Hg(OTf)2-Catalyzed Instantaneous Hydration of β- and δ-Hydroxy Internal Alkynes with Complete Regioselectivity

Abstract

The hydration of band d-hydroxy internal alkynes catalyzed by Hg(OTf) 2 took place instantaneously to give ketones with complete regioselectivity under mild conditions, whereas the hydration of internal alkyne without hydroxy moiety was very slow and gave a mixture of ketones. If the hydroxy group is located more than five carbons from the triple bond it has no significant effect upon the hydration reaction.

A new catalyst for organic synthesis: mercuric triflate

“…The reaction was very slow and the ketone 29 was obtained in only 42% yield after 24 h. Although Hg(OTf) 2 itself showed higher reactivity (Hg-acetylide formation does not take place from the internal alkyne), a yield of only 73% was obtained after 24 h. In contrast, the reaction of a d-hydroxy internal alkyne, dec-5-yn-1-ol (30), was completed almost instantaneously, even using 1 mol% of Hg(OTf) 2 at room temperature, leading to the selective generation of 31 in 90% yield. 21 The reaction should involve a rapid equilibrium between the exo-cyclization leading to 32 and the endo-cyclization leading to 33, with the favoured 32 producing the cation 34 via the rate determining protonation by the in situ generated TfOH. Subsequent demercuration to form 35 and hydration results in the selective formation of 31.…”

“…The hydration of b-hydroxyl internal alkyne, oct-3-yn-1-ol (36), is also completed within 5 min to selectively give 1-hydroxyoctan-4-one (37) in 82% yield via a five membered ring intermediate. 21 Uchimoto has reported the hydration of hydroxy-substituted internal alkynes to give hydroxy ketones using PdCl 2 . 22 Scheme 5 Hg(OTf) 2 -catalyzed hydration of internal alkynes.…”

Platinum‐Catalyzed Regioselective Formation of β‐Alkoxy Ketones from Internal Alkynes

“…[11] Mercury triflate displayed the same unfavorable regioselectivity, preferring an 8-endo-dig product (Table 1, Entry 5). The Nishizawa group reported a regioselective Hg(OTf) 2 -catalyzed hydration of undec-6-yn-1-ol in aqueous MeCN, favoring a 7-exo-dig product (3:1, 7-exo/ 8-endo); [12] however, when we treated compound 1a under identical reaction conditions to those used by the Nishizawa group, ketone 3a was obtained as the dominant 8-endo-dig product (1:5, 2a/3a). This shows that the oxygen atom at the homopropargyl position in 1a plays a very important role in the catalytic process.…”

Hydration of Unsymmetrical Internal Alkynes: Factors Governing the Regioselectivity

“…The hydration of β-hydroxy internal alkynes (homopropargylic alcohols) 42 was studied by Nishizawa in the presence of Hg(OTf) 2 as the catalyst and H 2 O. 62 Hydration of 42 using Hg(OTf) 2 (1 mol %) and H 2 O (3 equiv) in MeCN at rt furnished ketones 43 with a total β-regioselectivity (Scheme 28).…”