Hi friend...... These are highlights of chemical rxn starting with C. Hope u find them useful..
Cadiot-Chodkiewicz Coupling
The copper(I)-catalyzed coupling of a terminal alkyne and an alkynyl halide offers access to unsymmetrical bisacetylenes.
Mechanism
Recent Literature
Bulky Trialkylsilyl Acetylenes in the Cadiot-Chodkiewicz Cross-Coupling Reaction
J. P. Marino, H. N. Nguyen,
J. Org. Chem., 2002,
67, 6841-6844.
Cannizzaro Reaction
This redox disproportionation of non-enolizable aldehydes to carboxylic acids and alcohols is conducted in concentrated base.
?-Keto aldehydes give the product of an intramolecular disproportionation in excellent yields.
Mechanism
An interesting variant, the Crossed Cannizzaro Reaction, uses formaldehyde as reducing agent:
At the present time, various oxidizing and reducing agents can be used to carry out such conversions (with higher yields), so that today the Cannizzaro Reaction has limited synthetic utility except for the abovementioned conversion of ?-keto aldehydes.
The Cannizzaro Reaction should be kept in mind as a source of potential side products when aldehydes are treated under basic conditions.
Recent Literature
Ytterbium Triflate-Promoted Tandem One-Pot Oxidation-Cannizzaro Reaction of Aryl Methyl Ketones
M. Curini, F. Epifano, S. Genovese, M. C. Marcotullio, O. Rosati,
Org. Lett.,
2005,
7, 1331-1333.
Corey-Bakshi-Shibata Reduction
The enantioselective reduction of ketones using
borane and a chiral oxazaborolidine as catalyst (CBS Catalyst). Usually, MeCBS is used (R'' = Me, but selectivity may be increased by varying this substituent).
Mechanism
The mechanism depicted portrays the rationale for the enantioselectivity and high reaction rates, which are influenced only by the CBS catalyst. This catalyst is a combination of both a Lewis acid and a chiral auxiliary!
Recent Literature
An Efficient and Catalytically Enantioselective Route to (
S)-(-)-Phenyloxirane
E. J. Corey, S. Shibata, R. K. Bakshi,
J. Org, Chem.,
1988,
53, 2861-2863.
A Catalytic Enantioselective Synthesis of the Endothelin Receptor Antagonists SB-209670 and SB-217242. A Base-Catalyzed Stereospecific Formal 1,3-Hydrogen Transfer of a Chiral 3-Arylindenol
W. M. Clark, A. M. Tickner-Eldridge, G. K. Huang, L. N. Pridgen, M. A. Olsen, R. J. Mills, I. Lantos, N. H. Baine,
J. Am. Chem. Soc.,
1998,
120, 4550-4551.
Acetoacetic-Ester Condensation
Claisen Condensation
The Claisen Condensation between esters containing ?-hydrogens, promoted by a base such as sodium ethoxide, affords ?-ketoesters. The driving force is the formation of the stabilized anion of the ?-keto ester. If two different esters are used, an essentially statistical mixture of all four products is generally obtained, and the preparation does not have high synthetic utility.
However, if one of the ester partners has enolizable ?-hydrogens and the other does not (e.g., aromatic esters or carbonates), the mixed reaction (or crossed Claisen) can be synthetically useful. If ketones or nitriles are used as the donor in this condensation reaction, a ?-diketone or a ?-ketonitrile is obtained, respectively.
The use of stronger bases, e.g. sodium amide or sodium hydride instead of sodium ethoxide, often increases the yield.
Mechanism
Recent Literature
Powerful Ti-Crossed Claisen Condensation between Ketene Silyl Acetals or Thioacetals and Acid Chlorides or Acids
A. Iida, S. Nakazawa, T. Okabayashi, A. Horii, T. Misako, Y. Tanabe,
Org. Lett.,
2006,
8, 5215-5218.
Claisen Rearrangement
The aliphatic Claisen Rearrangement is a [3,3]-sigmatropic rearrangement in which an allyl vinyl ether is converted thermally to an unsaturated carbonyl compound.
The aromatic Claisen Rearrangement is accompanied by a rearomatization:
The etherification of alcohols or phenols and their subsequent Claisen Rearrangement under thermal conditions makes possible an extension of the carbon chain of the molecule.
Mechanism
Mechanism of the Cope Rearrangement
Mechanism of the Claisen Rearrangement
The reaction proceeds preferably via a chair transition state. Chiral, enantiomerically enriched starting materials give products of high optical purity.
A boat transition state is also possible, and can lead to side products:
The aromatic Claisen Rearrangement is followed by a rearomatization:
When the
ortho-position is substituted, rearomatization cannot take place. The allyl group must first undergo a
Cope Rearrangement to the
para-position before tautomerization is possible.
