Oxidative addition of aldehydes

The familiar standard de carbonyl at ion mechanism ( 3, 5) involving a concerted oxidative-addition of aldehyde, CO migration (with subsequent elimination), and reductive-elimination of product, would seem with metalloporphyrins to require coordination numbers higher than six, and in this case Ru(IV) intermediates. Although this is plausible, the data overall strongly suggest a radical mechanism and Ru(III) intermediates. 

Acylmetal hydride is formed by the oxidative addition of aldehyde, and hydroacylation occurs by insertion of alkene or alkyne. The Ni-catalysed hydroacylation of internal alkyne 600 with aldehyde gave rise to the v./l-unsaturated ketone 601 [230]. The Ru-catalysed hydroacylation of cyclohexene with aldehyde 602 under CO pressure at high temperature gives the ketone 603 [231]. 

Facile decarbonylation of aldehydes with the Rh complex (Wilkinson complex) is known [43,44], The reaction is explained by the oxidative addition of aldehyde to Rh, followed by decarbonylation and reductive elimination. However, the Rh-catalysed intramolecular reaction of some unsaturated aldehydes proceeds without the decarbonylation, and cyclic ketones are obtained. Treatment of unsaturated aldehyde... 

Properly placed donor atoms in aldehyde ligands can stabilize the acyl metal hydrides that result from oxidative addition of aldehyde C—H bonds. In all reported examples, the donor atom (which can be nitrogen, oxygen or phosphorous) must be located so that a five-membered chelate is formed upon oxidative addition. Stabilization... 

Through the oxidative addition of aldehydes, hydridoformyl and -acyl compounds are formed. Formaldehyde adds to the reactive complex, [Ir(PMe3) ]PF, to afford [HIr(CHO)(PMe3) ]PF. Cyclometallation of an aldehyde H—C bond results from treating RhCl(PPh3)3 with 8-quinolinecarboxaldehyde in CH2CI2 in 10 min to yield (95%) HRh(CRO)Cl(PPh3)2 (R = 8-carboxyquinoline). ... 

When [Rh(Cl)(Nbd)(bipy)] (Nbd = norbornadiene) 92 prepared in situ was treated with Ph2P(o-C6H4CHO) 93 in methanol, oxidative addition of aldehyde to rhodium followed by insertion of norbornadiene into the Rh-H bond occurred (Equation 29) <2005EJI1671 >. NMR spectroscopy, including two-dimensional (2-D) experiments, allowed complete characterization of complex 94. 

Oxidative addition of aldehydes is expected from the mechanism of their decarbonylation reactions, catalyzed by rhodium and palladium catalysts 9-10>. Harvie and Kemmitt reported the formation of the following diacyl complex by the reactions of the aldehydes with Pt(PPh3)4 u). 

The mechanism of hydroformylation by Pt/Sn systems has been investigated with the help of model complexes (Scheme 42). Only platinum SnCla complexes react with H2 to give EtCHO and close the cycle. 4-Pentenal is cyclized to cyclopentanone by cationic rhodium catalyst such as [Rh(dppe)2] in nitromethane or dichloromethane at 20 °C. The initiating step of the process is the oxidative addition of aldehyde-CH to the Rh(I) centre, a reversal of the final step in an olefin hydroformylation sequence. The mechanism was probed by deuteration studies direct evidence for the catalytic intermediates by NMR was unobtainable. The intermediates are involved in the reversible formation of side products, although selectivity to cyclopentanone can be as high as 98%. The essential features of the reaction are outlined in Scheme 43. ... 

The only Rh(III) intermediate that has been isolated from the reaction sequence is given below. This compound was prepared by reacting an aldehyde, 8-quinoline carboxaldehyde, with RhCl(PPh3)3. The ability of the aldehyde to form a chelate after oxidative addition has occurred (termed chelate trapping by the author) imparted sufficient stability to the compound to allow isolation and characterization. Prolonged heating in refluxing xylene yields the expected decarbonylation products. Other examples of oxidative addition of aldehydes to Rh(I) complexes are presented in Chapter 7, Section 4. 

There is good precedence for the oxidative addition of aldehydes to... 

When these reaction conditions were employed to oxidative addition of aldehydes with 1,3-indandione 149, different type of product was obtained, the bispiro-snbstituted cyclopropanes 150 exclusively and in good yields (Scheme 2.52, Table 2.46). Model reactions carried out in dioxane solvent gave the same prodncts, but after 1 h the yields were lower. The reaction mechanism is thought to start with Knoevenagel condensation, followed by iodination, and intramolecnlar nucleophilic 0-attack with HI elimination to dihydrofurans. When intramolecnlar nucleophilic C-attack occurs, with subsequent elimination of HI, cyclopropanes were prodnced. 

