Patai S., Rappoport Z. 2000 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 2 / 0329 479

Simple chiral tartrate derived dioxaborolidine 386 induced a moderate enantioselectivity in the cycloaddition reaction of cyclopentadiene with ˛-bromoacrolein (equation 115)243.

Yamamoto and colleagues showed that very high enantioselectivities and yields were obtained in the cycloadditions of cyclopentadiene with several ˛-substituted acrylic aldehydes using binaphthol catalyst 387 (equation 116).

The high stereopreference was rationalized by considering complex 388 in which an attractive – donor–acceptor interaction favors co-ordination of the dienophile to the face of the boron center which is cis to the 2-hydroxyphenyl substituent. Hydrogen bonding of the hydroxyl proton of the 2-hydroxyphenyl group to an oxygen of the adjacent B O bond played an important role in the asymmetric induction. Protection of this hydroxy functionality with a benzyl group caused reversal of enantioselectivity in the cycloaddition of cyclopentadiene with methacrolein (model 389)244.

Further improvement of catalyst 387 resulted in the development of catalyst 393, as demonstrated by the formation of 391 and 392 from dienophiles 390 and cyclic dienes which gave good results with less reactive dienes and dienophiles (equation 117, Table 21)245.

Reilly and Oh explored the asymmetric induction of chiral catalysts derived from bis(dichloroborane) 397 in the cycloaddition of cyclopentadiene with ˛-bromoacrolein and methacrolein. N-Tosyltryptophan (394) and chiral diols 395 and 396 were employed as chiral ligands246,247. The application of chiral N-tosyltryptophan afforded the best results (equation 118, Table 22).

Corey and coworkers205b,c,248 reported the reactive cationic oxazaborinane catalyst and afforded 398a which promoted cycloadditions between cyclopentadiene and several ˛,ˇ-enals good enantioselectivities. Excellent results were obtained in cycloadditions of several modestly reactive dienes to ˛-bromoacrolein in the presence of catalyst 398b having tetra[3,5-bis(trifluoromethyl)phenyl]borate as the counterion (Table 23).

Later, Corey and colleagues reported that this reaction (with R D TMS, TES, Me2PhSi and Bu3Sn) was effectively catalyzed by 398b, with which ee values of 80 –87% were

TABLE 21. Reaction data for equation 117

3. Chiral titanium catalysts

Chiral titanium catalysts have generally been derived from chiral diols. Narasaka and colleagues251 developed an efficient catalyst, 406, prepared from TiCl2(OPr-i)2 and a (C)- tartaric acid derived 1,4-diol. These authors found that N-crotonyl-1,3-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.

CHO

(2S-405; 91% ee)

The catalyst was prepared from the corresponding chiral diol and TiCl2(OPr-i)2 at room temperature in the presence of 4 A˚ molecular sieves. Without molecular sieves, stoichiometric amounts of the titanium complex were required to obtain an equally high enantioselectivity. A remarkable solvent effect was observed. Various cycloadducts were only obtained with high optical yields when non-polar solvents were employed252,253. For example, 4-substituted 4-cyclohexene-1,2-dicarboxylate derivatives 408 were obtained with ee values ranging from 91 to 94% in the reactions of 91a, 399 and 407 with 17b in toluene/

petroleum ether (equation 121).

Narasaka and Yamamoto applied catalyst 406 in the cycloaddition of 1-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 (C)-paniculide254.

Corey and colleagues studied the chiral induction of various analogs of 406 in which the phenyl groups on the tertiary carbinol unit were replaced by other aromatic groups. The use of 3,5-xylyl groups (catalyst 412) gave the best results. Cycloadduct 413, for example, was obtained with 95% ee in the presence of this catalyst255.

O

Chiral metallocene complex [(S)-1,2-ethylenebis( 5-tetrahydroindenoyl)]Ti(OTf)2 422a and its zirconium analog 422b efficiently catalyzed the cycloadditions of 1,3-oxazolidin- 2-one based dienophiles 17a and 404 with cyclopentadiene which gave 421 and 405, respectively261. The endo selectivity was highest in dichloromethane, whereas the enantioselectivity was higher in nitroalkane solvents (equation 127, Table 24).

4. Chiral copper(II) catalysts

Evans and coworkers262 demonstrated the utility of bis(oxazolidine)copper(II) complexes 425 as Lewis acid catalysts in Diels –Alder reactions of N-enoyl-1,3-oxazolidin-2- ones 423 with cyclopentadiene, which gave adducts 424 (equation 128, Table 25). Their best results were obtained using catalyst 425c. Surprisingly, only 30% ee was obtained in the reaction between cyclopentadiene and 17a when catalyzed by 425a. Similar results were obtained for the thiazolidine-2-thione analogs of the N-enoyl-1,3-oxazolidin-2-ones.

The enantioselectivities observed were rationalized by the transition state depicted in Figure 7. Copper(II) has a high propensity for 4-co-ordinacy. In this case, two coordination sites are occupied by the bidentate ligand, the substrate binding to the two remaining binding sites. Cyclopentadiene approaches the dienophile from the side opposite

428 with several dienes, e.g. with cyclopentadiene to form 429 (equation 129, Table 26)264. This counterion effect had already been observed in the utilization of tridentate bis(oxazolidinyl)pyridine based copper(II) complexes 427 in Diels –Alder reactions of ˛-substituted acroleins. Catalyst 427d, for example, proved about 20 times more reactive and induced higher ee values than catalyst 427a (equation 130, Table 27)264,265. Catalysts 425c and 427 were compared with their Zn(II) analogs265. It was concluded that they are superior to their Zn(II) counterparts as chiral Lewis acids in the Diels –Alder reactions of cyclopentadiene with substituted N-acryloyl-1,3-oxazolidin-2-ones.

Davies and colleagues266 studied the use of copper(II) complexes of chiral bis(oxazolidine) 430 as catalysts in the cycloadditions of cyclopentadiene to substituted N-acryloyl- 1,3-oxazolidin-2-ones. They observed high endo and enantioselectivities. Again, the highest enantioselectivities were observed using SbF6 as the counterion, although differences were small this time: ee values of 92 and 95% were obtained for the triflate and SbF6 based catalysts, respectively.

The effect of the ligand bite angle on the enantioselectivity in the copper(II) catalyzed Diels –Alder reaction of cyclopentadiene with N-acryloyl-1,3-oxazolidin-2-one was studied using spiro bis(oxazolidine) based complexes 431a–d (Table 28)267,268. The data show that the enantioselectivity and endo selectivity increase with increasing bite angle which is related to the angle and n. Substitution of the dimethyl moiety on 430 with a cyclopropyl moiety (431a) induced an increase in enantioselectivity, which is in agreement with the expected increase in bite angle.