All Claisen Rearrangement reactions described to date require temperatures of > 100 °C if uncatalyzed. The observation that electron withdrawing groups at C-1 of the vinyl moiety exert a positive influence on the reaction rate and the yield has led to the development of the following variations:
Ireland-Claisen Rearrangement
Eschenmoser-Claisen Rearrangement
Johnson-Claisen Rearrangement
Recent Literature
Using Geminal Dicationic Ionic Liquids as Solvents for High-Temperature Organic Reactions
X. Han, D. W. Armstrong,
Org. Lett.,
2005,
7, 4205-4208.
A Domino Copper-Catalyzed C-O Coupling-Claisen Rearrangement Process
G. Nordmann, S. L. Buchwald,
J. Am. Chem. Soc.,
2003,
125, 4978-4979.
Tandem Horner-Wadsworth-Emmons Olefination/Claisen Rearrangement/Hydrolysis Sequence: Remarkable Acceleration in Water with Microwave Irradiation
E. Quesada, R. J. K. Taylor,
Synthesis,
2005, 3193-3195.
Asymmetric Claisen Rearrangements Enabled by Catalytic Asymmetric Di(allyl) Ether Synthesis
S. G. Nelson, K. Wang,
J. Am. Chem. Soc.,
2006,
128, 4232-4233.
Gold(I)-Catalyzed Propargyl Claisen Rearrangement
B. D. Sherry, F. D. Toste,
J. Am. Chem. Soc.,
2004,
126, 15978-15979.
Gold(I)-Catalyzed Synthesis of Highly Substituted Furans
M. H. Suhre, M. Reif, S. F. Kirsch,
Org. Lett.,
2005,
7, 3873-3876.
Clemmensen Reduction
The Clemmensen Reduction allows the deoxygenation of aldehydes or ketones, to produce the corresponding hydrocarbon.
The substrate must be stable to strong acid. The Clemmensen Reduction is complementary to the
Wolff-Kishner Reduction, which is run under strongly basic conditions. Acid-labile molecules should be reduced by the Wolff-Kishner protocol.
Mechanism
The reduction takes place at the surface of the zinc catalyst. In this reaction, alcohols are not postulated as intermediates, because subjection of the corresponding alcohols to these same reaction conditions does not lead to alkanes. The following proposal employs the intermediacy of zinc carbenoids to rationalize the mechanism of the Clemmensen Reduction:
Cope Elimination
N-Oxides give alkenes via a
syn-elimination under heating. This reaction obeys
Hofmann's Rule.
Mechanism
Cope Rearrangement
(Anionic) Oxy-Cope Rearrangement
The Cope Rearrangement is the thermal isomerization of a 1,5-diene leading to a regioisomeric 1,5-diene. The main product is the thermodynamically more stable regioisomer. The Oxy-Cope has a hydroxyl substituent on an sp3-hybridized carbon of the starting isomer.
The driving force for the neutral or anionic Oxy-Cope Rearrangement is that the product is an enol or enolate (resp.), which can tautomerize to the corresponding carbonyl compound. This product will not equilibrate back to the other regioisomer.
The Oxy-Cope Rearrangement proceeds at a much faster rate when the starting alcohol is deprotonated, e.g. with KH. The reaction is then up to 1017 times faster, and may be conducted at room temperature. Aqueous work up then gives the carbonyl compound.
Mechanism
Two transition states are possible, and the outcome of the reaction can be predicted on the basis of the most favorable overlap of the orbitals of the double bond, as influenced by stereoelectronic factors:
Recent Literature
Reverse Aromatic Cope Rearrangement of 2-Allyl-3-alkylideneindolines Driven by Olefination of 2-Allylindolin-3-ones: Synthesis of ?-Allyl-3-indole Acetate Derivatives
T. Kawasaki, Y. Nonaka, K. Watanabe, A. Ogawa, K. Higuchi, R. Terashima, K. Masuda, M. Sakamoto,
J. Org. Chem.,
2001,
66, 1200 - 1204.
Corey-Bakshi-Shibata Reduction
The enantioselective reduction of ketones using
borane and a chiral oxazaborolidine as catalyst (CBS Catalyst). Usually, MeCBS is used (R'' = Me, but selectivity may be increased by varying this substituent).
Mechanism
The mechanism depicted portrays the rationale for the enantioselectivity and high reaction rates, which are influenced only by the CBS catalyst. This catalyst is a combination of both a Lewis acid and a chiral auxiliary!
Recent Literature
An Efficient and Catalytically Enantioselective Route to (
S)-(-)-Phenyloxirane
E. J. Corey, S. Shibata, R. K. Bakshi,
J. Org, Chem.,
1988,
53, 2861-2863.
