Mukaiyama aldol addition

Last updated

educts
Mukaiyama Aldehyde V1.svg
aldehyde (R1 = Alkyl, Aryl)
or formate (R1 = OR)
Mukaiyama Enol V1.svg
silyl enol ether (R1 = Alkyl, Aryl, H;
R2 = Alkyl, Aryl, H, OR, SR)

In organic chemistry, the Mukaiyama aldol addition is an organic reaction and a type of aldol reaction between a silyl enol ether (R2C=CR−O−Si(CH3)3) and an aldehyde (R−CH=O) or formate (R−O−CH=O). [1] The reaction was discovered by Teruaki Mukaiyama in 1973. [2] His choice of reactants allows for a crossed aldol reaction between an aldehyde and a ketone (>C=O), or a different aldehyde without self-condensation of the aldehyde. For this reason the reaction is used extensively in organic synthesis.

Contents

General reaction scheme

The Mukaiyama aldol addition is a Lewis acid-mediated addition of enol silanes to carbonyl (C=O) compounds. In this reaction, compounds with various organic groups can be used (see educts). [3] A basic version (R2 = H) without the presence of chiral catalysts is shown below.

Simplified overview with a stereocenter Mukaiyama Aldol-Ubersichtsreaktion1.svg
Simplified overview with a stereocenter

A racemic mix of enantiomers is built. If Z- or E-enol silanes are used in this reaction a mixture of four products occurs, yielding two racemates.

Overview of reaction with consideration of stereochemistry Mukaiyama Aldol-Ubersichtsreaktion2.svg
Overview of reaction with consideration of stereochemistry

Whether the anti-diastereomer or the syn-diastereomer is built depends largely on reaction conditions, substrates and Lewis acids.

The archetypical reaction is that of the silyl enol ether of cyclohexanone, (CH2)5CO, with benzaldehyde, C6H5CHO. At room temperature it produces a diastereomeric mixture of threo (63%) and erythro (19%) β-hydroxyketone as well as 6% of the exocyclic, enone condensation product. In its original scope the Lewis acid (titanium tetrachloride, TiCl4) was used in stoichiometric amounts but truly catalytic systems exist as well. The reaction is also optimized for asymmetric synthesis.

MakaiyamaAldolReaction.svg

Mechanism

Below, the reaction mechanism is shown with R2 = H:

Mukaiyama Aldol-MechanismusV7 en Mukaiyama Aldol-MechanismusV7 en.svg
Mukaiyama Aldol-MechanismusV7 en

In the cited example the Lewis acid TiCl4 is used. First, the Lewis acid activates the aldehyde component followed by carbon-carbon bond formation between the enol silane and the activated aldehyde. With the loss of a chlorosilane the compound 1 is built. The desired product, a racemate of 2 and 3, is obtained by aqueous work-up. [3]

Scope

A typical reaction involving two ketones is that between acetophenone as the enol and acetone: [4]

Mukaiyama aldol between two ketones MukaiyamaAldolKetones.svg
Mukaiyama aldol between two ketones

Ketone reactions of this type require higher reaction temperatures. For this work Mukaiyama was inspired by earlier work done by Georg Wittig in 1966 on crossed aldol reactions with lithiated imines. [5] [6] Competing work with lithium enolate aldol reactions was published also in 1973 by Herbert O. House. [7]

Mukaiyama employed in his rendition of taxol total synthesis (1999) two aldol additions, [8] [9] one with a ketene silyl acetal and excess magnesium bromide:

Mukaiyama aldol in taxol synthesis MukaiyamaAldolInTaxolSynthesis.svg
Mukaiyama aldol in taxol synthesis

and a second one with an amine chiral ligand and a triflate salt catalyst:

Mukaiyama asymmetric aldol taxol MukaiyamaAsymmetricAldolTaxol.svg
Mukaiyama asymmetric aldol taxol

Utilization of chiral Lewis acid complexes and Lewis bases in asymmetric catalytic processes is the fastest-growing area in the usage of the Mukaiyama aldol reaction. [3]

Related Research Articles

The following outline is provided as an overview of and topical guide to organic chemistry:

The aldol reaction is a reaction that combines two carbonyl compounds to form a new β-hydroxy carbonyl compound.

<span class="mw-page-title-main">Enamine</span> Class of chemical compounds

An enamine is an unsaturated compound derived by the condensation of an aldehyde or ketone with a secondary amine. Enamines are versatile intermediates.

<span class="mw-page-title-main">Aldol condensation</span> Type of chemical reaction

An aldol condensation is a condensation reaction in organic chemistry in which two carbonyl moieties react to form a β-hydroxyaldehyde or β-hydroxyketone, and this is then followed by dehydration to give a conjugated enone.

