Boron catalysts

Perfluoroallyl fluorosulfate is prepared by the treatment oiperfluoropropene with sulfur tnoxide m the presence of boron catalysts [2, 3, 4, 5, 6, 7] (equation 2) Perfluoroisopropyl allyl ether reacts similarly to give 58% polyfluoroallyl fluorosulfate in a cis/trans ratio of 6 4 [S] Sultones are the exclusive products without catalyst. Polyfluoroolefins such as 2-hydropentafluoropropylene [9], (2,3-dichloropropyl)tri-fluoroethylene [70], perfluoropropene [2, i], perfluoroisopropyl alkenyl ethers [S], and acyclic polyfluoroallyl ethers [77] undergo sulfur trioxidation to regioselectively produce the corresponding P-sultones in high yield... 

A series of chiral boron catalysts prepared from, e.g., N-sulfonyl a-amino acids has also been developed and used in a variety of cycloaddition reactions [18]. Corey et al. have applied the chiral (S)-tryptophan-derived oxazaborolidine-boron catalyst 11 and used it for the conversion of, e.g., benzaldehyde la to the cycloaddition product 3a by reaction with Danishefsky s diene 2a [18h]. This reaction la affords mainly the Mukaiyama aldol product 10, which, after isolation, was converted to 3a by treatment with TFA (Scheme 4.11). It was observed that no cycloaddition product was produced in the initial step, providing evidence for the two-step process. 

Chiral boron(III) complexes can catalyze the cycloaddition reaction of glyoxy-lates with Danishefsky s diene (Scheme 4.18) [27]. Two classes of chiral boron catalyst were tested, the / -amino alcohol-derived complex 18 and bis-sulfonamide complexes. The former catalyst gave the best results for the reaction of methyl glyoxylate 4b with diene 2a the cycloaddition product 6b was isolated in 69% yield and 94% ee, while the chiral bis-sulfonamide boron complex resulted in only... 

Scheeren et al. reported the first enantioselective metal-catalyzed 1,3-dipolar cycloaddition reaction of nitrones with alkenes in 1994 [26]. Their approach involved C,N-diphenylnitrone la and ketene acetals 2, in the presence of the amino acid-derived oxazaborolidinones 3 as the catalyst (Scheme 6.8). This type of boron catalyst has been used successfully for asymmetric Diels-Alder reactions [27, 28]. In this reaction the nitrone is activated, according to the inverse electron-demand, for a 1,3-dipolar cycloaddition with the electron-rich alkene. The reaction is thus controlled by the LUMO inone-HOMOaikene interaction. They found that coordination of the nitrone to the boron Lewis acid strongly accelerated the 1,3-dipolar cycloaddition reaction with ketene acetals. The reactions of la with 2a,b, catalyzed by 20 mol% of oxazaborolidinones such as 3a,b were carried out at -78 °C. In some reactions fair enantioselectivities were induced by the catalysts, thus, 4a was obtained with an optical purity of 74% ee, however, in a low yield. The reaction involving 2b gave the C-3, C-4-cis isomer 4b as the only diastereomer of the product with 62% ee. 

As for boron catalysts, the aluminum catalysts have exclusively been applied for the inverse electron-demand 1,3-dipolar cycloaddition between alkenes and nitrones. The first contribution to this field was published by j0rgensen et al. in... 

Scheme 10.35 Aromatic Claisen rearrangements of catechol mono allylic ethers with sulfur-containing boron catalyst.

Scheme 10.36 Claisen rearrangements of difluorovinyl allyl ethers with sulfurcontaining boron catalyst.

Another cyclic boron catalyst K, derived from ra x-2-aminocyclohexanc-methanol, can be prepared with a quaternary nitrogen that enhances activity.101 This particular catalyst is not very stable, but it is highly active. 

Chiral Boron Catalysts for Diels-Alder reaction... 

