Facial Stereoselectivity in the Diels−Alder Reactions of (Arylethynyl)ethoxycarbenes of Group 6 with 1,2,3,4,5-Pentamethylcyclopenta-1,3-diene and Unusual Rearrangement of Strained Norbornadienyl Fischer Carbene Complexes of Chromium(0)

Abstract

A study of reactivity and facial selectivity in Diels−Alder reactions of the Fischer carbene complexes (CO)5MC(C⋮CC6H4R)OCH2CH3 (1a, M = Cr, R = H; 1b, M = Cr, R = p-OCH3; 1c, M = Cr, R = p-CH3; 2, M = W, R = H; 3, M = Mo, R = H), with 1,2,3,4,5-pentamethylcyclopenta-1,3-diene (4) was carried out. The cycloadditions were found to be highly π-facial stereoselective, favoring the anti adduct. Among these complexes, 3 was the most reactive dienophile. The stereochemical assignment of the adducts 5−7 was supported by NOE measurements; structures of the anti cycloadducts 5a and 6 were established by single-crystal X-ray diffraction. A [4 + 2] cycloaddition/benzannulation/rearrangement cascade reaction was observed for the chromium carbene complexes 1a−c with 4 under thermal conditions, resulting in the naphthofurandione derivatives 9−11.

Regioselective Multicomponent Synthesis of 2,4,6‐Trisubstituted Phenols from Fischer Alkynyl Carbene Complexes

“…All this evidence, coupled with the finding that pentafulvenes lead exclusively to [4+3] cyclization in the presence of CO, [43] confirms that alkynyl carbenes are strongly influenced by the type of diene used. However, unlike the thermal rearrangement reported by our group, [40] [4+2] cycloadducts derived from pentafulvenes and alkynylcarbenes do not show this behavior. [43] In this paper, we present a new method for the synthesis of 2,4,6-trisubtituted phenols based on the reactivity of alkynylalk-oxy carbenes of chromium(0).…”

“…This step should be carried out under appropriate reaction conditions to avoid the formation of the 1,2-and 1,4-naphthofuranedione by-products. [40] Finally, reduction of the carbonyl group would lead to the desired alcohols (i.e., 6a-6q). First, a series of cycloadducts 4a-4g was prepared following the previously reported method in which only the trans adduct was obtained.…”

“…First, a series of cycloadducts 4a-4g was prepared following the previously reported method in which only the trans adduct was obtained. [40] After purification, the products were subjected to [3+2+1] cycloaddition reactions with various terminal alkynes 3a-3l. This reaction was also tested using 4a with internal alkynes (e.g., diphenylacetylene and phenylpropylacetylene), but neither [3+2+1] cycloadduct was observed; only naphthofurandione isomers were formed, demonstrating a clear steric effect of the substituents.…”

“…On the other hand, the use of less hindered dienes than the diene whose use we report here (2) is fundamental to changing the reactivity of the species, as other cases have demonstrated (compare, for instance, refs. [40,42] ). Thus, Dötz reactions are extremely versatile, as demonstrated by Barluenga et al, [32] who reported the synthesis of aminophenols from electron-poor alkenyl carbene complexes in a benzannulation reaction.…”

“…[39] Our group has previously reported the unexpected cascade reaction between alkynyl ethoxycarbenes of group 6 and 1,2,3,4,5-pentamethylcyclopentadiene (2) to produce 1,2-and 1,4-naphthofuranediones. [40] These compounds can be used as starting materials for the total synthesis of many natural products. [41] This result was particularly surprising, as Merlic, who studied an analogous reaction with cyclopentadiene, had not detected the formation of quinones in addition to the benzannulation cycloaddition product.…”