A Study of the Stereochemical Course of β‐Oxygen Elimination with a Rhodium(I) Complex

Abstract

Dedicated to Professor Dieter Seebach on the occasion of his 65th birthdayThe stereochemical course of b-oxygen elimination of an organorhodium(I) complex was investigated through the Rh-catalyzed addition of phenylboronic acid to a chiral propargyl acetate to produce an allene. The degree of chirality transfer suggests that the b-oxygen elimination takes place in both syn and anti modes.Introduction. ± Transition metal catalyzed organic transformations constitute an important class of synthetic tools. b-Elimination is often involved in these processes. For example, b-H elimination is a crucial step in the Pd-catalyzed Mizoroki-Heck reaction. b-H Elimination is known to proceed in a syn mode with a variety of transition metals [1]. On the other hand, little is known about the stereochemical course of b-O elimination, although this is also a common process [2]. Hacksell and Daves found that b-OH elimination took place via a syn mode [3] and b-AcO elimination via an anti mode [4]. Zhu and Lu, on the other hand, found only anti eliminations for b-Cl, b-AcO, and b-OH groups in Pd-catalyzed ene-yne cyclization reactions [5]. This paper provides experimental results that suggest that the b-AcO elimination of an organorhodium(I) complex takes place via both syn and anti modes.

Rh‐Catalyzed Reaction of Propargylic Alcohols with Aryl Boronic Acids–Switch from β‐OH Elimination to Protodemetalation

“…When R was the phenyl group, the desired allylic alcohols E ‐ 3fa , E ‐ 3fb , and E ‐ 3fi were also obtained with moderate yields. It is exciting to note that such Rh(III)‐catalyzed hydroarylation of problematic secondary propargylic alcohols with different arylboronic acids also worked in the presence of B(OH) 3 to afford the corresponding allylic alcohols in an excellent stereoselectivity with moderate yields and high regioselectivity (entries 8—12). Furthermore, the reaction may also be extended to homopropargylic alcohols to form the desired products with high regioselectivity (Table ).…”

“…Consequently, enormous attention has been paid to their highly selective synthesis . Transition metal‐catalyzed hydroarylation of alkynes represents a powerful tool to construct substituted olefins, however, the regioselectivity for non‐symmetric alkynes including propargylic alcohols is still a challenge . On the other hand, it has been reported the C—H activation based Rh(III)‐catalyzed reaction of arenes with propargylic alcohols afforded allene products (Scheme a) .…”

[3]Dendralene Synthesis: Rhodium(III)‐Catalyzed Alkenyl CH Activation and Coupling Reaction with Allenyl Carbinol Carbonate

“…The reaction takes place exclusively at the less sterically hindered, electron-rich diene moiety. Analogously, the use of dimethyl acetylenedicarboxylate (14) furnished the cyclohexadiene 15 in good yields.…”

“…[13] The reaction was believed to proceed by means of a novel olefin insertion/b-oxygen elimination reaction mechanism (Scheme 2 a). [14] Intriguingly, this type of reaction pathway has been rarely exploited in C À H activation reactions. In addition, we developed an oxidative annulation of allenes with N-(pivaloyloxy)benzamides in the presence of a Rh III catalyst, wherein the carborhodation of monosubstituted allenes delivered an intermediate in which a C À C bond has been formed at the central carbon atom (Scheme 2 b).…”

Stereoselective Synthesis of α‐Allenols by Rhodium‐Catalyzed Reaction of Alkynyl Oxiranes with Arylboronic Acids

“…[3] In an extension of this work we set out to examine the use of alkynyl oxiranes as acceptors for arylboronic acids owing to the considerable interest in the resulting a-allenols as building blocks for the construction of oxygenated heterocycles of biological and pharmacological relevance.[4] We report herein on the rhodium-catalyzed reaction of alkynyl oxiranes with arylboronic acids which yields a-allenols with excellent diastereoselectivity.Alkynyl oxirane 1 a (1.0 equiv) was treated with phenylboronic acid (2 a, 1.5 equiv) in the presence of [{RhCl(nbd)} 2 ] (5 mol % of Rh, nbd = norborna-2,5-diene) [5] and KOH (0.6 equiv) in THF (0.1m) at room temperature. The reaction was completed in 2 h, and an extractive workup followed by chromatographic isolation afforded the a-allenol 3 aa in 81 % yield with excellent diastereoselectivity (syn/anti = 99:1)…”