Rhodium-Catalyzed Addition–Spirocyclization of Arylboronic Esters Containing β-Aryl α,β-Unsaturated Ester Moiety

Abstract

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

Recent Methodologies That Exploit C–C Single-Bond Cleavage of Strained Ring Systems by Transition Metal Complexes

“…Examples involving other alkylidenecyclobutanes were also given. Subsequent studies directly generated intermediates related to 260 by transmetalation from arylboronic esters, which resulted in the same downstream pathway . The process is a striking example of the mechanistic complexity on offer with such strategies.…”

1,4‐Migration of Transition Metals in Organic Synthesis

“…An intramolecular 1,4‐addition of 115 under the catalysis of Rh(I) catalyst would generate alkyl‐Rh species M106 , which is very similar to M103 (Scheme ) . The 1,4‐Rh migration of M106 yielded aryl‐Rh M107 .…”

Aryloxymethyltrifluoroborates for Rhodium-Catalyzed Asymmetric Conjugate Arylation. o-Methoxyarylation through 1,4-Rhodium Shift

“…Considering the very wide availability of substituted phenols and their easy conversion into aryloxymethyltrifluoroborates, , this 1,4-shift/arylation sequential reaction is of great advantage over the reaction of o -methoxyarylboron reagents. The 1,4-Rh shift from alkyl-Rh to aryl-Rh intermediates has been reported to be involved as a key step in the catalytic cycle of several types of rhodium-catalyzed reactions, which have been developed since Miura’s report in 2000 . As a reaction related to the present study, there has been a report where the Rh shift between aryloxymethyl and o -methoxyaryl is proposed to explain a methoxy-directed aryl-to-aryl 1,3-Rh migration …”

“…On the other hand, Molander reported synthesis of alkoxymethyltrifluoroborates (ROCH 2 BF 3 K) from alcohols (ROH) and their use as alkoxymethyl nucleophiles in the palladium-catalyzed cross-coupling reactions , (Scheme , eq 2). Herein, we report that the reaction of aryloxymethyltrifluoroborates (ArOCH 2 BF 3 K) with α,β-unsaturated carbonyl compounds in the presence of a chiral diene–rhodium catalyst introduces o -methoxyaryl groups at the β-position in high yields with high enantioselectivity, where the reaction proceeds through the 1,4-shift of rhodium , from aryloxymethyl to o -methoxyaryl intermediates (Scheme , eq 3). Considering the very wide availability of substituted phenols and their easy conversion into aryloxymethyltrifluoroborates, , this 1,4-shift/arylation sequential reaction is of great advantage over the reaction of o -methoxyarylboron reagents.…”