A Catalytic Enantioselective Synthesis of the Endothelin Receptor Antagonists SB-209670 and SB-217242. A Base-Catalyzed Stereospecific Formal 1,3-Hydrogen Transfer of a Chiral 3-Arylindenol
W. M. Clark, A. M. Tickner-Eldridge, G. K. Huang, L. N. Pridgen, M. A. Olsen, R. J. Mills, I. Lantos, N. H. Baine,
J. Am. Chem. Soc.,
1998,
120, 4550-4551.
Corey-Chaykovsky Reaction
The reaction of sulfur ylides with carbonyl compounds such as ketones or the related imines leads to the corresponding epoxides or aziridines.
Corey-Chaykovsky Epoxidation
Corey-Chaykovsky Aziridination
The reaction of sulfur ylides with enones gives cyclopropanes.
Corey-Chaykovsky Cyclopropanation
Mechanism
The ylides are generated in situ by the deprotonation of sulfonium halides with strong bases.
Dimethyloxosulfonium methylide - known as the Corey-Chaykovsky Reagent - is a valuable alternative to dimethylsulfonium methylide and can be generated from trimethylsulfoxonium iodide.
Higher substituted ylides can be generated selectively if one substituent is preferably deprotonated over the others, for example when the negative charge is stabilized or the environment is sterically less demanding:
Such ylides are able to transfer more than just a methylene group, and enantioselective induction can be observed if the ylide is chiral:
The ylide initially acts as a nucleophile toward the carbonyl compound. The resulting oxygen anion then reacts as an intramolecular nucleophile toward the now electrophilic ylide carbon, which bears a sulfonium cation as a good leaving group:
The reaction of the Corey-Chaykovsky Reagent with enones is a 1,4-addition that is followed by ring closure to give a cyclopropane:
As sulfides are readily alkylated, it is even possible to use them catalytically. Such methods can give very interesting results when expensive chiral sulfides are used for the generation of chiral epoxides.
For a review of enantioselective methods see: V. K. Aggarwal, J. Richardson,
Chem. Commun. 2003, 2644.
DOIWith phosphorus ylides as used for the
Wittig Reaction, the phosphorus atom forms a strong double bond with oxygen. This leads the mechanism in a different direction, to effect olefination instead of epoxidation through intermediate oxaphosphetanes.
Recent Literature
Design of Sulfides with a Locked Conformation as Promoters of Catalytic and Asymmetric Sulfonium Ylide Epoxidation
M. Davoust, J.-F. Briere, P.-A. Jaffres, P. Metzner,
J. Org. Chem.,
2005,
70, 4166-4169.
The first Corey-Chaykovsky epoxidation and cyclopropanation in ionic liquids
S. Chandrasekhar, Ch. Narasihmulu, V. Jagadeshwar, K. Venkatram Reddy,
Tetrahedron Lett.,
2003,
44, 3629-3630.
A Novel Procedure for the Synthesis of Epoxides: Application of Simmons-Smith Reagents toward Epoxidation
V. K. Aggarwal, A. Ali, M. P. Coogan,
J. Org. Chem.,
1997,
125, 8628-8629.
Corey-Fuchs Reaction
This two step methodology allows the preparation of terminal alkynes by one-carbon homologation of an aldehyde. The first step is comparable to a
Wittig Reaction, and leads to a dibromoalkene. Treatment with a lithium base (BuLi, LDA) generates a bromoalkyne intermediate via dehydrohalogenation, which undergoes metal-halogen exchange under the reaction conditions and yields the terminal alkyne upon work-up.
A modification of the Corey-Fuchs Reaction involves the reaction of the intermediate alkynyllithium with an electrophile prior to aqueous work-up, giving a chain extension product:
Mechanism
In the formation of the ylide from CBr4, two equivalents of triphenylphosphine are used. One equivalent forms the ylide while the other acts as reducing agent and bromine scavenger.
The addition of the ylide to the aldehyde:
Reaction of the dibromoalkene with BuLi:
Recent Literature
End-Cap Stabilized Oligoynes: Model Compounds for the Linear sp Carbon Allotrope Carbyne
T. Gibtner, F. Hampel, J.-P.Gisselbrecht, A. Hirsch,
Chem. Eur. J.,
2002,
68, 408-432.
Corey-Kim Oxidation
The Corey-Kim Oxidation allows the synthesis of aldehydes and ketones from primary alcohols and secondary alcohols, respectively.
