Iridium alkyl, reaction

The most fundamental reaction is the alkylation of benzene with ethene.38,38a-38c Arylation of inactivated alkenes with inactivated arenes proceeds with the aid of a binuclear Ir(m) catalyst, [Ir(/x-acac-0,0,C3)(acac-0,0)(acac-C3)]2, to afford anti-Markovnikov hydroarylation products (Equation (33)). The iridium-catalyzed reaction of benzene with ethene at 180 °G for 3 h gives ethylbenzene (TN = 455, TOF = 0.0421 s 1). The reaction of benzene with propene leads to the formation of /z-propylbenzene and isopropylbenzene in 61% and 39% selectivities (TN = 13, TOF = 0.0110s-1). The catalytic reaction of the dinuclear Ir complex is shown to proceed via the formation of a mononuclear bis-acac-0,0 phenyl-Ir(m) species.388 The interesting aspect is the lack of /3-hydride elimination from the aryliridium intermediates giving the olefinic products. The reaction of substituted arenes with olefins provides a mixture of regioisomers. For example, the reaction of toluene with ethene affords m- and />-isomers in 63% and 37% selectivity, respectively. 

The iridium catalytic system can also be applied to the a-alkylation of active methylene compounds. The alkylation of cyanoacetates 93 with primary alcohols 94 was achieved by using [lrCl(coe)2]2 and PPhs to afford saturated a-alkylated products 95 (Equation 10.20) [42]. Here, the alkylation reaction was efficiently accomplished, without the need for any base. 

A special type of reaction is observed with the platinum(IV) complex [PtI(Me)3] which cleaves the Af,N,Af, A -tetraphenyltetraaminoethylene under reduction to form the dimeric cyclometallated mono(NHC) complex of platinum(II) iodide [Eq. (31)]. Cyclometallation with the same ligand is also observed for ruthe-nium. Additional cyclometallations with various substituents of NHCs have been reported for ruthenium(II), rhodium(III), iridium(I), palladium(II), " and platinum(II). In the case of iridium, alkyl groups can be activated twice. In rare cases like for nickel(II) /x-bridging NHCs have been obtained. ... 

The formation of a branched chiral product from the alkylation of monosubstituted substrates is not limited to the catalysis of metals described thus far. Allylic alkylation reactions catalyzed with rhodium [211] and iridium [212] complexes have been shown to occur at the more... 

In 2008, Shibata and co-workers reported a cationic iridium bidentate phosphine complex catalyzed C—H bond alkylation reaction of 2-methy-lacetophenone with alkynes and alkenes (Scheme 5.61). Compared with alkynes, the use of styrenes as acceptors requires the weakly coordinated tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF) as a counteranion of the iridium complex and a higher temperature to achieve good yields. When a norbornene was used as the acceptor, the sole example of an asymmetric ortho C—H bond alkylation product 175 was obtained in 58% yield and 70% ee. 

More specifically, calculations have suggested that the ruthenium-alkyl complex in Equation 6.53 reacts with arene to exchange covalent ligands by a process closely related to a a-bond metathesis mechanism. Computational studies of the reactions of a simple iridium-alkyl and alkoxo complex with alkanes to generate new metal-alkyl complexes have also suggested that a mechanism is followed that involves many of the characteristics of a classic a-bond metathesis transition state. However, calculations of the mechanisms of these two processes imply that the transition state contains some degree of M-H bonding. 

This catalytic system also performed effectively under solvent-free conditions, where it afforded the desired C-alkylated products in high yields. Moreover, the same authors reported that the analogous benzothienyl-iridium complexes afforded high levels of catalytic activity in the C-alkylation reactions of primary and secondary alcohols, as well as the reaction of ketones and primary alcohols through transfer hydrogen processes [107-109]. 

Scheme 8 Benzoxazolyl-iridium complex-catalyzed C-alkylation reactions...

In addition to recent advances in iridium- and ruthenium-based catalysts for the C-alkylation reactions, osmium has also been identified as an efficient catalyst for this... 

In the recent literature, Ramon and co-workers reported the successful Guerbet cross-alkylation reaction of two different alcohols using an iridium/magnetite catalyst [183]. In a typical example of this reaction, a mixture of 2-phenylethanol (1.0 mmol) and benzyl alcohol (2.0 mmol) was reacted with a catalytic amount of iridium/magnetite nanoparticles of Ir02-Fe304 (0.14 mol%) combined with KOH... 

In 2012, Fristrup and Madsen studied the mechanism of the same Af-alkylation reaction as above by combining experimental and theoretical methods [58]. In contrast to Eisenstein s proposal [57], Fristrup and Madsen s results suggested that both aldehyde and hemiaminal intermediates stay coordinated to the iridium catalyst... 

