Chirality transfer from silicon to carbon

The exploitation of the asymmetry at silicon in stereoselective synthesis is an exceptionally challenging task. Initially, silicon-stereogenic silanes have been utilized to elucidate the stereochemical course of substitution reactions at silicon. Apart from these mechanistic investigations, only a handful of synthetic applications with an asymmetrically substituted silicon as the stereochemical controller have been reported to date. In these transformations the chiral silicon functions as a chiral auxiliary. Conversely, a direct transfer of chirality from silicon to carbon during bond formation and cleavage at silicon has remained open until its recent realization in both inter- and intramolecular reactions. In this Concept, the pivotal considerations in relation to the nature of suitable silanes as well as mechanistic prerequisites for an efficient chirality transfer will be discussed.     

From an α‐Functionalized Silicon‐Stereogenic N,O‐Silane to a Monomeric and Tetracoordinate tBuLi Adduct with Lithium‐Centered Chirality

Donor‐functionalized silanes with stereogenic silicon centers are extremely rare. A convenient stereocontrolled route to a nitrogen‐oxygen‐functionalized silicon‐chiral compound with an additional aminomethyl function is presented. This silane was directly achieved in stereochemically pure form by a simple nucleophilic substitution reaction. Owing to the unique asymmetry of this silane and the presence of three donor functions, the first monomeric butyllithium compound with lithium‐centered chirality could be isolated; the configuration was assigned by X‐ray crystallography. This [silane? tBuLi] complex undergoes an unexpected deprotonation/stereospecific substitution sequence in toluene, leading to the development of a convenient one‐pot synthesis of a functionalized silicon‐chiral benzylsilane, which proceeds with inversion of configuration and complete preservation of the stereochemical integrity at silicon.     

Palladium-catalyzed asymmetric synthesis of axially chiral (allenylmethyl)silanes and chirality transfer to stereogenic carbon centers in S(E)' reactions

[reaction: see text] Novel stereoselective reactions of 4-substituted-1-trimethylsilyl-2,3-butadienes ((allenylmethyl)silanes) were developed. The axially chiral (allenylmethyl)silanes were prepared from (3-bromopenta-2,4-dienyl)trimethylsilane by a Pd-catalyzed asymmetric reaction with soft nucleophiles with up to 88% enantioselectivity. The (allenylmethyl)silanes reacted with acetals in the presence of a TiCl(4) promoter to give 1,3-diene derivatives via an S(E)' pathway. The 1,3-dienyl products have (E)-geometry exclusively and up to 88%( )()chirality transfer from the axially chiral allenes to the centrally chiral 1,3-dienes was observed in the S(E)' reaction.     

Preparation of stereodefined homoallylic amines from the reductive cross-coupling of allylic alcohols with imines

Regio-, diastereo-, and enantioselective coupling reactions between imines and allylic alcohols have been developed. These coupling reactions deliver complex homoallylic amine products through a convergent C-C bond forming process that does not proceed through intermediate allylic organometallic reagents. In general, convergent coupling, by exposure of an allylic alkoxide to a preformed Ti-imine complex, occurs with allylic transposition in a predictable and stereocontrolled manner. While simple diastereoselection in these reactions is high, delivering anti-products with ≥20:1 selectivity, the organometallic transformation described is compatible with a diverse range of functionality and substrates (including aliphatic and aromatic imines, allylic silanes, trisubstituted alkenes, vinyl- and aryl halides, trifluoromethyl groups, thioethers, and aromatic heterocycles). Alkene geometry of the products is a complex function of the allylic alcohol structure and is consistent with a mechanistic proposal based on syn-carbometalation followed by syn-elimination by way of a boat-like transition state geometry. Single asymmetric coupling reactions provide a means to translate the stereochemical information of the allylic alcohol to the homoallylic amine or to control diastereoselection in the coupling reactions of achiral allylic alcohols with chiral imines. Double asymmetric coupling reactions are also described that afford a unique means to control stereoselection in these complex convergent coupling processes. Finally, empirical models are proposed that are consistent with the observed stereochemical course of these coupling reactions en route to chiral homoallylic amines possessing di- or trisubstituted alkenes and anti- or syn- relative stereochemistry at the allylic and homoallylic positions.     