Mechanism
Dimethylchlorosulphonium ion is generated in situ from NCS and DMS:
Recent Literature
New odorless method for the Corey?Kim and Swern oxidations utilizing dodecyl methyl sulfide (Dod-S-Me)
S.-I. Ohsugia, K. Nishidea, K. Oonob, K. Okuyamab, M. Fudesakaa, S. Kodamaa, M. Node,
Tetrahedron,
2003,
59, 8393-8398.
The fluorous Swern and Corey-Kim reaction: scope and mechanism
D. Crich, S. Neelamkavil,
Tetrahedron,
2002,
58, 3865-3870.
Cross Metathesis
The transalkylidenation of two terminal alkenes under release of ethene, catalyzed by ruthenium carbenoids (Grubbs Catalyst). Statistically, the reaction can lead to three possible pairs of geometric isomers, i.e. E/Z pairs for two homocouplings and the cross-coupling (R-CH=CH-R, R'-CH=CH-R', and R-CH=CH-R') - a total of 6 products.
The selectivity of this reaction is currently undergoing further study, but various examples exist in which two alkenes with different reactivity give the cross-coupled product with excellent yields and excellent selectivity.
Mechanism
Recent Literature
Regioselective Cross-Metathesis Reaction Induced by Steric Hindrance
S. BouzBouz, R. Simmons, J. Cossy,
Org. Lett., 2004,
6, 3465-3467.
Selective Synthesis of (2
Z,4
E)-Dienyl Esters by Ene-Diene Cross Metathesis
G. Moura-Letts, D. P. Curran,
Org. Lett.,
2007,
9, 5-8.
Advanced Fine-Tuning of Grubbs/Hoveyda Olefin Metathesis Catalysts: A Further Step toward an Optimum Balance between Antinomic Properties
M. Bieniek, R. Bujok, M. Cabaj, N. Lugan, G. Lavigne, D. Arlt, K. Grela,
J. Am. Chem. Soc.,
2006,
128, 13652-13653.
A Good Bargain: An Inexpensive, Air-Stable Ruthenium Metathesis Catalyst Derived from ?-Asarone
K. Grela, M. Kim,
Eur. J. Org. Chem.,
2003, 963-966.
The facile preparation of alkenyl metathesis synthons
T. W. Baughman, J. C. Sworen, K. B. Wagener,
Tetrahedron, 2004,
60, 10943-10948.
Ruthenium-Catalyzed Tandem Olefin Metathesis-Oxidations
A. A. Scholte, M. H. An, M. L. Snapper,
Org. Lett.,
2006,
8, 4759-4762.
Preparation of Aliphatic Ketones through a Ruthenium-Catalyzed Tandem Cross-Metathesis/Allylic Alcohol Isomerization
D. Finnegan, B. A. Seigal, M. L. Snapper,
Org. Lett.,
2006,
8, 2603-2606.
Corey-Winter Olefin Synthesis
Conversion of 1,2-diols to alkenes. The cyclic thiocarbonate is available from reaction of the diol with thiophosgene or thiocarbonyldiimidazole, and reacts with added trimethylphosphite via a syn-elimination to the alkene.
Mechanism
It is assumed that the reaction proceeds with attack of phosphite on sulphur leading to a carbene, which may react with a second equivalent of phosphite during the cycloreversion that splits out carbon dioxide and yields the product alkene.
Curtius Rearrangement
The Curtius Rearrangement is the thermal decomposition of carboxylic azides to produce an isocyanate. These intermediates may be isolated, or their corresponding reaction or hydrolysis products may be obtained.
The reaction sequence - including subsequent reaction with water which leads to amines - is named the Curtius Reaction. This reaction is similar to the
Schmidt Reaction with acids, differing in that the acyl azide in the present case is prepared from the acyl halide and an azide salt.
Mechanism
Preparation of azides:
Decomposition:
Reaction with water to the unstable carbamic acid derivative which will undergo spontaneous decarboxylation:
Isocyanates are versatile starting materials:
Isocyanates are also of high interest as monomers for polymerization work and in the derivatisation of biomacromolecules.
Recent Literature
Boc-Protected Amines via a Mild and Efficient One-Pot Curtius Rearrangement
H. Lebel, O. Leogane,
Org. Lett.,
2005,
7, 4107-4110.
Radical Azidonation of Aldehydes
L. Marinescu, J. Thinggaard, I. B. Thomsen, M. Bols,
J. Org. Chem., 2003,
68, 9453-9455.
Radical Azidonation of Aldehydes
L. Marinescu, J. Thinggaard, I. B. Thomsen, M. Bols,
J. Org. Chem., 2003,
68, 9453-9455.
Back to Community Shelf
Awaiting Review for Nickels![[Post New]](/templates/default/images/icon_minipost_new.gif){kind=icon}
156
