopy (see Experimental Section) and by X-ray analysis. On the basis of the known trapping reactions of transient diphosphanylcarbenes with alcohols,5a the formation of 6 could proceed through insertion of carbene 2 into a OH bond of water, followed by a 1,2-shift of the OH group to a phosphorus atom and further proton migration to the central carbon atom. To investigate the iodide abstraction leading to carbene 2 we treated 1 with some soluble silver(I) and gold(I) complexes. With the silver derivative Ag(ClO4)(PPh3), the interaction with iodide is so strong that formation of [{AgI(PPh3)}4] is immediately observed with parallel formation of [Ru(CNtBu)4{(PPh2)2CH2}]2+ as decomposition product of the carbene. Fortunately, treatment of 1 with [AuCl(tht)] allowed the isolation of the complex [Ru(CNtBu)4(PPh2)2C(AuCl)I]+ (7; Scheme 3). The structure of this cation (Figure 3) shows the AuCl unit bonded to the methanide carbon atom, but also a close Au⋅⋅⋅I contact of 3.35 Å which is shorter than the sum of van der Waals radii of both atoms (3.64 Å). Some activation is also observed in the CI bond in 7 (2.177(18) Å) compared to coordinated diphosphanyliodomethanide (2.115(4) Å).16 All these structural data indicate that 7 can be considered an isolated intermediate for the formation of transient carbene 2 by halide abstraction from 1 with metal fragments. Additionally the solid structure of 7 is of interest in forming supramolecular triangular aggregates, with central I⋅⋅⋅I contacts and peripheral CH⋅⋅⋅Cl hydrogen bonds (Figure 3). A view of the structure of the cation 7 (thermal ellipsoids set at 20 % probability) showing a three-cation aggregate. Phenyl groups are omitted for clarity. Selected bond lengths [Å] and angles [°]: Au1-C1 2.100(19), Au1-Cl1 2.326(5), C1-I1 2.171(18), C1-P1 1.837(12); P1-Ru1-P1 71.48(18), C1-Au1-Cl1 178.7(5), P1-C1-P1 97.4(9), P1-C1-Au1 104.4(7), P1-C1-I1 122.4(6), Au1-C1-I1 103.7(8). Formation of the heterometallic ruthenium(II)/gold(I) complex 7. R=tBu. Table 1 shows some selected results of the calculations performed on the carbene [Ru(CNH)4(PH2)2C:]2+ (2 b), a model structure obtained from 2 by replacing tBu and Ph substituents by hydrogen atoms.9 The optimized structure shows a perfect C2v symmetry in the highest levels of calculation, both for singlet and triplet states, although for the triplet state PC distances are clearly longer (1.788 Å) and P-C-P angles are much smaller (113.1°) than for the singlet ground state (the values of which are shown in Table 1). These results are much in line with those found for other symmetrically substituted cyclic diphosphanylcarbenes,6b and clearly differ from symmetrically substituted acyclic diphosphanylcarbenes,6a which prefer an asymmetric PC multiple-bond phosphorus vinyl ylide structure. In conclusion we have present an approach to generate a unique dicationic singlet carbene (2), whose electrophilic character has been confirmed experimentally and by DFT calculations. This new type of carbene, as well as its isolated pyridine and tetrahydrothiophene adducts, show promise as reaction intermediates for the preparation of new functionalized diphosphane ligands, as exemplified with the synthesis of the diphosphanylketone derivative 5. All reactions and manipulations were performed under a atmosphere of dry nitrogen by using standard Schlenk and glovebox techniques. Solvents were distilled over appropriate drying agents under dry nitrogen prior to use. Chemical shifts of the NMR spectra are referenced to internal SiMe4 (1H and 13C) or external H3PO4 (31P). 