Catalysts A -heterocyclic carbene

Grasa GA, Gueveli T, Singh R, Nolan SP (2003) Efficient transesterification/ acylation reactions mediated by A-heterocyclic carbene catalysts. J Qrg Chem 68 2812-2819... 

Singh R, Kissling RM, Letellier MA, Nolan SP (2004) Transesterification/ acylation of secondary alcohols mediated by A-heterocyclic carbene catalysts. J Org Chem 69 209-212... 

The combined features of charge and catalyst stabilisation find their expression in markedly enhanced recyclability. Using catalyst loadings of 2.5 - 5 mol%, the activity decreased by only a few percent after 10 cycles and remained above 90% of the initial value. Furthermore, the catalyst solution could be kept for months without measurable loss in activity. By substituting the tricyclohexylphosphine ligand for a A -heterocyclic carbene, catalyst stability was further improved which finds its expression in very low residual ruthenium levels in the isolated product.[44] As these are only the... 

Inamoto, K., Kuroda, J., Danjo, T. and Sakamoto, T. (2005) Highly efficient nickel-catalyzed Heck reaction using Ni(acac)2/A-heterocyclic carbene catalyst. Synlett, 1624-6. 

An alternative method for the formation of enantioenriched a-chloroesters, using A-heterocyclic carbene catalysts, was reported by Reynolds and Rovis (Scheme 13.14) [34]. In a similar mechanism to that presented in Scheme 13.12, initial attack of the carbene to the aldehyde and loss of HCl generated a chiral enolate. Asymmetric protonation of this enolate followed by displacement of the azolium species by a phenol produced enantiopure a-chloroesters. In contrast to the approach to chiral a-chloroesters presented in Scheme 13.13, a variety of aryl esters can be incorporated into the product by using different aryl alcohols (ArOH). Additionally, a carbon-chlorine bond is not formed in this reaction. Rather the introduction of a stereocenter in the chlorinated products is achieved via asymmetric protonation. This method was elaborated to use water as the proton/alcohol source to produce chiral a-chloro carboxylic acids (i.e., as in Scheme 13.12) [28]. Moreover, the use of D2O generated chiral a-chloro-a-deutero carboxylic acids. 

The simplest [2+2+2] reaction of isocyanates is their trimerization (see Section 3.3.I.3.). Numerous triple and double bonded substrates are known to undergo this reaction. The stoichiometry of the reagents sometimes determines when 2 1 or 1 2 adducts are formed. For example, reaction of isocyanates in the presence suitable catalysts form [2+2+2] cycloadducts 179, consisting of two equivalents of the acetylene derivative and one equivalent of the isocyanate. This reaction proceeds in high yield using Ni(o) or aluminum catalysts. The same cycloadducts are also obtained when the reaction is conducted in the presence of Co(acac)3/Et3Al catalysts Even better yields are obtained when a heterocyclic carbene catalyst is used in conjunction with triethylphosphine. ... 

The asymmetric hydroxylation of enals has been achieved using a resolved A -heterocyclic carbene catalyst (Scheme 2.9) [11]. The chemistry was carried out under very mild conditions using 4-nitropyridine Af-oxide as the oxidant and was highly selective (up to 92% ee). An array of enals bearing a range of alkyl and aryl groups was successfully functionahzed in this work. The authors proposed a radical pathway for the chemistry based upon a series of mechanistic investigations. 

Reaction of a chiral A-heterocyclic carbene catalyst and a,/ -unsaturated aldehyde gives , -unsaturated acyl azolium that participates in enantioselective annulation via a Coates-Claisen rearrangement that invokes the formation of a hemiacetal before a sigmatropic rearrangement to give dihydropyranone products (Scheme 14)7 ... 

The coupling of electron-neutral, electron-rich, and electron-deficient triaryl phosphates (92) has been achieved using a Ni(II)-(a-Aryl) complex/ A-heterocyclic carbenes catalyst system in dioxane at 110 °C, in the presence... 

Abstract The use of A-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation. 

Many parallel developments in organometallic chemistry focusing on various transition metals have eventually led to the use of A-heterocyclic carbenes in metathesis catalysts. A new episode of a successful story has been started. 

