Carbene complexes with phosphines

Diaminocarbene complexes were reported as early as 1968 [152], Preparation and applications of such complexes have been reviewed [153], Because of 7t-electron donation by both nitrogen atoms, diaminocarbenes are very weak tt-acceptors and have binding properties towards low-valent transition metals similar to those of phosphines or pyridines [18,153]. For this reason diaminocarbenes form complexes with a broad range of different metals, including those of the titanium group. Titanium does not usually form stable donor-substituted carbene complexes, but rather ylide-like, nucleophilic carbene complexes with non-heteroatom-substituted carbenes (Chapter 3). 

Ruthenium complexes B are stable in the presence of alcohols, amines, or water, even at 60 °C. Olefin metathesis can be realized even in water as solvent, either using ruthenium carbene complexes with water-soluble phosphine ligands [815], or in emulsions. These complexes are also stable in air [584]. No olefination of aldehydes, ketones, or derivatives of carboxylic acids has been observed [582]. During catalysis of olefin metathesis replacement of one phosphine ligand by an olefin can occur [598,809]. 

Scheme 16. Effect of phosphine ligands on the benzannulation of methoxy phenyl carbene complexes with diphenylethyne.

Interestingly these complexes showed high activity without addition of alkyl aluminum compounds in the ionic liquid while they are almost inactive in toluene. These results are interpretable in terms of catalyst stabilization by the imidazolium-based ionic liquid. Reductive elimination of imidazolium is also possible as in toluene as in the ionic liquid but in the ionic liquid, a rapid reoxidation via addition of the solvent imidazolium cation seems possible and may prevent the formation of Ni deposits associated with catalyst deactivation. The carbene complex with R = n-Bu showed the highest activity with a dimer yield of 70.2% (TOF = 7020 h ). The preferred product of the nickel-catalyzed reaction is methylpentene. Additional phosphine ligand had no significant influence on the distribution of the products in this case. 

Phosphine substitution on the phosphonium carbene was found to affect the initiation rate. Phosphine bulk helps stabilize the carbene complexes with respect to decomposition and kinetic deactivation by dimerization pathways [41]. All the complexes were synthesized through the trichloride intermediate 29 to prevent the decomposition of the complexes bearing the less bulky phosphine groups. The active, 14-electron complexes were then generated via the addition of B(C5F5)3 to abstract the chloride Hgand (Scheme 9.3). In solution, precatalysts bearing bulky phosphines were all monomeric, while the mixed phosphine cases tended to reversibly dimerize in solution. An illustrative dimerization is shown for catalyst 30, which possesses intermediate steric bulk at the phosphonium moiety. 

Dimesitylimidazolium chloride in the presence of potassium tert-butylate reacts with [RuCl2(PCy3)2(=CHPh)] or [RuCl2(PCy3)2(=CHCH=CMe2)] to yield the mixed phosphine-carbene complexes [RuCL(L)(PCy3)(=CHPh)] or... 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

Asymmetric versions of the cyclopropanation reaction of electron-deficient olefins using chirally modified Fischer carbene complexes, prepared by exchange of CO ligands with chiral bisphosphites [21a] or phosphines [21b], have been tested. However, the asymmetric inductions are rather modest [21a] or not quantified (only the observation that the cyclopropane is optically active is reported) [21b]. Much better facial selectivities are reached in the cyclopropanation of enantiopure alkenyl oxazolines with aryl- or alkyl-substituted alkoxy-carbene complexes of chromium [22] (Scheme 5). 

Ruthenium hydride complexes, e.g., the dimer 34, have been used by Hofmann et al. for the preparation of ruthenium carbene complexes [19]. Reaction of 34 with two equivalents of propargyl chloride 35 gives carbene complex 36 with a chelating diphosphane ligand (Eq. 3). Complex 36 is a remarkable example because its phosphine ligands are, in contrast to the other ruthenium carbene complexes described so far, arranged in a fixed cis stereochemistry. Although 36 was found to be less active than conventional metathesis catalysts, it catalyzes the ROMP of norbornene or cyclopentene. 

