Hydroformylation of alkynes

Few studies have been conducted on the hydroformylation of alkynes. Terminal acetylenes give saturated isomeric aldehydes supposedly through the initially formed alkenes.73 Conclusive evidence was obtained that internal alkynes, in contrast, first undergo hydroformylation to yield a,p-unsaturated aldehydes, which are subsequently hydrogenated to give the saturated aldehyde end products.74... 

Alkynes. New catalysts have been reported to effectively catalyze hydroformylation of alkynes. [Rh(CO)2(acac)] and a special biphosphate ligand enables the... 

The reduction of intermediately formed, isolable unsaturated nitriles opens up an easy route to regio- and stereo-chemically pure unsaturated aldehydes. The overall reaction is a regio- and stereo-selective hydroformylation of alkynes [61]. 

Table 8. Asymmetric Hydroformylation of Alkynes (with Subsequent Hydrogenation)22...

In contrast to facile hydroformylation of alkenes, only a few successful examples of hydroformylation of alkynes have been reported. Hidai and eo-workers have found that PdCl2(PCy3)2 is an effective catalyst for hydroformylation of alkynes. Furthermore, a bimetallic catalyst of PdCl2(PCy3)2 and Co2(CO)g showed remarkably high catalytic activity. Reaction of 4-octyne under CO and H2 pressure (35 atm each) at 150 °C in the presence of Et3N produced the 2-n-propyl-2-hexenal (2) in 95% yield [1]. Co2(CO)g alone is inactive. Also no hydroformylation of alkenes occurred with this bimetallic catalyst. 

The interest of alkyne and allene hydroformylation is the formation of a,) -unsamrated aldehydes, which are valuable intermediates in fine chemical and pharmacy. In contrast to alkenes, the studies on the hydroformylation of alkynes are relatively scant. The first case of acetylene hydroformylation was reported by Natta and Pino in 1951 [79]. It was found that in the presence of metallic cobalt at 120-150°C and 200-300 atm, acetylene reacted with synthesis gas yielding a mixture of high-boiling, unidentified products. During the following 40 years, hydroformylation of alkynes to a,/l-unsaturated aldehydes had little success [80-85]. The early investigations usually resulted in low selectivity and/or low yield of unsaturated aldehydes, primarily because the formation of the corresponding saturated aldehydes and non-carbonylated olefins could hardly be suppressed. 

The first systematic B3LYP DPT investigation into the hydroformylation of alkynes and allene was carried out by Huo et al. [92,93]. It was fotmd that acetylene hydroformylation has very similar catalytic mechanism as that of propene, therefore only the differences are emphasized, and no general comparison is made. Scheme 17 shows the activation and reaction free energies. 

This chapter discusses some general aspects of the hydroformylation of alkenes in organic synthesis. It focuses mainly on regio- and stereoselective processes, and analyzes the influence of the substrate and the catalysts. Practical methods which provide high yields and selectivities, and short-cuts compared to classical organic routes will be described. Particular attention will be paid to recent advances that have helped to enlarge the synthetic application of this reaction. Section 6.7 deals with the hydroformylation of alkynes, and such key aspects as hydroformylation in water-gas shift conditions and silylformylation, particularly efficient catalytic systems and the application ofhydroformylation in organic synthesis. 

The hydroformylation of alkynes with palladium, in particular, has had limited success. A bimetallic catalyst system consisting of PdCl2(PCy3)2-C02(CO)8 was developed for the hydroformylation of internal acetylenes (Scheme Although the reaction would... 

The insertion of CO in the Pd-C bond to form an acylpalladium species can be supported by Co2(CO)g as co-catalyst [9]. This finding helped improve the chemoselectivity of the Pd-catalyzed hydroformylation of alkynes [10]. Under the conditions shown in Scheme 1.42, almost no hydrogenation products, such as saturated aldehydes or nonfunctionalized olefins, were formed. 

Scheme 3.8 Hydroformylation of alkynes with formaldehyde as syngas source in aqueous medium.

In general, rhodium-catalyzed hydroformylation of alkynes proceeds much slower than the reaction with olefins. It should be remembered that homogeneously catalyzed hydroformylation of olefins with unmodified rhodium catalysts can be irreversibly poisoned by the presence of even trace quantities of alkynes. As Liu and Garland [94, 95] found by means of in situ IR spectroscopy, the reason is likely the formation of dinuclear rhodium-carbonyl complexes I, which are stable even in the presence of hydrogen (Scheme 4.17). Therefore, alternative pathways for the production of a,P-unsaturated aldehydes have been suggested, consisting of Ni-catalyzed hydrocyanation followed by chemoselective hydrogenation [96]. 

Scheme 4.18 Hydroformylation of alkynes with a self-assembling Rh catalyst.

Also, palladium-catalyzed hydroformylation of alkynes is known [98]. Particularly, Beller s group showed that a wide range of symmetrically and unsymmet-rically substituted alkynes can be converted into the unsaturated aldehydes by the effect of a palladium complex modified with a bidentate 2-pyrrolyl phosphine (Scheme 4.20) [103]. 

Recently, a Pd-catalyzed hydroformylation of alkynes was discovered (Scheme 6.78) [204]. This protocol gives access to a,P-unsaturated aldehydes. 

Scheme 6.78 Pd-catalyzed hydroformylation of alkynes and potential chemical targets.

Hydroformylation of alkynes catalysed by rhodium normally proceeds reluctantly and in poor yields. However, hydroformylation of but-1- and... 

Hydroformylation of alkynes by sol-gel catalysts (e.g.. Scheme 24-12) has been extensively studied in several laboratories. Examples are (Cauzzi, 1997 Lindner, 2000 de Campos, 2003 van Leeuwen, 2003). In many cases the sol-gel entrapped catalysts lead to a very high ratio of lineanbranched aldehydes. Notable examples of catalytic carbony-lation are the manufacture of acetic acid from methanol in the presence of entrapped... 

The catalytic performance of 1 was examined for hydroformylation of alkyne (as a catalyst additive) and methoxycarbonylation of alkyl halide, and 1 is also used as a stoichiometric reagent for an Ni-catalyzed ketone synthesis from alkyl halide. " Heterogeneous catalysts derived from 1 are used for hydrogenation of GO and coal tar pitch, crude oil upgrading, and coal liquefaction (preparation of the pyrrhotite catalyst). An Fe-Ru-Se catalyst dispersed on a glassy carbon-supported Nafion film prepared from 1 is reported to be effective for electro-catalytic oxygen reduction. ... 

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