Activity of carbonylation

Activation of Carbonyl Compounds by Chiral Lewis Acids... 

Scheme 54 Complementary benzannulation via photo activation of carbonyl ligands...

The LPDE system is applied to several reactions in which the metal ions coordinate to the lone pairs of heteroatoms, thereby activating the substrate. Initially, the effectiveness was shown in Diels Alder reactions (Scheme 1). In a highly concentrated (5.0 M) LPDE solution, Diels- Alder reactions proceeded smoothly.6-7 Generally, a catalytic amount of LiC104 is not effective in this reaction. In some cases, a catalytic amount of an additional Bronsted acid, such as camphorsulphonic acid (CSA), gives better results.8 An interesting double activation of carbonyl moieties by using dilithium compounds has been reported (compound... 

Activation of carbonyl compounds by double hydrogen bonding an emerging tool in asymmetric catalysis (P. M. Pihko, 2004) [lb]. 

Scheme 2.4 Activation of carbonyl compound by Bronsted acid.

In addition to the activation of carbonyl compounds and imines, Schreiner studied on thiourea-catalyzed acetalization reaction, in which ortho esters were activated by hydrogen bond [19]. Jacobsen has utilized the hydrogen-bond catalysis in reactions with acyliminium ions, wherein hydrogen bond activates the acylim-inium salt through complexation with chloride [20]. 

Activation, of carbonyl clusters, 30 144-146 Activation analysis, charged particles and, 1 339-341... 

Maruoka reported the use of the didentate catalyst 8 for double electrophilic activation of carbonyl compounds [70], but since no comparison with monofunctional phenolates was given it is not clear whether having two aluminium centres in the same catalyst offers any special advantages. They used this catalyst to effect transfer hydrogenation between remote aldehyde and alcohol groups in the same molecule [71], but again it is not clear whether the transfer is truly intramolecular or in any way different from that of reduction by an external alcohol using 8 or a monuclear aluminium catalyst. 

The turn over frequency (TOF) and the activity of carbonylation are defined by Equations 3 and 4, respectively. 

In all cases, superelectorophilic dicationic intermediates3 5 were suggested to be involved in the activation of carbonyl compounds based on the observation that protonated /V-heterocycles significantly enhance the reactivity of adjacent carbo-cationic centers. For example, cyclohexanone and acetophenone are unreactive toward benzene in triflic acid, whereas 4-piperidones252 and acetylpyridines254 react readily. Likewise, 3-pyridinecarboxaldehyde is able to alkylate deactivated... 

The activation of carbonyl compounds by Lewis acid sites in the tin species has been proposed to play an important role in the oxidation reaction (Figure 5.11). The pore size of beta zeolite ( 6.5 A x 7.5 A) may pose some restrictions on the substrate size. For much bulkier substrates, a new tin-MCM-41 (pore size around 20 A) catalyst has been developed by the same research group [64]. 

Figure 5.11 Activation of carbonyl compounds by Lewis acid sites of tin species during oxidation.

Figure 8.1 Activation of carbonyl compounds by iron (111) salts.

Employing a silyl ether instead of 38 provided a connective assembly of homoallylic ethers. This three-component reaction leads to the formation of homoallylic ethers 45 via activation of carbonyl 6 by Lewis acid 17. The in situ generated oxo-nium cation 43 can then be trapped by the nucleophilic silyl ether 42 affording 44. The new species can then react with allyltrimethylsilane 1, to form the desired ether 45 with subsequent regeneration of the catalyst and loss of TMSOTMS 47 (Scheme 13.16). 

Activation of Michael Acceptors by Iminium Ion Formation, Activation of Carbonyl Donors by Enamine Formation... 

Ruthenium-catalyzed ortho-Activation of Carbonyl-substituted Arenes... 

