TiCl₄-promoted Baylis−Hillman reaction: mechanistic rationale toward product distribution and stereoselectivity

The mechanism of TiCl4-promoted Baylis−Hillman reaction between Methyl Vinyl Ketone (MVK) and acetaldehyde, in the absence of any base, is studied using the mPW1K density functional theory. The study focuses on several mechanistic intricacies as well as selectivity issues at each step of the reaction. The minimum energy pathway for this reaction involves three major steps such as a chloride transfer resulting in a chloro-enolate, titanium-mediated aldol reaction, and elimination of HCl or HOTiCl3. Both s-cis and s-trans conformers of MVK are considered along with various modes of chloride transfer involving different complexes between TiCl4, aldehyde and MVK. Chloride transfer is found to be kinetically more favored for s-cis-MVK than for s-trans-MVK. The diastereoselectivity in the next step, i.e. Ti-mediated aldol reaction between the enolate and aldehyde, is found to be dependent on the geometry of the enolate, wherein anti and syn BH products are predicted for Z and E enolates, respectively. An interesting secondary orbital interaction between the oxygen atoms of the enolate and aldehyde moieties in the transition states for the C−C bond formation is identified as one of the contributing factors toward the predicted diastereoselectivity in the formation of the α-chloromethyl aldol product (P2). It has earlier been reported that under different experimental conditions, any of the three products such as (i) a normal BH product (P1), (ii) 2-(chloromethyl)vinyl ketones (P3) and (iii) α-chloro methyl aldol could be generated (Scheme 1). The present study offers valuable insights toward rationalizing the observed product distribution as well as diastereoselectivity in TiCl4-promoted BH reaction under base-free conditions. The computed energetics indicate that when MVK is employed as the Michael acceptor, the formation of 2-(choromethyl)vinyl ketone is the preferred product rather than the corresponding normal BH product, consistent with the known experimental reports