Wrintig Reaction of Semi-Stabilised Ylides with Benzaldehydes Studies on the Effect of ortho Position
- Category: Samples
Modern mechanistic interpretation
The Z/E ratio in reactions involving semi-stabilized ylides with Benzaldehydes have been investigated with intend of establishing the kinetic OPA selectivity during the formation step in Wittig reactions . As seen from Eq. 6 above, the Z/E ratio must be reflective of the cis/trans ratio during OPA kinetic control.
The general consensus is that the cis/trans ratio in OPA formation step in Wittig reaction is reflective of the alkene Z/E ratio. Decomposition of OPA is usually irreversible and streospecific, with cis-OPAs having relatively higher energy compared to trans-OPAs. The kinetic control mechanism operates in Wittig reactions involving semi-stabilized ylide derivative of methyldipheyl-phosphine (see Scheme 1.2). The decomposition has been demonstrated to be stereospecific for OPAs and betaines that are transiently formed in experimental β-HPS deprotonation [8, 9, 10].
Scheme 1.2 The kinetic control mechanism involving semi-stabilized ylide derivative of methyldipheyl-phosphine
From the Wittig reaction scheme above, the different ylides and corresponding adehydes react under specified conditions resulting to varying Z/E ratio as shown in Table 1 bellow.
Table 1 Z/E selectivity for different Witting reactions and varying conditions
(The ratioc e.g 11c, 12c, 22c, and 27c denotes Ylides generated at 20oC from the parent phosphorium salts by dry THF while under NaHMDs treatment, the rest were at treated at -78oC)
Comparing reactions from Eq. 8 and table 1 Z/E rations, the following deductions were made:
- If both benzaldehyde and benzylidenemethyldiphenylphosphorane are unsubstituted, the ratio of stilbene produced is 85: 15 respectively.
- If benzaladhyde with ortho-heteroatom substituent reacts with benzylidenemethyl-diphenylphosphorane there is a higher selectivity of Z-isomer where benzaldehyde reactivity is higher at carbonyl while 2-methoxybenzaldehyde has less reactivity, while iodobenzaldehyde displays similar reactivity as benzaldehyde.
- Reaction of ylides bearing ortho-heteroatom substituent with benzaladehyde demonstrates a relatively low E-selectivity
- On the other hand, reactions of ortho-substituent bearing ylides on the benzylide with benzaldehyde display a relatively moderate to higher E-selectivity. The observed increase in E-selectivity is almost parallel to increase in ortho-subsituent activating ability. This means ylides that are more reactive gives higher E-selectivity.
- For ortho-substituted benzylides, reaction with benzaladhydes bearing ortho-heteroatom substituent displays even higher Z-selectivity compared to (b) above. This can be explained based on the electron withdrawal bearing aldehydes hence higher reactive compared to benzaldehyde which is electron donor hence less reactive.
The general deduction from the cooperative effect of ortho-positioned substituent as seen in (a) to (d) is that unsubstituted benzylides show greater z-selectivity attributed to the steric effect. This is because whether the ortho-subsituted ylide is a lone-pair or not, the ylide still operates. On the other hand, the same ortho-subsituted bensylide can react with benzaldehyde at moderate to greater E-selectivity because of the latter’s TSs geometry which is different compared to ortho-heteroatom substituent benzaldehydes .
The TS geometry diagram showing Wittig reaction coordinates during cis-OPA formation
As seen from the activation energy diagram above, the cycloreversion barrier for cis-OPA formation when ylide reacts with aldehyde must be surmountable. This is because, aldehydes bearing β-heteroatom substituents must gain sufficient free energy similar to the OPA intermediates in the analogous reaction species where the aldehyde lacks the substituted group. The assumption is that the cis-OPA is disfavoured thermodynamically compared to its trans-isomer is such reactions hence activation energy for cis-selective TS is lower during OPA formation hence the E-selectivity .
On the other hand, presence of ortho-heteroatom on benzaladehyde acts as inducer for higher Z-selectivity due to electronic effect but not steric . Similarly, the electronic effect acts via space, not through electron donation or withdrawl in the benzaladehyde aromatic ring . Hence, we conclude that, presence of ortho-substituent alone on benzaladehyde partner may be necessary but has no capacity of inducing Wittig reactions Z-selectivity. The following scheme 1.2 summarizes these observations.
Scheme 1.2 Reactions of aldehydes with a ortho-substituent
Where: (a) and (b) show varying perspectives for cis-selectivity due to the phosphorus and hetero-atom bonding. In this case, the ortho-substituent is oriented in ways that minimizes steric interactions at 2-3 by avoiding R3 phosphorus substituents. (c) Shows a trans-selective Wittig reaction where bonding between heteroatom and phosphorus greatly suffers from the 2-3 steric effects .