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((multiple issues|((missing information|many important aspects of the field|date=November 2013))
((more citations needed|date=November 2013))))
((Acids and bases))

'''Chiral Lewis acids''' (CLAs) are a type of [[Lewis acid catalysis|Lewis acid catalyst]]. These acids affect the [[Chirality (chemistry)|chirality]] of the substrate as they react with it. In such reactions, synthesis favors the formation of a specific [[enantiomer]] or [[diastereomer]]. The method is an enantioselective [[asymmetric synthesis]] reaction. Since they affect chirality, they produce [[optically active]] products from optically inactive or mixed starting materials. This type of preferential formation of one [[enantiomer]] or [[diastereomer]] over the other is formally known as [[asymmetric induction]]. In this kind of Lewis acid, the electron-accepting atom is typically a metal, such as [[indium]], [[zinc]], [[lithium]], [[aluminium]], [[titanium]], or [[boron]]. The [[chirality (chemistry)|chiral]]-altering ligands employed for synthesizing these acids often have multiple [[Lewis base|Lewis basic]] sites (often a [[diol]] or a dinitrogen structure) that allow the formation of a ring structure involving the metal atom.<ref>((Cite book|last=Yamamoto|first=Hisashi|url=http://worldcat.org/oclc/315587750|title=Lewis acid reagents a practical approach|date=2007|publisher=Knovel|isbn=978-1-60119-442-8|oclc=315587750))</ref><ref>((Cite journal|last1=Corey|first1=E. J.|last2=Imwinkelried|first2=Rene|last3=Pikul|first3=Stanislaw|last4=Xiang|first4=Yi Bin|date=July 1989|title=Practical enantioselective Diels-Alder and aldol reactions using a new chiral controller system|url=http://dx.doi.org/10.1021/ja00196a081|journal=Journal of the American Chemical Society|volume=111|issue=14|pages=5493–5495|doi=10.1021/ja00196a081|issn=0002-7863))</ref>

Achiral Lewis acids have been used for decades to promote the synthesis of [[racemic mixtures]] in myriad different reactions. Since the 1960s, chemists have used Chiral Lewis acids to induce enantioselective reactions. This is useful when the desired product is a specific enantiomer, as is common in drug synthesis. Common reaction types include [[Diels–Alder reaction]]s, the [[ene reaction]], [2+2] [[cycloaddition]] reactions, [[hydrocyanation]] of [[aldehydes]], and most notably, [[Sharpless epoxidation]]s.<ref>((Cite journal|last=Narasaka|first=Koichi|date=1991|title=Chiral Lewis Acids in Catalytic Asymmetric Reactions|url=http://dx.doi.org/10.1055/s-1991-26364|journal=Synthesis|volume=1991|issue=1|pages=1–11|doi=10.1055/s-1991-26364|issn=0039-7881))</ref>

==Theory==
[[File: Chiral and Achiral Lewis Acid Catalyzed reactions.jpg|thumb|Figure 2. Top: Gibbs Free Energy diagram depicting single-step reaction where an achiral Lewis acid is catalyzing the formation of a racemic mixture of products from racemic starting materials. Bottom: Gibbs free energy diagram depicting the same reaction when a chiral Lewis acid is used as the catalyst |400px]]

The enantioselectivity of CLAs derives from their ability to perturb the free [[energy barrier]] along with the [[reaction coordinate]] pathway that leads to either the ''R''- or ''S''- enantiomer. Ground state diastereomers and enantiomers are of equal energy in the ground state, and when reacted with an achiral Lewis acid, their diastereomeric intermediates, [[transition state]]s, and products are also of equal energy. This leads to the production of [[racemic mixtures]]. However, when a CLA is used in the same reaction, the energetic barrier of formation of one diastereomer is less than that of another; the reaction is under [[kinetic control]]. If the difference in the energy barriers between the diastereomeric transition states are of sufficient magnitude, then a high [[enantiomeric excess]] of one isomer is observed.<ref>((Cite book | title = Asymmetric Organic Reactions | author = Morrison, J.D., [[Harry Stone Mosher|Mosher, H.S.]] | publisher = Prentice-Hall, Inc. | date = 1971 | isbn = 978-0-13-049551-8))</ref>

==Asymmetric synthesis==

=== Diels–Alder reaction ===

[[Diels–Alder reaction]]s occur between a conjugated [[diene]] and  an [[alkene]] (commonly known as the [[dienophile]]). This [[cycloaddition]] process allows for the stereoselective formation of [[cyclohexene]] rings capable of possessing as many as four contiguous stereogenic centers.