The catalyst RuH2(PPh3)4 itself has no catalytic activity in the C-H activation of carbonyl compounds, and a more basic catalyst such as RuCpH(PPh3)2 and RuCp H(PPh3)2 is required for activation of 1,3-dicarbonyl compounds. The catalytic Michael addition of 3-methyl-2,4-pentanedione to methyl vinyl ketone gave the corresponding adduct in 79% yield [2]. 

Recently, direct catalytic asymmetric Mannich-type reactions were reported via C-H activation of carbonyl compounds. The transformations are catalyzed both by organometallic complexes and metal-free organic catalysts. 

Maruoka has reported that chiral bimetallic Lewis acid catalysts 9-11, prepared from (S)-BINOL, M(0-i -Pr)4 (M=Ti, Zr, Hf), and the corresponding spacer, strongly enhance the reactivity of aldehydes or ketones toward allyl transfer from allylstannanes [18-20]. For example, treatment of acetophenone (42) with tetraallyltin (41) in the presence of 30 mol% of the chiral bidentate Ti(IV) catalyst 10 provided the (S)-enriched homoallylic alcohol 43 in 95% yield with 90% ee (Scheme 2) [19]. A suggested reaction mechanism involves double activation of carbonyls owing to the simultaneous coordination of two Ti atoms to a carbonyl oxygen atom. 

ZnBr2 also is an effective catalyst for the carbonyl insertion (Equation (75)).290 The Zn-catalyzed reaction is applicable to various aldehydes and ketones including aliphatic compounds. In sharp contrast to the Cu-catalyzed reaction, the carbonyl insertion occurs on the less substituted side with high regioselectivity. ZnBr2 most likely serves as electrophilic activation of carbonyl compounds. 

In almost any case of in situ production of iV-acyliminium ions from an aldehyde and a nitrile, a mixture of mineral acid with glacial acetic acid serves as the acid catalyst5-7. Probably the acetic acid is the source of the acylium ions which are necessary for the activation of carbonyl component (see Section III.B.3). 

Recently, a new effective method for electrophilic activation of carbonyl compounds was proposed in order to enable the latter to react with weak nucleophiles such as nitriles102. This method involves the conversion of aldehydes and ketones into highly active acyloxycarbenium ions 163. This new type of carboxonium ions is related to the hydroxycarbenium and alkoxycarbenium ions 105, whose high stability is well known76. 

The in situ generation of an iminium ion from a carbonyl compound lowers the LUMO energy of the system. Iminium catalysis is comparable to Brpnsted- or Lewis acid activation of carbonyl compounds. The LUMO energy is lowered, the a-CH acidity increases, and nucleophilic additions including conjugate additions as well as pericyclic reactions are facilitated (Eq. 34). 

The Lewis acidity of gold, related to relativistic effects, allows not only the coordination to carbon itt-systems but also other functional groups. Among them, the most exploited field is the activation of carbonyls and imines toward different nucleophiles. In these cases, Au(ni) species are many times the catalysts of choice over Au(I). An explanation for this may be found in the thermodynamic coordination preference of AuCft for aldehydes over other moieties such as alkynes, accounting for the fimctional group discrimination in these reactions. Nevertheless, Au(I) is still used in some of the transformations described in this section. 

Examples of gold-catalyzed carbonylation of amines and olefins exist related to the activation of carbonyl. The first case involves the formation of carbamates from anilines and CO in the presence of alcohols or the production of acetamides from aliphatic amines. In these examples, Au(I) (usually [AuCl(PPh3)]) catalysts are preferred. For the carbonylation of olefins, gold(I) carbonyls are prepared in situ in sulfiu ic acid media to afford carboxyhc acids. ... 

Asymmetric Ene Reaction. The enantioselective activation of carbonyl groups with the chiral aluminum reagent also enabled the asymmetric ene reaction of electron-deficient aldehydes with various alkenes (eq 3). In the presence of powdered 4 X molecular sieves, the chiral aluminum reagent can be utilized as a catalyst without loss of enantioselectivity. 

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