Intermediate carbamoyl complex

The main differences are concerned with the role of iodide cocatalyst, not yet well understood, and the DPU formation mechanism. It is suggested that the iodide promotes the formation in situ of molecular iodine (reaction 7) which then reacts with the intermediate carbamoyl complex coming from reaction (8) to afford iodoformamide (reaction 9). The in situ reaction of the last one with aniline gives DPU (reaction 10). 

It is well known that the oxidative carbonylation of aniline and the reductive carbonylation of nitrocompounds to give DPU or MPC occur according to the stoichiometry of reactions (1-2) and (4-5). Alkoxycarbonyl complexes (M-COOR 1) and carbamoyl complexes (M-CONHR 2) which then evolve into the final products, are believed to be key intermediates for these reactions. The two accepted different mechanisms for the formation of 1 and 2 along with their catalytic cycles are illustrated in the schemes 1 and 2 for the oxidative carbonylation of amines catalyzed by noble metals. Both the cycles involve a two electron redox process. 

The electrochemical reduction of CO2 in the presence of dimethylamine and [Ru(bipy)2(CO)2] as the catalyst in anhydrous acetonitrile gives dimethylform-amide and formate. Ruthenium carbamoyl complexes are likely intermediates, and the formation of [(bipy)2Ru(CO)(CONMe2)] was confirmed by FT-IR and H Dimethyl formamides are also formed if CO2 is reduced by H2 in the presence of dimethylamine. The catalysts employed were [RuH(Cl)(CO)L3], [RUCI2L3], [OsH(C1)L3], [Os(H)2C1(CO)L3], [IrCl(CO)L2], [Pt2(dppm)3],... 

Nucleophilic attack of the aryl amine on the complex could lead to displacement of the metal with direct formation of the product or to formation of the intermediate carbamoyl ligand. The former mechanism is ruled out because it would produce the wrong product, a carbamate. The formation of the carbamoyl ligand by nucleophilic attack on a metal carbonyl or on a methoxycarbonyl cannot be dismissed by any of our data. Unfortunately the site of nucleophilic attack cannot be established by our kinetics or any of our direct evidence. In a separate study of the transesterification of Ru(dppe)(C0)2[C(0)CXIH3]2, the reactivity patterns could be best explained by invoking nucleophilic attack on a metal carbonyl.(18) It is reasonable to expect a similar mechanism of nucleophilic attack with the aryl amine. 

Alkoxides and amines also attack coordinated carbon monoxide to give alkoxycarbonyl and aminocarbonyl complexes, respectively (Equations 11.5 and 11.6). Stable complexes of both types have been isolated and thoroughly characterized. Less stable alkoxycarbonyl and carbamoyl complexes are intermediates in a number of synthetically important carbo-nylation processes discussed in Chapters 17 and 19. 

The proposal that nitrobenzene oxidises an intermediate Pd carbamoyl complex, rather than a Pd° complex formed after elimination of urea, is stimulating. However, there is no experimental evidence supporting this proposal, which must be considered as speculative. 

Under appropriate conditions, alcohols and amines can undergo an oxidative double carbonylation process, with formation of oxalate esters (Eq. 34), oxamate esters (Eq. 35) or oxamides (Eq. 36). These reactions are usually catalyzed by Pd(II) species and take place trough the intermediate formation of bis(alkoxycarbonyl)palladium, (alkoxycarbonyl)(carbamoyl)palladium or bis(carbamoyl)palladium complexes, as shown in Scheme 29 (NuH, Nu H = alcohol or amine) [227,231,267,293-300]. 

Carbamoyl or alkoxycarbonyl complexes 87 are obtained by the attack of amines or alkoxides to metal carbonyls. They are important intermediates of carbonylation reactions and undergo insertion of alkene and alkyne. 

There is no doubt that most of the mono- and polynuclear metal carbonyls react with liquid NH3 via nonisolable carbonyl carbamoyl intermediate complexes in which the acid amide group —CONH2 is directly bound to the transition metal. For the reaction of Mn2(CO)10 with liquid NH3, we have been able to show that, at -78°C, quantitative formation of ciJ-Mn(CO)4(NH3)—CONH2 and HMn(CO)5 occurs (105) according to the following reaction ... 

Quite a range of P-lactams have been made by methodologies following disconnection a with carbamoyl radicals (aminoacyls) as intermediates. Pattenden and co-workers made carbamoyl cobalt salophen complexes and showed that on photolysis carbamoyl radicals were released and underwent 4-exo cyclisations [75-77]. For example, carbamyl chloride derivative 59 was converted to cobalt complex 60, which on photolysis yielded the cyclised cobalt-azetidinone complex 61. The free azetidinone 62 was released by heating the cobalt complex in toluene and was transformed into thienamycin in several subsequent steps (Scheme 15). 

Anionic ortho-Vries, rearrangement which involves a 1,3-transposition of a carbamoyl group occurred also in the chromium complex 256 on wanning the Uthinm intermediate 257 to —20°C (equation 118). The lithio benzo[i>]thiophene 258 obtained at —78°C was allowed to attain room temperature, and when it was left stirring for 12 hours it gave the salicylamide 259 (equation 119). ... 

On the contrary, scheme 2 is accepted for systems working under drastic conditions and in heterogeneous phase. In these cases, the reduced form of catalyst undergoes oxidative addition by amine and produces an imine-complex, leading to the carbamoyl intermediate by subsequent CO insertion into the M-N bond [laj. 

From a methodological point of view, it should be pointed out the formation of 51, which is a result of the addition of acetone to an allenylidene ligand. Heteroatom-containing cyclic metal-carbene complexes [24] have been conveniently prepared via metal co-haloacyl, carbamoyl, alkoxycarbonyl, or imido intermediates [25], opening of epoxides by deprotonated Fischer-type carbene complexes [26], and activation of homopropargylic alcohols with low-valent d complexes [27], including ruthenium(II) derivatives [28]. In general, the preparation of unsaturated cyclic carbene complexes requires the previous preparation of functional carbenes to react with P-dicarbonyl derivatives, acrylates, and enol ethers [29]. 

An unusual rearrangement of the carbamoyl moiety was reported by Hill et The reaction of the iron trifluoromethyl complex Fe( -OCNTr2)(CF3)(CO)2(PPh3) with KTp led to the formation of the novel ferraoxetene TpFe /c = C(NTr2) 0CF2 (C0) in 73% yield with loss of CO and PPh3 via a postulated difluorocarbene intermediate (Scheme 5). 

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