Wittig reaction - Essay Example

? Wittig Reaction Wittig Reaction is also known as the witting olefination, the witting reaction is a chemical test that examines the chemical reaction that occurs when the chemical components of aldehyde also known as ketone and triphenyl phoshonium are combined. The latter chemical agent is commonly known as the witting reagent, it results into chemical agents known as alkene as well as triphenylphosphine oxide (Carrutthers, 1971). Discovered in 1954 by Georg Witting, it continues to be vastly used in the organic synthesis of alkene preparation. This procedure should not be misunderstood with, the Witting rearrangement chemical procedure which is based on a different theory. The witting reaction is normally applicable in the coupling of aldehydes and kentones, is the single substitution of phosphine ylides. The out coming data from the ylides test results is nearly exclusive with the z-alkene chemical agent product. As such for one to obtain the E-alkene chemical agent, there is need to apply ylides under stabilization which can also be substituted with unstablised ylides. This can be undertaken with the application of the Schlosser modification chemical tests, after which the Witting reaction chemical test can be performed (Vedejs et al, 2000). The witting reaction has a variation known as the classical mechanism; this is the established theoratical procedure of the witting reaction chemical test. It involves the bulk steric of ylide, this interacts with sterochemicals to produce nucleophilic addittives. This gives rise to betaine, the carbon-carbon bond rotation produces betaine tht in turn produces oxaphosphetane. By eliminating the desireable z-alkene in addittion to triphenylphosphine oxide components, the simplified witting reagents are used in a sequenced procedure. The first sequence of this procedure begins with a combination of aldehyes and ketones, this is followed by the decomposition of betaine. This decomposition occurs to the fifth form, this stage is also known as the rate-determination level. However, with ylides under stablization the initial sequence is noticed to be the slowest. As such the general alkene formation rate is reduced with time, this results into a sizeable proportion of the akene product in which case being the E-isomer. This creates an understanding of the reasons, behind the failure of the stablizing reagents in proper reaction with sterical hindered ketones (Vedejs et al. 2000). Witting reagents such as phosphorus ylides, are prepared in a formulated procedure. Phosphium salt is the known derivative of preparation; it is also a resulting chemical agent from the reaction of triphenylphosphine and alkyl halide. As such in order to create the witting reagent being ylide, phosphonium salt must undergo suspension in a solvent such like diethyl ether with treatment of strong base chemicals like phenyllithium which can also be substituted with butyllitium. This can be shown with the following chemical equation Ph3P+CH2R X? + C4H9Li > Ph3P=CH?R + LiX + C4H10, in this chemical procedure methylenetriphenylphosphorane is the simplified ylide in use (Vedejs and Marth, 1998). This yield is also a precursor to a more defined elaboration of the witting reagents, alkylation occurs as seen in this chemical equation Ph3P=CH2 by the main alkyl halide that creates a phosphonium salt substitution. The formulation of these salts, is deprotonated in the normal matter resulting into a chemical agent as identified by the following chemical equation Ph3P=CH?CH2R. The ylide which is the witting reagent is structured in such a way that is identified as phosphorane in written form. This is an established representation of the ylide form, being a vital contributor as carbon remains mildly nucleophilic. Its chemical structure is comprised of a ball-and-stick model arrangement, that is takes the physical form of a crystal structure. In terms of its reactivity, simplified phosphoranes are highly reactive and very unstable in moisturized and oxygenated atmospheric environments (Vedejs and Marth 1998). Concerning the reactivity of the witting reaction, this involves simplified phosphoranes that are highly reactive in addition to being unstable in environments of high moisture with oxygenated air. As such there preparation is undertaken in super dry solvent, with argon or nitrogen with the addittion of carbonyl compounds at the moment of formation of phosphorane. For more stabilized phosphoranes these can be obtained, when the yilde compositions contains a certain collection that can undergo stabilization such that the resulting negative charge from the carbanion reacts as shown in the following chemical reaction Ph3P=CHPh. While phosphonium salts which exist as reagents, their formation is of more readiness with only a requirement of NaOh. These salts are of increased