Lewis acid, Bulky aluminum

The smooth palladium-catalyzed conversion of aryl triflates into aryl bromides and chlorides has been described by the Buchwald group in 2010. The protocol makes use of a bulky Lewis acid (triisobutyl aluminum) combined with a PEG as a phase transfer catalyst (Scheme 5-171). Fluorides can also be synthesized from triflates, also shown by the Buchwald group. ... 

Maruoka, K. Synthetic Utility of Bulky Aluminum Reagents as Lewis Acid Receptors. In Lewis Acid Reagents-, Yamamoto, H., Ed. Oxford University Press Oxford, 1999 pp 5-29. 

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. ... 

Regioselective aldol addition of a,/ -unsaturated aldehydes has been achieved using a method in which the enal and the carbonyl acceptor are treated first with a bulky Lewis acid aluminum /r/.v-(2,6-diphenoxidc), and then LDA is added. 

Catalytic activity of the aluminum-Schiff base system is dramatically enhanced by adding a bulky Lewis acid (Table 2). Inoue et al. reported that a combination of 3 with 2c led to over 1000 times acceleration in the polymerization of PO at room temperature compared with the polymerization in the absence of 2c.The resulting polymers have narrow MWDs, molecular weights close to those estimated, assuming that every molecule of 3 forms one polymer chain. The same accelerating effect of 2c is also demonstrated in the polymerization of PO by using aluminum-phthalocyanine and aluminum-tetraazaannulene complexes, 4 and 5, which exhibit very low catalytic activities without 2c. 

Selective alkylation of the y position of a,/3-unsaturated esters, aldehydes, or ketones can be achieved if a sterically demanding Lewis acid is used to coordinate to the carbonyl group and inhibit a-alkylation by steric shielding [123, 125, 126] (Scheme 5.11). This method not only results in high regioselectivity but also enables highly stereoselective aldol-type additions to be performed in good yields, even with sensitive substrates, such as a,/3-unsaturated aldehydes [126]. Thus, when two dia-stereotopic y positions are available, the addition of a bulky aluminum phenolate leads to the clean formation of the Z-alkene (second and third examples, Scheme 5.11). 

A two-coordinate base-free alumenium ion (42) was prepared by Wehmschulte using bulky terphenyl groups to provide steric protection of the aluminum center and protect against aryl transfer. There is no interaction of the anion with the cationic aluminum center in the solid state. The shortest Al-F distance is 5.498 A. There are, however, contacts between the Lewis acidic aluminum and carbons of the mesityl groups as short as 2.353(1) A. The C-Al-C angle in (42) is 159.17(5)°. 

Enolates, which are associated with an optically active bulky Lewis acid (equation 11), can be alkylated in a regio- and enantioselective manner as reported by Yamamoto and coworkers (Table 5). After precomplexation of the ketones 31a or 31b with aluminum tris((/ )-l-Q -naphthyl-3-phenylnaphthoxide) (32 ) in toluene and treatment with a THF-pentane solution of LDA/f-BuLi, addition of TBSOTf caused ring-opening of THE and butylation at the more congested a-site of the enolate, to give 32 predominantly with good enantiomeric excesses. Thereby, the productivity of the reaction depends strongly on the molar ratio of the ketone, the Lewis aeid (32 ) and the base used. 

How can highly selective processes be developed using Lewis-acid reagents I would like to offer one typical example in this field of research—the chemistry of bulky aluminum reagents (Eqs 4 and 5) [10]. 

Benzaldehyde and the bulky aluminum reagent ATPH, for example, form a relatively stable complex which when exposed to an alkyllithium reagent from outside the system generates the cyclohexadiene derivative in high yield. The reaction proceeds not via the usual 1,2-addition pattern but through the unique 1,6-addition process, which is very difficult in the absence of such a bulky Lewis-acid catalyst (Eq. 6) [11]. 

Based on statistical analyses it may be reasonable to propose that this structure reflects the idealized or low energy mode of coordination to carbonyls for aluminum-centered Lewis acids. It would be interesting to see how distortions away hrom such an arrangement may come about Imposition of steric congestion through bulky substituents is one way to address this question. Ibe crystal suucture of (2,6-di-f-butyl-4-methyl)phenoxydiethylaluminum/methyl toluate was recently reported (Figure 22). ... 

It may be recalled that an opposite stereochemical result is obtained by employing the bulky aluminum reagents MAD and MAT. This observation has been explained by invoking out-of-plane complexation of the Lewis acid in a direction which would prevent equatorial attack (Figure 48). The X-ray crystal structure of methyl toluate complexed with a bulky aluminum Lewis acid is fully consistent with tius model7 However, it is worth mentioning that a six-membered transition state, perhaps involving [Me2( ArO)Al] Li, has not been considered as an alternative mechanism. 

Furthermore, saturated aldehydes are somewhat less basic than saturated ketones or esters, resulting in reversible complexation even with bulky aluminum reagents. However, whether the equilibrium [Lewis acid -i- base Lewis acid-base complex] is reversible or irreversible, the selective functionalization of more labile or sterically less-encumbered aldehydes is facile using bulky or mild Lewis acids. 

Few examples have been reported demonstrating enantioselective cyclization methodology. One known example, however, is similar to the diastereoselective cyclization of 175, which uses a menthol-derived chiral auxiliary and a bulky aluminum Lewis acid (see Eq. (13.55)). The enantioselective variant simply utilizes an achiral template 188 in conjunction with a bulky chiral binol-derived aluminum Lewis acid 189 (Eq. (13.59)) [75]. Once again the steric bulk of the chiral aluminum Lewis acid complex favors the s-trans rotamer of the acceptor olefin. Facial selectivity of the radical addition can then be controlled by the chiral Lewis acid. The highest selectivity (48% ee) was achieved with 4 equivalents of chiral Lewis acid, providing a yield of 63%. 

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