Cycloadditions cobalt-rhodium

The NHCs have been used as ligands of different metal catalysts (i.e. copper, nickel, gold, cobalt, palladium, rhodium) in a wide range of cycloaddition reactions such as [4-1-2] (see Section 5.6), [3h-2], [2h-2h-2] and others. These NHC-metal catalysts have allowed reactions to occur at lower temperature and pressure. Furthermore, some NHC-TM catalysts even promote previously unknown reactions. One of the most popular reactions to generate 1,2,3-triazoles is the 1,3-dipolar Huisgen cycloaddition (reaction between azides and alkynes) [8]. Lately, this [3h-2] cycloaddition reaction has been aided by different [Cu(NHC)JX complexes [9]. The reactions between electron-rich, electron-poor and/or hindered alkynes 16 and azides 17 in the presence of low NHC-copper 18-20 loadings (in some cases even ppm amounts were used) afforded the 1,2,3-triazoles 21 regioselectively (Scheme 5.5 Table 5.2). 

Oxidative additions are frequently observed with transition metal d8 systems such as iron(0), osmium(O), cobalt(I), rhodium(I), iridium(I), nickel(II), palladium(II) and platinum(II). The reactivity of d8 systems towards oxidative addition increases from right to left in the periodic table and from top to down within a triad. The concerted mechanism is most important and resembles a concerted cycloaddition in organic chemistry (Scheme 1.1). The reactivity of metal complexes is influenced by their... 

In this section, we will mainly emphasize cascade sequences where cycloaddition, cycloisomerization or ene-type reactions take part. Most of these reaction sequences involve a transition metal-catalyzed step (mostly cobalt and rhodium), which is either followed by another reaction promoted by the same catalyst or by a purely thermal reaction. A simple change in the temperature of the reaction mixture is often the only technical requirement to go from one step to another. This part will be divided into sections according to the number of rings produced during the cascade reactions. 

The Pauson-Khand reaction is a well-known method for preparing cydopente-nones by the [2 + 2 + 1] cycloaddition reaction of alkyne, alkene and CO. While reactions using stoichiometric amounts of Co2(CO)g were initially examined, catalytic versions with cobalt, titanium, rhodium, iridium, and ruthenium complexes have recently been developed. Whilst the intramolecular version is rather easy, the inter-molecular version is a very difficult problem that has not yet been solved [76]. 

A [2 + 2 + 2]-cycloaddition reaction is also known, facilitated by Ni(cod)2 or a cobalt catalyst. " " [2 + 2 + 1]-Cycloaddition is known.A cobalt catalyst is used for a [4- -2- -2]-cycloaddition of 1,3-butadiene and bicyclo[2.2.2]octa-2,5-diene. " " " Eight-membered rings are products by a rhodium catalyzed [4- -2- -2]-cycloaddition. " " Chromium catalysts are available for [6-1-4]-cycloadditions... 

Instead of a second or third alkyne, an alkene C=C double bond may be incorporated into the cyclotrimerization reaction. Iron [65], rhodium [66], nickel [67], palladium [68], or cobalt [69] catalysts have been used to form cyclohexa-dienes. However, the preparative use of this catalytic co-cyclization is disturbed by consecutive side reactions of the resulting dienes such as cycloaddition or dehydrogenation. Itoh, Ibers and co-workers [70] have reported the straight palladium-catalyzed co-cyclization reaction of C2(C02Me)2 and norbomene (eq. (24)). 

The catalytic [2 + 2 + 1]-cycloaddition reaction of two carbon—carbon multiple bonds with carbon monoxide has become a general synthetic method for five-membered cyclic carbonyl compounds. In particular, the Pauson-Khand reaction has been widely investigated and established as a powerful tool to synthesize cyclopentenone derivatives.110 Various kinds of transition metals, such as cobalt, titanium, ruthenium, rhodium, and iridium, are used as a catalyst for the Pauson-Khand reaction. The intramolecular Pauson-Khand reaction of the allyl propargyl ether and amine 91 produces the bicyclic ketones 93, which bear a heterocyclic ring as shown in Scheme 31. The reaction proceeds through formation of the bicyclic metallacyclopentene intermediate 92, which subsequently undergoes insertion of CO to give 93. 

