Chiral Taddols

A frequently used catalytic system used for the catalytic enantioselective carbo-Diels-Alder reaction of N-alkenoyl-l,3-oxazolidin-2-one 4 is the chiral TADDOL-Ti(IV) 6 [14] complexes (Scheme 8.2 see Ghapter 1 in this book, by Hayashi) [15]. 

An X-ray structure of the complex formed between 3-cinnamoyl-l,3-oxazohdin-2-one and a chiral TADDOL-Ti(IV) complex (see Chapters 1 and 6 by Hayashi and Gothelf, respectively) has been characterized [16]. The structure of this complex has the chiral TADDOLate and cinnamoyloxazohdinone ligands coordinated to titanium in the equatorial plane and the two chloride ligands in the axial plane and is similar to A in Fig. 8.8. The chiral discrimination was proposed to be due to... 

In contrast to the limited success with vinyl sulfides as components of [2 + 2] cycloadditions, allenyl sulfides show wide applicability. As illustrated in Scheme 8.91, Lewis acid-catalyzed [2 + 2] cycloadditions of l-trimethylsilyl-l-methylthio-1,2-propadiene (333) with a variety of electron-deficient olefins 336 provide cycloadducts 337 with excellent regioselectivity but with moderate stereoselectivity [175c], Nara-saka and co-workers reported the first Lewis acid-catalyzed asymmetric [2 + 2] cycloaddition of C-l-substituted allenyl sulfides 319 with a,/3-unsaturated compounds 338 using a chiral TADDOL-titanium catalyst. The corresponding cycloadducts 339 were obtained with 88-98% ee, but a low level of trans/cis selectivity (Scheme 8.92) [169,175d[. 

The BIPOLato-Ti-TADDOLato catalysts prepared by addition of BIPOL and TADDOL to Ti(0 Pr)4, catalyze methylation with an achiral methyltitanium reagent to give highly enantiomerically pure methylcarbinol. Since the sterically bulky 3,3 -substituents leads to an increase in enantioselectivity, the chirality of BlPOLato-Ti(0 Pr)2 catalyst 30 can be dynamically controlled by the chiral TADDOL moiety (Scheme 8.25). 3,3 -Dimethoxy derivative affords complete enantioselectivity (100% ee), while the moderate enantioselectivity is obtained with the parent BIPOL (73% ee). 

Using these optimized conditions, the conjugate addition of 45 to the malonates 49 in the presence of the solid base provided access to the Michael adducts in good yields and high diastereoselectivities. The removal of the chiral TADDOL auxiliary could be accomplished without epimerization or racemization in this case, too. After esterification of the very polar acid intermediates with diazomethane the corresponding dimethyl esters 52 were obtained with yields of 72-94% and enantiomeric excesses ee of 84-94% (Scheme 1.1.13). 

A related approach has recently been reported by Belokon and Kagan et al. These workers used chiral TADDOL-type diols, derived from tartaric acid and 2-amino-2 -hydroxy-1,1 -binaphthyl (NOBIN), as catalysts to obtain yields of up to 95% and enantioselectivity up to 93% ee [59-61], The catalytically active species seem to be the sodium salts of the diols. 

Narasaka has demonstrated that TADDOL-Ti dichloride prepared from TADDOL and Cl2Ti(OPr )2 in the presence of MS 4A acts as an efficient catalyst in asymmetric catalytic Diels-Alder reactions with oxazolidinone derivatives of acrylates, a results in extremely high enantioselectivity (Sch. 45) [112]. Narasaka reported an intramolecular version of the Diels-Alder reaction, the product of which can be transformed into key intermediates for the syntheses of dihydrocompactin and dihydromevinolin (Sch. 46) [113]. Seebach and Chapuis/Jurczak [114] independently reported asymmetric Diels-Alder reactions promoted by chiral TADDOL- and 3,3 -diphenyl BINOL-derived titanium alkoxides. Other types of chiral diol ligands were also explored by Hermann [115] and Oh [116]. 

Not unexpectedly, a chiral TADDOL dendritic catalyst292 demonstrated a slight reduction in enan-... 

We have also explored the use of vanadium substituted Qi2 WZn(VO)2 (ZnWg034)2], prepared via metal exchange [Eq (16.2)], as a catalyst for allylic alcohol epoxidations with nonchiral and chiral organic hydroperoxides instead of PI2O2 as the terminal oxidant [8,17]. These conversions are assumed to proceed via V-alkylperoxido species. Besides the substantial as Tnmetric induction for a number of substrates with a chiral TADDOL-derived hydroperoxide, a particularly... 

During the past two decades the homogeneous and heterogeneous catalytic enan-hoselective addition of organozinc compounds to aldehydes has attracted much attention because of its potential in the preparation of optically active secondary alcohols [69]. Chiral amino alcohols (such as prolinol) and titanium complexes of chiral diols (such as TADDOL and BINOL) have proved to be very effective chiral catalysts for such reactions. The important early examples included Bolm s flexible chiral pyridyl alcohol-cored dendrimers [70], Seebach s chiral TADDOL-cored Frechet-type dendrimers [28], Yoshida s BINOL-cored Frechet-type dendrimers [71] and Pu s structurally rigid and optically active BlNOL-functionalized dendrimers [72]. All of these dendrimers were used successfully in the asymmetric addition of diethylzinc (or allyltributylstannane) to aldehydes. 

Figure 4.41 Chiral TADDOL-cored dendrimer with styryl groups located at the periphery.

Note that the diastereoselective synthesis of cis-l,2-dialkyicyclopropanol is considerably improved by using (i-PrOljTiCl or (i-PrO) Ti with RCHjCH MgBr (where R > H). The procedure is further simplified by adding an alkyl bromide to a mixture of the ester, (i-PrOljTiCl, and Mg in THF at room temperature, thus obviating the preparation of the Grignard reagent. In the presence of a chiral TADDOLate, the reaction becomes enantioselective. 

Seebach and coworker have reported the enantioselective desymmetrization of meso esters, anhydrides and sulfonylimides using TADDOL-Ti reagents [334]. In the desymmetrization of anhydrides, a catalytic amount of the chiral TADDOL-Ti... 

A significant problem associated with the conventional polystyrene supports is the low mechanical strength and often poor thermo-oxidative stability. Degni and CO workers developed the facile anchoring method of chiral TADDOL on... 

Narasaka advanced the use of chiral TADDOL-titanium complexes as catalysts for enantioselective Diels-Alder reactions [75-78], In order to minimize the conformational flexibility of the substrates, dienophiles 136 were employed because of their ability to participate in chelate formation with Lewis acids (Equation 15). In the presence of titanium catalyst 137 [76], isoprene (135) undergoes cycloaddition with 136 to give adduct 138 as one diastereomer in 96 % ee and 93 % yield [77]. 

The ruthenium catalyst generated in situ from H2Ru(CO)(PPh3)3, (S)-SEGPHOS and the TADDOL-derived phosphoric acid (169) promoted butadiene hydrohydroxyalkylation to form enantiomerically enriched products (168) (Scheme 61). Match/mismatch effects between the chiral ligand and the chiral TADDOL-phosphate counterion have been described. For the first time, single-crystal X-ray diffraction data for a ruthenium complex modified by a chiral phosphate counterion has been reported. ... 

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