Efficient Baylis--Hillman reaction using stoichiometric base catalyst and an aqueous medium

Aqueous Morita-Baylis-Hillman reaction of unprotected isatins with cyclic enones

The readily available bicyclic imidazolyl alcohol 1 is a unique catalyst for the aqueous Morita-Baylis-Hillman (MBH) reaction between unprotected isatins and cyclic enones that gives access to a variety of potentially very useful 3-substituted 3-hydroxy-2-oxindoles in an operationally simple, efficient, and environmentally friendly way. The hydroxyl group of the catalyst is believed to stabilize the betaine intermediate formed in the first step of the MBH reaction.

The aza-Morita-Baylis-Hillman reaction: a mechanistic and kinetic study

The aza-Morita-Baylis-Hillman (aza-MBH) reaction has been studied in a variety of solvents, a selection of imine substrates and with various combinations of PPh3 and para-nitrophenol as the catalyst system. The measured kinetic data indicates that the effects of solvent and protic co-catalyst are strongly interdependent. These results are most easily reconciled with a mechanistic model involving the reversible protonation of zwitterionic intermediates in the catalytic cycle, which is also supported by (31)P NMR spectroscopy and quantum chemical studies.

Remarkable influence of microwave heating on Morita-baylis-Hillman reaction in PEG-200

Background

Morita Baylis Hillman (MBH) reaction is used to introduce carbon-carbon or carbon-heteroatom bond in a molecule. The major drawback of this reaction is the relatively low product yield and long reaction time. Though notable changes have been made to improve the reaction rate and yield of MBH adduct by various groups, a reliable synthetic procedure under ambient temperature in presence moisture and air is remain unsolved. Continuing the effort to improve the rate and yield, we report here an eco-friendly and cost-effective method to generate MBH adducts. Non-volatile polyethylene glycol-200 is used as reusable solvents and the reaction was carried out under the influence of microwave energy.

Results

Microwave irradiation have a remarkable influence on PEG suspended 4-Diazabicyclo [2.2.2] octane (DABCO) catalysed MBH reaction between aldehydes and ethyl acrylate. Molecular weight of the PEG is found to have a significant influence on the reaction yield. PEG-200 was the most efficient solvent and in combination with DABCO, the medium can be recycled upto three more runs. This reaction condition is successfully applied to obtain MBH adduct of five different aldehydes in very short time with excellent yield and the required catalyst concentration was very low compared to standard MBH reaction. Since the MBH adduct is an important reactive intermediates for many complex organic syntheses, this approach can be successfully utilised as an alternative to existing reaction conditions.

Conclusion

A new method was developed to improve the reaction rate and yield of MBH reaction The PEG 200-DABCO combination provides a sustainable, non-volatile, recyclable and environment friendly solvent medium to produce MBH adducts. This medium in combination with microwave energy proved to be very effective to introduce a new carbon-carbon or a carbon-heteroatom bond in a molecule.

The aza-Morita-Baylis-Hillman reaction of electronically and sterically deactivated substrates

The aza-Morita-Baylis-Hillman (azaMBH) reaction has been studied for electronically and sterically deactivated Michael acceptors. It is found that electronically deactivated systems can be converted with electron-rich phosphanes and pyridines as catalysts equally well. For sterically deactivated systems clearly better catalytic turnover can be achieved with pyridine catalysts. This is in accordance with the calculated affinities of the catalysts towards different Michael-acceptors.

A tandem 1,3-H-shift-6π-electrocyclization-cyclic 2-amido-diene intramolecular Diels-Alder cycloaddition approach to BCD-Ring of atropurpuran

An approach toward the BCD-ring of atropurpuran via a sequence of allenic 1,3-H shift, 6π-electron pericyclic ring closure, and intramolecular Diels-Alder cycloaddition of cyclic 2-amidodiene is described.

Baylis–Hillman reaction under solvent-free conditions — Remarkable rate acceleration and yield enhancement

A simple and efficient method has been developed for remarkable rate acceleration and yield enhancement of the Baylis–Hillman reaction under solvent-free “neat conditions” and solvent-less isolation of products. Reaction of equimolar quantities of aldehyde and olefin in the presence of 20 mol% of DABCO under neat conditions affords the highest yield in most cases within the shortest reaction time, giving support to the mechanisms of proton transfer in protic and aprotic solvents. Solvent-free conditions are found to be especially fast, selective, and high yielding for aromatic aldehydes.

Highly chemoselective Baylis-Hillman and aldol reactions of 2H-thiopyran-4(3H)-one using tertiary amine catalysts in aqueous media

For the first time, the Baylis-Hillman (BH) reaction of 2H-thiopyran-4(3H)-one is investigated, and surprisingly, the reaction of 2H-thiopyran-4(3H)-one with aldehydes in the presence of different tertiary amines shows excellent chemo- and regioselectivity in water. At room temperature, DBU affords BH adducts, but with DABCO, aldol products were obtained. In the case of DABCO, Et(3)N, or DMAP, domino aldol-rearrangement reactions occurred at 45-50 °C.

