Imine complexes nucleophilic attack

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. 

First, deprotonation of dimethyl phosphite accompanied by coordination of oxygen to the oxophilic lanthanide gives 33. Nucleophilic attack of P on the imine carbon along with N-coordination gives 34 proton transfer followed by product de-complexation regenerates the catalyst [33],... 

With a YbPB catalyst at room temperature, 86% yield and 98% ee were obtained. After extensive optimization of the catalyst, solvent, temperature, pressure, and catalytic loading, 98% yield and 98% ee was achieved using YbPB (5 mol%) at 50°C for 48 h in 1 7 THFitoluene. The active catalyst was isolated its structure is similar to that shown in Scheme 5-46, and a similar mechanism was proposed. Additional spectroscopic studies suggested that complexation of the phosphite to the lanthanide center was a plausible first step, and that the P-C bond is formed by nucleophilic attack of phosphoms on an N-complexed imine [34]. 

Recently, Ohe and IJemura reported a novel approach to the catalytic cyclopropanation of alkenes via 2-furyl178 179 or 2-pyrrolyl carbenoids180 that originate from the intramolecular nucleophilic attack of a carbonyl oxygen or an imine nitrogen (ene-yne-ketone and ene-yne-imine precursor, respectively) on a 7t-alkyne complex or a cationic cr-vinyl complex. Initially, the group 6 complexes like Cr(CO)s were used. Soon it was found that a series of late transition... 

The hydrogen-bond complex 5 and ion pair 6 are activated form of the carbonyl compounds. The nucleophilic addihon of carbon nucleophile to carbonyl compounds and imines may be accelerated by acid catalysis. Nucleophilic attack to carbonyl compounds or imine took place either by way of 5 or 6 to furnish addihon product. If HX activates carbonyl compound by forming hydrogen-bond complex 5 and nucleophilic addition takes place to give an adduct, the reaction is a hydro-gen-bond catalyzed reaction (Scheme 2.5). In contrast, when ion pair 6 is formed and nucleophilic addihon occurs, the reachon is a Br0nsted-acid-catalyzed reachon. 

For the formation of the 4-ethynylquinoline complexes a mechanism was proposed involving nucleophilic attack of the terminal carbon of the butatrienylidene ligand at the imine carbon, followed by C—C bond formation between the ortho carbon of the N-aryl group and C3 of the butatrienylidene ligand. Deprotonation finally affords 4-ethynylquinoline complexes (Scheme 3.27). Some preference was observed for quinoline formation with the more electron-rich metal centers, whereas... 

The most important reaction of this type is the formation of imine bonds and Schiff bases. For example, salicylaldehyde and a variety of primary amines undergo reaction to yield the related imines, which can be used as ligands in the formation of metal complexes. However, it is often more desirable to prepare such metal complexes directly by reaction of the amine and the aldehyde in the presence of the metal ion, rather than preform the imine.113 As shown in Scheme 31, imine formation is a reversible process and isolation of the metal complex results from its stability, which in turn controls the equilibrium. It is possible, and quite likely, that prior coordination of the salicylaldehyde to the metal ion results in activation of the carbonyl carbon to amine nucleophilic attack. But it would be impossible for a precoordinated amine to act as a nucleophile and consequently no kinetic template effect could be involved. Numerous macrocyclic chelate systems have been prepared by means of imine bond formation (see Section 61.1.2.1). In mechanistic terms, the whole multistep process could occur without any geometrical influence on the part of the metal ion, which could merely act to stabilize the macrocycle in complex formation. On the other hand,... 

New chelate rings can be formed by the nucleophilic addition of alcohols to imine complexes. For example, the nickel(II) TAAB complex is susceptible to attack by bis-alkoxides (equation 31).127 It is not clear whether or not a kinetic template effect operates by prior coordination of the central oxygen or sulfur atom. However, such an effect is not necessary, as simple alkoxides undergo a similar addition reaction.128... 

The formation of imine or Schiff base metal complexes can be effected very readily by arranging for reaction of the amine with the carbonyl compound to take place in the presence of the metal ion. Nucleophilic attack of the amine on the carbonyl group is enhanced by polarization of the carbonyl group by coordination of the oxygen atom to the metal ion. Furthermore, the reaction is reversible unless steps are taken to effect dehydration of the intermediate hydroxy amine, and coordination of the product imine is an important factor in the promotion of the reaction by suitable metal ions. 

Despite its formal simplicity, the stepwise mechanism of the reaction between ketenes and imines raises a complex stereochemical situation since ketenes can be unsymmetrically substituted and imines can exist in either (E)- or -configurations [49]. As far as the first step of the reaction is concerned, the nucleophilic attack of the nitrogen atom of the imine can occur through the less hindered exo face, namely that which has the shortest substituent, or through the endo face, which incorporates the largest substituent (Scheme 8). In principle, the exo attack leads to second transition structures that exhibit the largest substituents at the 3-out position. 

