Question

The Wittig reaction can be used for the synthesis of conjugated dienes, as, for example, 1-phenyl-1,3-pentadiene. Propose two sets of reagents that might be combined in a Wittig reaction to give this conjugated diene.

Short answer

Answer: The two sets of reagents are: 1. Benzylidene(phenyl)phosphorane and acrolein 2. (buta-2,3-dienylidene)(phenyl)phosphorane and benzaldehyde

Step-by-step solution

Understanding the Wittig Reaction Mechanism

The Wittig reaction is a chemical reaction used to convert aldehydes or ketones into alkenes. It involves the reaction of a carbonyl compound with a phosphonium ylide. The general mechanism can be described as follows: 1. Formation of a phosphonium ylide from an alkyl halide and triphenylphosphine. 2. Reaction of the ylide with an aldehyde or ketone to form a betaine intermediate. 3. Rearrangement and elimination of the betaine intermediate to form a new alkene and a triphenylphosphine oxide byproduct.

Identifying the Desired Product and Possible Precursors

We want to synthesize 1-phenyl-1,3-pentadiene, which has the following structure: C6H5-CH=CH-CH=CH2. To get this product from a Wittig reaction, we need to propose two sets of reagents that can be combined to create the double bond between the phenyl group and the first carbon of the pentadiene chain.

Proposing the First Set of Reagents

For the first set of reagents, we can use benzylidene(phenyl)phosphorane as the phosphonium ylide and acrolein (propenal) as the carbonyl compound. Benzylidene(phenyl)phosphorane can be prepared by reacting benzyl bromide (C6H5-CH2-Br) with triphenylphosphine and an alkali metal base: C6H5-CH2-Br + PPh3 -> C6H5-CH=PPh3+ + Br- Acrolein is an aldehyde with the following structure: CH2=CH-CHO. When the ylide reacts with acrolein, a betaine intermediate is formed, which rearranges and eliminates to give the desired product, 1-phenyl-1,3-pentadiene, and triphenylphosphine oxide: C6H5-CH=PPh3+ + CH2=CH-CHO -> C6H5-CH=CH-CH=CH2 + Ph3PO

Proposing the Second Set of Reagents

For the second set of reagents, we can use (buta-2,3-dienylidene)(phenyl)phosphorane as the phosphonium ylide and benzaldehyde (phenylmethanal) as the carbonyl compound. (buta-2,3-dienylidene)(phenyl)phosphorane can be prepared by reacting 1-bromo-3-butene with triphenylphosphine and an alkali metal base: CH3-CH=CH-CH2-Br + PPh3 -> CH3-CH=CH-CH=PPh3+ + Br- Benzaldehyde is an aldehyde with the following structure: C6H5-CHO. When the ylide reacts with benzaldehyde, a betaine intermediate is formed, which rearranges and eliminates to give the desired product, 1-phenyl-1,3-pentadiene, and triphenylphosphine oxide: CH3-CH=CH-CH=PPh3+ + C6H5-CHO -> C6H5-CH=CH-CH=CH2 + Ph3PO In conclusion, we have proposed two sets of reagents for the Wittig reaction to synthesize 1-phenyl-1,3-pentadiene: 1. Benzylidene(phenyl)phosphorane and acrolein 2. (buta-2,3-dienylidene)(phenyl)phosphorane and benzaldehyde