Reactions similar to the PKR that provide cyclopentenones from the [2 + 2 + 1] cycloaddition of an alkyne, an alkene, and carbon monoxide can be achieved with metals other than cobalt. Chromium, iron, iridium, molybdenum, nickel, palladium, rhodium, ruthenium, titanium, tungsten and zirconium have all been reported to catalyze the cycloaddition. The mechanism, selectivity, and functional group compatibility varies with each metal, making their discussion beyond the scope of this chapter. 

Cycloadditions Several metals are known to trigger stereoselective [2-I-2-I-2] cycloaddition of polyunsaturated systems [3] and this approach has been applied to different types of unsaturated substrates. In this general overview, most cited examples will focus on cobalt, nickel, and rhodium catalysis. 

For example, access to axial chirality can be realized under cobalt catalysis using a chiral cobalt(I) complex [4], However, the use of chiral iridium and rhodium species dramatically improved the scope and enantioselectivities obtained for this cycloaddition. Tanaka and coworkers synthesized an atropoisomeric diphosphine oxide in 97% ee, by treatment of the suitable hexayne with [Rh(cod)2]BF4 in the presence of (7 )-TolBINAP as source of chirality (double [2-1-2-1-2] cycloaddition). Subsequent reduction afforded an axially chiral bidentate ligand as a single enantiomer (Scheme 7.1) [5]. 

Cycloaddition of an enediyne leads to the corresponding cyclohexadiene along with the possibility of creating one to two consecutive stereogenic centers, depending on the substitution of the alkene inserted. Cobalt, albeit stoichiometric, is a reactant of choice to perform these reactions [8], Thus, the presence of a chiral motif on the enediyne substrate is crucial to achieve a satisfactory level of diastereoselectivity (Scheme 7.4). Stereoselective catalytic versions were also achieved by switching to cationic rhodium in the presence of atropisomeric ligands [9]. Various enan-tioenriched cyclohexadienes were obtained with enantioselectivities up to 97% (Scheme 7.5). 

Recently, the [64-2] cycloaddition of CHT with internal alkynes was described with rhodium catalysts (Scheme 8.37) [58]. The optimal catalytic system consisted of a combination of [Rh(cod)Cl]2, Cul, and PPh3. A mechanism involving rhodacycles similar to the one reported with cobalt complexes [55] was proposed and supported by density functional theory (DFT) calculations. 

The [2 -I- 2 -I- 2] cycloaddition reaction can give rise to chiral compounds, especially biaryls [3q]. Control of the enantioselectivity in such transformations is of prime importance, notably because biaryls can be used as ligands in asymmetric catalysis. This topic is covered in detail in Chapter 9. Nowadays, cobalt still looks like a poor relation in this field, which is largely dominated by rhodium. Nevertheless, a report from Heller et al. shows for the first time that phosphorus-bearing axially chiral biaryls 9 can be formed by enantioselective benzene formation using the neomenthyl-indenyl cobalt complex II as a catalyst (Scheme 1.3) [7]. Good yields... 

The synthesis of chiral racemic atropisomeric pyridines by cobalt-catalyzed [2 + 2 + 2] cycloaddition between diynes and nitriles was reported in 2006 by Hrdina et al. using standard CpCo catalysts [CpCo(CO)2, CpCo(C2H4)2, CpCo(COD)] [34], On the other hand, chiral complexes of type II were used by Gutnov et al. in 2004 [35] and by Hapke et al. in 2010 [36] for the synthesis of enantiomerically enriched atropisomers of 2-arylpyridines (Scheme 1.18). This topic is described in detail in Chapter 9. It is noteworthy that the 2004 paper contains the first examples of asymmetric cobalt-catalyzed [2 - - 2 - - 2] cycloadditions. At that time, it had been preceded by only three articles dealing with asymmetric nickel-catalyzed transformations [37]. Then enantioselective metal-catalyzed [2 -i- 2 - - 2] cycloadditions gained popularity, mostly with iridium- and rhodium-based catalysts, as shown in Chapter 9. 

