Prins reaction, mechanism

The Prins reaction often yields stereospecifically the and-addition product this observation is not rationalized by the above mechanism. Investigations of the sulfuric acid-catalyzed reaction of cyclohexene 8 with formaldehyde in acetic acid as solvent suggest that the carbenium ion species 7 is stabilized by a neighboring-group effect as shown in 9. The further reaction then proceeds from the face opposite to the coordinating OH-group " ... 

This type of polyhalo alkane adds to halogenated alkenes in the presence of AICI3 by an electrophilic mechanism. This is called the Prins reaction (not to be confused with the other Prins reaction, 16-53). ... 

The addition of an alkene to formaldehyde in the presence of an acid catalyst is called the Prins reaction.Three main products are possible which one predominates depends on the alkene and the conditions. When the product is the 1,3-diol or the dioxane, the reaction involves addition to the C=C as well as to the C=0. The mechanism is one of electrophilic attack on both double bonds. The acid first protonates the C=0, and the resulting carbocation attacks the C=C ... 

For example, a simple and possibly too naive picture of the HDS reaction would be that a sulfur vacancy at the edge of an MoS2 slab is the active site. A sulfur-containing hydrocarbon such as thiophene adsorbs with its sulfur atom towards the exposed molybdenum. Next, the molecule becomes hydrogenated, the two C-S bonds in thiophene break, the sulfur-free hydrocarbon desorbs and the catalytic site is regenerated by the removal of sulfur by hydrogen. We refer to Prins et al. [50] for a discussion of reaction mechanisms. 

During the coverage period of this chapter, reviews have appeared on the following topics reactions of electrophiles with polyfluorinated alkenes, the mechanisms of intramolecular hydroacylation and hydrosilylation, Prins reaction (reviewed and redefined), synthesis of esters of /3-amino acids by Michael addition of amines and metal amides to esters of a,/3-unsaturated carboxylic acids," the 1,4-addition of benzotriazole-stabilized carbanions to Michael acceptors, control of asymmetry in Michael additions via the use of nucleophiles bearing chiral centres, a-unsaturated systems with the chirality at the y-position, and the presence of chiral ligands or other chiral mediators, syntheses of carbo- and hetero-cyclic compounds via Michael addition of enolates and activated phenols, respectively, to o ,jS-unsaturated nitriles, and transition metal catalysis of the Michael addition of 1,3-dicarbonyl compounds. ... 

The formation of the products could be explained by hemiacetal formation followed by Prins cyclization and subsequent Ritter amidation. A tentative reaction mechanism to realize the cis selectivity is given in Fig. 20 and could be explained by assuming the formation of an (L )-oxocarbenium ion via a chair-like transition state, which has an increased stability relative to the open oxocarbenium ion owing to electron delocalization. The optimal geometry for this delocalization places the hydrogen atom at C4 in a pseudoaxial position, which favors equatorial attack of the nucleophiles. 

Dolby, L. J., Schwarz, M. J. The mechanism of the Prins reaction. IV. Evidence against acetoxonium ion intermediates. J. Org. Chem. 

Watanabe, S. Thermal Prins reaction. II. The mechanism of the thermal Prins reaction. Kogakubu Kenkyu Hokoku (Chiba Daigaku) 1966, 17, 17-21. 

Wilkins, C. L., Marianelli, R. S. Mechanism of the Prins reaction of styrenes. Prins reaction of trans- -deuterostyrene. Tetrahedron 1970, 26,4131-4138. 

The formation of tetrahydropyrans by the Prins reaction The oxo-ene mechanism Stereoselectivity in the Prins reaction... 

The intramolecular Prins reaction can be divided into three types8a depending on the mechanism of the cvclization step and each pathway results in a different product. Several studies involving the Prins reaction in ring-closure processes of larger systems failed37. 

While several mechanisms for the Prins reaction have been proposed,7,10 there is general agreement that the mechanism shown below is reasonable.7 Attack of olefin 1 on protonated aldehyde 10 gives carbocation 11. This carbocation can then be trapped with another molecule of aldehyde, generating adduct 12. Cyclization of 12 affords dioxane 3. Carbocation 11 can also be captured with water giving diol 4. Finally, elimination of a proton from carbocation 11 affords homoallylic alcohol 5. It is possible that the formation of 5 could also proceed via an ene-type mechanism.8... 

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