In addition to the above computational studies on the mechanism of Ir-catalyzed IV-alkylation reactions, in 2015 Martm-Matute and co-workers [60] also investigated a bifunctional iridium complex-catalyzed iV-alkylation reaction of amines and alcohols by a combination of experimental and computational methods. The mechanisms of Ru- [61], Cu- [62], and Pd-catalyzed [63] IV-alkylation reactions have also been smdied by DFT calculations. 

By using certain ligands to improve the activity of the metal catalyst, the N-alkylation reactions could also be performed under milder conditions. In 2009, Kempe and co-workers reported that a [Ir(cod)Cl]2/P,N-ligand complex could enable the A -alkylation of (hetero)arylamines with alcohols under mild conditions of only 70 °C with a catalyst loading as low as 0.1 mol% (Eq. 23) [110]. In 2010, the same group designed a new P,N-ligand stabilized iridium complex 12 (Scheme 22) for efficient alkylation of anilines with alcohols under mild conditions... 

Scheme 35 Boron-iridium heterobimetallic complex (32)-catalyzed A alkylation reaction of ammonia and primary, secondary, and cyclic amines with alcohols under air...

Later in the same year, Uozumi, Yamada, and co-workers also reported an inwater dehydrative A(-alkylation reaction under air using a heterogeneous boron-iridium heterobimetallic Polymeric Catalyst 34 (Scheme 35) [184]. This method is suitable for the reactions of benzylic and aliphatic alcohols with ammonia, and primary, secondary, and cyclic amines. Different to usual hydrogen autotransfer reactions, which require basic conditions, the reactions of benzylic, aliphatic, and secondary amines and ammonia require a pH 4 aqueous buffer solution under microwave conditions. The catalyst can be recovered and reused at least twice without loss of activity. 

Interest in iridium hydrocarbyl complexes has been fueled by the higher thermodynamic stability of both Ir-C and Ir-H bonds in comparison to related compounds of rhodium. Therefore, iridium alkyl and aryl complexes have been frequently and successfully used as kinetically inert models for a variety of rhodium-catalyzed reactions allowing to collect intriguing mechanistic information on important industrial processes and organometallic reactions of academic interest. 

Hydrosoluble iridium alkyl complexes may also be prepared by hydrolysis of alkynes and alkenes promoted by water-soluble precursors. The reaction with alkynes follows the well-known mechanism demonstrated by Bianchini et al. for ruthenium complexes.A reasonable mechanism, related to that of hydrolytic breakage, has been proposed by Chin etal. to account for the hydrolysis of ethene promoted by [Cp"lr(TPPMS)Cl2] 344 in the presence of silver salts in water. Scheme 37 describes the Chin s hydrolysis of alkynes, leading to [Cp Ir(TPPMS)(CO)(CH2R)] (69 R=Ph, Bz, Bu, />-Tol) via the aquo complex [Cp Ir(TPPMS)(OH2)OTf 345... 

Therefore, direct functionalization by iridium-catalyzed reactions involving C-H bond activation would provide an alternative protocol to the existing multi-step organic synthesis [116-130]. To date, iridium-catalyzed C-H activation of aromatic rings for reactions such as borylation [131 -136], alkylation/alkenylation [137-143] and silylation [144-146], and cross-coupling [147, 148] has been investigated. 

Then, the resulting 2-(4-hydroxybutyl)indole was reacted in the same cationic Ir(III) catalyst system with KOH at 110 °C to afford 1,2,3,4-tetrahydrocarbazole as the product (Scheme 11.15). Here, the iridium catalyst bearing a methyl-bridged bipyrazoyl or a pyrazoyl-1,2,3-triazole (N-N) ligand [Ir(N-N)Cp Cl]BAr 4 is much more effective than [Cp IrCl2]2 (TOF = 0.2h ). This reaction has been proposed to proceed by a C3-alkylation reaction. 

Palladium remains the most widely recognized transition metal to effect stereoselective allylic alkylation reactions. Consequently, diastereoselective and enantioselective Pd-catalyzed processes are extensively discussed in Sections 14.2 and 14.3. More recent advances in the field of metal-catalyzed al-lylation reactions include the use of chiral iridium complexes, dealt with in Section 14.4 [33, 34]. Section 14.5 describes selected stereoselective copper-catalyzed SN2 -allylation reactions [33, 35-37], while a brief presentation of allylation reactions with other transition metals including Mo and Rh is given in Section 14.6 [8, 13, 33, 38, 39]. The closing Section 14.7 deals with selected methods for asymmetric ring-opening reactions of unsaturated heterocycles [38, 40, 41]. 

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