Abstract
We found that elemental iodine and bromine are converted to trihalide nucleophiles (triiodine and tribromide anion, respectively) in the presence of catalytic amounts of meso-tetraphenylporphyrins (H2TPP). Therefore a highly regioselective method for the synthesis of β-haloalcohols through direct ring opening of epoxides with elemental iodine and bromine in the presence of H2TPPs as new catalysts is described. At room temperature a series of epoxide derivatives were converted into the corresponding halohydrins resulting from an attack of trihalide species anion atoms at the less substituted carbon atom. This method occurs under neutral and mild conditions with high yields in various aprotic solvents, even when sensitive functional groups are present.
Highlights
Oxiranes are among the most versatile intermediates in organic synthesis, as they can be prepared from a variety of other functional groups [1] and due to their ring strain and high reactivity, their reactions with various nucleophiles lead to highly regio and stereoselective ring opened products [2,3,4]
In particular metal halides such as Li/Ti [2], Sn [20], P [13], Cu [21], and Ni [10] induce epoxide-opening, in which the use of a stronger Lewis acid and a metal ion in structure of catalyst often results in low yields of the halohydrins when other sensitive functional groups are present [22]
In the second step [Equation (2)] this complex is further decomposed to release the X3− nucleophile ion into the solution. In this way, molecular iodine or bromine is converted to a nucleophilic halogen species in the presence of H2TPP and, in the third step Equations (3) and (4), this ion participates in the ring opening reaction of the epoxides, and the catalyst is reproduced and is used in the first step again
Summary
Oxiranes are among the most versatile intermediates in organic synthesis, as they can be prepared from a variety of other functional groups [1] and due to their ring strain and high reactivity, their reactions with various nucleophiles lead to highly regio and stereoselective ring opened products [2,3,4]. In particular metal halides such as Li/Ti [2], Sn [20], P [13], Cu [21], and Ni [10] induce epoxide-opening, in which the use of a stronger Lewis acid and a metal ion in structure of catalyst often results in low yields of the halohydrins when other sensitive functional groups are present [22]. In these approaches we encountered with some limitations, such as the need for strong Lewis acid and protic media that certainly are unsuitable conditions for complex epoxide compounds. The catalysts are recovered and can be reused several times
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