Bulky Lewis Acids

The rate of polymerization may be dramatically accelerated upon addition of a bulky Lewis acid. For example, addition of (184) to a sample of living PMMA generated by irradiation of (181)/MMA causes an increase in polymerization rate by a factor of >45,000.444 The dualcomponent systems (181)/(184), and (181)/(185), have been used to prepare monodisperse, ultra-high-molecular-weight samples of PMMA (Mn > 106, Mw/Mn= 1.2).445... 

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. 

Immortal polymerization of epoxides with la and an alcohol is also accelerated by co-use of bulky Lewis acid 2a. The polymerization of PO with la/2-propanol system ([PO]/[la]/[2-propanol] = 1000/1/49) in the presence of 2a ([PO]/[2a] = 1000/1) proceeds rapidly to achieve 86% conversion in 1.5 h, while the polymerization in the absence of 2a requires 380 h to reach 84% conversion (Table 1). The polyether produced in the presence of 2a has an of 900 gmoP and an MJM of 1.10, which indicates that almost all of la and 2-propanol participate in the initiation of the polymerization. Other protic chain-transfer reagents, such as methanol, benzyl alcohol, and 4-/ r/-butylphenol, are also applicable to the high-speed immortal polymerization to give similar results as 2-propanol. As a substrate, ECH is also employable. Polymerization of ECH ([EGH]/[la]/[2-propanol]/[2a] = 1000/1/49/1) gives a polymer with and/n of 1100gmol close to the value estimated from the conversion and [PO]/([la] + [2-propanol]) ratio, and a narrow M IM of 1.10, while the conversion is lower than the case of PO. 

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. 

Sometimes the Lewis acid that coordinates with the carbonyl oxygen is sufficiently bulky that it seriously influences the stereochemistry of attack. Sometimes these reaction products, which seem opposite of the expected Cram Rule analysis, are termed "anti-Cram" products. Compare the "normal" situation with the influence of a sterically bulky Lewis acid ... 

MAD is a bulky Lewis acid used to complex with functional groups to provide a steric discrimination to a number of organic transformations. 

Figure 3.34 a Formation of the various products in the second hydrocyanation step, co-catalyzed by Lewis acids (LA) b bulky Lewis acids such as BPh3 shift the linear/branched product equilibrium toward the desired linear product. 

Of interest is the fact that a Lewis acid with bulky substituents, such as methylaluminium di(2,6-di-t-butyl-4-methylphenoxide), also has an accelerating effect on the polymerisation of /1-lactone, the extent of acceleration being dependent on the mode of lactone ring cleavage. The polymerisation of /i-butyrolactone in the presence of (tpp)AlOMe [scheme (9)] was slower than that in the presence of (tpp)AlCl [scheme (10)], but the accelerating effect of the bulky Lewis acid was more significant for the (tpp)A10Me catalyst [125]. Thus, the acceleration effect is considered [125] to be due to the coordination of... 

T. Ooi, K. Maruoka, Exceptionally Bulky Lewis Acidic Reagent, MAD, Rev. Heteroatom Chem., 1998,... 

Cyclic nitrones generated by [4+ 2]-cycloaddition of nitroalkenes undergo various, synthetically very valuable reactions. Thus, Denmark et al. have developed an elegant access to different enantiopure, 3- and 3,4-substituted pyrrolidine derivatives by reductive ring contraction of the cyclic nitrone resulting from a hetero Diels-Alder reaction [389,390]. Upon reaction of -2-nitrostyrene 4-51 with the chiral enol ether 4-52 in the presence of the bulky Lewis acid MAPh (4-53), three diastereomeric cycloadducts 4-54, 4-55 and 4-56 were formed. Hydrogenolysis of the main product 4-54 yielded the desired pyrrolidine 4-57 in excellent optical purity and allowed nearly quantitative recovery of the chiral auxiliary (Fig. 4-12) [391]. It is noteworthy that the nature of the Lewis acid catalyst, especially its steric demand, decisively influences the stereochemical course of such cycloadditions [392]. 

Similarly, precomplexation of the unsymmetrical ketone by a bulky Lewis acid prior to deprotonation results in the inversion of the regioselectivity by alkylation at the more hindered site, probably via a preferred coordination with one of the lone pairs anti to the more hindered side of the ketone (Scheme 84)396. Interestingly, an aldehyde in the same conditions is not deprotonated (vide infra). 

A communication by Yamamoto and co-workers details the use of a bulky Lewis acid additive which serves to direct the addition in a 1,4-fashion rather than the typical 1.2-fashion, e.g. formation of 3. 

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. 

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]. 

The exceptionally bulky Lewis acid MAD has a distinct steric effect on stereoselectivity in the Diels-Alder reaction of cyclic dienes and a,/3-unsaturated aldehydes, as exemplified by the MAD-mediated highly exo-selective cycloaddition of methacrolein and cyclopentadiene (Sch. 130) [169],... 

Addition of MeLi to (1) at -78 C in diethyl ether requires 60 min to reach completion and affords a 6S 3S mixture of axial equatorial alcohols (Figure 1). - In the presence of LiC104, however, the same addition is complete within 5 s and proceeds with higher stereoselectivity to give predominantly the axial alcohol in a 92 8 ratio. Precomplexation of the ketone with the bulky Lewis acid, methylaluminum(2,6-di-r-butyl-4-methylphenoxide) (MAD), on the other hand, affords the equatorial alcohol almost exclusively. ... 

B). Although facial differentiation would not be feasible in this case due to the planar alignment, the Lewis acid will be syn or anti to particular substituents (R ). The third, a bent non-planar mode of bonding results from movement of the metal out of the carbonyl n nodal plane (C), which is often adopted by the bulky Lewis acids. The last mode is a coordination of metal to the C=0 n bond, where the carbonyl is formally the donor, but back-bonding into the C=0 n orbital occurs (D), This mode has been reported for transition metal complexes [5J. The metal center is... 

Recognition of Carbonyl Substrate with Bulky Lewis Acid... 

Table 2-1. The X-ray crystal structure properties of bulky Lewis acid-base complexes. ...

---