Interestingly, HR of < -bromobenzaldehyde (93) with acrylate gave the doubly substituted product 94 and the expected product 95 under Jeffery s ligandless conditions [60]. Eormation of 94 is explained by the following mechanism. Insertion of acrylate to 93, followed by oxidative addition of aldehyde generates 96. The palladacycle 97 is formed by decarbonylation, and its reductive elimination gives 98. The final product 94 is obtained by HR of 98 with acrylate. 

The indenone 70 is obtained by the reaction of o-iodobenzaldehyde (66) with alkyne [22,23], Two mechanisms are suggested. One of them involves the formation of Pd(IV) species 68 from 67 by oxidative addition of aldehyde, and its reductive elimination affords the indenone 70. Another possibility is the insertion of carbonyl group (or nucleophilic attack) to form the indenyloxypalladium 69, and /i-H elimination gives the indenone 70. 

The oxidative addition of aldehydes and related species to tetrakis(triphenyl-phosphine)platinum(o) has been investigated and the product is the square planar platinium(n) diacyl species ... 

The knowledge of enthalpies of dissociations for M —H, M —R, M —C(0)R, and Y —C(0)R (Y = H, R) bonds allows the estimation of enthalpies of various organome-tallic reactions (Table 4.4). This table shows that the following reactions are thermodynamically favorable oxidative addition of hydrogen, CO insertion into the M —R bond, olefin insertion into the M —H bond, and olefin insertion into the M —R bond. Oxidative addition of aldehydes to the transition metal complexes is possible. However, due to the near-zero value of the enthalpy of this reaction, oxidative addition of aldehydes does not occur easily in the above-mentioned processes. Thermodynamically unfavorable are oxidative addition of the unstrained C —C bonds, oxidative addition of C — H bonds, and CO insertion into the M — H bonds. 

A mechanism for this cyclization has been proposed (Scheme 8.26). It initiates with the oxidative addition of aldehyde C-H bond to the Rh(I) cationic species to form a hydroacylrhodium A. The insertion of the carbon-carbon double bond of the allene moiety leads to oxo-rhodacycle B, which upon stereoselective 1,3-niigration of carbon-Rh bond gives C. The subsequent insertion of alkyne gives the rhodacy-cle D, which upon reductive elimination releases the final bicyclic ketone and the cationic active Rh(I) species. The stereoselective axial/center chirality transfer is in accordance with the proposed mechanism. Moreover, supporting this mechanism, an experiment with the deuterium-labeled substrate aldehyde C(0)-D affords the cyclic compounds with a deuterium on the alkene moiety. 

Interesting formation of the fulvene 422 takes place by the reaction of the alkenyl bromide 421 with a disubstituted alkyne[288]. The indenone 425 is prepared by the reaction of o-iodobenzaldehyde (423) with internal alkyne. The intermediate 424 is formed by oxidative addition of the C—H bond of the aldehyde and its reductive elimination affords the enone 425(289,290]. 

Perfluoroalkyltin halides can be prepared via oxidative addition of perfluo-roalkyl iodides to tin(II) halides in dimethylformamide (DMF) [12] The per-fluoroalkyltin(IV) dihalide could not be isolated, but in DMF solution, the tin(lV) compound did react with aldehydes and ketones in the presence of pyndine [12] (equation 8) Typical perfluoroalkylcarbinols prepared by this method are shown in Table 1 [12]... 

A plausible mechanism accounting for the catalytic role of nickel(n) chloride has been advanced (see Scheme 4).10 The process may be initiated by reduction of nickel(n) chloride to nickel(o) by two equivalents of chromium(n) chloride, followed by oxidative addition of the vinyl iodide (or related substrate) to give a vinyl nickel(n) reagent. The latter species may then undergo transmetala-tion with a chromium(m) salt leading to a vinyl chromium(m) reagent which then reacts with the aldehyde. The nickel(n) produced in the oxidative addition step reenters the catalytic cycle. 

Employing ketones or aldehydes as starting materials, the corresponding silylethers are obtained. Thereafter, the oxidation or hydrolysis of the obtained silylethers gives the corresponding alcohols (Scheme 17). In most cases, a hydride (silyl) metal complex H-M-Si (M = transition-metal), which is generated by an oxidative addition of H-Si bond to the low-valent metal center, is a key intermediate in the hydrosilylation reaction. 