Diels–Alder reactions can lead to the formation of a variety of structural isomers and stereoisomers. [[Molecular orbital theory]] considers that the endo transition state, instead of the exo transition state, is favored (endo addition rule). Also, augmented secondary orbital interactions have been postulated as the source of enhanced endo diastereoselection.

[[File:CLA1.png|350px]]

Usually, CLAs are employed to activate the dienophile.  A typical CLA catalyst is derived from a Mg<sup>2+</sup> center made chiral by attachment of a [[binol]]- phosphate ester. CLAs have been applied to a number of intramolecular Diels–Alder reactions.<ref>((cite book |doi=10.1007/3418_2015_141|chapter=The Future of Catalysis by Chiral Lewis Acids|title=Chiral Lewis Acids|series=Topics in Organometallic Chemistry|year=2015|last1=Sha|first1=Qiang|last2=Deng|first2=Yongming|last3=Doyle|first3=Michael P.|volume=62|pages=1–25|isbn=978-3-319-70804-1))</ref>

A complex derived from [[diethylaluminium chloride]] and a “vaulted” biaryl ligand below catalyzes the enantioselective Diels–Alder reaction between cyclopentadiene and methacrolein. The chiral ligand is recovered quantitatively by silica gel [[chromatography]].<ref>((Cite journal|last1=Bao|first1=Jianming|last2=Wulff|first2=William D.|last3=Rheingold|first3=Arnold L.|date=May 1993|title=Vaulted biaryls as chiral ligands for asymmetric catalytic Diels-Alder reactions|url=http://dx.doi.org/10.1021/ja00062a073|journal=Journal of the American Chemical Society|volume=115|issue=9|pages=3814–3815|doi=10.1021/ja00062a073|issn=0002-7863))</ref>

[[File:CLA5.png|550px]]

The chiral (acyloxy) borane (CAB) complex is effective in catalyzing a number of aldehyde Diels–Alder reactions. NMR spectroscopic experiments have indicated close proximity of the aldehyde and the aryl ring. [[Pi-Stacking (chemistry)|Pi stacking]] between the aryl group and aldehyde has been suggested as an organizational feature that imparts high enantioselectivity to the cycloaddition.<ref>((Cite journal|last1=Ishihara|first1=Kazuaki|last2=Gao|first2=Qingzhi|last3=Yamamoto|first3=Hisashi|date=November 1993|title=Mechanistic studies of a CAB-catalyzed asymmetric Diels-Alder reaction|url=http://dx.doi.org/10.1021/ja00075a088|journal=Journal of the American Chemical Society|volume=115|issue=22|pages=10412–10413|doi=10.1021/ja00075a088|issn=0002-7863))</ref>

[[File:CLA6.png|550px]]

Bronsted acid-assisted chiral Lewis acid (BLA) catalyzes a number of diene-aldehyde cycloaddition reactions.<ref>((Cite journal|last1=Ishihara|first1=Kazuaki|last2=Yamamoto|first2=Hisashi|date=February 1994|title=Bronsted Acid Assisted Chiral Lewis Acid (BLA) Catalyst for Asymmetric Diels-Alder Reaction|url=http://dx.doi.org/10.1021/ja00083a048|journal=Journal of the American Chemical Society|volume=116|issue=4|pages=1561–1562|doi=10.1021/ja00083a048|issn=0002-7863))</ref>

[[File:CLA7-3.png|550px]]

