Borane-derived catalysts

Asymmetric Diels-AUer reactions The observation that simple acyloxy-boranes such as H2BOCOCH=CH2, prepared by reaction of BH3 with acrylic acid, can serve as Lewis acid catalysts for reactions of the a,P-unsaturated acids with cyclopentadiene (15, 2) has been extended to the preparation of chiral acyloxy-boranes derived from tartaric acid. The complex formulated as 3, prepared by reaction of BH3 with the monoacylated tartaric acid 2, catalyzes asymmetric Diels-Alder reactions of a,P-enals with cyclopentadiene with high enantioselectivity. The process is applicable to various dienes and aldehydes with enantioselectivities generally of 80-97 % ee. 

The excellent enantioselectivity and wide scope of the CBS reduction have motivated researchers to make new chiral auxiliaries [3]. Figure 1 depicts examples of in situ prepared and preformed catalyst systems reported since 1997. Most of these amino-alcohol-derived catalysts were used for the reduction of a-halogenated ketones and/or for the double reduction of diketones [16-28]. Sulfonamides [29,30], phosphinamides [31], phosphoramides [32], and amine oxides [33] derived from chiral amino alcohols were also applied. The reduction of aromatic ketones with a chiral 1,2-diamine [34] and an a-hydroxythiol [35] gave good optical yields. Acetophenone was reduced with borane-THF in the presence of a chiral phosphoramidite with an optical yield of 96% [36]. 

Mono- and bicyclic phosphorus heterocycles 199, 200, 202, and 203 were synthesized starting from the bifunctional phosphorylating agent bis(diisopropylamino)ethynyl phosphine 195 via ring-closing enyne metathesis using 4,5-dihydroimidazol-2-ylidene ruthenium benzylidene complex, as a catalyst. Bicyclic phosphorus oxides 199 were obtained in 66-83% yield, whereas phosphorus borane derivative 202 was isolated in 74% yield <2001TL8231>. 

Moreover, it has been clearly demonstrated that catalyst 18 is converted at elevated temperature to the derivative 19 through ring opening of the heterocyclic ring upon exposure to borane. Derivative 19 appeared to be the effective catalyst in the enantioselective reduction of the ketone by the borane. 

In 1988, Yamamoto and coworkers reported the use of enantiomerically pure (acyloxy)borane (CAB) catalysts (8.16) for the enantioselective Diels-Alder reaction. These catalysts are derived from tartaric acid, and again, their reactions have been particularly selective with a-substituted enals as substrate. The dienophile... 

Yamamoto developed a remarkable boron-derived catalyst for enantioselec-tive Diels-Alder reactions which is easily assembled from monoacylated tartaric acid and borane. Spectroscopic data provided evidence that supports the proposed catalyst structure 144 depicted in Equation 16 [79, 80]. Such chiral (acyloxy)borane (CAB) catalysts have been employed in numerous cyclo-additions with unsaturated aldehydes to afford the corresponding products, such as 145, with high selectivity (98% ee, endo exo > 99 1) [80]. 

The most successful of the Lewis acid catalysts are oxazaborolidines prepared from chiral amino alcohols and boranes. These compounds lead to enantioselective reduction of acetophenone by an external reductant, usually diborane. The chiral environment established in the complex leads to facial selectivity. The most widely known example of these reagents is derived from the amino acid proline. Several other examples of this type of reagent have been developed, and these will be discussed more completely in Section 5.2 of part B. 

The hydride-donor class of reductants has not yet been successfully paired with enantioselective catalysts. However, a number of chiral reagents that are used in stoichiometric quantity can effect enantioselective reduction of acetophenone and other prochiral ketones. One class of reagents consists of derivatives of LiAlH4 in which some of die hydrides have been replaced by chiral ligands. Section C of Scheme 2.13 shows some examples where chiral diols or amino alcohols have been introduced. Another type of reagent represented in Scheme 2.13 is chiral trialkylborohydrides. Chiral boranes are quite readily available (see Section 4.9 in Part B) and easily converted to borohydrides. 