Stereoselective Alcohol Silylation by Dehydrogenative Si–O Coupling: Scope,Limitations, and Mechanism of the Cu–H‐Catalyzed Non‐Enzymatic Kinetic Resolution with Silicon‐Stereogenic Silanes

Ligand‐stabilized copper(I)–hydride catalyzes the dehydrogenative Si–O coupling of alcohols and silanes—a process that was found to proceed without racemization at the silicon atom if asymmetrically substituted. The present investigation starts from this pivotal observation since silicon‐stereogenic silanes are thereby suitable for the reagent‐controlled kinetic resolution of racemic alcohols, in which asymmetry at the silicon atom enables discrimination of enantiomeric alcohols. In this full account, we summarize our efforts to systematically examine this unusual strategy of diastereoselective alcohol silylation. Ligand (sufficient reactivity with moderately electron‐rich monophosphines), silane (reasonable diastereocontrol with cyclic silanes having a distinct substitution pattern) as well as substrate identification (chelating donor as a requirement) are introductorily described. With these basic data at hand, the substrate scope was defined employing enantiomerically enriched tert‐butyl‐substituted 1‐silatetraline and highly reactive 1‐silaindane. The synthetic part is complemented by the determination of the stereochemical course at the silicon atom in the Si–O coupling step followed by its quantum‐chemical analysis thus providing a solid mechanistic picture of this remarkable transformation.     

The Catalytic Asymmetric Aldol Reaction

The construction of C-C bonds with complete control of the stereochemical course of a reaction is of utmost importance for organic synthesis. The aldol reaction-the simple addition of an enolate donor to a carbonyl acceptor-is one of the most powerful reactions available to the synthetic chemist. In general, control of the relative and absolute configuration of the newly formed stereogenic centers has been achieved through the use of chiral starting materials or chiral auxiliaries. In recent years the search for catalytic methods that efficiently and effectively transfer chirality information has become a major effort in synthetic organic chemistry. Two different approaches have been taken toward the catalytic asymmetric aldol reaction: biocatalysis and catalysis with small molecules. Both approaches have specific advantages and limitations, and as a result are complementary to each other. The important efforts toward both approaches are reviewed in this article.     

Illuminating the Mechanism of the Borane‐Catalyzed Hydrosilylation of Imines with Both an Axially Chiral Borane and Silane

The reduction of C?O groups with silanes catalyzed by electron‐deficient boranes follows a counterintuitive mechanism in which the Si? H bond is activated by the boron Lewis acid prior to nucleophilic attack of the carbonyl oxygen atom at the silicon atom. The borohydride thus formed is the actual reductant. These steps were elucidated by using a silicon‐stereogenic silane, but applying the same technique to the related reduction of C?N groups was inconclusive due to racemization of the silicon atom. The present investigation now proves by the deliberate combination of our axially chiral borane catalyst and axially chiral silane reagents (in both enantiomeric forms) that the mechanisms of these hydrosilylations are essentially identical. Unmistakable stereochemical outcomes for the borane/silane pairs show that both participate in the enantioselectivity‐determining hydride‐transfer step. These experiments became possible after the discovery that our axially chiral C₆F₅‐substituted borane induces appreciable levels of enantioinduction in the imine hydrosilylation.     

OPTICALLY PURE CHALCOGENURANES: SYNTHESIS AND STEREOCHEMISTRY OF THEIR REACTIONS

Abstract

An overview of our recent studies on the asymmetric syntheses, stereochemical studies, reactions and applications of chiral chalcogenuranes is described. Chiral chalcogenuranes, including halooxachalcogenuranes and spirooxachalcogenuranes, have been synthesized by highly diastereoselective oxidation of the 2-exo-hydroxy- 10-bornyl chalcogenides. The stereochemistry of chalcogenuranes have been confirmed as with a trigonal bipyramidal (TBP) geometry. Nucleophilic substitution reactions of these compounds provided a good method to prepare the chiral chalcogenonium (IV) compounds with excellent diastereoselectivity. Our results indicated that the reactivity of the axial bonds of chalcogenuranes plays an important role in the control of the stereochemistry of the reactions. The mechanistic and stereochemical research on the nucleophilic substitution reactions have been carried out which indicated that two kinds of pathways, dissociative and associative routes, might exist. The applications of the reactions through the asymmetric [2, 3] sigmatropic rearrangement and enantioselective protonation with optically active selenonium (IV) compounds have been investigated to give good to high selectivities.     

Asymmetric benzoin condensation catalyzed by chiral rotaxanes tethering a thiazolium salt moiety via the cooperation of the component: can rotaxane be an effective reaction field?

