Lewis acid co-catalysts

Scheme 77.8 Strategies to prepare cationic polymerization catalysts (a) using oxide supports that have high Bransted and Lewis acidity (b) the addition of a co-catalyst to a neutral supported species (c) modification of the surface with Lewis acid co-catalysts prior to the grafting of the organometallic species [91, 96, 98].

Finally, a few examples of the Morita-Baylis-Hilhnan reaction are provided, where a silyl species functions as a Lewis acid co-catalyst. These examples could have been presented in the previous section about silyl cation-based catalysts. Since the enantiomeric induction originated in the present examples from a Lewis base, we have listed these examples in this section. 

The low reaction rates usually associated with the MBH reaction can be increased either by pressure [15a, 22, 34], by the use of ultrasound [35] and micro-wave radiation [14a], or by the addition of co-catalysts. Various intra- or inter-molecular Lewis acid co-catalysts have been tested [26, 36, 37] in particular, mild Bronsted acids such as methanol [36, 57d], formamide [38], diarylureas and thioureas [39] and water [27a, 40] were examined and found to provide an additional acceleration of the MBH reaction rate (Table 5.1). 

The yields and regioselectivity of the reactions of substituted methylenecyclopropanes with 2-cy-clopentenone are improved with the use of triethylborane as a Lewis acid co-catalyst (equations 95, 96). No reaction with cyclopentenone is observed in the case of (82) without this co-catalyst. ... 

In the first reported direct A -carbonylation of nitroaromatics to isocyanates, simple Pd- or Rh-based systems were used to catalyze the reaction of aromatic mononitro compounds with carbon monoxide [11, 12]. Later, it became possible to work without the drastic reaction conditions that had been required initially, by using Lewis acid co-catalysts [13], Various catalysts and catalyst mixtures, normally based on Ru, Rh, or Pd complexes with co-catalysts, were described in numerous patents and publications [1, 3, 14—16], The careful choice of the composition of the triad consisting of metal salt, co-catalysts and ligand (preferably aromatic amines) led to efficient catalyst systems [14a-e] for the direct reductive carbonylation process. A quite active Pd-phenanthroline-H system with noncoordinating carboxylic acids such as 2,4,6-trimethlybenzoic acid as proton source is worth mentioning [14 d]. 

Rather interestingly, radical cations have also been identified in styrene polymerizations catalysed by more conventional Lewis acid/co-catalyst systems [BF3/0(C2Hs)2, AlCla/PhOH, BFs/PhOH] by employing 2,4,6-tri-t-butyl-nitrosobenzene as a trap. One wonders once again therefore if the mechanisms which are generally accepted to operate do indeed represent a total picture. 

Huser and Perron have extended this work to the isomerization of 2-methyl-3-butenenitrile (2M3 BN) to 3-PN (isomerization step Eq. (6) 92% yield) [17]. This patent mentions the use of iron and palladium catalysts but does not provide examples beyond nickel. In other work these same inventors discuss the use of other water-soluble ligands such as those containing carboxylate, phosphate, and alkyl-sulfonate substituents [18], while also exploring a wide range of Lewis acid co-catalysts for the addition of HCN to 3-pentenenitrile (Eq. 7) [19]. In general, the addi-... 

The q -phenol complex undergoes conjugate addition at C4 with a variety of Michael acceptors (Fig. 7), including those with p substituents [44]. In most cases, the addition reaction is accomplished with an amine base as catalyst (see 25). Less reactive electrophiles, such as methyl acrylate or acrylonitrile, require a Lewis acid co-catalyst (e.g., 24). An example of the versatility of this reaction is shown in Fig. 7, where the aromatic steroid p-estradiol (26) is complexed (27) and subsequently alkylated exclusively at CIO (i.e.,para) at -40 °C. Since the osmium preferentially binds the a face of the steroid in 27, conjugate addition occurs from the p face, yielding the stereochemistry found in testosterone [26]. The overall yield of this transformation after decomplexation of the dienone product 29 is 69%. 

Fig. 1 Boron Lewis acid co-catalyst effect in hydrogenations of 1-hexene catalyzed by 17a at 23°C under 10 bar H2...

In spite of the extremely efficient catalytic performance exhibited by the transphosphine Re(I) mononitrosyl complexes in the presence of a Lewis acid co-catalyst, the system was seen to suffer versatility in hydrogenation catalyses, exactly due to the employment of Lewis acids that prohibited the hydrogenation of alkenes bearing functional groups, such as carbonyl functions. The well-defined diphosphine complexes 20c-f and 21 turned out to overcome this disadvantage. 

Lewis acids/co-catalyst combinations (TiCl4, AICI3, BF3/H2O) stable carbenium cations generated by the inifo- mediod Monomers 1,1 -substituted alkenes, styrenes, vinyl-ediers,... 

In 2001, we extended this chemistry to the use of multinucleus BINOL-aluminum catalysts, which could be regarded as LLA catalysts, for a number of Diels-Alder reactions (Fig. 10) [51]. These complex structures have been elucidated on the basis of NMR studies, as well as gas generation measurements. Such kind of aluminoxide catalysts can be viewed as chiral versions of methyl aluminoxane (MAO), a very powerful Lewis acid co-catalyst for cationic polymerization of olefins. We thus believed further extension of this chemistry should be very promising. 

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