Nitriles catalysts, rhodium complexes

We (79TH1 81GEP3117363 84USP4588815) and others (87MI1) have studied acetylacetonato and rj -cp-rhodium complexes as catalysts in the pyridine formation [Eq.(l)]. Resin-attached cp-rhodium complexes are also active in the cocyclization of alkynes and nitriles, and the activity is... 

A brief review of the cotrimerization of alkynes with nitriles to give pyridines has appeared. Pyridine itself is produced from C2H2 and HCN in benzene at 110 C/60 min under 23 bar pressure. The borabenzene complex (54) as the catalyst gives 103 turnovers. The catalytic formation of substituted thiophenes from alkynes and elemental sulfur in the presence of [CPC0L2] catalysts is also mentioned. Analogous rhodium complexes [Cp RhL2] also catalyze the formation of pyridines. 

Many other catalysts based on cobalt, ruthenium, rhodium, and platinum are now known to catalyze the hydrosilylation of alkenes, and the types of products can be controlled by the choice of catalyst and silane. Rhodium complexes, such as Wilkinson s catalyst, have been used frequently. A comprehensive treatment of selectivities is beyond the scope of this chapter, but several reviews provide information on the products formed from different catalysts and silanes.As one example, crotononitrile undergoes hydrosilylation in the presence of Wilkinson s catalyst to form the a-silyl nitrile product (Equation 16.23), and this regioselectivity contrasts with that for the reaction of the related acrylic acid ester in Equation 16.19 conducted with Speier s catalyst. 

Axially chiral biaryls are an important class of molecules for both biologically active compounds and chiral ligands (78-80). The most common approach to obtain biaryls is by aryl coupling followed by resolution of the racemic product to afford enantiopure biaryls. Even though enantioselective partial intramolecular cyclotrimerization of diyne with alkynes (81,82) or nitriles (83) were developed with various transitional metals, it was difficult to carry out complete intermolecular reaction. Using a cationic chiral rhodium complex as catalyst, a regioselective intermolecular cross-cyclotrimerization of alkynes 72 and 73 for... 

There are rather few water-soluble hydrogenation catalysts. A number of rhodium complexes derived from ligands such as (PH2PCH2CH2)2NC0C6H4S03" are active in this way. " Catalysts have been developed for hydrogenation of ketonescarboxylic acids/ esters/ nitriles/ and nitro compounds. 

Cationic rhodium(I)/chiral biaryl bisphosphine complexes are suitable for the enantioselective [2-I-2-1-2] cycloaddition reactions between electron-rich alkynes and electron-deficient unsaturated compounds (alkynes, nitriles, and isocyanates). Alkynylcarbonyl compounds, alkynylphosphonates, and alkynylphosphine oxides are the best-suited electron-deficient unsaturated compounds for these catalysts. These complexes are also able to catalyze the highly enantioselective synthesis of axially chiral anilides and bezamides. 

Rhodium,3 osmium4 and ruthenium5 based catalyst systems are affected by nitrile in a similar way. This arises from the relatively high affinity of complexes of these metals towards nitrile group coordination.11 The resulting equilibrium between free catalyst and catalyst with bound nitrile reduces the effective catalyst concentration and hence reaction rate for a given set of conditions. 

In contrast to carbocyclic alkyne cyclotrimerizations, the catalytic pyridine synthesis from alkynes and nitriles relies exclusively on cobalt catalysts with a few exceptions where rhodium [16] and iron complexes [17] could be applied. The cobalt-catalyzed pyridine synthesis can even be carried out in a one-potreac-tion generating the catalyst from C0CI2 6 H20/NaBH4 -1- nitrile/alkyne in situ [18]. 

NBR can be partially or even completely hydrogenated (to eliminate carbon-carbon double bonds) in nonaqueous solution by using suitable catalysts (e.g., cobalt, rhodium, mthenium, iridium, or palladium complexes) to give hydrogenated nitrile rubbers (H-NBRs). Completely saturated H-NBR grades are cross-linked with peroxides. 

Complexes of ruthenium and rhodium have been identified that catalyze the hydrogenation of nitriles under mild conditions with very good selectivities to the primary amine. In particular, Otsuka and co-workers showed that [RhHfP Prj) ] catalyzes the hydrogenation of a variety of nitriles to amines under mild conditions (1 atm pressure, 20 °C, 2 h Equation 15.119). The reaction is reversible primary amines undergo dehydrogenation in the presence of [Rl iHfP Prj) ] to form nitriles (Equation 15.120). Iridium catalysts for the dehydrogenation of nitriles have also been reported recently. ... 

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. 

Cationic rhodium(I)/biaryl bisphosphine complexes are an effective catalyst for intermolecular [2 + 2 + 2] cycloaddition of alkynes with nitriles under mild conditions. The [2 + 2 + 2] cycloaddition of 1-dodecyne with ethyl cyanoacetate proceeded at 60 °C in the presence of the cationic rhodium(I)/BINAP catalyst to give trisubstituted pyridines in high yield with moderate regioselectivity (Scheme 4.53) [55]. 

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