Computational investigation on the mechanism and the stereoselectivity of Morita-Baylis-Hillman reaction and the effect of the bifunctional catalyst N-methylprolinol

The mechanism of the Morita-Baylis-Hillman (MBH) reaction between formaldehyde and methyl vinyl ketone (MVK) catalyzed by N-methylprolinol was investigated using density functional theory (DFT) method. The overall reaction includes two steps: C-C bond formation and hydrogen migration. In the presence of water, the hydrogen migration occurs via a six-membered ring transition state and the corresponding energy barrier decreases dramatically, and therefore the RDS is the C-C bond formation step. The calculations indicate that the C-C bond formation step controls the stereochemistry of the reaction. In this step, the hydrogen bonding induces the direction of the attack of enamine to aldehyde from the -OH group side of N-methylprolinol. The energy-favored transition states are mainly stabilized by hydrogen bonding, while the chirality of the products is affected by the hydrogen bonding and the steric hindrance. The calculations correctly reproduce the major product in (R)-configuration, which is consistent with the experimental observation.

Exploring Morita-Baylis-Hillman reactions of p-quinols

The Morita-Baylis-Hillman reaction of p-methylquinols with activated aromatic aldehydes has been studied. Depending on the reaction conditions (solvent and additives), three different products were formed. A mono or double Morita-Baylis-Hillman adduct and a fused 1,3-dioxolane could be obtained in good chemical yields. The use of non-nucleophilic bases to promote the reaction suggested an autocatalytic mechanism.

Enantioselective, organocatalytic Morita-Baylis-Hillman and Aza-Morita-Baylis-Hillman reactions: stereochemical issues

Conscious of the importance that stereochemical issues may have on the design of efficient organocatalyts for both Morita-Baylis-Hillman and aza-Morita-Baylis-Hillman reaction we have analyzed them in this minireview. The so-called standard reactions involve "naked" enolates which therefore should lead to the syn adducts as the major products, irrespective of the E, Z stereochemistry of the enolate. Accordingly, provided the second step is rate determining step, the design of successful bifunctional or polyfunctional catalysts has to consider the geometrical requirements imposed by the transition structures of the second step of these reactions. On the other hand, MBH and aza-MBH reactions co-catalyzed by (S)-proline and a secondary or tertiary amine (co-catalyst) involve the aldol-type condensation of either a 3-amino-substituted enamine, dienamine, or both, depending on the cases. A Zimmerman-Traxler mechanism defines the stereochemical issues regarding these co-catalyzed condensations which parallel those of the well established (S)-proline catalyzed aldol-like reactions.

Mechanistic insights and the role of cocatalysts in Aza-Morita-Baylis-hillman and Morita-Baylis-Hillman reactions

The mechanism of the trimethylamine or trimethylphosphine catalyzed aza-Morita-Baylis-Hillman (MBH) reaction between acrolein and mesyl imine is investigated by using ab initio and density functional methods. All key transition states are located at the CBS-4M as well as at the mPW1K/6-31+G** levels of theories. To account for the experimentally known rate enhancements through the use of polar protic cocatalysts, transition state models with explicit cocatalysts are considered. Inclusion of polar protic cocatalysts is found to have a profound influence in decreasing the activation barriers associated with the key elementary steps. The protic cocatalysts such as water, methanol, and formic acid are identified as effective in promoting a relay proton transfer. Interestingly, the efficiency of the relay mechanism results in relatively better stabilization of the proton transfer transition state as compared to the addition of enolate to the electrophile (C-C bond formation). The cocatalyst bound models suggest that the proton transfer could become the rate-determining step in the aza-MBH reaction under polar protic conditions. A comparison of the aza-MBH reaction with the analogous MBH reaction is also attempted to bring out the subtle differences between these two reactions. Enhanced kinetic advantages arising from the nature of the activated electrophile are noticed for the aza-MBH reaction. The difference in the relative energies between the transition states for the proton transfer and the C-C bond formation steps with bound cocatalyst(s) is found to be more pronounced in the aza-MBH reaction. In general, the reported results underscore the importance of considering explicit solvents/cocatalysts in order to account for the likely role of the specific interactions between reactants and solvents/cocatalysts.

A concise total synthesis of saliniketal B

We report a concise, enantioselective, and highly efficient synthesis of the marine actinomycete-derived natural product saliniketal B. Our approach was motivated with an eye toward future structure-function studies of this inhibitor of phorbol ester-mediated ornithine decarboxylase induction via an unknown mechanism. Our strategy highlights the utility of Pt(II)-mediated cycloisomerization of alkynediols developed in our laboratory to construct the dioxabicyclo[3.2.1]octane ring system, a highly selective aldol fragment coupling whose stereochemical outcome is influenced by a gamma-stereogenic methyl group, and an interesting one-pot desilylation/dihydropyranone fragmentation/amidation sequence. As such, saliniketal B was obtained in 11 steps and 23% overall yield from commercially available starting material via a convergent coupling of two equally complex fragments assembled in seven and eight steps (39 and 45%), respectively.