General Method for Preparation of Allenic Zinc Reagents by Three-Carbon Homologation of Triorganozincates:  Convergent Three-Component Coupling of Propargylic Substrates, Triorganozincates, and Electrophilic Reagents

Abstract

Allenic zinc reagents (R1R2CCC(R3)ZnL) are efficiently prepared by the reaction of propargylic substrates (R1R2C(X)C⋮CH, X = MeSO2O, Cl, R2NCO2) with a variety of triorganozincates ((R3)3ZnM; R3 = alkyl, alkenyl, aryl, M = Li, MgCl). Treatment of the allenic zinc reagents with D2O gives the corresponding deuteroallenes with high deuterium incorporation. The allenic zinc reagents thus prepared undergo a coupling reaction with a variety of electrophiles (aldehydes, acyl chlorides, I2, NCS, and chlorosilanes) regioselectively at the γ position to afford the corresponding propargylic derivatives (R1R2C(El)C⋮CR3, El = RCH(OH), RCO, I, Cl, R3Si) in high yields. Silicon containing allenic zinc reagents R1CHCC(CH2SiMe3)ZnL and R1CHCC(SiPhMe2)ZnL are readily prepared by the reaction of propargylic carbamates (R1CH(OCONPh2)C⋮CH) with (TMSCH2)3ZnLi and by the reaction of bromoproparglic mesylates R1CH(OSO2Me)C⋮CBr with (PhMe2Si)3ZnLi, respectively. Successive treatment of these reagents with electrophiles affords functionalized organosilicon compounds of high synthetic utility.

Ring‐Opening of N‐tert‐Butanesulfinylethynylaziridines with Lithium Tris(dimethylphenylsilyl)zincate: Stereoselective Access to 4‐Amino‐1‐allenylsilanes

“…This result indicates that ( n‐ Bu) 3 ZnLi is able to afford acetylenic zincate Zn ‐ 1a (through deprotonation and subsequent transmetallation), but that the transfer of an n‐ Bu group via an S N i mechanism does not take place in contrast with the reactivity observed by Oku et al. with propargylic electrophiles 19,22. On the other hand, two supplementary experiments were performed with M2 .…”

“…Being unable to avoid the formation of disilylated side‐product 3 ,18 we thus planned to employ zincate M2 19 instead of cuprate M1 . To our great delight, when carried out at −80 °C in THF, the reaction with aziridine 1a was complete within 2 h and afforded 2a in 66% isolated yield as a single isomer (Table 1, entry 4).…”

Preparation of Propargylic Carbenoids and Reactions with Carbonyl Compounds − A Stereoselective Synthesis of Propargylic Halohydrins and Oxiranes

“…The values of these coupling constants are listed in Table 2. The excellent diastereoselectivity of this reaction with aliphatic aldehydes [21,22] can be explained by a chelate-type transition state in which the allenyl moiety and the CϭO bond are eclipsed (Scheme 4). In this transition state, the R group of the aldehyde and the chlorine atom adopt an antiposition, to minimize steric interactions.…”

“…We chose zinc salts, because propargylic and allenic zinc compounds have afforded very good regioselectivities in their additions to aldehydes. [21,22] Dropwise addition of LDA to an equimolar mixture of 3-chloro-1-trimethylsilylpropyne (1) [23,24] and zinc bromide in THF at Ϫ78°C, followed by treatment with n-butanal, gave the corresponding desired halohydrin 2a in 50% yield. To our pleasure, the use of 2 equiv.…”

Radical β-Elimination of a Sulfinyl Group to Afford Allenes

“…Among them, the S N 2′ displacement of a propargylic leaving group (pathway a) has attracted a lot of attention,2 and has allowed the preparation of enantiopure allenes 3,4. Propargylic precursors can also be used to generate propargylic metallic entities that can be trapped in the allenic form (pathway b) 5−7. Two alternative approaches have relied on monounsaturated precursors.…”