Insertion into transition metal amides

Chisolm [85-87] has extensively studied the reactirai of CO2 with amides of early transitimi metals to give metal carbamates (4.21). Insertion of the heterocumulene into the metal-amido bond (4.21) 

Energetics of (4.22a) seems to depend markedly on the substituents on the N atom [88-95]. Gaseous carbamic acid H2NC(0)0H has been reported to be unstable [82, 88] with respect to NH3(g and C02(g —5S kJ/mol  

AG°r = —93 kJ/mol [82]). However, theoretical calculations show that in other cases, wherein R H, reactimi (4.22a) may be close to thermoneutral or may exhibit negative reaction 

In general, M(NMe2) (02CNMe2)m adducts are labile with respect to CO2 exchange (4.25). The activation energy required for the exchange process was 

The Ti- and Zr-carbamato complexes can also undergo exchange according to (4.27) and (4.28) [87]. The latter exchange processes ((4.27) and (4.28)) were also found to be catalyzed by trace amount of fortuitous amine [86, 87]. 

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The hydroamination of olefins has been shown to occur by the sequence of oxidative addition, migratory insertion, and reductive elimination in only one case. Because amines are nucleophilic, pathways are available for the additions of amines to olefins and alkynes that are unavailable for the additions of HCN, silanes, and boranes. For example, hydroaminations catalyzed by late transition metals are thought to occur in many cases by nucleophilic attack on coordinated alkenes and alkynes or by nucleophilic attack on ir-allyl, iT-benzyl, or TT-arene complexes. Hydroaminations catalyzed by lanthanide and actinide complexes occur by insertion of an olefin into a metal-amide bond. Finally, hydroamination catalyzed by dP group 4 metals have been shown to occur through imido complexes. In this case, a [2+2] cycloaddition forms the C-N bond, and protonolysis of the resulting metallacycle releases the organic product. 

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. 

Insertion into Main Group and Post-Transition Metal Amides... 

The insertion of unsaturated molecules into metal-carbon bonds is a critically important step in many transition-metal catalyzed organic transformations. The difference in insertion propensity of carbon-carbon and carbon-nitrogen multiple bonds can be attributed to the coordination characteristics of the respective molecules. The difficulty in achieving a to it isomerization may be the reason for the paucity of imine insertions. The synthesis of amides by the insertion of imines into palladium(II)-acyl bonds is the first direct observation of the insertion of imines into bonds between transition metals and carbon (see Scheme 7). The alternating copolymerization of imines with carbon monoxide (in which the insertion of the imine into palladium-acyl bonds would be the key step in the chain growth sequence), if successful, should constitute a new procedure for the synthesis of polypeptides (see Scheme 7).348... 

The transition metal-catalyzed C-H insertion reaction of carbenes to organic compounds is a well-established synthetic method, as shown in the first two sections in this chapter. However, nitrene C-H insertion, the corresponding reaction of carbene analog, is much less known. In the past decade, considerable advances have been made in the development of this chemistry into a generally useful C-H amination process by using improved catalysts and protocols, in which readily available amines or amides are used as the starting substrates. Moreover,... 

Although insertion of CO into a metal-Me bond is well known in transition metal chemistry, among metal amides it appears to have been unambiguously observed only for alkali and actinide metal derivatives. In the NaN(SiMe3)2/CO system the expected product Na[CON(SiMe3)2], if formed, decomposes ... 

Following the pioneering work of Breslow and Gellman, transition metal-catalyzed inter- and intramolecular C-H functionalizations (aminations or amidations) have advanced tremendously in recent decades/ """ Characteristic examples are Rh-catalyzed insertions into allylic or benzylic C-H bonds [Eq. (6.134)] and the transformation of carbamates to oxazolidi-nones [Eq. (6.135)] ... 

Scheme 15.18 General mechanism of late-transition-metal-catalyzed hydroamination with insertion into metal-amide or metal-hydride bonds.

Alkyl halides are usually considered to be less suitable for double carbonylation because of the possibility of the direct reaction of alkyl halides with nucleophiles and of instability of alkyl-transition metal complexes involved in the catalytic process. However, allylic halides were found amenable to double carbonylation promoted by zerovalent palladium complex. It is well known that allylic halides undergo ready oxidative addition with a Pd(0) species to produce Tj -allylpalladium halide complexes. Thus, it was reasoned that the double carbonylation process might be realized if CO insertion into the aUyl-palladium bond proceeds before attack of amine on the 17 -allylpaUadium halide takes place. On the basis of fundamental studies on the behavior of i7 -allylpalladium halide complexes with CO and secondary amines, double carbonylation processes of substituted aUyl halides to give a-keto amides in high yields have recently been achieved (Eqs. 15 and... 

An alternative mechanism starts from the coordination of an amine, and the successive deprotonation gives a metal amide species (Scheme 8b). Coordination of a C-C multiple bond to this metal center is followed by migratory insertion into the M-N bond. The newly formed M-C bond is cleaved by protonolysis to regenerate the active metal species. The advantage of this pathway is that it does not require the change of oxidation number of metal, and it looks similar in mechanism to hydroamination of other group metals (for group 4 metals, metathet-ical reaction takes place at the step of C-N bond formation) and partially similar in mechanism to oxidative amination of late transition metals. However, so far, most hydroamination reactions catalyzed by late transition metals can be explained by the mechanisms discussed in Sects. 3.1 and 3.2.2. If the activation of the C-C... 

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