Chemistry and catalysis advances in organometallic chemistry and catalysis
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50 M = Mo, W
(a, b) Stabilization of 2-aminoethyl isocyanide and 2-aminophenyl isocyanide at metal templates. 9.4 COMPLEXES WITH NR,NH-NHCS BY OXIDATIVE ADDITION OF AZOLES In 1997, the reaction of an ethylenediamine-tethered adenine with complex anion [Ru III Cl
(DMSO) 2 ] − was described to proceed under C8-metallation of the adenine moiety and formation of a tether-substituted protic NHC ligand. The resulting complex 54 featured a neutral tridentate NR,NH-NHC/diamine ligand Scheme 9.17 [65]. Formally, this reaction can be considered to be a tautomerization of the purine moiety (C8 −H→N7−H) generating a neutral NR,NH-NHC. The metal center did not change its oxidation state. This unusual reactivity was attributed to the tethered chelating diamine group. In the absence of the tether, the more typical metal coordination at N7 of the nucleobase was observed [66]. As the oxidation state of the metal center did not change, an oxidative addition of the C8 −H bond does appear unlikely but the COMPLEXES WITH NR,NH-NHCS BY OXIDATIVE ADDITION OF AZOLES 125 N N N N
H Ru
NH 2 S O Me Me HN NH 2 Cl Cl N N N N NH 2 N NH 2 H [Ru III
Cl 4 (DMSO) 2 ] + *HCl MeOH
S Ru II S O Me Me HN NH 2 Cl Cl C N N N N NH 2 Me Me O H H C 8 N 7 H-shift
Scheme 9.17 Metallation at C8 of a diamine-tethered adenine to give 54. simple C8 −H→N7−H tautomerization generating the NR,NH-NHC is also very unlikely. No mechanistic proposal for the formation of 55 has been put forward. Related chelate-assisted formal tautomerization reactions of neutral C2 −H-substituted azoles have been described. Bergman and coworkers [67] showed that an N-olefin substituted benzimidazole reacts with [RhCl(coe) 2 ]
to yield the NR,NH-NHC/olefin chelate complex 55 (Scheme 9.18a). Ikariya and Kuwata [68] used N1-(2-pyridyl)benzimidazole for the synthesis of the C2ˆN(pyridyl) chelate complex 56 featuring also a protic NHC ligand (Scheme 9.18b). Grothjahn and coworkers reacted an N-ethylenephosphine-substituted imidazole with Ru II or Ir
III to prepare the CˆP chelate complexes 57 [69a] and 58 (Scheme 9.18c) [69b]. Apart from their unusual mode of formation, complexes 56 −58 show an interesting follow-up chemistry. For example, the deprotonation of the NR,NH-NHC moiety in these complexes is possible leading to anionic NHC ligands with an unsubstituted ring-nitrogen atom. The formation of complexes 55 −58 has been attributed to a “tautomerization” reaction of the azoles, possibly aided by the tethered donor function. While this description represents the reaction products correctly, a simple tautomerization of an azole to an NR,NH-NHC is altogether not very likely. The exact mechanism for the formation of these complexes has not been established at the time of publication. We became interested in the reaction of N-donor-functionalized benzimidazoles with transition metals and prepared the precursors for bidentate C(NHC)ˆP(phosphine) ligands 59 −62. The benzimidazoles 59 and 60 react with [RuCp*Cl] 4 to give complexes 63 and 64, respectively (Scheme 9.16a) [70]. The N-donor-substituted benzimidazoles 61 and 62 react with [RhCl(coe) 2 ]