<span class="mw-page-title-main">Enol</span> Organic compound with a C=C–OH group

In organic chemistry, alkenols are a type of reactive structure or intermediate in organic chemistry that is represented as an alkene (olefin) with a hydroxyl group attached to one end of the alkene double bond. The terms enol and alkenol are portmanteaus deriving from "-ene"/"alkene" and the "-ol" suffix indicating the hydroxyl group of alcohols, dropping the terminal "-e" of the first term. Generation of enols often involves deprotonation at the α position to the carbonyl group—i.e., removal of the hydrogen atom there as a proton H+. When this proton is not returned at the end of the stepwise process, the result is an anion termed an enolate. The enolate structures shown are schematic; a more modern representation considers the molecular orbitals that are formed and occupied by electrons in the enolate. Similarly, generation of the enol often is accompanied by "trapping" or masking of the hydroxy group as an ether, such as a silyl enol ether.

<span class="mw-page-title-main">Michael addition reaction</span> Reaction in organic chemistry

In organic chemistry, the Michael reaction or Michael 1,4 addition is a reaction between a Michael donor and a Michael acceptor to produce a Michael adduct by creating a carbon-carbon bond at the acceptor's β-carbon. It belongs to the larger class of conjugate additions and is widely used for the mild formation of carbon-carbon bonds.

<span class="mw-page-title-main">Enolate</span> Organic anion formed by deprotonating a carbonyl (>C=O) compound

In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds.

In organic chemistry, self-condensation is an organic reaction in which a chemical compound containing a carbonyl group acts both as the electrophile and the nucleophile in an aldol condensation. It is also called a symmetrical aldol condensation as opposed to a mixed aldol condensation in which the electrophile and nucleophile are different species.

<span class="mw-page-title-main">Trimethylsilyl group</span> Functional group

A trimethylsilyl group (abbreviated TMS) is a functional group in organic chemistry. This group consists of three methyl groups bonded to a silicon atom [−Si(CH3)3], which is in turn bonded to the rest of a molecule. This structural group is characterized by chemical inertness and a large molecular volume, which makes it useful in a number of applications.

The Reformatsky reaction is an organic reaction which condenses aldehydes or ketones with α-halo esters using metallic zinc to form β-hydroxy-esters:

<span class="mw-page-title-main">Danishefsky Taxol total synthesis</span>

The Danishefsky Taxol total synthesis in organic chemistry is an important third Taxol synthesis published by the group of Samuel Danishefsky in 1996 two years after the first two efforts described in the Holton Taxol total synthesis and the Nicolaou Taxol total synthesis. Combined they provide a good insight in the application of organic chemistry in total synthesis.

<span class="mw-page-title-main">Holton Taxol total synthesis</span>

The Holton Taxol total synthesis, published by Robert A. Holton and his group at Florida State University in 1994, was the first total synthesis of Taxol.

In organosilicon chemistry, silyl enol ethers are a class of organic compounds that share the common functional group R3Si−O−CR=CR2, composed of an enolate bonded to a silane through its oxygen end and an ethene group as its carbon end. They are important intermediates in organic synthesis.

The Rubottom oxidation is a useful, high-yielding chemical reaction between silyl enol ethers and peroxyacids to give the corresponding α-hydroxy carbonyl product. The mechanism of the reaction was proposed in its original disclosure by A.G. Brook with further evidence later supplied by George M. Rubottom. After a Prilezhaev-type oxidation of the silyl enol ether with the peroxyacid to form the siloxy oxirane intermediate, acid-catalyzed ring-opening yields an oxocarbenium ion. This intermediate then participates in a 1,4-silyl migration to give an α-siloxy carbonyl derivative that can be readily converted to the α-hydroxy carbonyl compound in the presence of acid, base, or a fluoride source.

Lanthanide triflates are triflate salts of the lanthanides. These salts have been investigated for application in organic synthesis as Lewis acid catalysts. These catalysts function similarly to aluminium chloride or ferric chloride, but they are water-tolerant (stable in water). Commonly written as Ln(OTf)3·(H2O)9 the nine waters are bound to the lanthanide, and the triflates are counteranions, so more accurately lanthanide triflate nonahydrate is written as [Ln(H2O)9](OTf)3.

<span class="mw-page-title-main">Kuwajima Taxol total synthesis</span>

The Kuwajima Taxol total synthesis by the group of Isao Kuwajima of the Tokyo Institute of Technology is one of several efforts in taxol total synthesis published in the 1990s. The total synthesis of Taxol is considered a landmark in organic synthesis.

<span class="mw-page-title-main">Mukaiyama Taxol total synthesis</span>

The Mukaiyama taxol total synthesis published by the group of Teruaki Mukaiyama of the Tokyo University of Science between 1997 and 1999 was the 6th successful taxol total synthesis. The total synthesis of Taxol is considered a hallmark in organic synthesis.

