Wittig Reaction – Synthesis of trans-9-(2-phenylethenyl)

Introduction The Wittig reaction is one of the most useful mechanisms in organic chemistry for its ability to convert aldehydes and ketones to alkenes. Its ability to convert carbonyl compounds to alkenes, though, is limited as this reaction cannot convert esters or amides.2 This reaction is carried through with the aid of a Wittig reagent (termed, ylid), which is synthesized through use of a phosphonium salt with strong base.1 The ylid species holds opposite charges on adjacent atoms and is hence, the reason why it acts so well as a nucleophile. The Wittig reagent is important in its ability to act as a nucleophile and attack the pi bond of the carbonyl compound to form an alkoxide intermediate. This alkoxide intermediate is then able to form a oxaphosphetane, whose fragmentation results in the final alkene product. In this experiment, the final alkene product, trans-9-(2-phenylethenyl)anthracene is formed from the benzyltriphenylphosphonium chloride electrophile. Another reaction, the Horner-Wadsworth-Emmons modification is almost similar in comparison to the Wittig reaction, although it holds some minor differences in their waste products, reactive intermediates, reactivity, and electron withdrawing group limitations. The Horner-Wadsworth-Emmons modification does not produce triphenylphosphine as its waste product, but instead, forms a phosphate ester, which is washed from the product with much more ease than the triphenylphosphine counterpart. The intermediate of the Horner-WadsworthEmmons modification also gets its reputation for its higher nucleophilicity than the ylid in the Wittig reaction, as its intermediate is an anion. The HWE modification also creates a phosphonate ester that can be alkylated with the combination of a base and alkyl halide before undergoing the main reaction. This difference also puts a restriction on this mechanism (HWE) as the phosphonate ester must contain an additional electron withdrawing group to perform correctly and carry to completion.3 Main Mechanism Side Reactions Experimental To a 5-mL conical vial, add 0.3 g benzyltriphenylphophonium chloride, 0.115 g 9anthraldehyde, 1 mL dichloromethane, a magnetic stir bar, TLC, and 0.4 mL NaOH. Stir quickly upon addition of NaOH Stir for 30 minutes. Take a TLC of the reaction to determine if the reaction was successfully finished. To create the reaction mixture, add 1.5 mL DCM, 1.5 mL H2O, cap and shake. Drain the organic layer into the reaction tube. Once crystals are obtained, collect weight, percent yield, and IR spectrum of final product. To the crude product, add 3 mL 1- propanol. Heat the solution and then cool to room temperature. Once the product has reached room temperature, cool even further to 0°C. To the reaction mixture, add 1.5 mL DCM. Cap and shake, again. Drain aqueous layer and combine the organic layer with the previous organic layer. Add calcium chloride pellets and decant the solution Table of Chemicals Chemical Formula Mola r Mass Densit y Meltin g Point Boiling Point Appearan ce C25H22Cl P 388.8 6 g/mol 1.21 g/cm3 314C N/A Off-white powder solid C15H10O 206.2 4 g/mol 1.217 g/cm3 103107C 405.7C Yellowgreen crystalline powder CH2Cl2 84.93 g/mol 1.33 g/cm3 -97C 40C Colorless liquid NaOH 40.0 g/mol -2.13 g/cm3 318C 1390C White solid C3H8O 60.10 g/mol 8040 g/cm3 -127C 97C Clear, colorless liquid C22H16 280.3 6 g/mol 1.163 g/cm3 130132C 476C N/A Results Solvent Front 9-anthraldehyde Alkene (product) TLC 4.1 cm 2.8 cm 3.7 Table 1: This table is indicative of the results following the TLC of the reaction for 9anthraldehyde and the final alkene product – trans-9-(2-phenylethenyl) anthracene Mass of trans-9-(2-phenylethenyl) anthracene 0.25 g Melting Point of Final Product 128-131°C Table 2: This table is indicative of the final qualitative results of the final alkene product following experimentation including the final mass obtained and final melting point. IR Spectrum Final Product Experimental 1H NMR Theoretical 1H NMR Percent Yield Limiting Reagent 0.3 g C25H22ClP x 1mol C 22 H 16 1 mol C 25 H 22ClP x =0.00077 mol C22H16 388.86 g C 25 H 22 ClP 1 mol C 25 H 22ClP 0.115 g C15H10O x 1 mol C 15 H 10 O 1mol C 22 H 16 =¿0.00056 mol C22H16 x 206.24 g C 15 H 10 O 1 mol C C 15 H 10 O Theoretical Yield 0.115 g C15H10O x 1 mol C 15 H 10 O 1 mol C 22 H 16 280. 36 g C 22 H 16 x x =¿ 0.02 g trans-9206.24 g C 15 H 10 O 1 mol C 15 H 10O 1mol C 22 H 16 (2-phenylethenyl)anthracene Actual Yield 0.25 g x 100 = 156% 0.16 g Rf Rf (starting materials) = Rf (final product) = Distancetravelled by compound 2.8 = = 0.68 4.1 Distance traveled by solvent Distancetravelled by compound 3.7 = = 0.9 4.1 Distance traveled by solvent Discussion When comparing the final products melting point with the theoretical melting point of trans-9-(2-phenylethenyl)anthracene, the identity of the final product is indeed able to be confirmed as the expected product with high confidence. With an experimental melting point of 128-131 and a theoretical value of 130-132, there is no evidence of contamination or possibility of side products as the values correlate precisely with one another. To confirm that a reaction went to completion, a TLC is taken of both the starting materials and final products to compare the Rf values. With the starting materials Rf value (9anthracene, 0.68) being lower than that of the final product ((trans-9-(2phenylethenyl)anthracene, 0.9)), one can conclude that this reaction went to completion. The formation of trans-9-(2-phenylethenyl)anthracene was deemed to be successful when analyzing the percentage yield. This reaction produced a very high percentage yield of 156%. This could be due to impurities in the final product or the presence of excess water not taken out during the final drying process, as this yield is higher than the expected 100%. Looking at the IR spectrum, peaks around 1250 cm-1 and 3100 cm-1 can be observed. The disappearance of a peak at 1710 for the carbonyl stretching of the aldehyde allows for the assumption that the starting material was indeed converted into the expected product. However, the intensity of the sp2 carbon-hydrogen stretching at 3100 cm-1 is at very low intensity. However, the disappearance of a peak at 1710 cm-1 confirms that the expected product was prepared. The olefin trans product that is obtained should theoretically show a sharp singlet peak for the two olefin protons that are not aromatic. This peak appears around 7 PPM. The presence of this singlet at this shift value confirms the formation of the desired product. Therefore, looking at the experimental 1H NMR of the final product, the peak present at 7 PPM is indicative of the expected product. Conclusion The goal of this experiment was to synthesize trans-9-(2-phenylethenyl)anthracene from a carbonyl compound using the Wittig reaction. The melting point, Rf and 1H NMR spectrum coincided almost precisely with literature values of the final product. Overall, with a melting point, percent yield, Rf, IR and 1H NMR spectrum confirming the identity of the final product as, indeed, trans-9-(2-phenylethenyl)anthracene, this experiment was successful through use of the well-known and widely used Wittig reaction. The Wittig reaction has been used in the total synthesis of natural products including those related to medicinal purposes -- lactone, pyrone and lactam.4