in the presence of different tertiary phosphines (Scheme 9.16b) to give complexes cis/trans-65 and 66 bearing protic NHC ligands [71]. Here, the length of the linker between the NHC-nitrogen atom and the phosphine as well as the steric demand of the used monodentate phosphines determine the geometry of the resulting complexes. In the presence of the sterically demanding tricyclohexyl phosphine, ligand precursor 61 yields a mixture of the cis-P,P (cis-65, major 77%) and the trans-P,P complexes (trans-65, minor 23%). The PˆC ligand precursor 62 with an ethylene spacer between the ring-nitrogen atom and the di(tBu)phosphine donor yields in the presence of PCy 3 exclusively the trans-P,P complex 66. The introduction of a longer space with the concurrent expansion of the C(NHC) −Rh−P(tBu) angle and the use of the sterically demanding PCy 3 apparently prevent the cis-coordination of the phosphine donors. The mechanism for the formation of complexes 63 −66 has not been established unambiguously yet. As mentioned before, related complexes bearing bidentate imidazolin-2-ylidene/donor or benzimidazolin-2-ylidene/donor ligands (Scheme 9.18) have been described as formed by tautomerization of the azole to the NR,NH-substituted NHC that then together with the tethered donor coordinates to the metal center. This rather simple type of reaction mechanism is not very likely to be operative. Although the oxidation state of the metal center does not change in the reactions depicted in Scheme 9.19, we still assumed that a C2 −H oxidative addition step, among others, is involved in the formation of complexes 63−66. To test this hypothesis, we have prepared C2 −X (X = Cl, I) substituted neutral benzimidazoles that are thought to be more reactive in oxidative addition reactions than their C2–H-substituted analogs. The oxidative addition of NR,NR-substituted benzimidazolium cations to transition metals is a well-established reaction [72]. Much less is known about the C2–X oxidative addition of neutral azoles. An early study indicated that 2-chloroazoles do indeed oxidatively add to transition metals and that the resulting negatively charged NHC ligand can be N-protonated to yield complexes with protic NHC ligands [73]. In the presence of NH 4 BF
, neutral 2-chloro-N-methylbenzimidazole oxidatively adds to Pd 0 and Pt 0 leading to complexes of type 69 [74a]. A similar one-pot oxidative addition was observed between 2-halogenato-N-methylbenzimidazoles and Ni 0 complexes in the presence of NH 4 BF 4 as a proton source [74b]. It was proposed that these reactions proceed by an initial oxidative addition of the C2 −X bond to the transition metal to give the intermediate 67 (Scheme 9.20 for M = Pd, pathway a).
126 COMPLEXES WITH PROTIC N-HETEROCYCLIC CARBENE (NR,NH-NHC) LIGANDS Ru N
N Cl H [RuCp*Cl] 4 + N N N THF N N [RhCl(coe) 2 ] 2 + N N Rh PCy 3 Cl H PCy 3 , THF [IrCp*Cl 2 ] 2 + N N Ph 2 P Ir N N Ph 2 P Cl H Ru N N Ph 2 P Cl H [RuCpCl(cod)] + Cl 55 56 57 58 (a)
(b) (c)
Scheme 9.18 (a–c) Reactions of donor-functionalized C2-H azoles with selected transition metals. This intermediate would feature an unusual anionic NHC ligand with an unsubstituted ring-nitrogen atom. Subsequent N- protonation leads to complex 69. Alternatively, but not very likely, the 2-chloro-N-methylbenzimidazole could first be N-protonated by NH 4 BF 4 giving an NR,NH-benzimidazolium cation 68 that then oxidatively adds as described [72] to the transition metal (Scheme 9.20, pathway b). While the protonation of benzimidazoles by NH 4 BF
is not very likely (pathway b in Scheme 9.20), the formation of 69 does not provide any information on which pathway is operative. Such information can be obtained if the oxidative addition is performed in the absence of a proton source. The reaction of 2-chloro-N-methylbenzimidazole with [Pt 0 (PPh 3 ) 4 ] in toluene yields a mixture of complexes 70 and 71 (Scheme 9.21). Complex 71 is formed by dimerization of the reaction product of the initial oxidative addition clearly demonstrating that the oxidative addition precedes the protonation step (pathway a in Scheme 9.20). Related dinuclear complexes have been obtained by N-deprotonation and dimerization of complexes bearing protic NHC ligands [32, 75]. Even more interesting is complex 70 that constitutes the initial product of the oxidative addition of the 