Although some reactions on rotaxanes have been reported, the characteristic features of the rotaxanes providing unique reaction fields have hardly been studied, especially as catalyst. In our continuous studies on interlocked molecules such as rotaxanes and catenanes, we have noticed the importance of such interlocked structures with high freedom in functionalized materials such as molecular catalyst. For catalytic asymmetric benzoin condensations, two optically active rotaxanes possessing thiazolium salt moieties were prepared using the binaphthyl group as the chiral auxiliary. The benzoin condensations of aromatic aldehydes catalyzed by the chiral rotaxanes as catalysts gave optically active benzoins with ca. 30% ee in moderate to high chemical yields depending upon the structure of rotaxane and the reaction conditions employed. From the results, two intrarotaxane chirality transfers are confirmed: (i) through-space chirality transfer from wheel to axle and (ii) through-bond chirality transfer controlled with an achiral wheel. Because these asymmetric reaction fields are specific to the rotaxane structure, the importance and possibility of the "rotaxane field" as a particular reaction field is demonstrated in this work.     

Chiral Memory in Silylium Ions

The configurational stability of the chiral silicon center in Lewis base‐stabilized silyliums has been studied. Complete retention of configuration at silicon is observed at low temperature in silylium ions stabilized by lone‐pair interactions. In contrast, loss of chiral memory is observed with systems involving π‐interactions. Epimerization of the silicon center was observed by simply allowing equilibration of the diastereomeric silyliums at RT or by adding a catalytic amount of a chelating solvent. Conditions for the transfer of axial chirality to Si‐centered chirality were shown to rely on the strength of the Lewis base‐silylium ion interaction.     

Stereocontrolled synthesis of tertiary silanes via optically pure 1,3,2-oxazasilolidine derivatives

Optically pure 1,3,2-oxazasilolidine derivatives were synthesized from a chiral 1,2-amino alcohol. These heterocyclic compounds containing a stereogenic silicon atom produced tertiary silanes with excellent optical purity through successive reactions with Grignard reagents and diisobutylaluminum hydride. Stereochemical course of the reactions of the oxazasilolidine at the chiral silicon atom was elucidated based on the absolute configurations of the products and the substrate which were determined by chiral HPLC and X-ray crystallographic analyses.     

Importance of planar chirality in chiral catalysts with three chiral elements: the role of planar chirality in 2'-substituted 1,1'-P,N-ferrocene ligands on the enantioselectivity in Pd-catalyzed allylic substitution.

A series of novel planar chiral 2'-substituted 1,1'-P,N-ferrocene ligands 9-11, 14, and 16 were prepared with diastereopurity >99:1 and found to be effective in asymmetric allylic alkylation and amination reactions. Ligand 14 furnished the highest enantiomeric excess, 98.5% and 96.5% ee in alkylation and amination reactions, respectively. The role of planar chirality in asymmetric reactions has been examined, and decisive effects on enantioselectivity as well as the control of absolute configuration in palladium-catalyzed allylic alkylation and amination reactions were observed. To clarify why and how the planar chirality governed the stereochemical outcome, X-ray crystallographic structures of eta(3)-diphenylallyl Pd complexes, (1)H NMR, (31)P NMR spectra of palladium dichloride complexes, and eta(3)-diphenylallyl Pd complexes of three 1,1'-P,N-ferrocene ligands were analyzed with the aid of COSY and 2D NOESY experiments. All results led to the conclusion that planar chirality influences the stereochemical outcome by changing or even inverting the ratio of two rotamers because of the steric interaction between a planar chiral group and the coordination site.     

新型过渡金属硅基化合物的合成、表征及反应性

在这篇综述中,我们将简要地介绍我们在新一类的金属硅基化合物(RCH₂)₃MSiR′₃(M=Ti, Zr), (RCH₂)₂Ta(=CHR)SiR′₃和(RCH₂)₂W(≡CR)SiR′₃方面的工作,并讨论亚烷基、氨基化合物与硅烷之间的反应。这些研究具有以下的两重意义：(1)探索从分子前体形成金属硅化物的机理。(2)研究新一类金属硅基化合物的合成、结构及它们与硅烷的反应性。我们还通过对烷基和硅基配体在形成亚烷基配合物时不同的反应机理的研究来比较元素碳和硅之间的反应性。在这方面的研究中,我们发现“(RCH₂)₂Ta(=CHR)Cl”为在(RCH₂)₃TaCl₂和Li(THF)₃SiR₃的反应中形成硅基亚烷基化合物(RCH₂)₂Ta(=CHR)SiR₃(R=SiMe₃)的中间体而Ta(CH₂R)₅(R=CMe₃, SiMe₃)则是从(RCH₂)₃TaCl₂生成烷基亚烷基化合物(RCH₂)₃Ta=CHR的中间体。除此之外,金属氨基(M-NR₂)化合物与硅烷之间的反应被证明可以可逆地生成氨基氢化物和氨基硅烷。我们将一并讨论这一结果对于阐明金属硅化物和M-Si-N三元体系物质的形成机理的意义。     

Chiral oxazolines and their legacy in asymmetric carbon-carbon bond-forming reactions

