Catalytic enantioselective allylation

The majority of catalytic enantioselective allylation reactions involve the chiral Lewis-acid-catalysed additions of allylsilanes or allylstannanes to carbonyl compounds. Monothiobinaphthol has been used by Woodward et al. as a chiral promoter in the enantioselective catalytic allylation of aryl ketones with impure Sn(allyl)4, prepared from allyl chloride, air-oxidised magnesium and SnCl4. Therefore, the allylation of arylketones in these conditions was achieved very efficiently, since the corresponding allylic alcohols were formed in... 

A chiral dinuclear Ti(IV) oxide 20 has been successfully designed by Maruoka and coworkers and can be used for the strong activation of aldehydes, thereby allowing a new catalytic enantioselective allylation of aldehydes with allyltributyltin (Scheme 12.18). ° The chiral catalyst 20 can be readily prepared either by treatment of bis(triisopropoxy)titanium oxide [(/-Pr0)3Ti-0-Ti(0/-Pr)3] with (S)-BINOL or by the reaction of ((5)-binaphthoxy)isopropoxytitanium chloride with silver(I) oxide. The reaction of 3-phenylpropanal with allyltributyltin (1.1 equiv) under the influence of 20 (10 mol%) gives l-phenyl-5-hexen-3-ol... 

In 1998, Yamamoto et al. reported the first catalytic enantioselective allylation of imines with allyltributylstannane in the presence of a chiral 7i-allylpalladium complex 23 (Scheme 9) [15]. The imines derived from aromatic aldehydes underwent the allylation with high ee values. Unfortunately, the allylation reaction of aliphatic imines resulted in modest enantioselectivities. They proposed that a bis-Jt-allylpalladium complex is a reactive intermediate for the allylation and reacts with imines as a nucleophile. The bis-Jt-allylpalladium complex seemed the most likely candidate for the Stille coupling [16]. Indeed, the Stille coupling reaction takes place in the presence of triphenylphosphine even if imines are present, whereas the allylation of imines occurs in the absence of the phosphine [17]. They suggested the phosphine ligand played a key role in controlling the... 

A similar chiral silver(I) catalyst 21 was applied to the asymmetric addition of allyltributyltin to various aldehydes in an aqueous medium [28]. Shi et al. have shown that chiral silver complex 22, prepared from chiral bidentate phospho-ramide and AgOTf, is also an effective chiral catalyst for the allylation [29]. Chiral bis(oxazoline) ligands have found widespread use in asymmetric reactions catalyzed by chiral metal complexes, and C2-symmetric chiral bis(oxazoline)-Zn(OTf)2 complex 23 has been applied to catalytic enantioselective allylation of aldehydes with allyltributyltin (44) however, satisfactory enantioselectivity was not observed [30]. 

Jorgensen et al. reported that C2-symmetric bis(oxazoline)-copper(II) complex 25 also acts as chiral Lewis acid catalyst for a reaction of allylic stannane with ethyl glyoxylate [37]. Meanwhile, p-Tol-BINAP-CuCl complex 26 was shown to be a promising chiral catalyst for a catalytic enantioselective allylation of ketones with allyltrimethoxysilane under the influence of the TBAT catalyst [38]. Evans and coworkers have developed (S,S)-Ph-pybox-Sc(OTf)3 complex 27 as a new chiral Lewis acid catalyst and shown that this scandium catalyst promotes enantioselective addition reactions of allenyltrimethylsilanes to ethyl glyoxylate [39]. But, when the silicon substituents become bulkier, nonracemic dihydrofurans are predominantly obtained as products of [3+2] cycloaddition. 

Chiral Bronsted acids can also promote the asymmetric addition of allylic tin reagents to carbonyl compounds. Baba and coworkers have found that a stoichiometric amount of (fl)-BINOL (37) acts a chiral promoter for the allylation of unactivated ketones with tetraallyltin and in the presence of MeOH, the corresponding nonracemic tertiary homoallylic alcohols are obtained with up to 60% ee [50]. Later, Woodward et al. improved this process and achieved a catalytic enantioselective allylation of aryl ketones by employing (fl)-monothio-binaphthol 36 as a chiral Bronsted catalyst [49]. For instance, in the presence of 20 mol% of the chiral acid 36 and 40 mol% of H20 in toluene, acetophenone (42) was allylated by a 0.7 0.3 mixture of tetraallyltin (41) and butyltriallylltin (55) to give the (jR)-enriched allylated product 56 almost quantitatively with 89-86% ee (Scheme 8). 

Catalytic enantioselective allylation of a-hydrazono esters with allyltrimethoxysilanes in aqueous media can be accomplished by ZnF2-chiral diamine complexes.132... 

Another major contribution of polydentate ligands is the creation of a chiral pocket around the catalytic center providing, an appropriate chiral environment. The chiral pocket concept has been introduced by Trost for catalytic enantioselective allylic alkylation with the tetradentate aminophosphine ligand 33 [133]. The nucleophile fits into the chiral environment created by the chiral ligand and the allyl Pd intermediate. As a result, the enantiocontrol of the newly formed chiral center is very effective. In addition, the chiral control is likely to be efficient even at positions remote from the chiral ligand. That auxiliary has been widely... 

