Intermediates 5-coordinate carbonyl

The mechanism of the cycloaddition reaction of benzaldehyde 2a with Danishefsky s diene 3a catalyzed by aluminum complexes has been investigated theoretically using semi-empirical calculations [14]. It was found that the reaction proceeds as a step-wise cycloaddition reaction with the first step being a nucleophilic-like attack of Danishefsky s diene 2a on the coordinated carbonyl compound leading to an aldol-like intermediate which is stabilized by interaction of the cation with the oxygen atom of the Lewis acid. The next step is the ring-closure step, giving the cycloaddition product. 

The ratios of these slopes for L- and D-esters are shown in Table 12. The kL/kD values of the acylation step in the CTAB micelle are very close to those in Table 9, as they should be. It is interesting to note that the second deacylation step also occurs enantioselectively. Presumably it is due to the deacylation ocurring by the attack of a zinc ion-coordinated hydroxide ion which, in principle, should be enantioselective as in the hydroxyl group of the ligand. Alternatively, the enantioselectivity is also expected when the free hydroxide ion attack the coordinated carbonyl groups of the acyl-intermediate with the zinc ion. At any rate, the rates of both steps of acylation and deacylation for the L-esters are larger than those for the D-esters in the CTAB micelle. However, in the Triton X-100 micelle, the deacylation step for the D-esters become faster than for the L-esters. 

Normally, the addition of C-nucleophiles to chiral a-alkoxyaldehydes in organic solvents is opposite to Cram s rule (Scheme 8.15). The anti-Cram selectivity has been rationalized on the basis of chelation control.142 The same anti preference was observed in the reactions of a-alkoxyaldehydes with allyl bromide/indium in water.143 However, for the allylation of a-hydroxyaldehydes with allyl bromide/indium, the syn isomer is the major product. The syn selectivity can be as high as 10 1 syn anti) in the reaction of arabinose. It is argued that in this case, the allylindium intermediate coordinates with both the hydroxy and the carbonyl function leading to the syn adduct. 

A prerequisite for CO activation in 6 is an O-attack of the silane at the coordinated carbonyl ligand. This activation step induces the reduction of CO by formal electron transfer Mn—>C and yields a highly reactive 17e intermediate siloxycarbyne complex which dimerizes to give 8. 

Additional support for the carbon monoxide-alkoxycobalt insertion reaction is found in the reaction of tert-butyl hypochlorite with sodium cobalt carbonyl. This reaction, if carried out at — 80°C. in ether solution under nitrogen or carbon monoxide, leads to about a 10% yield of /er/-butoxycarbonylcobalt tetracarbonyl which has been isolated as the monotriphenylphosphine derivative. The major product seems to be cobalt octacarbonyl, possibly formed by decomposition of an intermediate JerJ-butoxycobalt tetracarbonyl (34). This reaction appears to be a true insertion reaction although a direct attack of a coordinated carbonyl group upon oxygen cannot be ruled out. 

The reduction of metal ions in higher oxidation states by CO and H20 has been known for many years. Work on the reduction of Hg2+, Ag+, Ni2+, Cu2 +, and Pd2+ has been summarized recently (4). The reduction of these metal ions does not proceed via a stable intermediate carbonyl. Since a metal carbonyl must be an intermediate in this reaction, however, the coordinated carbonyl must be very susceptible to attack by water, reacting as soon as it is formed. The ability of a metal in a higher oxidation state to activate a coordinated carbonyl to attack by as weak a nucleophile as water was noted previously in the description of the work by James et al., on the reduction of rhodium(III) by carbon monoxide and water (62). Here a stable rhodium(III) carbonyl, Rh(CO)Cl2-, can be observed as the initial product of reaction of RhCl3 3HzO with CO. The Rh(III) is then efficiently reduced to the rhodium(I) anion [RhCl2(CO)2], even in nonaqueous solvents such as dimethylacetamide, where the only water available for reaction is the water of hydration of the starting rhodium chloride. 

The key intermediate in the reduction of metal ions by carbon monoxide and water is the hydroxycarbonyl (18). Initially (18) was proposed to form by a migratory insertion of CO into a M—OH bond, but more recent studies have favored a direct attack of water or hydroxide on a coordinated carbonyl (4,62). This latter view is in accord with the expected reactivity of coordinated CO toward nucleophiles. Intermediate (18) may then decarboxylate to give C02 and either a reduced metal ion or a metal hydride, as in (29) and (30), respectively. 

