From α,β‐Unsaturated Fischer Carbene Complexes to Highly Substituted 3‐Ethoxycyclopentadienes, Masked Cyclopentenones

Abstract

The β-amino-substituted α,β-unsaturated Fischer carbene complexes 3 are readily available by a four step one-pot procedure from terminal alkynes, chromium hexacarbonyl and secondary amines (24 examples with yields of 68−99% and 7 examples with yields of 26−63%). The formal [3+2] cycloadditions of complexes 3 with different alkynes including diynes and enynes performed in donor solvents such as pyridine or acetonitrile afforded highly substituted 5-(dialkylamino)-3-ethoxycyclopentadienes 7, generally in medium to excellent yields (25 examples with yields of 60−95% and 7 examples with yields of 18−53%). The steric and electronic effects of the substituents on the carbene complexes and the incorporated alkynes on the regio-and stereoselectivity of the ring-forming reaction have been elaborated. An interesting 1,5-or, more likely, 1,2-migration of the dimethylamino group was observed for 5-(dimethylamino)-3-ethoxycyclopentadienes with trimethylsilyl and iPr substituents at C-5. Attempted asymmetric syntheses of cyclopentadienes 7 from complexes 3 with chiral amino groups or substituents were only moderately successful. At a center of chirality in the secondary amino group, complexes of type 3 gave compounds 7 with diastereomeric excesses of, at best, 59% in yields of

Some recent applications of Fischer carbenemetal complexes in organic synthesis

“…The desired complexes 4 can be obtained as the sole (or at least as the major) products by careful choice of the reaction conditions. Eventually, the procedure to prepare the { [2-(dialkylamino)ethenyl]carbene}chro-mium complexes 4 was improved to the extent that good to excellent yields were obtained in a one-pot operation directly from lithiated terminal alkynes 1 (Table 1) [7].…”

“…Recently, a wide range of 5-dialkylamino-3-ethoxycyclopentadienes 8 accessible in high yields from b-aminosubstituted a,b-unsaturated Fischer carbene complexes 4 in pyridine, and terminal as well as internal alkynes, has been reported [7,10]. Since these [3 + 2] cocyclization products 8 essentially are highly functionalized protected cyclopentenones 9, they have meanwhile established themselves as extremely useful building blocks for the construction of various complex skeletons.…”

An experimental–theoretical approach to the kinetics and mechanism of Michael type addition: α,β-Unsaturated tungsten Fischer carbene complex as the substrate

“…They are involved in the formation of phenol (Dö tz product) [3] and indene derivatives [4]. They also act as the intermediates to pyridines [5], pyrroles [6], cyclopentenones [7], cyclopenta- [b]pyrans [8] and other heterocyclic complexes [9].…”

Syntheses, Structures and Electronic Properties of Cationic Hydroxy‐ and Methoxy‐Substituted Tricarbonyl(tetracyclopropylcyclopentadienyl)iron Complexes

“…The carbon-carbon bonds C 1 -C 2 , C 2 -C 6 and C 6 -C 7 in the newly formed ring of the tricycle 6 are significantly (3-6 pm) longer than the corresponding ones in the previously described structure of diethyl 4,7-dimethyl-5,10-dioxo-endo-tricyclo[5.2.1.0 2,6 ]deca-3,8-diene-3,8-dicarboxylate [13] with the same tricyclic skeleton.…”

“…The reaction of neat dicyclopropylacetylene (1) [2] with dodecacarbonyltriiron in a sealed tube gave a mixture of products from which, after flash chromatography on silica gel, on top of the previously described hexacyclopropylbenzene (2), [3] and tricarbonyl(tetracyclopropylcyclopentadienone)iron (3), [3] tetracyclopropyl-p-benzoquinone (4) [4] and tricarbonyl(tetracyclopropylcyclobutadiene)iron (5) [5] were isolated in 3 and 1 % yield, respectively (Scheme 1). Scheme 1. Uncomplexed cyclopentadienones are very reactive compounds, which are prone to immediately undergo [4+2] or [2+2] cyclodimerization [6] unless sufficiently sterically demanding substituents prevent it. Thus, tetraisopropyl- [7] tetra-tert-butoxy-and tetra-tert-butylcyclopentadienone [8] as well as four-, five-and six-ring-annelated 2,5-bis(tri-methylsilyl)cyclopentadienones [9] can be isolated as stable monomers.…”