Intramolecular C−H Insertion Reactions of (η⁵-Cyclopentadienyl)dicarbonyliron Carbene Complexes:  Scope of the Reactions and Application to the Synthesis of (±)-Sterpurene and (±)-Pentalenene

Abstract

(eta(5)-Cyclopentadienyl)dicarbonyliron carbene complexes, [(eta(5)-C(5)H(5))(CO)(2)Fe=CHR](+)BF(4)(-), are generated as reactive intermediates from thioether derivatives, (eta(5)-C(5)H(5))(CO)(2)FeCH(R)SPh, by S-alkylation with trimethyloxonium tetrafluoroborate and loss of thioanisole. The carbene complexes undergo intramolecular C-H insertion into appropriately situated side chains to form cyclopentane derivatives. The reaction has been developed into a general procedure employing cycloalkanones as scaffolds bearing the iron carbene moieties and the side chains at C(2) and C(3), respectively. The products of the intramolecular insertion reactions are substituted bicyclo[n.3.0]alkanones. The scope and limitations of the reaction are described. The reaction is applied to a total synthesis of sterpurene and to a formal synthesis of pentalenene. Overall, this approach to cyclopentane annulation complements the related metal-catalyzed insertion reactions of diazocarbonyl compounds, which are also believed to occur via metal carbene complexes.

C-H Functionalization via Iron-Catalyzed Carbene-Transfer Reactions

“…The same authors investigated the selectivity of this process in further studies and explained the observed reaction outcome via a concerted C-H insertion reaction that proceeds via a chair-like cyclic transition state (12, Scheme 5) [29,30]. (13,14) by Helquist and coworkers [29,30].…”

“…The direct C-H functionalization of benzene or alkyl benzenes (15) via carbene or metal-carbene intermediates can give access to three different products, which arise from (a) insertion into a C(sp 2 )-H bond (17), (b) insertion into a side-chain C(sp 3 )-H bond (18), or (c) a cycloaddition reaction Scheme 5. Intramolecular C-H insertion reactions for the synthesis of cyclopentane derivatives (13,14) by Helquist and coworkers [29,30].…”

“…Thereafter, Zhou and-coworkers reported on an enantioselective C-H functionalization of indole heterocycles. For this purpose, they studied the reaction of silyl-protected indole heterocycles (34) with α-aryl-α-diazoesters (29) in the presence of Fe(ClO 4 ) 2 and a chiral spirobisoxazoline ligand 36. This strategy allowed the C-H functionalization of indole heterocycles under mild reaction conditions with moderate enantioselectivity.…”

Functionalization of Carbon–Hydrogen Bonds Through Transition Metal Carbenoid Insertion

“…These include copper [36][37][38][39][40][41][42][43][44][45], silver [46,47], iron [48][49][50], gold [40], and magnesium [51,52], and most will be discussed in context, particularly in Sect. 3.1.…”

Iron‐Catalyzed Intermolecular Functionalization of Non‐Activated Aliphatic C−H Bonds via Carbene Transfer

“…[5][6][7][8][9][10] Seminal studies started with the intramolecular functionalization of CÀ H bonds using stoichiometric amounts of a preformed iron-carbene compound. [5] Subsequent reports allowed the functionalization of strong CÀ H bonds (> 95 kcal • mol À 1 ) in both stoichiometric and catalytic fashion but applying harsh reaction conditions (Scheme 1a) [6] that are also compatible with decomposition of the diazo reagent, resulting in uncatalyzed CÀ H insertion. [7] Most importantly, these studies pinpoint to high energetic barriers for the iron mediated Csp 3 À H insertion reactions, which have limited the scope to relatively weak CÀ H bonds.…”