In this study, we developed a rhodium(I)-catalyzed spirocyclization. The reaction includes 1,4-rhodium migration and provides a route for forming spirocyclic 1-indanones.1,4-Rhodium migration is an effective intramolecular C-H bond-activation process, 1-5 which has been used in the formation of carbo-and heterocyclic frameworks. 3 Recently, we reported that 1,1′-spirobi[indan]-3-ones were synthesized using the rhodium-catalyzed addition-ringexpansion reaction of (3-arylcyclobutylidene)acetates involving successive 1,4-rhodium migration tandem (Scheme 1). 4 The second 1,4-rhodium migration in this reaction occurred with (1-phenylindan-1-ylmethyl)rhodium(I) species A to generate 2-(indan-1-yl)phenylrhodium(I) B, which subsequently reacted with the ester group to provide a spirocyclic structure. 6 Rhodium(I)-catalyzed cyclization reactions of arylboronic acids and esters bearing electrophilic functionalities have been extensively studied. 7-9 In 2012, Sarpong et al. reported rhodium(I)-catalyzed asymmetric cyclization of arylboronic esters bearing a pendant ketone group (Scheme 2, a). 9 The reaction produced tertiary 1-indanols in good yields and enantioselectivities. We anticipated that the replacement of the ketone group with a β-aryl α,β-unsaturated ester group would cause spirocyclization through a mechanism analogous to our previous addition-ringexpansion of (3-arylcyclobutylidene)acetates (Scheme 2, b). Herein, we report rhodium(I)-catalyzed addition-spirocyclization of arylboronic esters containing a pendant β-aryl α,β-unsaturated ester moiety that produces 1,1′-spirobi[indan]-3-ones through 1,4-rhodium migration.Phenylboronic pinacol ester 1a 10 bearing a β-phenyl α,β-unsaturated ester with E stereochemistry was heated in refluxing toluene in the presence of 5 mol% [Rh(OH)(cod)] 2 (cod = cycloocta-1,5-diene; Table 1, entry 1). Spirobiindanone 2a was formed, as expected, in 52% yield, with an accompanying 24% yield of indanylacetate 3a, derived from the protonation of organorhodium(I) species C or D. The addition of base was subsequently investigated to facilitate the conversion of the rather robust pinacol ester. After screening several organic and inorganic bases, higher conversion of (E)-1a was achieved when two equivalents of tetramethylethylenediamine (TMEDA) was added to the reaction, albeit with a worse 2a/3a ratio (Table 1, entry 2). Further, we examined the effects of phosphine ligands on the Scheme 1 Ph CO 2 Me + NaBPh 4
