Aldehydes catalytic decarbonylation

We recently reported briefly on an extremely efficient thermal catalytic decarbonylation of aldehydes using a system based on Ru(TPP)(PPI13)2 (6), and report here further studies on this system and one based on Ru(TPP)(CO)(tBu2P0H). 

CO into a metal-hydrogen bond, apparently analogous to the common insertion of CO into a metal-alkyl bond (6). Step (c) is the reductive elimination of an acyl group and a hydride, observed in catalytic decarbonylation of aldehydes (7,8). Steps (d-f) correspond to catalytic hydrogenation of an organic carbonyl compound to an alcohol that can be achieved by several mononuclear complexes (9JO). Schemes similar to this one have been proposed for the mechanism of CO reduction by heterogeneous catalysts, the latter considered to consist of effectively separate, one-metal atom centers (11,12). As noted earlier, however, this may not be a reasonable model. 

Catalytic decarbonylation of aldehydes has been studied using mono- and bisdiphosphine complexes of Rh(I). The catalyst [Rh(dppp)2]BF4f where dppp = 1,3-bis( diphenylphosphino)propane, decarbonylates al-... 

It would be more interesting and useful if the reaction could be made catalytic. Actually, catalytic decarbonylation reaction was found to be possible by using chlorocarbonylbis(triphenylphosphine) rhodium (XII) (26). This complex is reasonably stable, and more importantly it is four-coordinated and coordinatedly unsaturated, so that it may expand to a six-coordinated complex by the oxidative addition of acyl halides or aldehydes. The oxidative addition of methyl iodide to similar complexes was reported by Heck (5). 

Since most of the elementary reactions involved in carbonylation chemistry are readily reversible (equation 4), it is not surprising that metal complexes which catalyze carbonylation also catalyze decarbonylation under certain conditions. The catalytic decarbonylation of aldehydes by [RhCl(CO)(PPh3)2], for example, is shown in Scheme 9. 

By heating above 200 C catalytic decarbonylation is possible using [RhCl(PPh3)3], and is particularly suitable for aromatic aldehydes since aliphatic aldehydes tend to dehalogenate under these conditions to form alkenes (equation 72). Cationic rhodium complexes, for example [Rh(Ph2P(CH2)2PPh2)2], are much more active catalysts and hence reactions can be carried out at below 100 Because of the milder conditions aliphatic aldehydes can be decarbonylated to the alkane using this catalyst system. Rhodium catalysts can also be used to decarbonylate a- and -diketones and keto esters (equations 73 and 74). ... 

In the foregoing, the formation of organic molecules on transition metal complexes is explained by stepwise processes of oxidative addition, insertion, and reductive elimination. One typical example, which can be clearly explained in this way, are the carbonylation and decarbonylation reactions catalyzed by rhodium complexes 10-137). Tsuji and Ohno found that RhCl(PPh3)3 decarbonylates aldehydes and acyl halides under mild conditions stoichiometrically. Also this complex and RhCl(CO) (PPh3)2 are active for the catalytic decarbonylation at high temperature. 

Catalytic decarbonylations of a few substrates other than aldehydes have been known for some time, e.g. conversion of benzoic anhydrides to fluorenones at high temperatures (ca. 225 °C). ... 

A plausible, but speculative, reaction scheme for the catalytic decarbonylation of acid chlorides (and aldehydes) that involves phosphine dissociation from one of the Intermediates has been postulated. It is clear, though, that there are presently not enough facts to substantiate this hypothesis. Future work on the mechanism of catalytic decarbonylation using RhCl(CO)(PPh3)2 and other catalysts which investigates phosphine inhibition could be very informative. 

CATALYTIC DECARBONYLATION OF ALDEHYDES WITH CATIONIC DIPHOSPHINE COMPLEXES OF Rh(l)... 

Table 5. Catalytic Decarbonylation of Aldehyde Using [RhidppplzlBFJ ...

Figure 11.3. Possible mechanism for the catalytic decarbonylation of aldehydes using [Rh(P-P)2] as catalyst where (P-P) = dppp or dppe.

The rates of catalytic decarbonylation of benzaldehyde using mono-diphosphine complexes of Rh and Ir provide an interesting comparison. First of all, the mono-diphosphine complexes of Rh and Ir are not robust under the conditions of the catalytic reaction and therefore are of little practical use. However, they do provide useful data for mechanistic arguments. With Rh, the bis-diphosphine catalysts [Rh(P-P)2] are always more active than their mono-diphosphine analog [Rh(P-P)] when neat aldehyde is used as solvent. Although Rh-P bond rupture is not necessary with the coordinatively unsaturated mono-diphosphine complexes, the rhodium may not be electron-rich enough to promote facile oxidative addition. In support of this argument, the presence of the diolefin cod in the coordination core, [Rh(cod)(dppp)]", increased the activity of decarbonylation by a factor of 6 compared with [Rh(dppp)]. With Ir... 

Hydrogenation of ketones generally proceeds with more difficulty than reduction of olefins. This is due in part to lesser stability of complexes with ketones (which activate hydrogen) compared to olefin complexes and to the ability to coordinate of the resulting alcohols in contrast to the alkanes. Moreover, the resulting secondary alcohols show a tendency to oxidize themselves back to ketones. Some rhodium complexes oxidize secondary alcohols to ketones. Wilkinson s complexes RhX(PR3)3 do not represent suitable catalysts for reduction of aldehydes, because decarbonylation of the substrate and the formation of compounds of the type RhCl(CO)(PR3)2 takes place these compounds are not catalytically active. However, hydrogenation of ketones is effectively accelerated by complexes such as [Rh(NBD)(PR3) ] CIO (w = 2, 3). The... 

The catalytic decarbonylation of aldehydes by lRh(triphos)COJBF4 illustrates the reductive elimination of an alkane ftom an octahedral Rh(lII) intermediate (Eq. 6.36, L = triphos) note some similarities with Fig. 6.1. 

Aldehydes are decarbonylated catalytically using solutions of bis(triphenylphosphine)(tetraphenylphorphyrinato)ruthenium(ii) at, or slightly above, room temperature. Decarbonylation of aromatic aldehydes takes place... 

In 1998, Wakatsuki et al. reported the first anti-Markonikov hydration of 1-alkynes to aldehydes by an Ru(II)/phosphine catalyst. Heating 1-alkynes in the presence of a catalytic amount of [RuCljlCgHs) (phosphine)] phosphine = PPh2(QF5) or P(3-C6H4S03Na)3 in 2-propanol at 60-100°C leads to predominantly anti-Markovnikov addition of water and yields aldehydes with only a small amount of methyl ketones (Eq. 6.47) [95]. They proposed the attack of water on an intermediate ruthenium vinylidene complex. The C-C bond cleavage or decarbonylation is expected to occur as a side reaction together with the main reaction leading to aldehyde formation. Indeed, olefins with one carbon atom less were always detected in the reaction mixtures (Scheme 6-21). 

Morimoto, Kakiuchi, and co-workers were the first to show that aldehydes are a useful source of CO in the catalytic PKR [68]. Based on 13C-labeling experiments, it was proposed that after decarbonylation of the aldehyde, an active metal catalyst is formed. This was proven by the absence of free carbon monoxide. As a consequence CO, which is directly generated by previous aldehyde decarbonylation, is incorporated in situ into the carbonylative coupling. The best results were obtained using C5F5CHO and cinnamaldehyde as CO source in combination with [RhCl(cod)]2/dppp as the catalyst system. In the presence of an excess of aldehyde the corresponding products were isolated in the range of 52-97%. 

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