Narasaka catalyst

The Lewis acid catalyst 53 is now referred to as the Narasaka catalyst. This catalyst can be generated in situ from the reaction of dichlorodiisopropoxy-titanium and a diol chiral ligand derived from tartaric acid. This compound can also catalyze [2+2] cycloaddition reactions with high enantioselectivity. For example, as depicted in Scheme 5-20, in the reaction of alkenes bearing al-kylthio groups (ketene dithioacetals, alkenyl sulfides, and alkynyl sulfides) with electron-deficient olefins, the corresponding cyclobutane or methylenecyclobu-tene derivatives can be obtained in high enantiomeric excess.18... 

Recently, Wada et al. [175,176] have observed an excellent enantioselectivity for the intermolecular hetero Diels-Alder reactions of ( )-2-oxo-l-phenylsul-fonyl-3-alkenes 2-197 - 2-199 with enol ethers 2-200 to give the dihydropyrans 2-201 - 2-203 using the Narasaka catalyst 2-204. The best results were obtained with the iso-propyl vinyl ether 2-200c (Fig. 2-56). 

In a similar system, namely the intermolecular cycloaddition of 8-29 to give the two enantiomers 8-30 and ent-8-30 also a pressure induced increase in enan-tioselectivity using the Narasaka catalyst 8-31 was observed. Whereas the reaction proceeds with 4.5% ee at 20 °C and 1 bar, an ee of 20.4% was observed at 20°C and 5 kbar (Fig. 8-10) [551],... 

The first example of a true positive high pressure effect on the enantioselectivity was found for the intramolecular hetero-Diels-Alder reaction of the l-oxa-1,3-butadiene (173) in the presence of the Narasaka catalyst (164) to give the two enantiomeric bridged cycloadducts 174 and 175 (Scheme 8.44) [80]. At atmospheric pressure the two enantiomers were formed with 4.5 % ee, whereas at 500 MPa an increase to 20.4 % ee was observed which corresponds to a AAV = —(1.7 + 0.2) cm mol . In addition, the yield was improved from 50 to 89 %. It was assumed that under high pressure complexes of different stoichiometry may be formed which are more favorable towards a facial selective addition. However, a clear interpretation of the results cannot be given at this point. 

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. 

I would like to thank Professors E. J. Corey and K. Narasaka for giving me a chance to work with super-reactive chiral catalyst 9 and TADDOL-based chiral titanium catalyst 31, respectively. 

Buchwald et al. have shown that 5-20 mol % Cp2Ti(CO)2 facilitates the PKR at 18 psi CO and 90 °C, giving yields in between 58 and 95% [38]. Moreover, Mitsudo et al. [39] and Murai et al. [40] reported independently on the employment of Ru3(CO)i2 as active catalyst. Cyclopentenones were isolated in moderate to excellent yields (41-95%). In addition, rhodium catalysts were successfully examined for use in the PKR. Narasaka et al. [41] carried out reactions at atmospheric CO pressure using the dimeric [RhCl(CO)2]2 complex. Also, in the presence of other rhodium complexes like Wilkinson catalyst RhCl(PPh3)3 and [RhCl(CO)(dppp)]2 [42] in combination with silver salts, cyclopentenones were obtained in yields in the range of 20-99%. Some representative examples of the catalytic PKR are shown in Eq. 2. 

Chiral titanium catalysts have generally been derived from chiral diols. Narasaka and colleagues251 developed an efficient catalyst, 406, prepared from TiCl2(OPr- )2 and a (+)-tartaric acid derived 1,4-diol. These authors found that Af-crotonyl-l,3-oxazolidin-2-one (404) reacted with cyclopentadiene in the presence of 10 mol% of 406 to give cycloadduct 405 with up to 91% ee (equation 120)252. 

Narasaka and Yamamoto applied catalyst 406 in the cycloaddition of l-acetoxy-3-methyl-1,3-butadiene (409) to 3-boryl propenoic acid derivative 410 (equation 122). Cycloadduct 411 was employed in the total synthesis of (-l-)-paniculide254. 