atmospheric stability, in addition to having a reduced reaction in comparison with simplified ylides. As such they normally fail in reaction with ketones, which necessitates the application of the Horner-Wadsworth chemical reaction test as a substitute test. The combination of these tests, results in the rise of the E alkene chemical product instead of the normal Z alkene product (Vedejs 1990). The alternative Schlosser modifcation chemical test, exists as a substitute test due to the crucial limitations in the known Witting reactio. The limitation in the witting reaction is the fact that it mainly occurs via the intermediary known as erythro betaine, this results in the Z alkene. It is from these findings that Schloeer and Christmann as the inventors of this chemical test deduced that the erythro betaine component can be changed to threo betaine with the application of phenyllithium at reduced tempratures which leads to the formation of betaine which results in the subsequent formation of HCl. This resulting chemical process, leads to the creation of the E-alkene product (Maryanoff et al. 1999). The mechanisms involved in the Witting reaction, have long been a contentious matter in the field of organic chemistry. In order to understand this organic chemistry concept, in a less simplified structure it is important to gather a formidable amount of theoretical chemistry evidence showing the Li salt free chemical agents related to the Witting reaction. Other aspects that need to be taken into consideration, include non-stabilization, semi-stabilization and stable ylides that are all formed within the kinetic controls of a common mechanism. It is this common mechanism that led to the development of oxaphosphetane, being an initial formation as well as also existing as an intermediary (Spinella & Soriente 1997). The varying current modifications to the subject with the inclusion of computation studies, combined with experimental data relevant to both the stages of reaction in regards to the reaction oxaphosphetane in its formation and subsequent decomposition. At present several acceptable explanations of stereoselectivity and the witting reaction, have been presented in addition prepositions on the mechanisms made from the historical tests of the Witting reaction. Furthermore, it has been shown of the difficulties in accounting for all evidence collected from experiments that is currently available (Maryanoff et al. 1989). Recent research developments of the witting reaction show that reactionary mechanisms, presented from the classical mechanism of the witting reaction being the original reaction procedure do not fully account for all experimental outcomes. Mechanism based studies have mostly focused on ylides, with stabilization as the intermediaries can be subsequently followed with NMR spectroscopy. This existence combined with the interconversion on betaine components, remains debateable being a subject of continued research. Evidence exists showing phosphonium ylide and their potential of reaction with carbonyl compounds, which happens via the cycloaddittion as a result of the direct formation of oxaphosphatanes. Its reaction structure is comprised of a ball-and-stick model arrangement, that is takes the physical form of a crystal structure. In terms of its reactivity. As such is is clear that the steerochemistry structure of the chemical product, is as a result of added ylides to the carbonyl component of the reaction in addition to the capability of intermediaries and being able to undergo equilibration. Notable researchers Maryanoff and Reitz have been able to identify the issues regarding equilibration of the witting reaction and its intermediaries, a process they have termed as a "stereochemica drift" For a long term the study and research field of stereochemistry concerning the witting reaction and the formation of the carbon-carbon bond formation, has been under the assumption of direct correspondence to the Z/E stereochemistry structure and reaction of the associated alkene products. However, it has been oberserved that specific reactant components do not conform to the simplified pattern. This has been seen in lithium salts which have been known to exert, a formidable effect on the outcome of stereochemical reactions. The witting chemical reaction test has a degree of scope and limitation, though popularized as established methodology for the synthesis of alkenes in addittion to being convincigly precise which has led to its broad application. This is contrary to other elimination based reaction tests for example the dehydrohalogenation of alkyl halides, which results in the productio of a mixture of alkene regiosomers which are determination of the Saytzeff rule which are based on the witting reaction as it creates the double bond in a singular position absent of any ambuguities. A large variation