Allylic C-0 bond cleavage

Utilizing the processes involving the allylic C-0 bond cleavage promoted by transition metal complexes and combining them with subsequent other processes, such as nucleophilic attack, CO insertion, hydrogenolysis, etc., one can de-... 

The catalytic processes for allylation reactions are composed of the allylic C-0 bond cleavage and nucleophilic attack on the r 3-allyltransition metal complexes formed to give various organic allylation compounds (Scheme 3). 

Combination of the allylic C-0 bond cleavage to form ri3-allyltransition metal complexes with nucleophilic attack by hydridic reagent gives alkenes as the reduction products of allylic compounds. 

The mechanism for the carbonylation of allylic substrates is considered to be composed of the following processes (a) oxidative addition involving allylic C-0 bond cleavage to form r 3-allyltransition metal complexes (b) CO insertion into the metal-allylic bond and (c) nucleophilic attack to liberate carboxylic acid derivatives (Scheme 6). 

As shown above, the anfi-elimination is normally the lower energy process for the allylic C-0 bond cleavage by low-valent palladium complexes. However, if... 

Keywords C-0 bond cleavage, Activation of C-0 bonds, Oxidative addition, Transition metal complexes, Allylic compounds, Esters, Ethers, Anhydrides, Alcohols... 

Other allylic compounds having C-0 bonds, such as allylic carboxylates, carbonates, phosphates, ethers, and alcohols, can also serve as a substrate in such oxidative addition reactions via the C-0 bond cleavage [1]. 

Examples of direct C-0 bond cleavage of allylic alcohols are relatively rare compared to other allylic compounds. Deoxygenation of allylic alcohols by WCl2(PMePh2)4 has been reported [ 17]. Several other examples of the C-O bond cleavage in allylic alcohols have been reviewed previously [1]. 

The experimental results that both branched and linear allylic ethers are obtained in the palladium-catalyzed decarboxylation of branched allylic carbonate indicate occurrence of direct oxidative addition involving the C-0 bond cleavage followed by the nucleophilic attack of the alkoxide liberated on either the substituted or non-substituted terminus of the allylic ligand (Scheme 4) [1]. 

Allylation of aromatic compounds with allylic alcohols and esters through C-0 bond cleavage catalyzed by molybdenum, tungsten, and palladium complexes has been reported recently [25,26]. In addition,molybdenum-catalyzed aromatic substitution with alcohols has been achieved [27]. 

Catalytic conversion of esters through the C-0 bond cleavage has been developed. Pd-catalyzed carbonucleophilic substitution of naphthylmethyl and 1-naphthylethyl esters has been achieved as shown in Eq. 25 [65]. The ease of the naphthylalkyl-0 bond cleavage may be partly due to the stability of the naphthylmethyl entity that can form an r 3-allylic-palladium bond. 

By combination of the C-0 bond cleavage in alkenyloxiranes and CO insertion processes, Pd-catalyzed carbonylation of alkenyloxiranes has been achieved to give unsaturated ester, (3-lactone, diene, and allylic alcohol, depending on the substrate as well as the nature of the substituents (Eq. 43) [98]. 

The metal inserts into the C—H bond and the reaction may be considered as a concerted oxidative addition. This is a rare example of an allyl-hydrido-metal complex being isolated from allylic C—H bond cleavage by the metal. Other examples are the reaction of Mo(0) and Ru(0) complexes with olefins where isolable hydrido-7t-allyl-metal(II) complexes are characterized (vide infra). 

In the palladium-catalyzed carbonylation process, allyl formate, prepared by the reaction of allyl alcohol with formic acid, oxidatively adds to Pd(0) species with the C-0 bond cleavage to give allyl palladium formate. The CO insertion into the allylpalladium bond produces butenoyl palladium formate, which reductively ehminates butenoic formic anhydride with regeneration of the catalytically active Pd(0) species. Spontaneous decarbonylation of the mixed anhydride yields 3-butenoic acid, which isomerizes to 2-butenoic acid [61]. The process to give the butenoic acid proceeds only under CO pressure, suggesting that the CO insertion into the allyl-Pd bond is favored under CO pressure. When the reaction is carried out under normal pressure of CO, decarboxylation of the formate to give palladium hydride takes place. Reductive elimination of the allylpalladium hydride yields hydrogenation product of the allyl moiety [62]. 

C-0 bond cleavage of alkenyl carboxylates such as vinyl and allyl carboxylates can also be achieved by transition metal hydrides. For example, a divalent ruthenium dihydride complex c 5-RuH2(PPh3)4 reacts with vinyl acetate to give c 5-Ru(H)(OAc)(PPh3)4 accompanied by evolution of ethylene (Scheme 3.31)... 

Similar to carboxylates and carbonates, the C-0 bond cleavage of allylic ethers with low-valent metal complexes relatively is well documented (Eq. 3.15). Typical example is demonstrated in the reaction of group 10 metal complexes such as Pd or Ni resulting in the clean allyl-0 bond cleavage [76]. 

The C-0 bond cleavage of alkenyl silyl ethers is known to take place at room temperature by transition metal hydride such as cw-RuH2(PPh3)4, CoH(N2)(PPh3)3 and RhH(PPh3)4 [84]. Since the isomerization of allyl silyl... 

In the C-0 bond cleavage reaction of vinyl epoxides by a palladium complex, formic acid acts as a good proton and hydride donor evolving CO2 (Eqs. 3.24 and 3.25). Zero-valent palladium complex favors the attack at an allylic carbon in an Sn2 manner to give i/ -allylpalladium(II) complex with inversion of configiuation, and formate anion coordinates to the palladium center (Scheme 3.50). Then, decarboxylation of the formate affords palladium hydride, which attacks the ry -allyl moiety from the endo side. Thus, 1,2-addition of hydrogen atoms takes place regioselectively with inversion at the allylic carbon [94]. 

Stoichiometric experiments were conducted with the functionalized alkenes to further explore iron-substrate interactions and gauge relative coordination affinities. Several bis(imino)pyridine iron amine and ketone compounds were isolated and studied using a combination of X-ray diffraction, NMR and Mossbauer spectroscopy and established electronic structures similar to that for ( PDI)Fe(N2)2-In the absence of H2, diallyl ether and allyl ethyl ether underwent facile C-0 bond cleavage and yielded a near equimolar mixture of the corresponding iron allyl and alkoxide complexes [89]. Our group has recently published a comprehensive study on these types of reachons and discovered rare examples of C-0 bond cleavage in saturated esters [89]. 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

Reductive cleavage of a C-0 bond requires activation of the bond such activation can be obtained in different ways. Conjugated carbinols, like allylic and benzylic alcohols, tt-electron-deficient heterocyclic carbinols, and a-hydroxyketones and the ethers of these compounds, may be reductively cleaved. Reductive elimination of two vicinal hydroxyl groups or derivatives thereof may also be possible. These reactions have been exploited in the electrochemical removal of protecting groups [40,41]. 

Allylpalladium(II) complexes are also formed by reaction of a Pd(0) precursor with allyl formates or carbonates. [Pd(PCy3)2] reacts with allylaryl ethers by oxidative addition of the C—O bond , and with A-allyltriethylammonium bromide by C—N bond cleavage ... 

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