The first examples utilising A-heterocyclic carbenes as ligands in the Buchwald-Hartwig amination involved the in situ formation of the catalyst from the corresponding imidazolium salt and a Pd(0) source. Nolan reported IPr-HCl/PdjCdbalj as a catalytic system for the amination of aryl chlorides in excellent yields, using different types of amines, anilines, and also imines or indoles [142,143] (Scheme 6.46). Hartwig showed later that in some cases the reactions could be performed at room temperature and without anhydrous conditions even for aryl chlorides [ 144]. This was later shown for the less challenging bromides and iodides [145,146]. 

By utilizing a solid support-based tetradentate A-heterocyclic carbene-palladium catalyst, cross couplings of aryl bromides with phenylboronic acid were achieved in neat water under air.121 A high ratio of substrate to catalyst was also realized. 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

Asymmetric induction has also been achieved in the cyclization of aliphatic alcohol substrates where the catalyst derived from a spirocyclic ligand differentiates enantiotopic alcohols and alkenes (Equation (114)).416 The catalyst system derived from Pd(TFA)2 and (—)-sparteine has recently been reported for a similar cyclization process (Equation (115)).417 In contrast to the previous cases, molecular oxygen was used as the stoichiometric oxidant, thereby eliminating the reliance on other co-oxidants such as GuCl or/>-benzoquinone. Additional aerobic Wacker-type cyclizations have also been reported employing a Pd(n) system supported by A-heterocyclic carbene (NHC) ligands.401,418... 

There have been many reports of the use of iridium-catalyzed transfer hydrogenation of carbonyl compounds, and this section focuses on more recent examples where the control of enantioselectivity is not considered. In particular, recent interest has been in the use of iridium A -heterocyclic carbene complexes as active catalysts for transfer hydrogenation. However, alternative iridium complexes are effective catalysts [1, 2] and the air-stable complex 1 has been shown to be exceptionally active for the transfer hydrogenation of ketones [3]. For example, acetophenone 2 was converted into the corresponding alcohol 3 using only 0.001 mol% of this... 

A -Heterocyclic carbene complexes of Ir(I) and Ir(III) have also demonstrated high reactivity in transfer hydrogenation reactions of ketones (Scheme 2) [4]. Complex 4 catalyzed the reduction of a range of ketones into the corresponding alcohols, including the reduction of pinacolone 7 into alcohol 8 with a low catalyst loading and short reaction time [5]. The chelating bis(Af-heterocyclic carbene) complex 5 was shown to catalyze the reduction of ketones, and in the case of the reduction of benzophenone 9 to alcohol 10, the reaction was complete within 4 min [6]. 

Triene 257 was transformed into the racemic cyclopentenone 258 by a ring closing metathesis using Grubbs iST-heterocyclic carbene catalyst (102) [55]. 

Abstract The use of iV-heterocychc carbenes as catalysts for organic transformations has received increased attention in the past 10 years. A discussion of catalyst development and nucleophilic characteristics precedes a description of recent advancements and new reactions using A-heterocyclic carbenes in catalysis. 

This review will focus on the use of chiral nucleophilic A-heterocyclic carbenes, commonly termed NHCs, as catalysts in organic transformations. Although other examples are known, by far the most common NHCs are thiazolylidene, imida-zolinylidene, imidazolylidene and triazolylidene, I-IV. Rather than simply presenting a laundry list of results, the focus of the current review will be to summarize and place in context the key advances made, with particular attention paid to recent and conceptual breakthroughs. These aspects, by definition, will include a heavy emphasis on mechanism. In a number of instances, the asymmetric version of the reaction has yet to be reported in those cases, we include the state-of-the-art in order to further illustrate the broad utility and reactivity of nucleophilic carbenes. 

It should be noted that asymmetric acyl transfer can also be catalyzed by chiral nucleophilic A-heterocyclic carbenes [27-32] and by certain chiral Lewis acid complexes [33-37] but these methods are outside the scope of this review. Additionally, although Type I and Type II tr-face selective acyl transfer processes have been reported to be catalyzed by some of the catalysts described in this review, these also lie outside the scope of this review. 

The MCR toward 2//-2-imidazolines (65) has found apphcation in the construction of A(-heterocyclic carbene (NHC) complexes (74). Alkylation of the sp Af-atom with an alkyl halide followed by abstraction of the proton at C2 with a strong base (NaH, KOtBu) resulted in the formation of the free carbene species, which could be trapped and isolated as the corresponding metal complexes (Ir or Rh) [160]. The corresponding Ru-complexes were shown to be active and selective catalysts for the transfer hydrogenaticm of furfural to furfurol using iPrOH as hydrogen source [161]. 

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