Zinc-catalyzed synthesis of 2-alkenylfurans via cross-coupling of enynones and diazo compounds

Abstract

Inexpensive, low-toxic ZnCl 2 serves as the catalyst for selective cross-coupling of enynones and diazocompounds, an unprecedented reaction pathway for zinc carbenoids. A cascade sequence comprising formal cyclization/crosscoupling leads to a variety of 2-alkenylfurans.

Zinc‐Catalyzed Synthesis of Conjugated Dienoates through Unusual Cross‐Couplings of Zinc Carbenes with Diazo Compounds

“…Since zinc salts do not promote N 2 extrusion in stabilized diazo compounds, we reasoned that they could act as nucleophiles to trap the electrophilic zinc carbenes, avoiding undesired homocoupling processes. In this manner, we described the synthesis of functionalized alkenes, namely β‐furylacrylates, by cross‐coupling of enynones with stabilized diazo compounds (Scheme b) …”

“…The synthesis of 1,3‐dienes is a relevant goal in chemistry due to the wide range of synthetic applications and the properties of this class of compounds . In view of this, we wondered whether applying our previously described methodology could produce extended π‐conjugated systems by using the corresponding vinyl diazo compounds . Herein, we disclose our findings.…”

“…Conjugated enynone 1 a and ethyl 2‐diazobut‐3‐enoate ( 2 a ) were selected at the outset (Scheme a). Under previously optimized reaction conditions (10.0 mol % of ZnCl 2 , 1.2 equiv of 2 a in CH 2 Cl 2 at ambient temperature), the formation of a new penta‐2,4‐dienoate 3 a arising from the cross‐coupling of 1 a and 2 a was observed in a 71 % isolated yield as a mixture of isomers ((2 E ,4 E ):(2 Z ,4 Z ):(2 Z ,4 E )=1:1.6:2.7). Importantly, NMR analysis of 3 a clearly revealed that the formation of a new double bond took place at the γ‐position (green dot) of the vinyl diazo moiety.…”

“…In this manner,w ed escribed the synthesis of functionalized alkenes, namely b-furylacrylates,b y cross-couplingo fe nynones with stabilized diazo compounds (Scheme 1b). [8] The synthesis of 1,3-dienesi sarelevant goal in chemistry due to the wide range of synthetic applicationsa nd the properties of this class of compounds. [9] In view of this, we wondered whether applyingo ur previously described methodology could produce extended p-conjugated systemsb yu sing the corresponding vinyl diazo compounds.…”

Zinc reagents as non-noble catalysts for alkyne activation

“…Making use of the inertness of stabilized diazocompounds toward zinc salts, López and Vicente use these compounds as nucleophiles to trap zinc carbenoids generated from enynones. 52 In this manner, a variety of 2-vinylfuran derivatives could be prepared in good yields and selectivities under very mild reaction conditions (Scheme 31). The process was compatible with diazocompounds bearing esters, ketones or even trifluoromethyl substituents as stabilizing groups.…”