Reactions of Fischer Carbene Complexes with Siloxydienes: Formation of Cycloheptadiene and Cyclopentene Derivatives—Formal [2+1] Cycloaddition Followed by Cope Rearrangement Versus Formal [3+2] Cycloaddition with or without Preceding Carbene-Ligand Metathesis

Abstract

Thermal reactions of the a,bunsaturated Fischer carbene complexes 3 ± 5 with the siloxydienes 6 and 7 mainly furnished the expected cyclohepta-1,4-dienes such as 9, 10, 12, 14, and 15, whose formation is explained by a formal [21] cycloaddition followed by Cope rearrangement. The rather electron-rich carbene complex 5 also afforded the cyclopentene derivatives 11 and 16 as by-products, which are probably formed by two distinct mechanisms. Single diastereomers of the cyclopentene derivatives 17 and 18 were isolated as the exclusive products of the reactions of 3 and 4, respectively, with the donor/ acceptor-substituted 1,3-diene 8. This process is interpreted as a metalla Diels ± Alder reaction to form a chromacyclohexene intermediate 25, which subsequently undergoes fragmentation to give cyclopentene derivatives with the required constitution. Most surprisingly, the reactions of simple carbene complexes 1 or 2 with the siloxydienes 7 or 8 yielded cyclopentene derivatives 19 and 20 that do not contain the carbene ligand of the precursor complexes. Instead, the constitution of the isolated products can be accounted for by a formal diene dimerization with fragmentation of a methylene unit. Their formation can be interpreted by an unprecedented sequence of carbene-ligand metathesis followed by a metalla Diels ± Alder reaction and reductive fragmentation. All these results illustrate the great importance of electronic fine-tuning in the precursors involved in reactions of Fischer carbene complexes with 1,3-dienes.

Tunable Properties of Inclusion Complexes Between Amylose and Polytetrahydrofuran

“…More importantly, when a metal–carbon bond is involved, these α,β‐unsaturated complexes can behave as one‐, three‐, or even five‐carbon synthetic equivalents. In this context, two‐component carbocyclization reactions—[2+1],4 [3+2],5 [3+3],6 [4+3],5b, 7 [5+1],8 [5+2],9 [6+3] [10] —have been reported, as well as multicomponent processes such as [3+2+1],11 [3+2+2],12 and [5+2+1]13 cyclization.…”

[3+2] and [4+1] Cycloaddition Reactions of Fischer Alkoxy(alkenyl)carbene Complexes with Electronically Neutral 1,3‐Dienes

“…Formation of products 3 and 4 can be rationalized, according to previous models,5, 7d, 10, 17d by means of a metalla‐Diels–Alder reaction, which presumably occurs after an initial thermally induced dissociation of a CO ligand, followed by reductive elimination of the metal moiety (Scheme ). The final [3+2] adduct 3 or [4+1] adduct 4 would involve the initial formal addition of the alkenylcarbene complex as a 1‐chroma‐1,3‐diene and as a CrC dienophile, respectively.…”

“…9 Nevertheless, the corresponding [4 D +1 C ] adduct was isolated when a Cr complex was mixed at room temperature with 2‐methyl‐1,3‐dimorpholino‐1,3‐butadiene 7c. In addition, the reaction of alkenylcarbene derivatives with a 1‐acceptor‐3‐donor‐substituted 1,3‐diene furnished cyclopentenes, which are the [3 C +2 D ] adducts of the corresponding carbene ligand to the electron‐rich terminal double bond of the silyloxydiene 7d. 10 Simple 1,3‐dienes have been shown to undergo either a thermally selective monocyclopropanation reaction ([2 D +1 C ] cycloaddition) with alkoxy(alkyl/aryl)carbene complexes of Mo and Cr,11 or a room‐temperature Diels–Alder reaction ([4 D +2 C ] annulation) with alkoxy(alkenyl)carbene complexes of Cr and W 8a,c.…”

Solvent‐Controlled Selectivity in the Synthesis of Five‐Membered Carbocycles from Carbene Complexes

“…The main types of product observed result from Diels–Alder reactions in which either the alkenyl group of the carbene complex acts as the dienophile to yield cyclohexenes A ,8 or in which the MC double bond or one of the double bonds of the diene acts as a dienophile to yield metallacyclohexenes, and, after reductive elimination, cyclopentenes such as B and C 9. Cyclopropanation of the diene, followed by a Cope rearrangement to yield cycloheptadienes D has also been reported 9b. 10…”