1
Carbon Monoxide

1.1 Hydroformylation of Alkenes and Alkynes

Hydroformylation is one of the most important reactions for the preparation of aldehydes and alcohols from alkenes and synthesis gas [1]. In 1938, Roelen (1897-1993) discovered the reaction between alkenes and an equimolar mixture of carbon monoxide (CO) and hydrogen to form aldehydes [2,3]. It is called "hydroformylation" and was originally called "oxo-reaction." Nowadays, homogeneous metal complexes commercially based on cobalt and rhodium are used as catalysts. With more than 10 million metric tons of oxo products per year, hydroformylation represents one of the most important industrial applications and achievements of homogeneous catalysis in the chemical industry [4].

The key consideration of hydroformylation is the selectivity of "normal" vs. "iso." For example, the hydroformylation of propylene can afford two isomeric products, butyraldehyde or isobutyraldehyde (Scheme 1.1).

Scheme 1.1 Example of hydroformylation.

These isomers are related to steric hindrance and the rate of CO migration insertion. In addition, they also reflect the regiochemistry of the insertion of alkene into the M-H bond. For example, the reaction mechanism begins with dissociation of CO from hydrido-metal-tetracarbonyl complex (1) to give the 16-electron species HM(CO)3 (2). Then, the alkene starts to coordinate with the HM(CO)3 complex. The p-complex (3) is converted into the corresponding s-complex (4); the 18 electron species are formed by adding CO (5). In the next step of the reaction cycle, the CO is inserted into the carbon-metal bond (6). Once again, CO is associated to end up in the 18 electron species (7). In the last step of the reaction cycle, the catalytically active hydrido-metal-tetracarbonyl complex (1) is released by adding hydrogen. Moreover, the aldehyde is formed by a final reductive elimination step (Scheme 1.2) [5]. In this section, we will summarize the history and recent advances of catalysts for hydroformylation.

Scheme 1.2 Mechanism of metal-catalyzed hydroformylation.

Source: Based on Heck and Breslow [5].

1.1.1 Co Catalysts

A generally accepted rough order of active metals in hydroformylation is given in Table 1.1 [6].

Table 1.1 Activity of metals in hydroformylation.

Source: Beller [6]. © 2006, Springer Nature.

Roelen first discovered and patented hydroformylation of straight-chain 1-olefins under the HCo(CO)4 catalyst system which generally leads to the formation of linear aldehydes and large amounts of branched aldehydes [7]. This laid a good foundation for all hydroformylation studies.

As early as in 1958, the heterogeneous cobalt catalyst for hydroformylation was reported by Aldridge et al. [8] With 0.879?wt% insoluble cobalt as a catalyst system, the conversion of C7 olefin (mixed isomers) using CO and H2 (about 1 : 1, 2750-2900?psig) at 177?°C is 83%. But, it was soon recognized that the real active species is the homogeneous complex hydridocobaltcarbonyl. HCo(CO)4 is a yellow liquid and strong acid, which is stable only under CO/H2 pressure above the melting point (-26?°C) [9]. Therefore, the mechanism of hydroformylation has been extensively studied.

In 1968, the hydroformylation of propene and 1-hexene has been investigated in a tertiary organophosphine-cobalt hydrocarbonyl catalyst system HCo2(CO)8(PBu3) by Tucci (Scheme 1.3) [10]. Trialkylphosphine complexes yielded 73% less branched isomer formation than the conventional HCo(CO)4 system. The activity of the cobalt hydrocarbonyl-catalyzed branched isomer formation decreases in the order HCo(CO)4 >?HCo(CO)3(PAr3)?>?HCo(CO)3(PR3).

Scheme 1.3 Ligand modification of the catalyst-catalyzed hydroformylation of olefins.

Source: Based on Tucci [10].

There has been an important breakthrough in the development of cobalt catalysts recently. Stanley's group found the [HCo(CO)n(P2)]+ catalyst showed nice activity at lower pressures for hydroformylation [11]. For example, using [Co(acac)(depe)](BF4) under standard conditions (1?mM catalyst, 1?M 1-hexene, dimethoxytetraglyme solvent, activate at 140?°C under 34?bar of 1 : 1 H2 : CO, then reduced to 100?°C and 10?bar), n/iso aldehyde ratio of 0.8% and 15.1% alkene isomerization can be obtained.