The serotonin antagonist LY426965 was synthesized using catalytic enantioselective allylation with a chiral biphosphoramide in the laboratory of S.E. Denmark. In order to prepare the necessary 3,3-disubstituted allyltrichlorosilane reagent, the ( -allylic alcohol was first converted by the Corey-Kim procedure to the corresponding chloride. 

Kii, S., Maruoka, K. Catalytic enantioselective allylation of ketones with novel chiral bis-titanium(IV) catalyst. Chirality 2003,15, 68-70. 

The allylation reactions of carbonyl compounds catalyzed by chiral Lewis acids represent a powerful new direction in allylmetal chemistry. Yamamoto and coworkers reported the first example of the catalytic enantioselective allylation reaction in 1991, using the chiral (acyloxy)borane (CAB) catalyst system (see below) [288]. Since then, several additional reports of the catalytic allylation reaction have appeared. To date, the most effective catalyst systems reported for the enantioselective reaction of aldehydes and Type II allyl- and crotylstannane and silane reagents include the Yamamoto CAB catalyst and catalysts complexes composed of various Lewis acidic metals and either the BINOL or BINAP chiral ligands [289-293]. Marshall and Cozzi have recently reviewed progress in the enantioselective catalytic allylation reaction [294, 295]. 

Selected Applications of the Catalytic Enantioselective Allylation Reaction in Natural Product Synthesis... 

The first example of truly catalytic, enantioselective allylation of imines has been reported by Yamamoto [46a]. These authors had first discovered that imines could undergo allylation reaction via a palladium-catalyzed allylstannane reaction to afford the corresponding homoallylamines in high yields [47a, 47b,... 

Catalytic enantioselective allylations of aldehydes already published can be classified into two methods carried out under the influence of chiral Lewis acid catalysts and chiral Lewis base catalysts. The process by chiral Lewis acid catalysts generally uses allyltrimethylsilane or allyltrialkylstannane as an allylating agent, both of which show low reactivity toward aldehydes without these catalysts. The process by chiral Lewis base catalysts employs allyltrichlorosilane or allylmetals possessing relatively higher reactivity. Both processes can be successfully applied to various substituted allylmetal compounds or allenylmetal compounds. 

Chiral bis(oxazoline) is an excellent chiral hgand as is BINAP for asymmetric reactions catalyzed by chiral metal complexes and has been applied to catalytic enantioselective allylation of aldehydes with allyltributyltin (33) by Cozzi, Um-ani-Ronchi and their colleagues [34]. Among various combinations of chiral bis(oxazohnes) and metal salts, they found that the zinc complex 13 is the most effective and the ee values are in the range of 40 to 46%. 

S. E. Denmark and J. Fu, Catalytic enantioselective allylation with chiral Lewis bases, Chem. Commun., (2003) 167-170. 

Yus M, Gonzalez-Gomez JC, Foubelo F (2011) Catalytic enantioselective allylation of carbonyl compounds and imines. Chem Rev 111 7774-7854... 

Gutmann, V. (1998). The Donor-Acceptor Approach to Molecular Interactions, Plenum Press, New York. Denmark, S. E., and Wynn, T. "Lewis Base Activation of Lewis Acids Catalytic Enantioselective Allylation and Pro-pargylation of Aldehydes."/. Am. Chem. Soc., 123,6199-6200(2001). 

Demnark SE, Fu J (2000) On the mechanism of catalytic, enantioselective allylation of aldehydes with chlorosilanes and chiral lewis bases. J Am Chem Soc 122 12021-12022... 

Denmark SE, Fu J (2003) Catalytic enantioselective allylation with chiral lewis bases. Chem Commnn 167—170... 

Denmark SE, Wynn T (2001) Lewis base activation of lewis acids catalytic enantioselective allylation and propargylation of aldehydes. J Am Chem Soc 123 6199-6200... 

Scheme 1.9 Catalytic enantioselective allyl and crotylboration of ketones by Schaus group.

Following the first report of catalytic enantioselective allylation reactions with (12) by Maruoka group in 2003 [24a], the synthetic utility of this type of chiral LLA was recently broadened. 

More than a decade ago, Yamamoto and coworkers demonstrated that the chiral acyloxyborane (6) is an excellent catalyst for the Sakurai-Hosomi allylation of aldehydes to furnish the homoallylic alcohols in good yields and enantioselectivities (Equation 60) [59]. This system remains to date, the most effective catalytic enantioselective allylation of aldehydes based on boron Lewis acids. 

Catalytic enantioselective allylation with allylsilanes has also been achieved with chiral Lewis acid catalysts such as Ti/binol " and chiral (acyloxy)borane complexes (Scheme 3-89). 

Scheme 3-89. Catalytic enantioselective allylation of aldehydes using...

In an innovative approach, Yamamoto developed the dimeric pinene-derived palladium catalyst 263 for catalytic enantioselective allylation of imi-nes (Equation 25) [170], A catalyst loading of 5 mol% promoted the enantioselective allylation of a host of imines including 261 in up to 82% ee. In subsequent investigations of the reaction conditions, the beneficial effect of addition of one equivalent of water was observed, with increases both in the reactivity of the system and in the enantioselectivity (up to 91 % ee)... 

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