Aluminum porphyrin

Endo, M., Aida, T., and Inoue, S., Immortal polymerization of e-caprolactone initiated by aluminum porphyrin in the presence of alcohol. Macromolecules, 20, 2982-2988, 1987. 

Equations 1 to 3 show some of fixation reactions of carbon dioxide. Equations la and lb present coupling reactions of CO2 with diene, triene, and alkyne affording lactone and similar molecules [2], in a process catalyzed by low valent transition metal compounds such as nickel(O) and palladium(O) complexes. Another interesting CO2 fixation reaction is copolymerization of CO2 and epoxide yielding polycarbonate (equation 2). This reaction is catalyzed by aluminum porphyrin and zinc diphenoxide [3],... 

Relatively few organometallic aluminum porphyrin complexes have been reported, unlike the heavier Group 13 elements for which more extensive series of compounds have been reported. However, the reaction chemistry of the aluminum porphyrins has been much more extensively studied and exhibits features not replicated by the heavier elements. For this reason the aluminum porphyrin complexes are discussed separately. 

Another very important visible light-initiated reaction of alkyl aluminum porphyrins is their 1,4-addition to alkyl methacrylates to produce ester enolate species [Eq. (4)]. This enolate then acts as the active species in the subsequent polymerization of the acrylate monomer. For example, Al(TPP)Me acts as a photocatalyst to produce polymethylmethacrylate with a narrow molecular weight distribution in a living polymerization process [Eq. (4)]. Visible light is essential for both the initiation step (addition of methylmethacrylate to Al(TPP)Me) and the propagation... 

Fig, 4, Schematic illustration of high-speed living polymerization of methacrylate esters accelerated by steric separation of the aluminum porphyrin nucleophile and bulky Lewis acid. ... 

Aluminum porphyrins first came to attention with the discovery that the simple alkyl complex Al(TPP)Et was capable of activating CO2 under atmospheric pressure. Both irradiation with visible light and addition of 1-methylimidazole were required for the reaction, which was proposed to proceed by initial coordination of the base to aluminum. The aluminum porphyrin containing direct product of CO2 insertion was not isolated, but was proposed on the basis of IR data to be (TPP)A10C(0)Et, which was then treated with HCl gas, presumably liberating propanoic acid, subsequently isolated as the butyl or methyl ester after reaction with 1-butanol or diazomethane, respectively [Eq. (5)]. Insertion of CO2 into the Al—C bond of an ethylaluminum phthalocyanine complex has also been reported. ... 

Aluminum porphyrins with alkoxide, carboxylate, or enolate can also activate CO2, some catalytically. For example, Al(TPP)OMe (prepared from Al(TPP)Et with methanol) can bring about the catalytic formation of cyclic carbonate or polycarbonate from CO2 and epoxide [Eq. (6)], ° - and Al(TPP)OAc catalyzes the formation of carbamic esters from CO2, dialkylamines, and epoxide. Neither of the reactions requires activation by visible light, in contrast to the reactions involving the alkylaluminum precursors. Another key difference is that the ethyl group in Al(TPP)Et remains in the propionate product after CO2 insertion, whereas the methoxide or acetate precursors in the other reactions do not, indicating that quite different mechanisms are possibly operating in these processes. Most of this chemistry has been followed via spectroscopic (IR and H NMR) observation of the aluminum porphyrin species, and by organic product analysis, and relatively little is known about the details of the CO2 activation steps. 

Both CO2 activation and enolate formation are combined in the preparation of malonic acid derivatives. The reaction of CO2 with methacrylic esters or methacry-lonitrile and under visible light irradiation produced the corresponding aluminum porphyrin malonate complex. When diethylzinc was added to this system, Al(TPP)Et could be regenerated by axial ligand exchange reactions, and the malonic acid derivatives were formed catalytically with respect to the aluminum porphyrins in a one-pot photosynthetic route (Scheme 1). The first step in this... 

The CO2 activation reactions seen for aluminum porphyrins are also observed for In(Por)Me (Por = OEP, TPP), which will insert CO2 in the presence of pyridine and under irradiation by visible light to give the acetato complex In(Por)OC(0)Me. The indium acetato product has been characterized by X-ray crystallography, whereas in the aluminum complex it was observed only by spectroscopy. An alternative synthesis of the acetato complex is by treatment of ln(Por)Cl by alumina and water, followed by acetic acid. For the indium and... 