2-chloro-N- methylbenzimidazole. This complex, which has been characterized by X-ray diffraction [76], bears an anionic NHC ligand with a naked, unsubstituted nitrogen atom. Density functional theory (DFT) calculations show that the negative charge is located at the unsubstituted ring-nitrogen atom. The electronic situation within the heterocycle of 70 explains its reactivity as a strong Lewis base. Preparation of complexes of type 70 in the presence of a proton source leads to the complex with the protonated NR,NH-NHC ligand (Scheme 9.20). In the absence of a proton source, complex 70 can react to give the dinuclear species 71 via interaction of the anionic nitrogen atom with the metal center of a neighboring molecule. The nucleophilic ring-nitrogen atom in 70 can also react with haloalkanes to yield complexes with classical NR,NR-NHC ligands. This type of reactivity was first observed when a dichloromethane solution of 70 was allowed to stand for a few weeks. The observed N-alkylation reaction could be reproduced and accelerated by dissolving 70 in a small amount of dichloromethane and heating the solution to 100 ◦ C in a
pressure tube for 16 h. After purification, 72 was obtained as a colorless solid in 67% yield (Scheme 9.21) [76]. It should be noted that the oxidative addition of the C2 −X bond (X = H, halogen) of neutral N-alkylazoles yields different reaction products depending on the substituent X. It was proposed that the reaction of C2 −H azoles (Scheme
COMPLEXES WITH NR,NH-NHCS BY OXIDATIVE ADDITION OF AZOLES 127 N N PPh 2 N N PPh
2 59 60 H H [Ru II (Cp*)Cl] 4 THF
64 [Ru
II (Cp*)Cl]
4 THF
Ru II N N P Ph Ph H Cl 63 Ru II N N P Ph Ph H Cl 61 N N PCy 2 N N Pt Bu 2
H H [Rh I Cl(coe)] 2 , PCy
3 THF
cis-65 major 77% N N
2 Rh I Cl PCy
3 N N PCy 2 Rh I PCy
3 Cl H H [RhCl
I (coe)]
2 , PCy
3 THF
66 N N Pt Bu 2 Rh I PCy
3 Cl H trans-65 minor 23% (a) (b)
Scheme 9.19 (a, b) Reactions of N-alkylphosphine functionalized benzimidazoles with Ru II and Rh
I . 9.18 and Scheme 9.19) most likely proceeds under initial C2 −H oxidative addition [76]. The initial reaction product, metal hydride complex G (Scheme 9.22), is not stable in the presence of the strongly basic anionic NR,N-NHC ligand and reacts further via reductive elimination of a proton that subsequently protonates the anionic ring-nitrogen atom of the NHC. The result is complex H featuring the metal center in the initial oxidation state. It is not clear yet if the proton shift proceeds in an intra- or intermolecular manner. The reductive elimination of a proton from metal hydride complexes after the oxidative addition of a C2 −H bond is a well-established reaction, at least for azolium salts [77]. Quantum chemical calculations have shown that this reaction sequence is also feasible for the C2 −H oxidative addition of neutral N-alkyl substituted azoles [78]. Thus, the “formal tautomerization” [67–69] observed in the reaction of neutral C2 −H azoles with transition metals is best described as a “redox-tautomerization” [71, 76] involving an oxidative addition/reductive elimination reaction sequence. This type of ligand shift from the metal to the coordinated NHC ligand is not observed after the oxidative addition of the C2 −Cl bond of 2-chloro-N-methylbenzimidazole to Pt 0 or Ni
0 , which (in the absence of a proton source) leads to isolable complexes of type I featuring an NHC ligand with an anionic unsubstituted ring-nitrogen atom (Scheme 9.22).
128 COMPLEXES WITH PROTIC N-HETEROCYCLIC CARBENE (NR,NH-NHC) LIGANDS N N
3 Cl N N CH 3 Pd PPh
3 Cl H N N CH 3 PdCl(PPh
3 ) 2 67 BF 4 69 PPh
3 NH 4 BF 4 N N CH 3 Cl H BF 4 [Pd(PPh
3 ) 4 ] toluene
NH 4 BF 4 toluene
68 pathway a pathway b [Pd(PPh
3 ) 4 ] toluene
Scheme 9.20 Alternative pathways for the oxidative addition of 2-chloro-N-methylbenzimidazole to Pd 0 .