Among the many chiral Lewis acid catalysts described so far, not many practical catalysts meet these criteria. For a,/ -unsaturated aldehydes, Corey s tryptophan-derived borane catalyst 4, and Yamamoto s CBA and BLA catalysts 3, 7, and 8 are excellent. Narasaka s chiral titanium catalyst 31 and Evans s chiral copper catalyst 24 are outstanding chiral Lewis acid catalysts of the reaction of 3-alkenoyl-l,2-oxazolidin-2-one as dienophile. These chiral Lewis acid catalysts have wide scope and generality compared with the others, as shown in their application to natural product syntheses. They are, however, still not perfect catalysts. We need to continue the endeavor to seek better catalysts which are more reactive, more selective, and have wider applicability. 

The dominant factors reversing the conventional ds-hydroboration to the trans-hydroboration are the use of alkyne in excess of catecholborane or pinacolborane and the presence of more than 1 equiv. of EtsN. The P-hydrogen in the ris-product unexpectedly does not derive from the borane reagents because a deuterium label at the terminal carbon selectively migrates to the P-carbon (Scheme 1-5). A vinylidene complex (17) [45] generated by the oxidative addition of the terminal C-H bond to the catalyst is proposed as a key intermediate of the formal trans-hydroboration. 

Stable aryl boronates derived from tartaric acid catalyze the reaction of cyclo-pentadiene with vinyl aldehyde with high selectivity. Chiral acyloxy borane (CAB), derived from tartaric acid, has proved to be a very powerful catalyst for the enantioselective Diels-Alder reaction and hetero Diels-Alder reaction. Scheme 5 23 presents an example of a CAB 73 (R = H) catalyzed Diels-Alder reaction of a-bromo-a,/i-cnal 74 with cyclopentadiene. The reaction product is another important intermediate for prostaglandin synthesis. In the presence of... 

ALKYLALUMINIUM DERIVATIVES, ALKYLBORANES, ALKYLHALOBORANES ALKYLHALOPHOSPHINES, ALKYLHALOSILANES, ALKYLMETALS ALKYLNON-METAL HYDRIDES, ALKYLPHOSPHINES, ALKYLSILANES ARYLMETALS, BORANES, CARBONYLMETALS, COMPLEX ACETYLIDES COMPLEX HYDRIDES, HALOACETYLENE DERIVATIVES HEXAMETHYLNITRATODIALUMINATE SALTS, METAL HYDRIDES NON-METAL HYDRIDES, ORGANOMETALLICS, PYROPHORIC ALLOYS PYROPHORIC CATALYSTS, PYROPHORIC IRON-SULFUR COMPOUNDS PYROPHORIC METALS... 

Chiral 1,2,3-oxazaborolidines simply obtained from a-amino acid derived sulfonamides and borane were first applied in Diels-Alder reactions by Taliasu and Yamamoto228, and Helmchen and colleagues229. Yamamoto prepared catalysts from a-aminobutyric acid derived arylsulfonamides and found that the enantioselectivity of the reaction between... 

S)-(-)-CITRONELLOL from geraniol. An asymmetrically catalyzed Diels-Alder reaction is used to prepare (1 R)-1,3,4-TRIMETHYL-3-C YCLOHEXENE-1 -CARBOXALDEHYDE with an (acyloxy)borane complex derived from L-(+)-tartaric acid as the catalyst. A high-yield procedure for the rearrangement of epoxides to carbonyl compounds catalyzed by METHYLALUMINUM BIS(4-BROMO-2,6-DI-tert-BUTYLPHENOXIDE) is demonstrated with a preparation of DIPHENYL-ACETALDEHYDE from stilbene oxide. A palladium/copper catalyst system is used to prepare (Z)-2-BROMO-5-(TRIMETHYLSILYL)-2-PENTEN-4-YNOIC ACID ETHYL ESTER. The coupling of vinyl and aryl halides with acetylenes is a powerful carbon-carbon bond-forming reaction, particularly valuable for the construction of such enyne systems. 

Borane reduction catalyzed by chiral oxazaborolidines (CBS reduction, CBS = Corey, Bakshi, and Shibata) exhibits excellent enantio- and chemoselectiv-ity for a wide variety of ketonic substrates (Figure 1.27). This reaction was originally developed as a stoichiometric system consisting of diphenylvalinol and borane, ° but was later extended to a useful catalytic method. Because of the high efficiency of this reaction, many chiral oxazaborolidines have been synthesized from p-amino alcohols.Among them the prolinol-derived oxazaboro-lidine is one of the most widely used catalysts. ... 

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