NHCs late transition metal complexes

Late Transition Metal Complexes with NHC Ligands. 104... 

For a recent review on Late Transition Metal-NHC complexes and catalysis see Dfez-Gonzalez S, Marion N, Nolan SP (2009) Chem Rev 109 3612-3676... 

There are a number of reports of NHC complexes of mid-late transition metals being used as catalysts for atom transfer radical polymerisation (ATRP) of acrylates and styrene. Grubbs reported Fe(II) complexes of a simple monodentate carbene. 

The development of CTOwn ether functionalised imidazolinm salts starts from the consideration that it is possible to link one polyether chain with two imidazolinm nnits at the end points. Since a transition metal can coordinate two imidazoUnm salts in trans fashion [131,162,208], two of these (poly)ether fnnctionaUsed bis-carbenes can form a macrocychc crown ether type hgand system with two transition metal carbene linkages. In favonrable cases, a pincer type C,0,C ms-coordination to the same transition metal is conceivable, bnt may not be very likely when the great affinity of late transition metals to NHC ligands and the aversion of these same late transition metals to ether donor Ugands is taken into account. However, hanilabile stabilisation of transition metal complexes in catalytic processes can certainly be hoped for. 

In contrast to the plethora of late-transition metal NHC complexes, reports on early-transition metal and more generally on oxophilic and electropositive metal NHC complexes are relatively scarce. This can be explained in part by the analogy between phosphine and Al-heterocyclic carbene ligands the soft nature of the phosphine prefers soft acids such as middle- or late-transition metals. 

To resolve this reactivity problem, we explored the possibility of employing a Lewis acid to activate the iV-acyl hydrazone by lowering its LUMO energy (Scheme 22) [145-147]. If successful, such an achievement would expand the possibilities for the development of new NHC reactions. The most important challenge to this idea is the known ability of carbenes to act as ligands and form stable complexes, particularly with late transition metals such as palladium and copper [148-151]. We hypothesized that early metals would react reversibly with NHCs, allowing for the development of a cooperative catalytic system. After... 

The direct oxidative addition of the C2-H to electron-rich late transition metals is another method for the preparation of NHC-metal complexes. Mild bases can also promote this reaction. Considering the large difference in between the imidazolium ion and the weak bases such as triethylamine or metal carbonates that are employed, it is likely that the base serves to deprotonate the oxidatively added imidazolium ion driving the reaction forwards. ... 

In contrast to these classical types of NHCs, isomers with less extensive heteroatom stabilisation, termed here non-classical carbenes , initially received much less attention/ Non-classical carbenes can contain just one, or even no heteroatom in positions a to the carbene carbon atom. Due to the lower number of heteroatoms, n stabilising effects are substantially diminished when compared to classical carbenes, which in turn reduces the stability of the free carbene. On the other hand, the electron-withdrawing inductive influence of the heteroatoms is significantly decreased. As a consequence, these non-classical carbenes are surmised to be stronger donors than their classical analogues. Lately, they have gained interest especially as ligands in transition metal complexes because of their distinct electronic properties, which open up new synthetic and catalytic opportunities. 

The first efficient catalytic applications of N-heterocyclic carbene ligands with late transition metals were reported for rhodium. Since the mid 1990s, NHC-rhodium chemistry has been extensively studied and may be comparable to palladium or nickel in terms of the number of catalytic applications. Outside the context of catalysis, NHC-Rh complexes have featured prominently in studies probing the electronic properties of NHCs, usually quantified by comparison of the pco frequencies in the IR spectra of [LRh(CO)2Cl] complexes. ... 

The aim of this Chapter is to examine the application of well-defined N-hetero-cyclic carbene (NHC) complexes as well as the systems prepared in situ which involve free NHCs or the precursor salt for the reduction of imsaturated organic molecules such as alkynes, alkenes and carbonyl compounds. The most active complexes for such reductions contain electron-rich, late transition metals in low oxidation states. Herein, reductions useful for organic synthesis will be classified into four types aeeording to reductants used (i) hydrogenations, (ii) transfer hydrogenation, (iii) hydrosilylation and (iv) hydroboration. For examples of reduction reactions with systems containing non-classical NHC ligands, the reader is referred to Chapter 5. 

Ruthenium s supremacy in the carbene chemistry of Group 8 elements is a direct consequence of the tremendous interest raised by NHC-Ru complexes as catalysts for olefin metathesis. Indeed, the synergy of a late transition metal tolerant of a wide variety of functional groups, together with a class of ligands whose physical and chemical properties are easily modulated to tailor the activity, selectivity, stability, water-solubility, recoverability, or latency of the resulting catalytic species, translated into an unprecedented success story of modern synthetic chemistry. Yet, the ability of ruthenium complexes to promote carbon-carbon bond formation goes well beyond... 

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