With Tartrate-Derived Chiral 1,4-Diol/Ti Complexes A catalytic asymmetric Diels-Alder reaction is promoted by the use of a chiral titanium catalyst prepared in situ from (Pr O TiC and a tartrate-derived (2.R,3.R)-l,l>4,4-tetraphenyl-2,3-0-(l-phenylethylidene)-l,2,3,4-butanetetrol. This chiral titanium catalyst, developed by Narasaka, has been successfully executed with oxazolidinone derivatives of 3-borylpropenoic acids as P-hydroxy acrylic acid equivalents [40] (Eq. 8A.21). The resulting chiral adduct can be utilized for the first asymmetric total synthesis of a highly oxygenated sesquiterpene, (-i-)-Paniculide. 

Narasaka s chiral titanium catalyst, prepared from (Pr 0)2TiCl2 and a tartrate-derived (2R,3R)-l,l,4,4-tetraphenyl-2,3-0-(l-phenylethylidene)-l,2,3,4-butanetetrol, is utilized for the asymmetric [2+2] cycloaddition of A-acyl oxazolidinones to 1,2-propadienyl sulfides possessing a-substituents, which afford methylenecyclobutane derivatives with high enantiomeric purity. These chiral adducts are readily transformed to seven- and eight-membered carbocycles with chiral side chains by the ring-cleavage reaction and subsequent cationic cyclization of the chiral cyclobutane derivative [68] (Eq. 8A.44). 

The oxidative radical ring opening of cyclopropanols 191 mediated by Mn(pic)3 was developed by Narasaka and coworkers. Their efforts culminated recently in the development of a silver-catalyzed method (see Part 3, Sect. 6.2). Kulinkovich et al. based a manganese-catalyzed process on it. Manganese abietate 192 (1—1.5 mol%) was used as the catalyst and oxygen as the terminal oxidant (Fig. 54) [289]. 

Narasaka and coworkers reported radical-polar crossover addition/cyclization reactions of phenacyl bromides 204 and electron-rich alkenes such as (silyl) enol ethers 205, catalyzed by the rhenium(I) complex 206 (Fig. 57) [302], The active catalyst 206A formed after thermal nitrogen elimination from 206 reduced 204 either directly or by oxidative addition/homolysis via rhenium enolate 204A to... 

Rhodium complexes are effective catalysts for the PKR and are receiving much attention. In addition to the studies by Narasaka with [RhCl (CO)2]2 [90], Jeong has introduced several species as new catalysts. Some of these rhodium complexes need activation with AgOTf. The reaction works well with non-terminal alkynes (36) and the scope and efficiency is dependent on the catalyst used. In the case of chiral species, a careful choice of conditions, including CO pressure, activation, solvent and ligands, is essential to obtain 37 with high enantioselectivity (Scheme 12) [91]. 

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]. 

Narasaka etal. first reported an excellent chiral titanium catalyst for this reaction Narasaka K, Iwasawa N, Inoue M, Yamada T, Nakashima M, Sugimori J (1989) J Am Chem Soc 111 5340... 

The catalyst was also found to be effective for the Diels-Alder reactions of an acrylic acid derivative (Narasaka et al. 1991). 3-Acryloyl-l,3-oxazolidin-2-one reacted with... 

Narasaka et al. developed a titanium catalyst generated by complexation with chiral diols.245 xhe dienophile must contain functionality that will coordinate with the metal catalyst to form a chiral complex, and these catalysts are less effective with dienes and dienophiles that do not contain heteroatoms. A related titanium-BINOL complex has been used to catalyze Diels-Alder reactions.246 Kelly prepared a transient... 

A small number of enantiomerically pure Lewis acid catalysts have been investigated in an effort to develop a catalytic asymmetric process. Initial work in this area was carried out by Narasaka and coworkers using the titanium complex derived from diol (8.216) in the cycloaddition of electron-deficient oxazolidinones such as (8.217) with ketene dithioacetal (8.218), alkenyl sulfides and alkynyl sulfides. Cyclic alkenes can be used in this reaction and up to 73% ee has been obtained in the [2- -2] cycloaddition ofthioacetylene (8.220) and derivatives with2-methoxycarbonyl-2-cyclopenten-l-one (8.221) usingthe copper catalyst generated with bis-pyridine (8.222). Furthermore, up to 99% ee has been obtained in the [2-1-2] cycloaddition of norbornene with alkynyl esters using rhodium/Hs-BINAP catalysts. This reaction is not restricted to the use of transition metal-based Lewis... 