Finally, the phosphine-functionalised carbene complex 40 (Fig. 4.14) has been tested following activation with [H(Et20)][B Ar "] [45], but again a very low activity was achieved (TON 129 after 2 h). The poor performance of these catalysts may also reflect their susceptibility to reductive elimination (of 2-acylimidazolium salt) and generation of Pd(0) [4, 5]. Interest in CO/aUcene seems to have dwindled recently, and as such no further reports on carbene complexes catalysing this reaction have appeared since 2003. 

Fig. 4.15), are active for ATRP of both styrene and methylmethacrylate (MMA) [46]. Polymerisation was well controlled with polydispersities ranging from 1.05 to 1.47. The rates of polymerisation 1 x 10 s ) showed the complexes to be more active than phosphine and amine ligated Fe complexes, and were said to rival Cu-based ATRP systems. It was quite recent that Cu(I) complexes of NHCs were tested as ATRP catalysts [47]. In this work, tetrahydropyrimidine-based carbenes were employed to yield mono-carbene and di-carbene complexes 42 and 43 (Fig. 4.15), which were tested for MMA polymerisation. The mono-carbene complex 42 gave relatively high polydispersities (1.4-1.8) and a low initiation efficiency (0.5), both indicative of poor catalyst control. The di-carbene complex 43 led to nncontrolled radical polymerisation, which was ascribed to the insolubility of the complex. 

An extension of the research on silver complexes with Lewis base-functionalized mono(A-heterocyclic carbene) ligands has been made toward the better-studied and stronger coordinating phosphine systems. The reaction of a diphenylphosphine-functionalized imidazolium salt with silver oxide in dichloromethane affords a trinuclear silver carbene complex 50, as confirmed by electrospray-ionization mass spectrometry.96,97 Metathesis reaction of 50 in methanol using silver nitrate gives 51 in 33% yield. The crystal structures of 51 were found to be different when different solvents were used during crystallization (Scheme 12).97 One NO3- anion was found to be chelated to... 

Complexes of the type (L)AuR have been isolated with a large variety of donors L, including predominantly tertiary phosphines and isocyanides. The dinuclear complex (dppm)(AuMe)2 has been prepared by treatment of (dppm)(AuCl)2 with MeLi and structurally characterized.17 Examples (L)AuR with a carbene ligand L are also known. A simple methyl compound was prepared from the chloride complex with dimethylmagnesium (Equation (l)).18... 

In marked contrast to the results of Gassman and Schrock, major differences were noted by Casey and co-workers in a series of studies utilizing phenylcarbene-substituted W(0) complexes in reactions with olefins. The H NMR spectra of new phenylcarbene tungsten and iron (69) complexes indicate a substantial positive charge residing on the carbene carbon, and as expected, these complexes readily form ylides on reaction with phosphines ... 

The olefin binding site is presumed to be cis to the carbene and trans to one of the chlorides. Subsequent dissociation of a phosphine paves the way for the formation of a 14-electron metallacycle G which upon cycloreversion generates a pro ductive intermediate [ 11 ]. The metallacycle formation is the rate determining step. The observed reactivity pattern of the pre-catalyst outlined above and the kinetic data presently available support this mechanistic picture. The fact that the catalytic activity of ruthenium carbene complexes 1 maybe significantly enhanced on addition of CuCl to the reaction mixture is also very well in line with this dissociative mechanism [11] Cu(I) is known to trap phosphines and its presence may therefore lead to a higher concentration of the catalytically active monophosphine metal fragments F and G in solution. 

Aqueous two-phase hydrogenations are dominated by platinum group metal catalysts containing water-soluble tertiary phosphine ligands. The extremely stable and versatile N-heterocyclic carbene complexes attracted only limited interest, despite the fact that such complexes were described in the literature [62-65]. Recently, it was reported that the water-soluble [RuXY(l-butyl-3-methylimi-dazol-2-ylidene) ( 76-p-cymene)]n+ (X=Ch, H20 Y = C1-, H20, pta) complexes preferentially hydrogenated cinnamaldehyde and benzylideneacetone at the C = C double bond (Scheme 38.5) with TOF values of 30 to 60 h 1 in water substrate biphasic mixtures (80 °C, lObar H2) [66]. 

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