N Pt Cl PPh 3 PPh 3 N N Cl [Pt(PPh
3 ) 4 ] toluene, Δ
N N Pt N N Pt PPh
3 PPh
3 Cl Cl 71 (minor, 20%) CH 2 Cl 2 , 100 °C N N Pt Cl PPh 3 PPh
3 72 CH 2 Cl + Cl Scheme 9.21 Oxidative addition of 2-chloro-N-methylbenzimidazole to Pt 0 in the absence of a proton source. CONCLUSION 129 N N M II L L N N H M n+ L x D N N M (n+2)+ D H N N M n+ D L H L
H N N Cl I M 0 L x Cl Scheme 9.22 Proposed mechanism for the oxidative addition of C2 −H and C2−X bonds of azoles to transition metals. N N N N O O Cl N N N N O O M PPh 3 Cl PPh 3 N N N N O O M PPh 3 Cl PPh 3 H [M(PPh 3 ) 4 ] toluene BF 4 [M(PPh 3 ) 4 ] NH 4 BF 4 HBF 4 , THF
toluene, Δ
73 74 Scheme 9.23 Synthesis of complexes bearing C8-metallated caffeine. Complexes of type G are rather rare and have so far only been obtained by deprotonation of coordinated NR,NH-NHCs [67, 68]. The direct access to complexes featuring NR,N-NHC ligands by the oxidative addition of the C2 −Cl bond of a 2-halogenato-N-alkylbenzimidazole is much more facile and allows a rich follow-up chemistry like the N-protonation or N-alkylation of the NR,N-NHC ligand. Particularly, complexes bearing “protic” NHC ligands can be obtained easily by this way.
The oxidative addition of neutral azoles can be utilized for the synthesis of C-metallated biomolecules such as caffeine or adenine. For example, 8-chlorocaffeine 73 reacts with Pd 0 or Pt
0 complexes to yield the complexes bearing a C8-metallated caffeine “carbene” ligand. Depending on the reaction conditions, both the N-protonated derivatives 74 and the complexes bearing a caffeine ligand with an unsubstituted nitrogen atom adjacent to the carbene carbon atom 75 have been isolated and crystallographically characterized (Scheme 9.23) [79].
Complexes bearing protic NHC ligands are accessible by various synthetic routes such as the deprotonation of azoles followed by reaction with a transition metal complex, the template-controlled cyclization of functionalized isocyanides, and the oxidative addition of different azoles to transition metal complexes. The complexes with simple monodentate NR,NH- NHCs often tend to tautomerize to give the N-bound azoles. This type of tautomerization is prevented in complexes with donor-functionalized NR,NH-NHCs. Recent studies demonstrate that complexes with protic NHCs obtained from C2 −H azoles are formed by an oxidative addition/reductive elimination reaction sequence. The N −H group in complexes with protic NR,NH-NHCs can serve as a hydrogen bond donor and thus as a molecular recognition unit and may enable various types of bifunctional catalysis. Recent studies indicate that even biomolecules such as caffeine can be C8-metallated. It
130 COMPLEXES WITH PROTIC N-HETEROCYCLIC CARBENE (NR,NH-NHC) LIGANDS should be possible to extend this chemistry to other biomolecules such as DNA building blocks (such as adenine), which would lead to new bioorganometallic chemistry. ACKNOWLEDGMENT The author thanks the Deutsche Forschungsgemeinschaft (SFB 858) for financial support. REFERENCES 1. Bruice, T. C.; Lightstone, F. C. Acc. Chem. Res. 1999, 32 , 127. 2. (a) Coolen, H. K. A. C.; van Leeuwen, P. W. N. M.; Nolte, R. J. M. Angew. Chem. Int. Ed Engl. 1992, 31 , 905. (b) Reetz, M. T.; Waldvogel, S. R. Angew. Chem. Int. Ed Engl. 1997, 36 , 865. (c) Reetz, M. T. Top. Catal. 1997, 4 , 187. (d) Rowlands, G. J. Tetrahedron 2001, 57 , 1865. (e) Wilkinson, M. J.; van Leeuwen, P. W. N. M.; Reek, J. N. H. Org. Biomol. Chem. 2005, 3 , 2371. 3. (a) Larsen, J.; Rasmussen, B. S.; Hazell, R. G.; Skrydstrup, T. Chem. Commun. 2004, 202. (b) Takacs, J. M.; Reddy, D. S.; Moteki, S. A.; Wu, D.; Palencia, H. J. Am. Chem. Soc. 2004, 126 , 4494. (c) Duckmanton, P. A.; Blake, A. J.; Love, J. B. Inorg. Chem. 2005,
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