Structural Characterization and Unique Catalytic Performance of Silyl-Group-Substituted Geminal Dichromiomethane Complexes Stabilized with a Diamine Ligand

Abstract

Stabilization by a silyl group on the methylene carbon and a diamine ligand led to the isolation of gem-dichromiomethane species. X-ray crystallography confirmed the identity of the structure of this rare example of reactive gem-dimetalloalkane species. The isolated gem-dichromiomethane complex acted as a storable silylmethylidene carbene equivalent, with reactivity that could be changed dramatically upon addition of a Lewis acid (ZnCl) and a base (TMEDA) to promote both silylalkylidenation of polar aldehydes and silylcyclopropanation of nonpolar alkenes. Identification of a key reactive species also identified the catalytic version of these transformations and provided insights into the reaction mechanism. In contrast to Simmons-Smith cyclopropanation, the real reactive species for the current cyclopropanation was a chromiocarbene species, not a chromium carbenoid species.

Synthesis of Borylcyclopropanes by Chromium-Promoted Cyclopropanation of Unactivated Alkenes

“…Our preliminary result for borylcyclopropanation was reported in 2007, in which two terminal alkenes (allyl benzyl ether and 1-dodecene) were demonstrated as substrates with I 2 CHBpin prepared in situ from the reaction of Cl 2 CHBpin and LiI, resulting in low stereoselectivity (up to 63:37) . During a detailed study to clarify the reactive chromium species and reaction mechanism, Charette and co-workers recently reported Simmons–Smith type borylcyclopropanation of olefins using a borylmethylzinc carbenoid (Scheme , eq 2) . As exemplified by this report, olefin substrates for most of the previously reported stereoselective cyclopropanation are limited to allylic ethers and aryl olefins .…”

“…We focused on the early transition metals as promoters, and developed chromium-mediated selective cyclopropanation of unactivated terminal alkenes (Scheme , eq 1). , This method provided cyclopropanes containing synthetically useful iodo and silyl , functionality in a single step from commercially available terminal alkenes. Recently, our mechanistic studies indicated that the reaction proceeded via generation of a gem -dichromiomethane (Cr–CHR–Cr) as a key reactive species from diiodomethane derivatives. , We envisioned that the use of diiodomethylboronate esters would provide borylcyclopropanes that could be utilized as efficient building blocks to incorporate a cyclopropyl ring into the target molecules through the Suzuki–Miyaura cross-coupling reaction . Several methods for providing borylcyclopropanes have been reported, including cyclopropanation of alkenylboranes with metal carbenoids, hydroboration of cyclopropenes, direct borylation of cyclopropanes having heteroatom-containing directing groups, and cross-coupling reactions with cyclopropyl halides .…”

“…Reduction of diiodomethylboronate ester with 2 equiv of [(tmeda)­CrCl 2 ] 2 A gave gem -dichromiomethylboronate ester C , along with (tmeda)­CrCl 2 I B . The stuctures of A , B , and the related complex of C , gem -(dichromiomethyl)­trimethylsilane, were confirmed umambiguously by X-ray crystallographic analysis in our recent work . Chromocarbene intermediate D was formed from C , which then converted into chromocyclobutane E via [2 + 2] cycloaddition with olefins.…”

A Rare-Earth-Metal Ensemble of the Tebbe Reagent: Scope of Coligands and Carbonyl Olefination

“…The feasibility of such “Schrock”-type nucleophilic carbenes and their applicability/superb performance in fundamental organic transformations such as olefin metathesis − and carbonyl methylenation (Wittig-type reactivity) , have triggered immense research activities. ,, Tebbe’s compound Cp 2 Ti­(μ-CH 2 )­(μ-Cl)­Al­(CH 3 ) 2 , also termed the Tebbe reagent, involving a Lewis acid (LA = AlMe 2 Cl) stabilized [TiCH 2 ] moiety, , has featured a most efficient (rapid CO to CCH 2 conversion at ambient temperatures) and hence prominent carbonyl methylenation reagent in organic synthesis ever since. − Crucially, in natural product synthesis the Tebbe reagent often outperforms the Wittig reagent CH 2 PPh 3 due to its enhanced functional group tolerance (e.g., ester or acetal) and also the Petasis reagent Cp 2 Ti­(CH 3 ) 2 , which requires elevated temperatures (65 °C) to form the [TiCH 2 ] moiety via methane elimination. − Drawbacks of the Tebbe reagent are its moisture sensitivity, the coformation of Ti­(III) species when Cp 2 TiCl 2 is reacted with the “reducing” reagent AlMe 3 , and the viability of only methylenation. , On the other hand, reduced titanium species such as those obtained from the ternary systems CH 2 Br 2 /TiCl 4 /Zn and CH 2 Br 2 /TiCl 4 /Mg have been employed according to the Takai–Lombardo reaction for the mild methylenation of ketones counteracting enolization (thus counteracting any epimerization of α-chiral centers; also displaying high functional group tolerance). , Moreover, an excess of divalent Cp 2 Ti­[P­(OEt) 3 ] 2 (via Cp 2 TiCl 2 /Mg/P­(OEt) 3 ) has been successfully used in the olefination of thioacetals with carbonyl compounds (Takeda reaction, also characterized by a wide substrate scope) . Switching to group 6 reagents, the iodo-methylidene Cr­(III) complex Cr 2 Cl 4 (CHI)­(THF) 4 (Takai olefination reagent) has been employed successfully to convert aldehydes to E -iodido-functionalized olefins accessible to further derivatization. − …”

Cyclization of 1,n-Enynes Initiated by the Addition Reaction ofgem-Dichromiomethane Reagents to Alkynes

“…Bench-stable gem -dimetalloalkanes (R 2 CM 1 M 2 ) possessing two metal atoms on the same carbon are classical but still important reagents in organic synthesis , Thanks to their mild reactivity, functional groups on the gem -dichromiomethane species were well-tolerated to convert various aldehydes into synthetically useful alkenyl iodides, silanes, and boranes with high regioselectivity . In addition, we have reported that proper Lewis base additives could change the reactivity of gem -dichromiomethane species as chromacarbene equivalents to promote cyclopropanation of unactivated alkenes (Figure (a)). , While the reactivity of gem -dimetallomethanes with double bonds of both polar CO and nonpolar CC have been studied, their use for the transformation of a triple bond of alkynyl CC is severely limited .…”