<span class="mw-page-title-main">Teruaki Mukaiyama</span> Japanese chemist (1927–2018)

Teruaki Mukaiyama was a Japanese organic chemist. One of the most prolific chemists of the 20th century in the field of organic synthesis, Mukaiyama helped establish the field of organic chemistry in Japan after World War II.

In organic chemistry, the Conia-ene reaction is an intramolecular cyclization reaction between an enolizable carbonyl such as an ester or ketone and an alkyne or alkene, giving a cyclic product with a new carbon-carbon bond. As initially reported by J. M. Conia and P. Le Perchec, the Conia-ene reaction is a heteroatom analog of the ene reaction that uses an enol as the ene component. Like other pericyclic reactions, the original Conia-ene reaction required high temperatures to proceed, limiting its wider application. However, subsequent improvements, particularly in metal catalysis, have led to significant expansion of reaction scope. Consequently, various forms of the Conia-ene reaction have been employed in the synthesis of complex molecules and natural products.

The ketimine Mannich reaction is an asymmetric synthetic technique using differences in starting material to push a Mannich reaction to create an enantiomeric product with steric and electronic effects, through the creation of a ketimine group. Typically, this is done with a reaction with proline or another nitrogen-containing heterocycle, which control chirality with that of the catalyst. This has been theorized to be caused by the restriction of undesired (E)-isomer by preventing the ketone from accessing non-reactive tautomers. Generally, a Mannich reaction is the combination of an amine, a ketone with a β-acidic proton and aldehyde to create a condensed product in a β-addition to the ketone. This occurs through an attack on the ketone with a suitable catalytic-amine unto its electron-starved carbon, from which an imine is created. This then undergoes electrophilic addition with a compound containing an acidic proton. It is theoretically possible for either of the carbonyl-containing molecules to create diastereomers, but with the addition of catalysts which restrict addition as of the enamine creation, it is possible to extract a single product with limited purification steps and in some cases as reported by List et al.; practical one-pot syntheses are possible. The process of selecting a carbonyl-group gives the reaction a direct versus indirect distinction, wherein the latter case represents pre-formed products restricting the reaction's pathway and the other does not. Ketimines selects a reaction group, and circumvent a requirement for indirect pathways.

References

  1. Mukaiyama, T.; Kobayashi, S. (1994). "Tin(II) Enolates in the Aldol, Michael, and Related Reactions". Org. React. 46: 1. doi:10.1002/0471264180.or046.01. ISBN   0471264180.
  2. New aldol type reaction Teruaki Mukaiyama, Koichi Narasaka and Kazuo Banno Chemistry Letters Vol.2 (1973), No.9 pp. 1011–1014 doi : 10.1246/cl.1973.1011
  3. 1 2 3 Kürti, László; Czakó, Barbara (2005). Strategic applications of named reactions in organic synthesis : background and detailed mechanisms . Elsevier Academic Press. pp.  298–299. ISBN   978-0-12-429785-2.
  4. Organic Syntheses, Coll. Vol. 8, p. 323 (1993); Vol. 65, p. 6 (1987). http://www.orgsynth.org/orgsyn/pdfs/CV8P0323.pdf
  5. Wittig, G.; Suchanek, P. (January 1966). "Über gezielte aldokondensationen—II". Tetrahedron. 22: 347–358. doi:10.1016/S0040-4020(01)82193-1.
  6. DIRECTED ALDOL CONDENSATIONS: β-PHENYLCINNAMALDEHYDE Organic Syntheses, Coll. Vol. 6, p. 901 (1988); Vol. 50, p. 66 (1970). G. Wittig, A. Hesse, Allan Y. Teranishi and Herbert O. House http://www.orgsynth.org/orgsyn/prep.asp?prep=cv6p0901
  7. House, Herbert O.; Crumrine, David S.; Teranishi, Allan Y.; Olmstead, Hugh D. (May 1973). "Chemistry of carbanions. XXIII. Use of metal complexes to control the aldol condensation". Journal of the American Chemical Society. 95 (10): 3310–3324. doi:10.1021/ja00791a039.
  8. Mukaiyama, Teruaki; Shiina, Isamu; Iwadare, Hayato; Saitoh, Masahiro; Nishimura, Toshihiro; Ohkawa, Naoto; Sakoh, Hiroki; Nishimura, Koji; Tani, Yu-ichirou; Hasegawa, Masatoshi; Yamada, Koji; Saitoh, Katsuyuki (4 January 1999). "Asymmetric Total Synthesis of Taxol\R". Chemistry – A European Journal. 5 (1): 121–161. doi: 10.1002/(SICI)1521-3765(19990104)5:1<121::AID-CHEM121>3.0.CO;2-O .
  9. TBS = t-butyldimethylsilyl, Bn = benzyl, PMB = p-methoxybenzyl ether