Asymmetric allenylation

Many noticeable examples of chiral Lewis base catalyzed allylation of carbonyl compounds have also appeared. Iseki and coworkers published a full paper on enantioselective addition of allyl- and crotyltrichlorosilanes to aliphatic aldehydes catalyzed by a chiral formamide 28 in the presence of HMPA as an additive [41]. This method was further applied to asymmetric allenylation of aliphatic aldehydes with propargyltrichlorosilane [40]. Nakajima and Hashi-moto have demonstrated the effectiveness of (S)-3,3 -dimethyl-2,2 -biquinoline N,AT-dioxide (29) as a chiral Lewis base catalyst for the allylation of aldehydes [42]. In the reaction of (fs)-enriched crotyltrichlorosilane (54 , E Z=97 3) with benzaldehyde (48), y-allylated anfi-homoallylic alcohol 55 was obtained exclusively with high ee while the corresponding syn-adduct was formed from its Z isomer 54Z (fs Z= 1 99) (Scheme 6). Catalytic amounts of chiral urea 30 also promote the asymmetric reaction in the presence of a silver(I) salt, although the enantioselectivity is low [43]. 

The catalytic asymmetric allenylation has been explored with a BINOL/Ti system, giving selectively the homoallenyl alcohol with up to 95% ee. Nevertheless, the lower reactivity of aUenyltin compared to allyltin necessitated a nearly stoichiometric amount of 8. Recently, an improvement of the reaction by using i-PrSBEt2 overcame that limitation, making the system truly catalytic, with ee in the range of 81 to 97% . Interestingly, this reaction showed an unexpected equilibration phenomenon, thus producing exclusively... 

Using the same catalyst system, Akiyama has demonstrated the catalytic asymmetric allenylation of a-amino ester 389 with propargylstannane 391 to synthesize allenic amine 392 in good yield and high enantiopurity (Scheme 5.2.87). 22... 

The 1,3-dipoles consist of elements from main groups IV, V, and VI. The parent 1,3-dipoles consist of elements from the second row and the central atom of the dipole is limited to N or O [10]. Thus, a limited number of structures can be formed by permutations of N, C, and O. If higher row elements are excluded twelve allyl anion type and six propargyl/allenyl anion type 1,3-dipoles can be obtained. However, metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions have only been explored for the five types of dipole shown in Scheme 6.2. 

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

A new type of asymmetric hydrosilylation which produces axially chiral allenylsilanes has been reported by use of a palladium catalyst coordinated with the bisPPFOMe ligand 51b.64 The hydrosilylation of l-buten-3-ynes substituted with bulky groups such as tert-butyl at the acetylene terminus took place in a 1,4-fashion to give allenyl(trichloro)-silanes with high selectivity. The highest enantioselectivity (90% ee) was observed in the reaction of 5,5-dimethyl-T hexen-3-yne with trichlorosilane catalyzed by the bisPPFOMe-palladium complex (Scheme 13). 

The use of tartrates as chiral auxiliaries in asymmetric reactions of allenyl bor-onic acid was first reported by Haruta et al.69 in 1982. However, it was not for several years that Roush et al.,70 after extensive study, achieved excellent results in the asymmetric aldol reactions induced by a new class of tartrate ester based allyl boronates. 

The allenyl carboxylate 35 was obtained in an enantiomerically enriched form by the palladium-catalyzed reduction of the racemic phosphate 34 using a chiral proton source [53]. The two enantiomers of the (allenyl)samarium(III) intermediate are in rapid equilibrium and thus dynamic kinetic resolution was achieved for the asymmetric preparation of (i )-35 (Scheme 3.18). 

Recently, Hiroi and co-workers reported a palladium-catalyzed asymmetric transformation of chiral 2-alkynyl sulfmates 142 into allenyl sulfones 145 (Scheme 4.38) [58], Treatment of 142 with Pd(OAc)2 in the presence of a phosphine ligand afforded allenylsulfones 145 with high stereospecificities (73-89%) in good yields, probably through intermediates 143 and 144. 

Scheme 4.52 Asymmetric synthesis of allenes via the allenyl- or propargylsilanes.