3-(BF4)2⋅0.5{[Ag(Py)2]BF4}: Pyridine (81 μL, 1 mmol) was added to a solution of [Ru(CNtBu)4{(PPh2)2CI}]I (1-I; 0.11 g, 0.1 mmol) in CH2Cl2 (20 mL). This solution was transferred by canula to a Schlenk containing AgBF4 (0.08 g, 0.4 mmol) previously cooled to −75 °C and covered to protect from the light. The mixture was stirred for 10 min and then allowed to warm to room temperature and stirred for a further hour. Filtration of the mixture gave a clear yellow solution. Half of the solvent was removed in vacuo and diethyl ether (15 mL) was carefully added. Storage at −18 °C for 4 days afforded yellow crystals, yield: 0.094 g (67 %). Elemental analysis (%) calcd for C55H66N7Ag0.5B2.5F10N6P2Ru: C 53.05, H 5.34, N 6.75; found: C 52.53, H 5.10, N 6.77; IR (CH2Cl2): =2215 (w), 2180 cm−1 (s) (CNtBu); 31P{1H} NMR (121.5 MHz, CD2Cl2): δ=10.5 ppm; 1H NMR (300 MHz, CD2Cl2): δ=8.1–7.3 (35 H), 1.53 (s, 18 H, CNtBu), 0.95 ppm (s, 18 H, CNtBu); 13C NMR (75.5 MHz, CD2Cl2): δ=133.5–126.3 (Ph and Py), 59.7 (s, CNC(CH3)3), 59.0 (s, CNC(CH3)3), 61.8 (t, 1J(P,C)=49, P2CPy, 30.7 (s, CNC(CH3)3) 29.9 ppm (s, CNC(CH3)3). Resonance for CNC(CH3)3 not observed. 4-(BF4)2: A solution of 1-I (0.1 g, 0.093 mmol) and THT (1 mL) in THF (30 mL) was added through a canula to a Schlenk containing AgBF4 (0.062 g, 0.318 mmol). The mixture was stirred for 20 min and then filtered and the filtrate concentrated to 15 mL to afford white crystals of the compound, yield: 0.056 g (56 %). Elemental analysis (%) calcd for C49H64N4B2F8P2RuS: C 54.61, H 5.99, N 5.20; Found: C, 54.32; H, 5.96; N, 5.09; IR (CH2Cl2): =2214 (m), 2172 cm−1 (vs) (CNtBu); 31P{1H} NMR (121.5 MHz, CD2Cl2): δ=0.38 ppm; 1H NMR (300 MHz, CD2Cl2): δ=7.7–7.6 (20 H, Ph), 3.46 (m, 2 H, tht), 1.85 (m, 2 H, tht), 1.82 (m, 4 H, tht) 1.55 (s, 18 H, CNtBu), 1.15 ppm (s, 18 H, CNtBu). 5-(ClO4)2: AgClO4 (0.025 g, 0.12 mmol) was added to a solution of 1-I (0.05 g, 0.048 mmol) in CH2Cl2 (10 mL). The mixture was stirred for 10 min in the absence of light. After filtration, the solvent was removed from the filtrate in vacuo and the residue recrystallized from CH2Cl2/diethyl ether affording yellow crystals, yield: 0.036 g (73 %). Elemental analysis (%) calcd for C45H56N4Cl2O9P2Ru: C 52.43, H 5.47, N 5.43; Found: C, 52.35; H, 5.31; N, 5.29; IR (CH2Cl2): =2230 (w), 2190 cm−1 (s) (CNtBu), 1721 cm−1 (m) (CO); 31P{1H} NMR (121.5 MHz, CD2Cl2): δ=76.2 ppm; 1H NMR (300 MHz, CD2Cl2): δ=7.8–7.4 (20 H, Ph), 1.76 (s, 18 H, CNtBu), 0.95 ppm (s, 18 H, CNtBu); 13C NMR (75.5 MHz, CD2Cl2): δ=134–126 (Ph), 216.6 (t, 1J(P,C)=16 Hz) P2CO, 61.4 (s, CNC(CH3)3), 60.6 (s, CNC(CH3)3), 30.5 (s, CNC(CH3)3) 29.3 ppm (s, CNC(CH3)3). Resonance for CNC(CH3)3 not observed. 6-(BPh4)2: A solution containing 1-I (0.09 g, 0.08 mmol) and water (4 μL, 0.22 mmol) in CH2Cl2 (20 mL) was added through a canula to AgBF4 (0.08 g, 0.4 mmol) placed in a Schlenk previously cooled to −75 °C and covered from the light. The mixture was stirred for 10 min and then allowed to warm to room temperature and stirred for a further hour. Filtration of the solution and evaporation of the solvent from the filtrate afforded a white solid of 6-(BF4)2 Yield: 0.086 g (70 %). To obtain suitable crystals for an X-ray diffraction study, a metathesis of the anion was performed by treating a CH2Cl2/MeOH solution of the compound with an excess of NaBPh4. Elemental analysis (%) calcd for C93H98N4B2OP2Ru: C 75.86, H 6.70, N 3.80; Found: C 75.79; H 6.45, N 4.02; IR (CH2Cl2): =2223 (w), 2181 (s), 2164 cm−1 (sh) (CNtBu); 31P{1H} NMR (121.5 MHz, CD2Cl2): δ=67.7 (d, 2J(P,P)=29 Hz), 41.4 ppm (d, 2J(P,P)=29 Hz); 1H NMR (300 MHz, CD2Cl2): δ=8.0–7.4 (60 H, Ph), 4.01 (dd, 2J(P,H)=11, 2J(P,H)=9 Hz, 2 H, CH2), 1.73 (s, 9 H, CNtBu), 1.53 (s, 9 H, CNtBu), 1.10 ppm (s, 18 H, CNtBu). 7-[Au(Cl)(I)]: [Au(Cl)(tht)] (0.031 g, 0.096 mmol) was added to a solution of 1-I (0.050 g, 0.048 mmol) in THF (12 mL) and the resulting mixture stirred for 10 min. The solution was then concentrated to 6 mL and let to stand over night to afford yellow crystals, yield: 0.065 g (90 %). IR (CH2Cl2):=2215 (s), 2177 cm−1 (vs) (CNtBu); 31P{1H} NMR (121.5 MHz, CD2Cl2): δ=13.4 ppm; 1H NMR (300 MHz, CD2Cl2): δ=7.7–7.4 (20 H, Ph), 1.63 (s, 18 H, CNtBu), 1.44 (s, 9 H, CNtBu), 0.99 ppm (s, 9 H, CNtBu).
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