Complexes of other transition metals have been reported to catalyze Pauson-Khand reactions. Buchwald reported intramolecular PKRs with 1.2 atm of CO at 90 °C in the presence of CpjTi(CO)2. " However, most other catalytic Pauson-Khand reactions have been conducted with late transition metal catalysts. Murai and Mitsudo simultaneously reported intramolecular PKRs catalyzed by ruthenium carbonyl clusters in dioxane or DMAc at 140-160 °C under 10-15 atm of CO. The first Rli-catalyzed PKR was reported by Narasaka. ° In this case, the reaction occurred with acceptable rates, even with CO pressures less than 1 atm. Shibata reported PKRs in refluxing xylenes under 1 atm of CO in the presence of catalytic amounts of PPli and [Ir(COD)Cl]2. Adrio and Carretero showed that the solvated molybdenum carbonyl complex Mo(DMF)3(CO)3 catalyzed intramolecular PKRs with monosubstituted olefins, as well as with disubstituted electron-poor olefins, and Hoye showed that W(CO)5(THF) catalyzes intramolecular PKRs. Iron and palladium complexes have also been reported to catalyze the PKR. 

Narasaka reported that TADDOL-TiCl2 was able to catalyze asymmetric DA reaction of cyclopentadiene with oxazolidinone derivatives of acrylates in the presence of 4A MS [148]. A remarkable solvent effect on the enantioselectivity was observed, and high enantioselectivity was attained using mesitylene as the solvent. Cycloadditions to oxazolidinone derivatives of acrylates were also efficiently catalyzed by dendritic or polymer-supported TADDOL-Ti catalysts [149]. From the structural determination of the 3-(( )-3-cinnamoyl)-l,3-oxazolidin-2-one adduct, it can be deduced that the transition state involves binding of the dienophile to the titanium catalyst via the N-acyl-oxazolidinone [19a] (Scheme 14.59). The diastereo-and enantioselectivity of this type of catalyst are thus probably owing to both electronic and steric effects from TADDOL ligand. 

The first catalytic asymmetric version of [2 + 2] cycloaddition reaction was realized by Narasaka in 1989 using chiral titanium catalyst derived from TADDOL. It was found that the reaction of ketene dithioacetal with acryloyloxazolidinone derivatives proceeded smoothly in the presence of 10 mol% of TADDOL-TiCl2 to give the cyclobutane derivatives in high yields (64-96%) and good to excellent enantioselectivities (80-98%) [183]. The reaction is presumed to proceed via a carbonyl substrate chelated TADDOL-TiCb intermediate although the exact reaction mechanism is unclear. Moreover, alkynyl, alkenyl, and 1,2-propadienyl... 

Since the pioneering work of Reetz and Narasaka, the use of various chiral titanium complexes as the catalysts for the enantioselective addition of trimethylsilylcyanide to the aldehydes has been an extensive topic of research. 

In the early 1960s, Brannock et al. reported a thermal [2+2] cycloaddition of enamines. Enamines react with a variety of electron-deficient alkenes such as acrylates, nitro-olefines, acetonitriles, vinylsulfones, fumarates, and malei-mides to give aminocyclobutanes [4]. The reaction generally does not require the assistance of an acid catalyst. Narasaka et al. exploited asymmetric thermal [2+2] cycloaddition of vinyl and aUenyl sulfides with electron-deficient alkenes catalyzed by Lewis acid [5]. Yamazaki et al. have reported that a stoichiometric amount of Lewis acid activates [2+2] cycloaddition of vinylselenides with highly electron-deficient olefins [6]. These reactions proceed via a stepwise annulation to give mercapto- and seleno-cyclobutanes, respectively. However, cyclobutane formation from silyl enol ethers, which are one of the most easily prepared ketone... 

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