===Aldol reaction===
In the [[aldol]] reaction, the diastereoselectivity of the product is often dictated by the geometry of the enolate. The [[Aldol reaction|Zimmerman–Traxler model]] predicts that the Z [[enolate]] will give [[syn addition|syn]] products, and that E enolates will give [[anti addition|anti]] products. Reactions catalyzed by tin-based CLAs allow products to deviate from this pattern.<ref>((Cite journal|last1=Kobayashi|first1=Shū|last2=Horibe|first2=Mineko|date=September 1997|title=Chiral Lewis Acid Controlled Synthesis (CLAC Synthesis): Chiral Lewis Acids Influence the Reaction Course in Asymmetric Aldol Reactions for the Synthesis of Enantiomeric Dihydroxythioester Derivatives in the Presence of Chiral Diamines Derived from L-Proline|url=http://dx.doi.org/10.1002/chem.19970030914|journal=Chemistry - A European Journal|volume=3|issue=9|pages=1472–1481|doi=10.1002/chem.19970030914|issn=0947-6539))</ref>

[[File:ald1.png|400px]]

The transition structures for reactions with both the R and S catalyst [[enantiomers]] are:

[[File:ald2.png|400px]]

=== Baylis–Hillman reaction ===
[[File:bhillmech.png|500px]]

The [[Baylis–Hillman reaction]] is a route for C-C bond formation between an alpha, beta-unsaturated [[carbonyl]] and an [[aldehyde]], which requires a [[nucleophilic]] catalyst, usually a tertiary [[amine]], for a Michael-type addition and elimination. The stereoselectivity of these reactions is usually poor.  [[Lanthanum]](III)-containing CLAs have been demonstrated to improve stereoselectivity. Similarly, a chiral amine may also be used to achieve stereoselectivity.<ref>((Cite journal|last1=Yang|first1=Kung-Shuo|last2=Lee|first2=Wei-Der|last3=Pan|first3=Jia-Fu|last4=Chen|first4=Kwunmin|date=February 2003|title=Chiral Lewis Acid-Catalyzed Asymmetric Baylis−Hillman Reactions|url=http://dx.doi.org/10.1021/jo026318m|journal=The Journal of Organic Chemistry|volume=68|issue=3|pages=915–919|doi=10.1021/jo026318m|pmid=12558416|issn=0022-3263))</ref>

[[File:bhillchiralcat.png|200px]]

The product obtained by the reaction using the chiral [[catalyst]] was obtained in good yield with excellent enantioselectivity.

[[File:bhillprod.png|200px]]

===Ene reaction===
Chiral Lewis acids have proven useful in the [[ene reaction]]. When catalyzed by an achiral Lewis acid, the reaction normally provides good diastereoselectivity.<ref>((Cite journal|last1=Yang|first1=Dan|last2=Yang|first2=Min|last3=Zhu|first3=Nian-Yong|date=October 2003|title=Chiral Lewis Acid-Catalyzed Enantioselective Intramolecular Carbonyl Ene Reactions of Unsaturated α-Keto Esters|url=http://dx.doi.org/10.1021/ol035486d|journal=Organic Letters|volume=5|issue=20|pages=3749–3752|doi=10.1021/ol035486d|pmid=14507221|issn=1523-7060))</ref>

[[File:enecatrxn.png|500px]]

Good enantioselectivity has been observed when a chiral Lewis acid catalyst is used.

[[File:enetstruct.png|500px]]

The enantioselectivity is believed to be due to the [[steric]] interactions between the [[methyl]] and [[phenyl]] group, which makes the transition structure of the iso product considerably more favorable.

==Achiral Lewis acids in stereoselective synthesis==
In some cases, an achiral Lewis acid may provide good stereoselectivity.  Kimura et al. demonstrated the regio- and diastereoselective coupling of 1,3-[[diene]]s with [[aldehydes]] using a [[nickel]] catalyst.<ref>((Cite journal|last1=Kimura|first1=Masanari|last2=Ezoe|first2=Akihiro|last3=Mori|first3=Masahiko|last4=Iwata|first4=Keisuke|last5=Tamaru|first5=Yoshinao|date=July 2006|title=Regio- and Stereoselective Nickel-Catalyzed Homoallylation of Aldehydes with 1,3-Dienes|url=http://dx.doi.org/10.1021/ja0608904|journal=Journal of the American Chemical Society|volume=128|issue=26|pages=8559–8568|doi=10.1021/ja0608904|pmid=16802822|issn=0002-7863))</ref>

==References==
((reflist))

[[Category:Stereochemistry]]
[[Category:Acids]]

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