Chiral boron catalysts had already been widely used in a variety of reactions before they were applied in Diels-Alder reactions220. Boron catalysts were first employed in the Diels-Alder reactions of quinones with electron-rich dienes. Kelly and coworkers221 found that stoichiometric amounts of a catalyst prepared from BH3, acetic acid and 3,3 -diphenyl-l,l/-bi-2-naphthol (344) catalyzed the reaction of 1-acetoxy-l,3-butadiene (341) with juglone (342) to afford cycloadduct 343 with 98% ee (equation 96). The reaction was supposed to proceed via a spirocyclic borate complex in which one face of the double bond of juglone was effectively shielded from attack by the diene. 

A similar approach was followed by Yamamoto and colleagues222. A chiral boron catalyst prepared from trimethyl borate and various (l ,l )-tartaric diamides 347 effectively catalyzed the cycloaddition of juglone to 1-triethylsilyloxy-1,3-butadiene (345) to give cycloadduct 346 with high enantioselectivity (equation 97). 

The application of chiral boron catalysts in the cycloadditions of ,/3-unsaturated aldehydes and acrylic acid derivatives has been investigated most. 

Yamamoto and colleagues developed achiral boron catalysts 379 and 380a-b derived from monoacylated tartaric acid and BH3 -THF as shown for 379 in equation 112. The cycloaddition of cyclopentadiene to acrylic acid (381) afforded endo 382 with 78% ee and 93% yield when catalyst 379 was employed (equation 113)239. 

Recently, silica supported nickel-boron catalyst was tested in the hydrogenation of cyclopentadiene and was found to be selective in giving cyclopentene47. 

Another semimetal is boron, which has been used for a long time as a Lewis acid, e.g. BFj, and of which enantiopure derivatives have been applied very successfully. Asymmetric boron catalysts have been reviewed [20-23] and will not be a part of this article. 

Fig. n.18 Critical friction energy of mixtures of AP + liquid organoiron and AP + boron catalysts is decreased with increasing mass fraction of these catalysts. 

Fig. n.19 Critical friction energy is decreased when the critical ignition temperature is lowered for both or-ganoiron and boron catalysts. 

Ashikari, N. Polymerization and copolymerization of olefins with trialkyl-boron catalysts. J. Polymer Sci. 28, 250—252 (1958). 

Acetaldehyde acetals are produced in 80-90% yields by the addition of primary aliphatic alcohols to vinyl acetate in the presence of an acidic mercury-boron catalyst. In a similar manner, acetone ketals are produced from isopropenyl acetate. ... 

Most amino acids are barely soluble in non-aqueous solvents. Nevertheless, their lactams can be prepared by this technique under heterogeneous conditions. For example, when 6-aminocaproic acid and 1 mol % boron catalyst 3,4,5-F3C6H2B(OH)2 are suspended in xylene under reflux the solid slowly dissolves and caprolactam is formed in 93 % yield. 

The process is quite general for simple dienes and aldehydes. For example, the reaction of acrolein with cyclopentadiene, cyclohexadiene, or 2,3-dimethyl-l,3-butadiene gives cycloadducts with 8(F-84 % ee and exolendo = 12/88-< 1/99. The a-substituent on the dienophile increases the enantioselectivity (acrolein compared with methacro-lein). When there is /3-substitution in the dienophile, as in crotonaldehyde, the cycloadduct is almost racemic. On the other hand, for a substrate with substituents at both a and ji positions, high ee is observed, as for 2-methylcrotonaldehyde and cyclopentadiene (90 % ee, exolendo = 97/3). The active boron catalyst is beheved to have the structure shown in Eq. (8), with a five-membered ring and a free carboxyl group. The latter seems not to be crucial for the enantioselectivity because eomparable results are obtained when the carboxylic group is transformed into an ester. 

Although the asymmetric aldol reaction of benzaldehyde and di ketene has been reported with a catalyst generated from di-iso-propyl tartrate and iso-propanol, low induction and low yields were observed for the d-hydroxyl-y5-keto ester 27 [8], Low induction was also observed for aldol reactions mediated by chiral aluminum catalysts generated from a-amino acids [9]. These types of catalyst have been very successful when employing boron as the Lewis acid, as illustrated in the aldol reaction of ketene acetal 10 with the boron catalyst 31 derived from (5)-valine (Sch. 4) [9,10]. Catalysts derived from A-tosyl-(5 )-valine and Et2AlCl and i-BuyAl were relatively ineffective (< 15 % ee) [9]. 

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