Palladium-Catalyzed Intermolecular Exchange between C–C and C–Si σ-Bonds

Abstract

A palladium-isocyanide complex opens the two four-membered rings of benzocyclobutenone and silacyclobutane to merge them into an eight-membered ring skeleton. The present reaction provides a unique example of an intermolecular cross metathesis-type reaction between covalent σ-bonds of low polarity.

Controllable Si−C Bond Activation Enables Stereocontrol in the Palladium‐Catalyzed [4+2] Annulation of Cyclopropenes with Benzosilacyclobutanes

“…TheO H-functionalized silacycle proved to be quite stable under the reaction conditions.M oreover,f luoride-anion-induced Si À Cb ond cleavage resulted in alternative access to multisubstituted cyclopropane derivatives with excellent diastereoselectivity; such compounds are difficult to synthesize with high diastereoselectivity by cyclopropanation reactions (Scheme 5bd). [18] On the basis of our experimental results (see Scheme S3 and Figure S1 in the Supporting Information) and literature precedent [5][6][7][8][9] as well as our DFT study on the palladiumcatalyzed desymmetrization of silacyclobutanes with alkynes, [13] two plausible reaction pathways are proposed for this ring-expansion/cycloaddition reaction of cyclopropenes with benzosilacyclobutanes ( Figure 2). To better understand the reaction mechanism, we performed simplified DFT calculations with PMe 3 as am odel ligand (see Table S3;t he potential-energy-surface diagram and the related energy parameters identified for two possible reaction pathways are summarized in the Supporting Information).…”

“…However,itis well-known that the development of an asymmetric version of at ransition-metal-catalyzed ring expansion based on SiÀC bond cleavage is quite difficult at present. [9][10][11] Encouraged by the success of using Pd(dba) 2 with phosphine ligand L7 to synthesize racemic silabicyclo [4.1.0]heptanes,w ec arried out careful optimization experiments with aw ide range of chiral phosphine ligands (see Tables S9-S15 for comprehensive results) to develop am ethod for the enantioselective construction of chiral silacycles 3 based on the tandem process of SiÀCbond cleavage and the formation of new SiÀCand CÀC bonds.A fter screening numerous solvents and palladium catalysts,aswell as the effect of concentration on the reaction of 1a and 2a,w ef ound that the TADDOL-derived phos-phoramidite L30 (Scheme 3) was the most efficient ligand in DCM at room temperature,generating product 3awith ahigh er value (91. 5:8.5 er).…”

“…[8] Thei ntermolecular cycloaddition of silacyclobutanes with alkynes ranks as the most attractive route to the construction of silacyclohexene six-membered-ring systems.Since Sakurai and Imai reported the first palladium-catalyzed cycloaddition reaction of silacyclobutanes with alkynes in 1975, the research groups of Oshima, Shintani, Hayashi, Song,a nd others have made important breakthroughs in developing regio-and stereoselective versions of the approach based on the Si À C bond cleavage of silacyclobutanes. [5][6][7][8][9][10][11] However,t he stereocontrol of ring expansion of silacyclobutanes (SCBs) with alkynes is still not easy;for example,inarecent study by Song et al an asymmetric rhodium-catalyzed reaction of SCBs with alkynes only gave 86:14 to 91:9 er. [8i] Intermolecular s-bond cross-exchange reactions between two small-ring systems are very interesting processes that enable the facile rearrangement of four chemical bonds to form larger ring systems with 100 %a tom economy (Scheme 1c-e).…”

“…In this regard, Murakami and co-workers reported ap ioneering example proceeding through nonpolar s-bond cross-exchange between ac yclobutanone C À Cb ond and as ilacyclobutane Si À Cb ond to give the corresponding silabicyclo [5.2.1]decanes with good diastereoselectivity (Scheme 1c). [9] Similarly,atransition-metal-catalyzed reaction between silacyclobutanes and three-membered-ring systems also provided ap ossible s-bond cross-exchange approach to give larger silacycles.I n2 008, Saito et al reported an ickel-catalyzed ring-expansion reaction of benzosilacyclobutanes with three-membered ethylcyclopropylideneacetate to give benzosilacycloheptenes containing aseven-membered ring (Scheme 1d). [10] However, the E/Z ratio for the corresponding product is not good.…”

Intermolecular σ‐Bond Cross‐Exchange Reaction between Cyclopropenones and (Benzo)silacyclobutanes: Straightforward Access towards Sila(benzo)cycloheptenones

“…[2] Thus,from adrug discovery viewpoint, investigations of sila(benzo)suberones are of great significance and in great demand (Scheme 1c). [6] Among these,transition metal catalyzed sbond-exchange reactions between C À C [7] and C À Si bonds, [8] pioneered by Murakami and co-workers,p rovides expedient access to cyclic compounds bearing asilicon atom at the ring junction (Scheme 1d). [3] Herein, we report anovel ring-expansion method to join cyclopropenones with (benzo)silacyclobutanes,t hus directly constructing sila-(benzo)suberones under mild reaction conditions.…”