of keton and aldehyde components have been known to be of effect in this reaction test, derivatives like carboxylic acid including derivatives like esters have failed in useful reactions As such mono, di and tri substituive alkenes are applicable in the preparation of positive yields in most situations (Maryanoff et al 1999). The carbonyly chemical compound is tolerant to varying groups lke OH, OR as well as other ester groups, problems can arise concerning ketones under sterical hinderance such that their is a reduced reaction that presents unsatisfactory results with focus on ylides under stablization These ylides donot face such outcomes as seen in other elimination reaction tests, such as the Horner-Wadsworth-Emmons reaction test that use phosphonate esters as a preference. Other limitations exist as liabilities in the natural structure of aldehydes that can undergo oxidation, polymerization or even decomposition for instance the Tandem Oxidation witting procedur is a formation of situ through the oxidation process of the alcohol as the associative component. The witting reagents are normally a derivative of the main alkyl halide, as seconday halides in the form of phosphomium salts are a formation of negative yields. As such tetrasubstituted alkenes are the most convincing means, by which can be used as an alternative for the wittting reaction test. However, this test is tolerant of the variations suc as alkenes aromatic rings, while remaining in compatibility with ether and ester groups. It has been observed that C-O and nitrile groups can exist if combined with ylides under stablisation. Bis ylides that comprise of P=C bonds, have been shown to be applicanle with success. Other limitations of the witting reaction test are in relation to the stereochemistry outcomes of the product, simplified ylides produce an outcome of Z-isomer as the main product though reduced amounts of the E-isomer are normally formed as well. When the E-isomer is the preferred product, the Schlosser modified test can be applied (Vedejs et al. 2000). With ylides under stabilization the resulting product is the E-isomer, being the isomer of similar attributes normally associated with the Horner-Wadsworth-Emmons reaction test. The witting reaction test has varied application, as a reliable and widely used chemical reaction test it has been established as a standardized tool in by chemists in synthetic organic chemistry. It use has been popularized in the introduction of methylene group with the application of methulenetriphenlphosphorane. For example a sterical kentone facing hinderance, for instance camphor can be converted with success to the resulting methylene derivative through heating it with a combination of methyltriphenylphosphonium bromide and potassium tert-butoxide that generates a witting reagent within situ. Another example would be the production of phosphorance, which is an outcoem of the production with application of sodium being a base chemical agent with successful conversion to aldehyde shown as a 62 percentage yield of the alkene (Maryanoff et al. 1989). This chemical reaction is undertaken in a cold THF environment that is sensitive to nitro and phenoxide groups that remain intact even after the reaction. This product is applicable in the incorporation of a photostabliser, to a polymer form so as to protect it from damage of ultra violet radiation (Reitz et al. 2009). References Carrutthers, W. 1971. Methods in organic synthesis. London: Cambridge University Press. Maryanoff, B. Reitzm B, Mutter, S. Inners, R. Almond, J. 1999. Stereochemistry mechanisims in the witting reaction and analysis of the reaction course. Journal of the American Chemical Society, 108, 7664-7678. Maryanoff, B. Reitzm B, Mutter, S. Inners, R. Almond, J. 1989. Detailed study on the witting reaction in non-stabilized phosphorus ylides via spectroscopy. Journal of the American Chemical Society, 107, 1068-1070. Reitz, B. Nortey, S. Jordan, Jr. Mutter, S Maryanoff, B. 2009. Dramatic Concentration of the dependence in stereochemistry of the Wittig Reaction. Examination of the Lithium-Salt Effect. Journal of the American Chemical Society, 51, 3302-3308. Spinella, A., F. & Soriente, A. 1997. Microwave acceleration of the witting reaction in stavilized phosphorus ylides with ketones in solvent free conditions. Synlett, 1:93-94. Vedejs, E., C. 1990. Witting reaction mechanisms with evidence against betaine intermediaries. Journal of the American Chemical Society, 10.3905-3909. Vedejs, E. & Marth, C. (1998). Witting reaction mechanisms and the role of substitutes with phosphorus, Journal of the American Chemical Society.12.3948-3958. Vedejs, E., Marth, F. Ruggeri, R. (2000). The substituent effects of the Wittig mechanism: a case of stereospecific oxaphosphetane decomposition, Journal of the American Chemical Society 12, 3940-3948. Read More