Wrintig Reaction of Semi-Stabilised Ylides with Benzaldehydes Studies on the Effect of ortho Position
- Category: Samples
In organic chemistry, Wittig reaction has attracted both immense interest and controversies among synthetic chemists. It continues to generate contentious issues especially where lithium salt-free reactions are involved. Much of these controversies arise from the original Wittig proposal that the reaction progresses via oxaphosphetane, an idea that was vehemently opposed almost immediately . The topic has received several excellent reviews concerning reaction mechanisms [2, 3]. Mutual consensus among researchers is that all Wittig reactions share a common experimental effect for all phosphonium-based ylide reactions. Similarly, selectivity is consistent for cis-oxaphosphetane together with its derivative products such aserythro-β-hydroxyphosphonium and Z-alkene salt in reactions where aldehydes bearing a heteroatom on the β-position as a substituent are involved. This effect usually operates for both aliphatic aldehydes and benzaldehydes, while in the absence of a substituent heteroatom they do not operate .
Following the discovery of shared common effect for ylide type reactions, strong arguments have emerged on whether there is also a common operational mechanism for all Wittig reactions involved in Lithium-salt free reactions. Different researchers [5, 6] have come to a conclusion that the reaction mechanism involves the [2+2] cycloaddition based on the strength of confirmatory experimental evidence supporting this argument. For instance, the most notable effect is ‘cooperation’ for ortho-substituted ylides particularly involving semi-stabilized cases. Therefore the cycloaddition mechanism is re-affirmed as the reason for the cooperative effect in semi-stabilized ylide Witting reaction. This effect has also been demonstrated in reactions involving ylide derivatives of triphenylphosphine under aqueous conditions . This finding suggests existence of a kinetic control during the reaction operation in both cases.
Synthetic chemists continue to face a permanent challenge regarding carbon-carbon bonding. The 1953 Wittig reaction (illustrated in Eq.1) succeeded in forming C=C by exploiting efficiency, reliability, and stereoselectivity.
The reaction in Eq.1 above was a revelation as it set the platform for synthesis of Vitamin A by the BASF Company soon afterwards. This co-joined academia and industry in a remarkable cooperation display.
Wittig reaction comprises two basic steps: Step 1 is the generation of a ‘phosphorous ylide’ using a base and phosphorium salt. Step 2 is a follow-up reaction of the generated phosphorus ylide reacts with a carbonyl group producing phosphane oxide and olefin. The produced alkene (olefin) is thus the product of ylide (generated from phosphonium salt) reaction with ketone or aldehyde . The resultant alkene geometry is largely dependent on the ylide reactivity as will be demonstrated in subsequent sections bellow. Equation 2 gives a summary of the two steps:
As aforementioned, the reactivity of the generated ylide depends mainly on the nature of the group that is attached to ylidic carbon atom. For example, substituent’s that exhibit strong conjugation such as CN, C(O)R stabilizes the phosphorous ylide thus making it easy to isolate due to less reactivity. Recent literature describes the stabilizing nature of ylides with growing consensus that the P=N and P=O bonding possessing polar σ-bonds that covalently combine with electrostatic interactions. However, new evidence suggests that the negative hyperconjugation witnessed in ylides may be due to the existence of lone pair on the ylides’ alpha-carbon within the σ* orbitals. The lone pair is believed to be responsible for the bonding with phosphorous and its substituent’s despite P-C bond being heavily polarized towards C [10, 11, 12] Because of this unique characteristics, ylides are categorized as “stabilized” ylides, “semi-stabilized” ylides (featuring substituent’s showing mild conjugation such as allyl, Ph), and “non-stabilized” ylides which lacks all the functional features named above. Both non-stabilized and semi-stabilized ylides are very unstable to isolate hence produced in situ following immediate reactions with ketone or aldehyde .
The three ylide classes
The present report focused more on the trends of Z/E variability of semi-stabilized ylides shown bellow.
The fact that Z alkene that is thermodynamically unstable is capable of producing unstabilizedylides has generated investigative interests. At present, Wittig reaction and the derivative products are widely researched for academia and industry. Among major focus by industry is development of aroma and fragrance compounds, carotenoids, hormones, steroids, fatty acid derivatives, pheromones, prostaglandins, terpenes, and many other synthetic and natural olefinic compounds of interest to chemists. However, Wittig reaction’s stereoselectivity control remains a major concern to chemists. The present study delves in the discussion of the importance of Wittig reaction withspecific focus onsteroselectivity and reaction mechanismpatterms .
- Next >>