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Wittig Reaction Lab Report

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Chapter Two – Results and Discussion

2.1 Synthesis of 1,3-butadienes

1,3-butadienes can be synthesised from aldehydes and ketones using the Wittig reaction. The Wittig reaction facilitates the synthesis of new carbon-carbon double bonds at specific locations in aldehydes and ketones (Bernard & Ford, 1983). The overall reaction mechanism is shown in Figure 4.

Figure 4. The Wittig Reaction - Formation of a transitional oxaphosphetane and resultant formation of a new carbon-carbon alkene bond resulting in the synthesis of 1,3-butadienes from aldehydes and ketones.

Protocols for synthesising 1,3-butadienes from aldehydes and ketones have been established in the literature. The synthesis protocol identified by Greatrex et al. (2014) was
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Scheme 2 – General reaction pathway for the synthesis of 1

1 was reacted in a prepared solution of potassium tert-butoxide and methyltriphenylphosphonium bromide for 18 hours. Reactions were performed in dried diethyl ether in an atmosphere of nitrogen at room temperature. Reaction products were isolated by column chromatography. The yield of 4 was determined by initially by weight, followed by purity determinations from the 1H NMR spectrum analysis, and found to be 20%. Results of six published articles for the synthesis of 4 with these reagents show varying yields from 36 (Radomkit et al., 2011) to 89% (Ventura & Taylor, 2014) under differing experimental
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034 100 5%
6 2.947 0.707 17 4%
a) Yield determined by purity determined from 1H NMR spectrum analysis following isolation by column chromatography.
Table 4 –Summary of 1,2-dioxines synthesis reaction outcomes

The general approach involved transforming each of the 1,3-butadienes at the site of the terminal alkene bond via reaction with the singlet oxygen. Meso-tetraphenylporphyrin was used as the photosensitiser to generate the singlet oxygen. The prepared butadienes were added to a volume of dichloromethane and cooled using a water-cooled jacketed flask. Reactions were followed by TLC and reaction times varied for each reaction based on observations. Reactions were ceased on the basis of an increasing prevalence of a new product appearing at the baseline of the TLC. Products of the reactions were purified via column chromatography. Two cycloaddition products were observed in each reaction. These were the 1,2-dioxine resulting from the Diels-Alder [4+2] cycloaddition reaction, and an aldehyde resulting from the ene cycloaddition
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