Catalytic enantioselective 1,6-conjugate additions of propargyl and allyl groups

• Published in: Nature (London) Vol. 537; no. 7620; pp. 387 - 393
• Main Authors: Meng, Fanke, Li, Xiben, Torker, Sebastian, Shi, Ying, Shen, Xiao, Hoveyda, Amir H.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 15-09-2016; Nature Publishing Group
• Subjects: 639/638/549; 639/638/77; Alkadienes - chemistry; Alkenes - chemistry; Alkyl groups; Allylic compounds; Anti-HIV Agents - chemical synthesis; Anti-HIV Agents - chemistry; Boron; Boron Compounds - chemistry; Carbon; Carbonyl compounds; Catalysis; Catalysts; Chemical properties; Chemical research; Chemistry Techniques, Synthetic - methods; Chemistry, Pharmaceutical - methods; Cobalt; Copper - chemistry; Humanities and Social Sciences; Iridium; letter; multidisciplinary; Organometallic Compounds - chemistry; Oxidation; Pyrrolidinones - chemical synthesis; Pyrrolidinones - chemistry; Science; Stereoisomerism; Tetrahydronaphthalenes - chemical synthesis; Tetrahydronaphthalenes - chemistry
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature19063

A difficult synthesis is described that uses an organocopper catalyst and commercially available starting materials to give high yield and the mechanics of the reaction are elucidated through density functional theory.
A difficult synthesis accomplished
One of the most useful sets of reactions in organic chemistry—with applications in both research and medicinal chemistry—is the 1,4-conjugate addition of a carbanionic species to α,β-unsaturated carbonyl compounds. The corresponding 1,6-conjugate additions would be similarly useful, but it is a 'difficult' synthesis. Catalytic enantioselective 1,6-conjugate additions are uncommon, and those that incorporate moieties that can be later functionalized, such as propargyl or allyl groups, into acyclic α,β-doubly unsaturated acceptors do not exist. Here Amir Hoveyda and colleagues describe a procedure for the 1,6-conjugate addition of propargyl and 2-boryl-substituted allyl groups to acyclic dienoates with high selectivity. The method uses an organocopper catalyst and commercially available starting materials to give high yield, and should be extendable to other readily available unsaturated organoboron and/or organocopper systems.
Conjugate (or 1,4-) additions of carbanionic species to α,β-unsaturated carbonyl compounds are vital to research in organic and medicinal chemistry, and there are several chiral catalysts that facilitate the catalytic enantioselective additions of nucleophiles to enoates
1
. Nonetheless, catalytic enantioselective 1,6-conjugate additions are uncommon, and ones that incorporate readily functionalizable moieties, such as propargyl or allyl groups, into acyclic α,β,γ,δ-doubly unsaturated acceptors are unknown
2
. Chemical transformations that could generate a new bond at the C6 position of a dienoate are particularly desirable because the resulting products could then be subjected to further modifications. However, such reactions, especially when dienoates contain two equally substituted olefins, are scarce
3
and are confined to reactions promoted by a phosphine–copper catalyst (with an alkyl Grignard reagent
4
,
5
, dialkylzinc or trialkylaluminium compounds
6
,
7
), a diene–iridium catalyst (with arylboroxines)
8
,
9
, or a bisphosphine–cobalt catalyst (with monosilyl-acetylenes)
10
. 1,6-Conjugate additions are otherwise limited to substrates where there is full substitution at the C4 position
11
. It is unclear why certain catalysts favour bond formation at C6, and—although there are a small number of catalytic enantioselective conjugate allyl additions
12
,
13
,
14
,
15
—related 1,6-additions and processes involving a propargyl unit are non-existent. Here we show that an easily accessible organocopper catalyst can promote 1,6-conjugate additions of propargyl and 2-boryl-substituted allyl groups to acyclic dienoates with high selectivity. A commercially available allenyl–boron compound or a monosubstituted allene may be used. Products can be obtained in up to 83 per cent yield, >98:2 diastereomeric ratio (for allyl additions) and 99:1 enantiomeric ratio. We elucidate the mechanistic details, including the origins of high site selectivity (1,6- versus 1,4-) and enantioselectivity as a function of the catalyst structure and reaction type, by means of density functional theory calculations. The utility of the approach is highlighted by an application towards enantioselective synthesis of the anti-HIV agent (−)-equisetin.