Although photochemically induced cleavage of Al—C bonds in the aluminum porphyrin complexes has been exploited in several applications, relatively little is known about the intimate mechanism of this process. Similar reactivity is observed for the organo-gallium and indium porphyrins, and for these elements... 

All three hydroxo species [M(OEP)(Me)(OH)] (M = P, As, Sb]) are sufficiently acidic to react with the aluminum porphyrin complex Al(OEP)Me, which is known to eliminate methane on reaction with protic reagents. Three novel binuclear... 

Aluminum porphyrins (Z = Cl, OR, SR) also initiate living polymerizations of methacrylates and acrylates without the need for low temperatures [Aida and Inoue, 1996 Inoue, 2000 ... 

Although anionic polymerization of cyclic ethers is generally limited to oxiranes, there are reports of successful oxetane and tetrahydrofuran polymerizations in the presence of a Lewis acid. Aluminum porphyrin alone does not polymerize oxetane, but polymerization proceeds in the presence of a Lewis acid [Sugimoto and Inoue, 1999]. Similarly, THF is polymerized by sodium triphenylmethyl in the presence of a Lewis acid such as aluminum alkoxide [Kubisa and Penczek, 1999]. The Lewis acid complexes at the ether oxygen, which weakens (polarizes) the carbon-oxygen bond and enhances nucleophilic attack. 

Aluminum-Porphyrin/Lewis Acid Catalyst System... 

PO proceeded in a living manner to yield highly regioregular polyethers with narrow MWDs. These authors also developed the immortal polymerization of epoxides where polymers with narrow MWDs were obtained with the number of polymer chains exceeding the number of initial aluminum-porphyrin complexes (Scheme I). The key in the immortal polymerization is a reversible chain transfer, which is much more rapid than the chain propagation. In the presence of an alcohol (R OH) as a chain-transfer reagent, an aluminum-porphyrin complex with a growing species reacts with R OH reversibly, so that the polymerization takes place from all the molecules of aluminum-porphyrin complex and R OH. 

Aluminum-porphyrin complex lb with an alkoxide ligand also demonstrates the same reactivity as la in the presence of only 0.1 mol.% of 2a. The polymerization rate with lb/2a catalyst system is dependent on the concentration of 2a in the range from 0.025 to 2.5 mol.%, the increase of 2a results in more rapid polymerization. On the other hand, molecular weight and the number of polymer chains are independent of the molar ratio of 2a to... 

Table 1 Polymerization of terminal epoxides catalyzed by aluminum-porphyrin complexes...

Accelerated Living Polymerization of Methacrylonitrile with Aluminum Porphyrin Initiators by the Activation of Monomer... 

Monomers with Aluminum Porphyrin-Organoaluminum Compound Systems. 79... 

Lewis Acid Driven Anionic Polymerization of a Monomer with High Cationic PolymerizabiHty Ring-Opening Polymerization of Oxetane with Aluminum Porphyrin in the Presence of a... 

Of key importance in the Lewis acid promoted living anionic polymerization of methacrylic esters with aluminum porphyrin is how to suppress the undesired reaction between the nucleophile (2j ) and the Lewis acid, leading to termination of polymerization (Fig. 11). As mentioned in previous sections, one of our approaches was to make use of sterically crowded Lewis acids such as methyla-luminum bis(ort/zo-substituted phenolates). This section focuses attention on the steric bulk of the nucleophile component (2 ), by using strategically designed aluminum porphyrins and some other methacrylates, for the purpose of understanding the scope and limitation of this method (Fig. 12). 

Living Polymerization of Methacrylic Ester with Aluminum Porphyrin-Organoboron Compound Systems [19]... 

For Lewis acid promoted living polymerization of MMA with (TPP)AlMe (1,X= Me) as initiator, a photoinitiation prior to the addition of the Lewis acid is required. This is because (1) 1 (X=Me) without irradiation does not have the ability to initiate the polymerization even in the presence of Lewis acid, and (2) all-at-once polymerization by direct irradiation of a mixture of MMA, 1 (X=Me), and the Lewis acid results in the formation of a relatively broad MWD PMMA with Mn much higher than expected. In this sense, the procedure using 1 (X= Me) as initiator is not convenient for practical application. In this section, we report on aluminum porphyrins with various axial ligands which were tested as initiators in order to realize a more convenient, one-shot high-speed living polymerization of methyl methacrylate with no need for irradiation with visible light. 

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