This reaction elucidates the mechanism of the photoreaction of 73 with dienes. In the first step, 73 loses one carbonyl ligand with formation of the reactive 16-electron species [0/5-C5Hj)Mo(CO)2CH3] (87) (109-113), which adds a diene molecule. jj2-Diene complexes [( 5-C5H5)Mo(CO)2CH3-( 72-diene)] (88) are quite likely as intermediates. Coordination of the free C=C double bond of the r 2-diene ligand causes insertion of CO into the Mo—C [Pg.338]

In a similar context Amdtsen developed a new pyrrole synthesis from alkynes, acid chlorides either imines or isoquinolines, based on the reactivity of isocyanides (Scheme 35a) [197]. Although all atoms from the isocyanide are excluded from the final structure, its role in the reaction mechanism is crucial. The process takes place through the activation of the imine (isoquinoline) by the acid chloride to generate the reactive M-acyliminium salt, which is then attacked by the isocyanide to furnish a nitrilium ion. This cationic intermediate coordinates with the neighboring carbonyl group to form a miinchnone derivative, which undergoes a [3+2] cycloaddition followed by subsequent cycloelimination of the isocyanate unit, to afford the pentasubstituted pyrrole adducts 243 and 244 (Scheme 35a, b). 

The intermediate acetyl derivative Nb(r -C5H5)2(COMe)CO, resulting from nucleophilic attack of the methyl carbanion on a coordinated carbonyl group, undergoes decarbonylation. 

Palladium nanoparticies can be easily prepared by displacement of the dba ligands by CO from Pd(dba)2. An intermediate unstable carbonyl complex forms, which collapses into clusters. By this method, palladium nanoparticies of sizes in the range 2-5 nm could be obtained using PVP or cellulose derivatives as stabilizers. Such particles were recently used to probe the mechanism of the Heck reaction. A correlation between the initial reaction rate and the particle sizes was established. The small particles proved more active in agreement with the presence of more surface palladium atoms with low metal-metal coordination number. ... 

A zerovalent platinum bimetallic complex Pt2(dppm)3 will also catalyze the oxidation of CO to CO2 using either O2 or NO as oxidant. " Again, no comment can be made regarding the reaction mechanism. Oxidation of CO on platinum metal surfaces involves the reaction of chemisorbed atomic oxygen, but it is unlikely that such species are formed under the mild conditions of temperature used in these examples. In the conversion of the coordinated carbonyl in IrCl(CO)PN (PN = o-(diphenylphosphino)-A,A-dimethylaniline) to CO2 though it has been tentatively suggested that a metallocyclic intermediate is formed (23) ... 

Such an intermediate can be formed because of the nucleophilic reactivity of a dioxygen molecule when complexed to iridium(I) and because of the known propensity of a coordinated carbonyl ligand to undergo such nucleophilic attack. 

A wide range of nucleophiles has been shown to attack coordinated carbonyl compounds. The products depend upon the fate of the tetrahedral hydroxy intermediate formed. In the reaction of carbonyl compounds and amines the loss of water results in the formation of imines, and this methodology has widespread synthetic application. A novel sexidentate macrocyclic ligand (26) has been prepared by template methods. The formation of the hydrazone from hydrazine and... 

According to the proposed mechanism, addition of the silyl-rhodium moiety to the coordinated carbonyl group converts it into the a-siloxyalkyl-rhodium complex (III), which most likely is an equilibrium mixture of complexes Ilk and Iltt. Then, transfer of the hydride ligand in III from the metal center to the alkyl carbon affords the products, IVfl and IV, respectively. The formation of the a-siloxyalkyl-rhodium intermediate is quite probable in view of the well-documented soft-hard conception , and must be characteristic of the ketone hydrosilylation. 

In acidic or neutral medium the alcohol reacts with the coordinated olefin thus yielding an alkoxy palladium intermediate subsequent carbonylation leads to the corresponding ether-ester. 

Cycloaddition of COj with the dimethyl-substituted methylenecyclopropane 75 proceeds smoothly above 100 °C under pressure, yielding the five-membered ring lactone 76. The regiocheraistry of this reaction is different from that of above-mentioned diphenyl-substituted methylenecyclopropanes 66 and 67[61], This allylic lactone 76 is another source of trimethylenemethane when it is treated with Pd(0) catalyst coordinated by dppe in refluxing toluene to generate 77, and its reaction with aldehydes or ketones affords the 3-methylenetetrahy-drofuran derivative 78 as expected for this intermediate. Also, the lactone 76 reacts with a, /3-unsaturated carbonyl compounds. The reaction of coumarin (79) with 76 to give the chroman-2-one derivative 80 is an example[62]. 

Oxidative cleavage of P-aminoacyl complexes can yield P-amino acid derivatives (320,321). The rhodium(I)-catalyzed carbonylation of substituted aziridines leads to P-lactams, presumably also via a P-aminoacyl—metal acycHc compound as intermediate. The substituent in the aziridine must have 7T or electrons for coordination with the rhodium (322,323). 

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