The studies performed with chiral oxazolines as vehicles for a number of C-C bond-forming reactions are traced back to the early 1970s. The main source of chirality in these studies was derived from chiral, nonracemic amino alcohols which, themselves, were prepared from amino acids. The formation of chiral oxazolines from these materials provided interesting and useful starting materials for these studies.The factors responsible for high optical yields during that era were attributed to rigidity or precomplexation of the reactants, thus providing a more well-defined topography during the reaction course. A number of C-C bond reaction types are described including alkylations, additions, and organometallic reactions. The ee's of many of the products arrived at are well above 90%. This level of ee was virtually unprecedented in the early to mid 1970s. In addition to the synthetic emphasis, several mechanistic questions, arising in other areas, were also undertaken due to the accessibility of the appropriate requisite chiral materials.     

Sequential Chiral Induction and Regulator-Assisted Chiral Memory of Pillar[5]arenes

Chirality transfer is widely observed in life processes, and many artificial chiral transfer systems have been developed. In these systems, chiral information is transferred from chiral inducers to chiral acceptors by a direct chiral induction process and a direct chiral memorization process. We have developed ternary nondirect chiral transfer systems based on pillar[5]arenes, in which a third factor was introduced as a regulator. The planar chirality of an acceptor was induced and memorized by a chiral inducer with precise control by a regulator. In the chiral induction period, the feed sequence of the chiral inducer and regulator affected the chiral induction behavior of the chiral acceptor. The chiral memory ability of the acceptor was substantially improved by the combined action of the chiral inducer and regulator.     

Asymmetric Synthesis of Silicon‐Stereogenic Vinylhydrosilanes by Cobalt‐Catalyzed Regio‐ and Enantioselective Alkyne Hydrosilylation with Dihydrosilanes

The strategic carbon‐to‐silicon substitution at a stereogenic center can produce chiral silanes with significantly improved properties relative to their carbon congeners. We herein report an unprecedented cobalt‐catalyzed asymmetric hydrosilylation of unsymmetric alkynes with dihydrosilanes that furnishes silicon‐stereogenic vinylhydrosilanes with high regio‐ and enantioselectivity. The absolute configurations of the products were determined by chiroptical methods in combination with DFT calculations. The synthetic versatility of the vinylhydrosilanes as chiral building blocks was further demonstrated by asymmetric Si?H insertion and catalytic hydroboration reactions.     

Mechanistic aspects of alkene oxidation using chiral hypervalent iodine reagents

Recent progress in the area of hypervalent iodine-induced enantioselective oxidation is reviewed with emphasis from a mechanistic point of view. Chiral lactate and lactamide sidechains in hypervalent iodine reagents induce herical chirality around the iodine reaction site, which provides a chiral environment suitable for enantioselective transformations. The stereochemical outcomes of alkene oxidation are also compiled and used for systematically understanding the reaction mechanism of oxidative double bond difunctionalization.     

Chirality transfer across length-scales in nematic liquid crystals: fundamentals and applications

When a chiral dopant is dissolved in an achiral liquid crystal medium, the whole sample organizes into a helical structure with a characteristic length-scale of the order of microns. The relation between chirality at these quite different length-scales can be rationalized by a relatively simple model, which retains the relevant factors coming into play: the molecular shape of the chiral dopant, which controls the chirality of short range intermolecular interactions, and the elastic properties of the nematic environment, which control the restoring torques opposing distortion of the director. In this tutorial review the relation between molecular and phase chirality will be reviewed and several applications of the chiral doping of nematic LCs will be discussed. These range from the exploitation of the amplified molecular chirality for stereochemical purposes (e.g., the determination of the absolute configuration or the enantiomeric excess), to newer applications in physico-chemical fields. The latter take advantage of the periodicity of the chiral field, with length-scales ranging from hundreds to thousands of nanometres, which characterise the cholesteric phase.     

Memory of chirality in diastereoselective alpha-alkylation of isoleucine and allo-isoleucine derivatives

[reaction: see text] alpha-Methylation of 3 gave 5 as a major product whereas 4 gave 6 predominantly, although both 3 and 4 have an (S)-chiral center at C(3). This indicates that chirality at C(2) in 3 and 4 was memorized in the corresponding intermediate enolates and the induced chirality made a major contribution in the stereochemical course of the reaction, while chirality at the adjacent chiral center C(3) had little effect.     

Synthesis and characterization of new chiral palladium β-diimine complexes

The synthesis and characterization of a range of chiral β-diimine ligands and their complexes with palladium(II) has been investigated. The introduction of chirality can be easily achieved through a combination of both achiral and chiral building blocks. The absolute configuration of the stereochemical centers has been determined. In addition, representative X-ray structures of both ligands and complexes have been determined.