CO2 insertion into M-N bonds

Numerous examples of carbon dioxide insertion into the M-N bond of transition metal amides [78-84] are described in the literature. In comparison, however, mechanistic studies in this area are relatively few. 

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Insertion Reactions CO2 insertion into M—H bonds also takes place, but of the two possible regiochemistries (Eq. 12.26), only the formate, either -q -, or n -bound, has been observed in a stable product (Eq. 12.27-... 

Substrates that contain polarized C-Y unsaturated moieties (Y = heteroatom) have been observed to insert into M-X bonds to give new metal complexes. This is the case of CO2, COS, and PhNCO, that insert in the Pt-N bond of [PtH(NHPh)(PEt3)2] [205], or CS2 into the Pd-OR bond of [Pd(Me)(OR)(dppe)j to give [Pd(Me) /cCsC(S)OR (dppe)] (R = CH(CF3)Ph) [211],... 

The reaction of CO2 with a metal hydride produces formate complexes M-0C(0)H, not formyl derivatives M-C(0)0H, and the insertion into M-C bonds gives the appropriate carboxylate compounds M-0C(0)R. In a similar fashion, the reactions with M-OH and M-OR (R = alkyl, aryl) generate the corresponding bicarbonate M-0C(0)0H and carbonate M-0C(0)0R species, respectively. The reaction of CO2 with a zinc hydroxide moiety is particularly important in biological systems, namely, for the reversible hydration of CO2 to HCOs catalyzed by Zn(ll) in carbonic anyhdrases. Moreover, it has been postulated that the insertion of CO2 into M-O bonds is essential in the co-polymerization of CO2 and epoxides and in the preparation of cyclic carbonates and polycarbo-In a similar vein, the insertion of CO2 into the M-N bond of both main group and transition metal... 

CO2 insertion into the N-Si bond of silylamido Ugands can play a key role in a few metathetical reactions of carbon dioxide with bis-silyl-amido complexes M[N (SiMe3)2] (Na, n = 2 Pr, Nd, n = 3 Ge, Sn, n = 2 Ti(IV), Zr(TV)) [82,83]. These metathetical reactions result in the oxo-transfer from CO2 to a silyl group of the bis-silylamido ligand and provide an entry into silyl isocyanates or l,3-bis(silyl) carbodiimides (Scheme 4.25). 

The cis-1,2-addition of M-X bonds to unsaturated A=B bonds and its reverse, the -elimination of X from M-B-A-X, are fundamental elementary steps of catalytic reactions such as hydrogenation, hydroformylation, oligomerization, polymerization, hydrosilation, hydrocyanation, or alkene isomerization processes, as well as the Heck reaction. Most of the reactions described in the literature involve M-H or M-C bonds, and alkenes or alkynes. Besides them there are processes where the unsaturated substrate is different from alkene or alkyne This includes CO2, CS2, aldehydes and ketones, imine, or nitrile. Also, there are processes involving M-Si, M-Sn, M-B, M-N, M-P, or M-M bonds. The insertion of alkenes into M-carbene bonds is not essentially different in their intimate mechanism, but it is not discussed in this chapter. 

The synthesis and coordination chemistry of carbon dioxide complexes has attracted a lot of attention over the years " and their reactivity, not surprisingly, includes several types of 1,2-migratoty insertion reactions. Thus, CO2 inserts into a variety of M-X bonds (X= H, C, O, N, P, S, Si) to yield the corresponding M-0C(0)X products, often following the initial formation of a Tt-complex (Scheme 10). 

Structure 4.3 Insertion of CO2 into M-N and M-P bonds (adapted from [148])... 

COa insertion into the M—N bonds of transition-metal dimethylamido-com-plexes [M(NMe2> ] (M=Ti, Zr, or V, /i=4 M=Nb or Ta, =5 M = W, =6) gives mixed dimethylamido-dimethylcarbamato-complexes which undergo easy CO2 exchange reactions, equations (5) and Originally the mechanism pro-... 

There has been a growing interest in the utilization of CO2 as a potential Cl source for chemicals and fuels to cope with the predictable oil shortage in the near future. Insertion reactions of CO2 into M-H, M-0, M-N, and M-C bonds are well documented, where these reactions are explained in terms of the electrophilicity of CO2 il, 2). Catalytic syntheses of lactones (3-9) and pyrones (10-16) are also established by incorporation of CO2 into dienes and alkynes activated on low-valent metal complexes. Carbon dioxide shows only an electrophilicity under usual reaction conditions, but it exhibits a nucleophilicity upon coordination to low-valent metals because of the intramolecular charge transfer from metals to CO2. Metal-C02 formation may be the key species in electro- and photochemical CO2 reductions. Since the first characterization of [Ni(PCy3)2(T) (C,0)-C02)] (17), a variety of metal... 

Nevertheless, although several reactions of CO2 can be categorized as insertion of CO2 into active M-E bonds (E = H, O, C, N, P, S and others), there are reactions which clearly require coordinated CO2 to occur. The following chapters make a detailed analysis of the reaction mechanism of CO2 conversion here we discuss a few examples of reactivity of coordinated CO2 with nucleophiles or electrophiles. 

Abstract This chapter deals with the insertion reactions of carbon dioxide (CO2) into E-X bonds, where E and X represent several different (sets of) atoms, such as M-H, M-OH, M-C, M-OR, M-O2, M-N, M-P, C-C, C-N, Si-H, and M-M (M = metal). Such reactions are relevant to catalysis for the formation of new bonds in which CO2 may be implied (C-C bonds or C-E bonds) and thus to the conversion of CO2 into added-value chemicals. The insertion product can be thermodynamically and kineticaUy stable or labile, offering in the latter case the opportunity of a catalytic path. 

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