Cycloaddition reactions using frequently employed transition-metal catalysts—cobalt, nickel, ruthenium, rhodium, and iridium— were described in Chapters 1 to 5. Other than these catalysts, several transition-metal complexes are able to catalyze [2 + 2 + 2] and related cycloaddition reactions. In this chapter we discuss synthetically useful examples. 

While application of the transition-metal-catalyzed [2 - - 2 - - 2] cycloaddition reaction and its variants for the construction of a benzene unit led to a plethora of natural products with different molecular structures and architectures, its use for the construction of a pyridine moiety within a natural product synthesis is less well developed. The reason for this is uncertain and should not account for the pyridine formation per se the co-cyclization of two alkynes with a nitrile unit to give a pyridine core can be catalyzed efficiently by cobalt, ruthenium, and cationic rhodium complexes, as shown in many methodology-oriented studies [35]. 

Originally, the pyridine construction reaction was based on cobalt catalysis and restricted to the use of acetonitrile or alkyl nitriles as one of the cycloaddition partners. However, recent advancements in this area have led to the development of certain ruthenium or rhodium catalysts, allowing the use of methylcyanoformate as an electron-deficient nitrile component in crossed [2 - - 2 - - 2]-cycloaddition reactions [39]. From the point of view of applications, the use of methylcyanoformate in transition-metal-catalyzed pyridine formation reaction is quite beneficial because the ester moiety might serve as a functional group for further manipulations. It might also serve as a protective group of the cyanide moiety, because cyanide itself cannot be used in this reaction. These considerations led to the design of a quite flexible approach to substituted 3-(130)- and y-carbolines (131) based on transition-metal-catalyzed [2 -f 2 -I- 2] cycloaddition reactions between functionalized yne-ynamides (129) and methylcyanoformate (Scheme 7.28) [40]. 

This chapter describes the synthesis of meta- and para-cyclophanes including pyridinophanes via the transition-metal-catalyzed [2 - - 2 - - 2] cycloaddition. Palladium, Cobalt, and rhodium-based catalysts are currently available for this purpose. The characteristic of these three catalysts may be summarized as follows. 

Complete intramolecular [2 + 2 + 2] cycloaddition of triynes is mediated by the cobalt(I), nickel(O), and rhodium(I) complexes. This method constructs three rings in one step, and double [2 + 2 + 2] cycloaddition of hexaynes enabled the long helicene synthesis through the formation of six rings. 

As a congener of cobalt in the periodic table, rhodium is involved in [5+1] cycloaddition even more widely (in the aforementioned cobalt-mediated carbonylation reported by Kurahashi and de Meijere [15], rhodium has been reported to be capable of catalyzing carbonylation transformation, but the substrate scope is very limited). Early examples include... 

The reaction of [2+2+2] cycloaddition of acetylenes to form benzene has been known since the mid-nineteenth century. The first transition metal (nickel) complex used as an intermediate in the [2+2+2] cycloaddition reaction of alkynes was published by Reppe [1]. Pioneering work by Yamazaki considered the use of cobalt complexes to initiate the trimer-ization of diphenylacetylene to produce hexasubstituted benzenes [54]. Vollhardt used cobalt complexes to catalyze the reactions of [2+2+2] cycloaddition for obtaining natural products [55]. Since then, a variety of transition complexes of 8-10 elements like rhodium, nickel, and palladium have been found to be efficient catalysts for this reaction. However, enantioselective cycloaddition is restricted to a few examples. Mori has published data on the use of a chiral nickel catalyst for the intermolecular reaction of triynes with acetylene leading to the generation of an asymmetric carbon atom [56]. Star has published data on a chiral cobalt complex catalyzing the intramolecular cycloaddition of triynes to generate a product with helical chirality [57]. 

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