It was found that one possible mechanism, denoted as (A) in Scheme 31, consists of the nucleophilic attack of imine (66) in the chromacyclopropanone complex (120) to yield intermediate (121). Isomerization of this [Cr]-C complex leads to [Cr]-0 zwitterionic intermediate (122), whose electrocyclic reaction yields O-coordinated (3-lactam (123). Alternatively, isomerization of (120) to the O-complexed ketene (124) permits the direct formation of zwitterionic intermediate... 

One of the paradoxes of metal-imine chemistry is the observation that in many cases the imine is stabilised with respect to nucleophilic attack by water upon co-ordination, but is still prone to attack by amines. We saw in Chapter 4 how the hydrolysis of imines may be either promoted or inhibited by co-ordination to a metal, and we also saw a number of examples involving nucleophilic attack on an imine by a variety of other nucleophiles. A special case of such a nucleophilic attack involves another amine. The consequence is a transimination reaction, as indicated in Fig. 5-53. Presumably, intermediates of type 5.26 are involved. The procedure is of some synthetic use for the preparation of imine complexes (Fig. 5-54). 

In earlier chapters we noted that metal ions could either activate or deactivate an imine with respect to addition of a nucleophile. We will now see an example of metal-ion activation in action. In fact, the complexes that are formed from 6.39 arise as a result of metal-initiated nucleophilic attack at the imine groups. The reaction of the free ligand 6.39 with methanolic cobalt(n) acetate results in the attack of methanol upon one of the imine bonds of the initially formed complex (Fig. 6-39). 

Figure 6-40. The complex 6.41 undergoes an intramolecular nucleophilic attack at a second imine to generate a new macrocycle with the correct cavity size for cobalt(n). The lower structure shows the complex cation as it is found in the solid state. The cobalt ion is actually seven-co-ordinate, with axial water and methanol ligands (omitted for clarity).

Weber and coworkers reported the synthesis of novel substituted borazoles. The reaction of boron trichloride with 1,2-diimines 367 provide a mixture of products. The formation of dimeric borazole 369 was explained based on a nucleophilic attack of the chloroborane 370 on the ate complex 368 followed by dehydrohalogenation. Alternatively, a controlled addition of BCR to the diimine 367 results in a clean formation of the imine-borane complex 373 that upon reduction with sodium amalgam and calcium hydride provided the chloroborazole 375 (Scheme 59) <2006EJI5048>. 

The conformation is proved by a significant NOE between the aldimine proton and the anomeric proton [17,24]. In polar solvents, free cyanide attacks the complex A, preferably from the unshielded Si-side. In unpolar solvents like chloroform, cyanide is not set free from the silyl derivative. The activation of the cyanide proceeds by an interaction between the exo chloride of the zinc complex and the silyl group. Thus, the cyanide is directed to the Re-side of the glycosyl imine (see Scheme 8). This nucleophilic attack produces L-aminonitriles with moderate or good stereoselectivity (S R 3-9 1) and high yields. 

However, when bound to a higher metal oxidation state center, with a limited tt-electron donor ability, the nitriles display v (N C) higher than in the free state and can be activated toward nucleophilic attack, as observed for the reactions of cfr-[ReCLi(NCMe)2] (formed by spontaneous reduction of ReCls in NCMe) with oximes, HON=CRR, amino-alkylated adenines, or alcohols, which behave as protic nucleophiles (reaction 2) to yield, for example, in the flrst case, the imine complexes cA-[ReCl4 NH=C(Me)ON=CRR 2]. 

Recent studies have shown that coordinated ammonia and amine ligands under basic conditions may effect nucleophilic attack at carbonyl centres in organic compounds, " These reactions occur due to formation of deprotonated amido species which can act as nucleophiles. For example, reaction of cobalt(III) and platinum(IV)ammines with ketones gives the corresponding Co and Pt imine complexes. A similar reaction between [Ru(NH3)6] and diones produces the corresponding Ru diimine (108). It has also been found that nitrilepentaammineruthenium(II)... 

Increasing the polarization of the imine double bond by complexation of 1-acetyl-3,5,5-trimethylpyrazoline (255) with Lewis acids is an effective method of inducing C-3 nucleophilic attack. LAH Reduction of 255 in acetic anhydride has the overall effect of a reductive alkylation to give 1 -ace-tyl-2-ethyl-3,5,5-trimethylpyrazolidine (256). Similar behavior has been ob-... 

The inactivation of Ala-R by trihaiogenated alanine analogues is shown in Figure 38. According to the mechanism proposed by Faraci and Walsh,the addition of the halogenated analogue leads to an initial transaldimination, followed by the elimination of the first fluoride ion and formation of the /3-difluoro-a,/3-unsaturated imine complex. The subsequent nucleophilic attack of the enzyme on the olefinic terminus induces the loss of the second fluoride ion and the formation of a stable inactive complex." ... 

---