Acyl imine insertion

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... 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

A series of complexes in which the cyanide ligands are modified or replaced arises from the decomposition of methyl and pyridiomethylcobalt(111) pentacyanide derivatives in acid solution. The reactions include protonation of a cyanide ligand, insertion of a cyanide ligand between the organic group and the cobalt atom to produce an imine (see Section VI,D), decomposition of this imine to an acyl product, and replacement of a cyanide ligand by water 100, 101). The products are listed in Table III, 29. 

Carbon monoxide rapidly inserts into the carbon—zirconium bond of alkyl- and alkenyl-zirconocene chlorides at low temperature with retention of configuration at carbon to give acylzirconocene chlorides 17 (Scheme 3.5). Acylzirconocene chlorides have found utility in synthesis, as described elsewhere in this volume [17]. Lewis acid catalyzed additions to enones, aldehydes, and imines, yielding a-keto allylic alcohols, a-hydroxy ketones, and a-amino ketones, respectively [18], and palladium-catalyzed addition to alkyl/aryl halides and a,[5-ynones [19] are examples. The acyl complex 18 formed by the insertion of carbon monoxide into dialkyl, alkylaryl, or diaryl zirconocenes may rearrange to a r 2-ketone complex 19 either thermally (particularly when R1 = R2 = Ph) or on addition of a Lewis acid [5,20,21]. The rearrangement proceeds through the less stable... 

Among reports related to radicals, ab initio calculations have been used to model intramolecular additions of acyl radicals to imines.83 Imines and oxazolines bearing a pendant acyl radical at carbon have been cyclized to give 2-piperidones through a selective 6-emfo-cyclization at nitrogen.84 The acyl radical is generated via CO (g) insertion into a suitable precursor. A diastereoselective example is also reported. 

An exception to the regiochemistry described above (R binds to C) is shown in Scheme 6.55 [169]. The imine moiety does not insert into the Pd-alkyl bond but inserts into the Pd-acyl bond and the authors attribute this different behavior to the formation of a strong amide bond in the reaction with the acyl derivative. This thermodynamic driving force and the electrophilic character of the acyl carbon may explain the different regiochemistry observed. 

The insertions of imines into late transition metal-carbon bonds are even less common. In one case, the insertion of an imine into Ni- and Pd-acyl bonds occurs with 2,1-regiochemistry to form an aminoalkyl product (Equation 9.80). Tlus reaction is likely to occur through a polar transition state formed by attack of a nucleophilic nitrogen at the electrophilic acyl carbon. One set of examples of 1,2-insertions of imines into late metal-carbon bonds have been reported. This example involves insertion of N-aryl aldimines into rhodium-aryl complexes containing a labile pyridine ligand (Equation 9.81). The rates of these reactions were inverse order in added pyridine, suggesting that the reaction occurs by an intramolecular migratory insertion mechanism after replacement of the coordinated pyridine by the imine. 

The intennolecular acylpalladation corresponds to the addition of an acyl-palladium bond onto a rr-bond system of another molecule this elementary step can also be referred to as an insertion (Scheme I). This produces another organopalladium complex, which can in principle participate in subsequent propagation or termination reactions. This excludes processes that involve alkoxycarbonylation (R— = R O—) and hydrocarbonyla-tion (R— = H—). This section will focus on nonpolymeric intermolecular reactions of acylpalladium complexes with different 7r-bond systems (alkenes, imines, dienes, and alkynes). 

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