Ruthenium complexes formyl

Research on intermolecular hydroacylation has also attracted considerable attention. The transition-metal-catalyzed addition of a formyl C-H bond to C-C multiple bonds gives the corresponding unsymmetrically substituted ketones. For the intermolecular hydroacylation of C-C multiple bonds, ruthenium complexes, as well as rhodium complexes, are effective [76-84]. In this section, intermolecular hydroacylation reactions of alkenes and alkynes using ruthenium catalysts are described. 

One of the most significant processes that involve CO in organic industrial chemistry is the hydroformylation of alkene, or the 0x0 process, in which rhodium and cobalt complex catalysts are used. Ruthenium is a strong candidate for replacing the very expensive rhodium catalyst. Further, ruthenium complexes are excellent catalysts for the addition of formyl groups of aldehydes, formates and formamides to alkenes. 

Systematic studies on the isomerization of W-allylamides 24 and -imides to aliphatic enamides 25 were carried out with iron, rhodium, and ruthenium complexes as catalysts, Eq. (8). Regrettably, no prochiral substrate was applied for the rhodium catalyst bearing polymer-anchored DIOP [33]. In the framework of a study on the conjugative interaction in the isomerization of 1-azabicyc-lo[3.2.2]non-2-ene 26 to orthogonal enamine 27, catalyzed by either f-BuOK or RuH(NO)(PPh3)3, the enamine formation was calculated to be favored by 4 kcal mob, Eq. (9) [34]. Recently, the palladium-catalyzed isomerization of the N-acyl-2,5-dihydropyrroles 28 to N-formyl-2,3-dihydropyrroles 29 was reported, Eq. (10) [35]. 

Substituted benzaldehydes, with a formyl substituent as directing group, were selectively arylated at their ortho-position with aryl bromides as electrophiles in the presence of palladium(O) catalysts [50]. The use of a ruthenium complexes within a cooperative multi-catalytic system [51] altered the chemoselectivity dramatically [52]. Thus, reactions of 8-formylquinoline (39) with iodoarenes proceeded regioselectivity at the formyl group itself to give the corresponding ketones in moderate to very good yields (Scheme 9.15) [52],... 

Two mechanistic pathways, which differed in the way of ruthenium-mediated initial cleavage of formyl C-H or amido N-H bond, were proposed for the catalytic cycle. As shown in Scheme 7.3, an irreversibly cleavage of formyl C-H bond by the active ruthenium complex was followed by reversible insertion of the olefin into the Ru-H bond, which afforded either six-membered or seven-membered ruthenacycle. After reductive elimination, indolin-2-ones or 3,4-dihydroquinolin-2-one was formed. According to isotopic studies, pathway leading to six-membered lactams is postulated to be less favored. Another cyclization process initiated by Ru-catalyzed oxidative addition of formyl N-H bond (Scheme 7.4) was similar to Carreira s proposal for their hydrocarbamoyla-tion reaction of allylic formamides under similar ruthenium catalysis conditions [7]. The 6-endo cyclization process is proposed to be favored under the catalytic system B. 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

The ruthenium carbonyl complexes [Ru(CO)2(OCOCH3)] n, Ru3(CO)12, and a new one, tentatively formulated [HRu-(CO)s ] n, homogeneously catalyze the carbonylation of cyclic secondary amines under mild conditions (1 atm, 75°C) to give exclusively the N-formyl products. The acetate polymer dissolves in amines to give [Ru(CO)2(OCOCH3)(amine)]2 dimers. Kinetic studies on piperidine carbonylation catalyzed by the acetate polymer (in neat amine) and the iiydride polymer (in toluene-amine solutions) indicate that a monomeric tricarbonyl species is involved in the mechanism in each case. 

The stoichiometric carbonylation observed using [HRu(CO)3] and the proposed catalytic schemes all involve tricarbonyl species as the active catalyst the relatively high activity of Ru3(CO)i2 is consistent with this. The relative activity of the complexes for piperidine carbonylation is [HRu(CO)3L Ru3(CO)12 > [Ru(CO)2(OCOMe)]n. The major cause of the decrease in carbonylation rates is the accumulation of formyl product although the decrease in amine concentration is also a contributing factor. This catalyst poisoning is likely attributable to com-plexation to the ruthenium, presumably via the carbonyl grouping as commonly found for formamide ligands (26). The product could compete with either amine or CO for a metal coordination site. 

Roper has been able to isolate another osmium formyl by rearrangement of an rj2-formaldehyde complex, as shown in Eq. (9) (54). Because of the unavailability of such precursors, this reaction also does not provide a general entry into neutral formyl complexes. However, Eq. (9) does lend support to the claim that the related ruthenium formyl, Ru(H)(solv) (PPh3)3(CHO) (40), can be isolated as an impure solid, contaminated with substantial quantities of an rf -formaldehyde precursor (55). 

After these pioneering studies, a number of other research groups reported on the cleavage of C-H bonds via the use of a stoichiometric amount of transition-metal complexes [7]. To date, several types of catalytic reactions involving C-H bond cleavage, for example, alkyl, alkenyl, aryl, formyl, and active methylene C-H bonds have been developed [8]. In many cases,for these types of catalytic reactions, ruthenium, rhodium, iridium, platinum, and palladium complexes all show catalytic activity. 

Ru( CHO)( CO)(dppe)][SbFg] have been used also " to identify more fully the radicals formed in the decomposition of cationic formyl complexes of ruthenium. The conclusion has been reached that a radical mechanism does not constitute major pathway for these decompositions ... 

The low-temperature hydride reductions of [Ru(CO)2(P—P)2][SbFg]2 (PP = PPh2(CH2) PPh2 n = l, dppm, n = 2, dppe) have been undertaken to elucidate the mechanism of the homogeneous hydrogenation of carbon monoxide to produce organic products for the petrochemical industry catalysed by ruthenium formyl complexes... 

A similar rate expression was first obtained by Natta et al. [19]. The problem with this system, however, is that the yield after 4 h reaction time is only 14 % at 110 °C with Ph, = Pco = 40 MPa and tetrahydrofuran as the solvent[20j. In order to obtain a higher yield, Hidai et al. [20] added a certain amount of Ru3(CO) 2 to the cobalt carbonyl-containing reaction system. Although the hydro-formylation activity of this carbonyl compound is relatively low (cf. Section 2.1.1.2.1), about one-third of that with the cobalt complex, the authors observed a marked increase of the initial reaction rate with increasing ruthenium content. They suggested that the ruthenium species opens an additional route for forming the final C-H bond. Thus, it is to be expected that there is a second reaction rate, 1-2, which can be given by eq. (5) ... 

RuH2(PPh3)4 reacts with aldehydes to give esters via Tishchenko-type dimerization. For example, benzaldehyde is converted to benzyl benzoate by RuH2(PPh3>4(eq (45)) [166-167]. This reaction involves C—H bond activation of the formyl proton followed by formation of a ruthenium acyl alkoxide complex Ru(OCH2Ph)(COPh)(PPh3)4. 

Further work on the use of the gcm-dimorpholine derivative of 2-thienylgly-oxal for the synthesis of gem-di(acylamino)- and other derivatives has appeared. In connection with work on surfactant complexes of ruthenium, thiophen-2-aldehydes were condensed with 4,4 -dimethyl-2,2 -bipyridyls. From the easily available 2,3,4-trichlorothiophen, the 5-formyl- and 5-acetyl-derivatives have been prepared, and from them a large number of derivatives. ... 

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