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

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 (ZnCl2) 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.

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

Treatment of 1,n-enynes with gem-dichromiomethane species prepared in situ from 1,1-diiodomethanes and CrCl2 gave alkenylbicycloalkanes. Alkenylcarbene species, which was generated via [2 + 2] cycloaddition with a triple bond, were intercepted by intramolecular cyclopropanation of a pendant double bond, and alkene metathesis did not proceed. This is a very rare example of transformation of alkynyl C≡C with gem-dimetalloalkanes. Silyl- and borylcarbene equivalents generated from gem-(dichromio)silyl- and borylmethanes promoted the coupling and cyclization of enediynes as well as 1,n-enynes.

Cyclization of 5-alkynones with chromium alkylidene equivalents generated in situ from gem-dichromiomethanes.

A rare example of cyclization with 5-alkynones, which possess non-polar alkynyl and less electrophilic polar keto carbonyl groups, was demonstrated. The key to promoting carbene/alkyne metathesis followed by alkylidenation with pendant C[double bond, length as m-dash]O double bonds was the Schrock-type nucleophilic reactivity of the generated chromium carbene equivalents from readily available diiodomethanes and CrCl2 by simple heating.

Correction: Cyclization of 5-alkynones with chromium alkylidene equivalents generated in situ from gem-dichromiomethanes.

Correction for 'Cyclization of 5-alkynones with chromium alkylidene equivalents generated in situ from gem-dichromiomethanes' by Masahito Murai et al., Chem. Commun., 2020, 56, 9711-9714, DOI: .

Chromium-Mediated Stannylcyclopropanation of Alkenes with (Diiodomethyl)stannanes.

A stannyl-group-substituted gem-dichromiomethane species, generated in situ from CrCl2, TMEDA, and tributyl(diiodomethyl)stannane, worked as an efficient stannylcarbene equivalent to promote cyclopropanation of alkenes. The reaction provided synthetically useful stannylcyclopropanes directly from commercially available unactivated alkenes without using potentially flammable alkylzinc and diazo compounds. Structural characterization of stannyl- and germyl-group-substituted gem-dichromiomethane complexes and the effect of group 14 elements containing substituents for cyclopropanation are also described.

Synthesis of Borylcyclopropanes by Chromium-Promoted Cyclopropanation of Unactivated Alkenes.

The combination of diiodomethylboronate ester, CrCl2 with TMEDA promoted borylcyclopropanation of unactivated alkenes under mild conditions. Compared with the typical Simmons-Smith cyclopropanation, the current protocol offers the following advantages: (1) the reaction proceeds stereoselectively with disubstituted alkenes even without hydroxy or alkoxy groups; (2) both electron-rich and electron-deficient alkenes can be applicable; and (3) the reaction does not require potentially flammable alkylzinc. These unique reactivity features result from the steric and electronic nature of the gem-dichromiomethane intermediates.