The catalytic asymmetric synthesis of allenes was first achieved by Elsevier and co-workers in 1989 [104]. A palladium-catalyzed cross-coupling reaction of an allenyl-metal compound 250 (M = ZnCl, MgCl or Cu) with iodobenzene in the presence of DIOP 251 gave 252 in 25% ee (Scheme 4.65). The synthesis of 252 by the reaction of 250 (M = Br) with phenylzinc chloride in the presence of a chiral palladium catalyst gave a quantitative conversion but very low enantiomeric excesses (3-9% ee). 

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[. 

Intramolecular [4+2]-cycloaddition reactions, which involve base-induced isomerization of a propargyl ether to an allenyl ether, have been extensively studied. Treatment of 157 with a base caused an intramolecular Diels-Alder reaction of the intermediate allenyl ether to give tricyclic compounds 158 [131]. An asymmetric synthesis of benzofuran lactone 159 was achieved by an analogous procedure [132],... 

Use of tartrate esters as chiral auxiliaries in the asymmetric reactions of allenyl boronic acid also have been reported Ikeda, N. Aral, I. Yamamoto, H. J. Am. Chem. Soc. 1986,108, 483 Haruta, R. Ishiguro, M. Ikeda, N. Yamamoto, H. Ibid. 1982,104, 7667. 

The small number of electrophiles that react well with Af-Boc-2-hthiopyrrolidine limits applications of the asymmetric deprotonation of Af-Boc-pyrrolidine. In a significant development. Dieter and coworkers have shown that transmetalation of the enantioen-riched 2-lithiopyrrolidine with CuCN-2LiCl forms a cuprate that reacts enantioselectively with vinyl and allenyl iodides (Scheme 34). Enantioselectivities in the 90-95% range were achieved, although the reaction is very sensitive to reagent purity. ... 

Asymmetric hydroboration of 1-phenyl-1,3-butadiene (95) catalyzed by Rh-BINAP gave the corresponding optically active 1,3-diol 155 with 72% ee [89,90] (Scheme 2.15). Palladium-MOP complex also exhibited catalytic activity for the asymmetric hydroboration of but-l-en-3-yne (156), giving an optically active allenyl borane 157 [91]. 

A chiral bis-oxazoline catalyses asymmetric Nozaki-Hiyama allenylation of aldehydes.179 For example, benzaldehyde is converted to silylated allene (62) using a bromoalkynylsilane, BrCH2-C = C-SiR R2 the product is readily desilylated quantitatively without loss of ee. 

Matsumura, S. Maeda, Y. Nishimura, T. Uemura, S. Palladium-catalyzed asymmetric arylation, vinylation, and allenylation of tert-cyclobutanols via enantioselective C—C bond cleavage./. Am. Chem. Soc. 2003, 125, 8862-8869. 

The enantioselective hydroaminations of allenes with chiral phosphine catalysts was accomplished with substrates that had a terminal symmetric substitution and with the amines protected as carbamates or sulfonamides. The same symmetric substituents were necessary for the enantioselective transformation nsing chiral counterions. However, very recently, high enantiomeric excesses were reached with trisubstituted asymmetric allenes by a dynamic kinetic enantioselective hydroamination of allenyl carbamates (eqnation 110), even thongh the E/Z ratio of the prodncts was not optimal. 

Scheme 26 Palladium-catalyzed asymmetric transformation of chiral 2-alkynyl sulfinates into allenyl sulfones...

A new perspective was opened up recently when Denmark demonstrated diat with chirally modified phosphoryl-activated allenes an asymmetric induction could be effected. From easily generated allenyl phosphoramidates containing an optically active amino alcohol, the diastereomeric adducts (37) and (39) could be obtained by addition of dlyl alcohol. When the separated adducts were employed in the carb-anionic Claisen rearrangement, a remarkable asymmetric induction (90 10) could be achieved with preferential formation of the diastereomers (38) or (40) respectively, whereas in a thermal reaction no stereoselection was observed (Scheme 63). Another example of an asymmetric induction in Claisen rearrangements is reported by Welch. ... 

The catalytic asymmetric propargylation [108] and allenylation [109] of achiral aldehydes has been performed with high levels of enantioselection. The asymmetric propargylation promoted by the chiral Lewis acid derived from bind and Ti(0/-FT)4 are representative. Between 50 and 100 mol% of titanium is required for these reactions to go to completion (Scheme 10-70). The reaction of benzalde-hyde with allenyltributylstannane 170 and the chiral promoter produced the homo-propargylic alcohol 171 in >99% ee and 48% yield (7% of the undesired allenyl alcohol was also obtained). 

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