The detailed decomposition (P-H ehminahon) mechanism of the hydrido(alkoxo) complexes, mer-crs-[lr(H)(OR)Cl(PR 3)3] (R = Me, Et, Pr R = Me, Et H trans to Cl) (56, 58, 60), forming the dihydrides mer-cis-[lr H)2Cl PR )2] (57, 59) along with the corresponding aldehyde or ketone was examined (Scheme 6-8). The hydrido(ethoxo) as well as the hydrido(isopropoxo) complexes 60 could also be prepared by oxidative addition of ethanol and isopropanol to the phosphine complexes 39 [44]. In the initial stage of the P-H elimination, a pre-equiUbrium is assumed in which an unsaturated pentacoordinated product is generated by an alcohol-assisted dissociation of the chloride. From this intermediate the transition state is reached, and the rate-determining step is an irreversible scission of the P-C-H bond. This process has a low... 

The catalytic process is also achieved in the Pd(0)/Cr(II)-mediated coupling of organic halides with aldehydes (Scheme 33) [74], Oxidative addition of a vinyl or aryl halide to a Pd(0) species, followed by transmetallation with a chromium salt and subsequent addition of the resulting organo chromate to an aldehyde, leads to the alcohol 54. The presence of an oxophile [Li(I) salts or MesSiCl] allows the cleavage of the Cr(III) - 0 bond to liberate Cr(III), which is reduced to active Cr(II) on the electrode surface. 

The reaction proceeds at room temperature and is rationalized invoking oxidative addition of a Pd(0) species upon the allylic C - O bond of 67, followed by decarboxylation to form an oxapalladacyclopentane intermediate 66 (Pd in place of Ni), which undergoes a facile b-C elimination to finally give an co-dienyl aldehyde 68 (Scheme 17). Recently, it has been revealed that a combination of Ni(cod)2 and a phosphine ligand also catalyzes the same... 

The event, oxidative addition of a Ni(0) species upon dienes and aldehydes activated by coordination with Lewis acids to provide oxanickellacycles 45, has proven to take place quite generally, and many variations making the best use of the intermediate 45 have been developed. The key issue of the reactions discussed in Sect. 3 is a regioselective and stereoselective hydrogen delivery... 

Recent advancements involving oxanickellacycles as common intermediates, which are formed by oxidative addition of a Ni(0) species upon dienes and aldehydes, is also reviewed very briefly. 

The development of the Grignard-type addition to carbonyl compounds mediated by transition metals would be of interest as the compatibility with a variety of functionality would be expected under the reaction conditions employed. One example has been reported on the addition of allyl halides to aldehydes in the presence of cobalt or nickel metal however, yields were low (up to 22%). Benzylic nickel halides prepared in situ by the oxidative addition of benzyl halides to metallic nickel were found to add to benzil and give the corresponding 3-hydroxyketones in high yields(46). The reaction appears to be quite general and will tolerate a wide range of functionality. 

An intermediate acylnickel halide is first formed by oxidative addition of acyl halides to zero-valent nickel. This intermediate can attack unsaturated ligands with subsequent proton attack from water. It can give rise to benzyl- or benzoin-type coupling products, partially decarbonylate to give ketones, or react with organic halides to give ketones as well. Protonation of certain complexes can give aldehydes. Nickel chloride also acts as catalyst for Friedel-Crafts-type reactions. 

The most plausible mechanism proceeds through oxidative addition of the aldehyde to an active Ru(0) species to form (acyl)(hydrido)ruthenium(ll) complex 155. Insertion of the less-substituted double bond of the 1,3-diene into the Ru-H bond occurs to generate an (acyl)( 73-allyl)ruthenmm(ll) intermediate of type 156. Successive regioselective reductive eliminations between the acyl and the 73-allyl ligands provide the desired product with regeneration of the... 

In relation to palladium enolates, Yamamoto and co-workers reported palladium-catalyzed addition of malononi-trile derivatives to imines or aldehydes (Equation (110)).466,466a Oxidative addition of the C-H bond of the malononitrile to Pd(0) followed by insertion of an electrophile is proposed. 

It is proposed that the reaction proceeds through (i) oxidative addition of a silylstannane to Ni(0) generating (silyl)(stannyl)nickel(n) complex 25, (ii) insertion of 1,3-diene into the nickel-tin bond of 25 giving 7r-allylnickel intermediate 26, (iii) inter- or intramolecular allylation of aldehydic carbonyl group forming alkoxy(silyl)nickel intermediate 27, and (iv) reductive elimination releasing the coupling product (Scheme 69). 

---