1.1.2 Rh Catalysts

Since the 1970s, most hydroformylation reactions rely on catalysts based on rhodium catalysts [12]. For example, Hanson and coworkers described the Rh/NaX and Rh/NaY catalyst system in a fixed bed reactor consisting of propylene : H2 : N2 : CO (3 : 3 : 2 : 1) at 1?atm [13]. The selectivity of n-butyraldehyde vs. iso-butyraldehyde is 2.0 : 1 and 1.9 : 1 for Rh/NaX and Rh/NaY at 150?°C, respectively (Scheme 1.4).

Scheme 1.4 Zeolite-catalyzed hydroformylation of propylene.

Water-soluble catalysts have been developed. They facilitate the separation of the products from the catalyst [14]. For example, Herrmann et al. reported the novel Rh(I)/BISBIS catalyst for hydroformylation of propene and 1-hexene [15]. The high activity (97.7) and productivity (1.26) at low phosphane/rhodium ratios (6.7 : 1) can be obtained in the hydroformylation of propene. Alper and coworker reported the first polymeric water-soluble metal complex Rh/PPA(Na+)/DPPEA for hydroformylation of aliphatic olefins (Scheme 1.5) [16]. For the hydroformylation of vinyl arenes, the complete conversion, high selectivity (>97%) and iso/n ratio (7.3-24) can be observed.

Scheme 1.5 Rh/PPA(Na+)/DPPEA-catalyzed hydroformylation of olefins.

Source: Based on Ajjou and Alper [16].

In 1989, Arhancet et al. described a novel supported aqueous-phase catalyst HRh(CO)[P(m-C6H4SO3Na)3]3/SiO2 (SAPC) for hydroformylation (Scheme 1.6) [17]. The hydroformylation of 1-octene yielded a nonanal/2-methyl octanal (n/iso) ratio ranging from 1.8 to 2.9 depending on the water content and ligand/rhodium ratio (which varied from 7 to 30).

Scheme 1.6 Hydroformylation of 1-octene using SAPC.

Source: Based on Arhancet et al. [17].

In 2000, Arya and Alper found a solid-phase synthetic approach to obtain dendritic ligands anchored onto beads for the hydroformylation of several olefins [18]. For example, the complete conversion of styrene (>99%) with a high selectivity for the branched isomer (branched : linear, 16 : 1) was obtained at 65?°C. The catalyst can be recycled five times without deactivation.

Chaudhari and coworkers reported HRh(CO)(PPh3)3 encapsulated and anchored in NaY, MCM-41 and MCM-48 for hydroformylation of olefins to aldehydes in 2003 (Scheme 1.7) [19]. 99.4% conversion of 1-octene and a 1.7 n/iso ratio of regioselectivity were obtained when using Rh-MCM-48 at 80?°C. This catalyst is recyclable and can be reused six times without obvious deactivation.

Scheme 1.7 Hydroformylation of olefins using HRh(CO)(PPh3)3-encapsulated catalysts.

Source: Based on Mukhopadhyay et al. [19].

Recently, Shi's group described the simple Rh black as a heterogeneous catalyst for hydroformylation of olefins [20]. This catalyst system has a broad substrate scope including the aliphatic and aromatic olefins, affording the desired aldehydes in good yields (Scheme 1.8). For example, 86% yield of propanal and 200?h-1 turnover frequency (TOF) were obtained when ethylene is the reactant. The catalyst could be recycled five times without loss of activity.

Scheme 1.8 Rh black-catalyzed hydroformylation of olefins.

For hydroformylation of alkynes, Breit and coworker designed and synthesized a series of new supramolecular ligands containing a functional guanidine group with increasing p-acceptor ability of the phosphine donor ligands in 2018 [21]. The desired aldehydes were obtained in 51-94% yields with regioselectivities up to 25 : 1 (Scheme 1.9).

Scheme 1.9 Hydroformylation of alkynes.

1.1.3 Au Catalysts

In 2008, Tokunaga...