Palladium-Catalyzed Regio- and Stereoselective Synthesis of (E)-1,3

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

Palladium-Catalyzed Regio- and Stereoselective Synthesis of (E)‑1,3Bissilyl-6-arylfulvenes from Aryl Iodides and Silylacetylenes Souta Suzuki, Hidenori Kinoshita,* and Katsukiyo Miura* Department of Applied Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-ohkubo, Sakura-ku, Saitama 338-8570, Japan

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S Supporting Information *

ABSTRACT: An efficient synthetic route to (E)-1,3-bissilyl-6-arylfulvenes has been developed. The reaction of aryl iodides with trimethylsilylacetylene in the presence of a catalytic amount of PdBr2 gives 6-aryl-1,3-bis(trimethylsilyl)fulvenes in good to excellent yields with complete regio- and stereoselectivity. The reaction involves trimerization of trimethylsilylacetylene and cleavage of one silyl group. The silylated fulvenes obtained could be converted into halogenated fulvenes by site-selective halodesilylation. The halogenated fulvenes underwent the Stille coupling leading to the corresponding arylated fulvenes.

F

Scheme 1. Syntheses of Fulvenes

ulvene, whose chemical formula is the same as that of benzene (C6H6), is a cyclic cross-conjugated compound and colored hydrocarbon. Since fulvene and its derivatives show unique physical and chemical properties,1 they appear as key structural motifs in a wide variety of systems ranging from biologically active compounds2 to organometallic compounds.3,4 Substituted fulvenes are also receiving much attention as organic molecular materials for use in semiconductors5 and solar cell applications.6 Conventional fulvene synthesis involves condensation of aldehydes or ketones with sodium cyclopentadienide (Scheme 1, eq 1).7 Pyrrolidine-promoted8a−c and catalyzed8d syntheses of fulvenes from cyclopentadiene itself have also been achieved (eq 2). However, these approaches are not efficient with respect to the regio-controlled introduction of substituents on the cyclopentadiene ring carbons. The Pd-catalyzed cycloaddition between alkynes and haloalkenes lends access to fulvenes bearing substituents on the ring carbons (eq 3).9 Syntheses of pentasubstituted fulvenes from symmetrical internal alkynes have also been reported (eq 4).10 Several intriguing syntheses of substituted fulvenes by Pd11 or other transition metal12 catalysts, such as Tanaka’s Rh catalyzed regioselective synthesis of substituted silylfulvenes from unsymmetrical internal alkynes (eq 5),13 have been reported to date. However, most of these reactions exhibit low to moderate efficiencies and the preparation of haloalkenes or more complicated compounds is required as substrates. Hence, developing efficient methods for regio-controlled syntheses of substituted fulvenes from commercially or readily available starting materials is desirable. Over the course of our study on Pd-catalyzed construction of multisubstituted benzenes,14 we unexpectedly found that the reaction of iodobenzene with an excess of trimethylsilylacetylene gave (E)-1,3-bis© XXXX American Chemical Society

(trimethylsilyl)-6-phenylfulvene as a single isomer (eq 6, R = Ph). Aryl iodides are commercially available compounds and more accessible substrates than the corresponding βhaloalkenes (eqs 3 and 4 vs eq 6). Therefore, the direct use of aryl iodides for the substituted fulvene synthesis is highly Received: January 13, 2019

A

DOI: 10.1021/acs.orglett.9b00144 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

the long reaction period. Next, the reaction time was scrutinized (entries 19−21). A prolonged reaction time dramatically improved the yield of 3aa, and the reaction carried out for 72 h gave fulvene 3aa in 85% isolated yield (entry 20). A slightly improved yield was observed when the reaction was conducted for 6 days (entry 21). Neither higher nor lower reaction temperatures accelerated the formation of 3aa. A large-scale reaction with 10 mmol of 1a also proceeded smoothly to give 3aa in 83% isolated yield (8.3 mmol, 2.5 g). With the optimized reaction conditions in hand, we tested the substrate scope (Table 2). The reaction of 1-iodonaph-

advantageous. Herein, we report our results on the novel palladium-catalyzed cycloaddition to form (E)-6-aryl-1,3bissilylfulvenes with complete regio- and stereoselectivity. We commenced optimization of the reaction using iodobenzene (1a) and trimethylsilylacetylene (2a) (Table 1). Table 1. Optimization of Reaction Conditionsa

Table 2. Synthesis of Arylfulvenes 3a entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

cat. Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2d Pd(OH)2 PdCl2 PdBr2 PdI2 PdCl2 + NaIe PdBr2 + NaIe PdI2 + NaIe PdBr2 + NaIf PdBr2 + NaIg PdBr2 + KIe PdBr2 + LiIe PdBr2 + TBAIe PdBr2 + NaIe PdBr2 + NaIe PdBr2 + NaIe

solvent i-Pr2NEt THF DMSO CH3CN DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF

c

time (h)

yield of 3aa (%)

24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 48 72 6 dh

17 22 22 27 35 39 trace 21 48 25 32 64(68) 30 41 30 31 28 36 (70) (85) (87)

b

isolated yield

a The reaction was carried out with 1a (0.50 mmol), 2a (2.5 mmol), catalyst (5 mol %), and i-Pr2NEt (0.75 mmol) in solvent (0.5 mL) at 30 °C. bNMR yield (Bn2O was used as an internal standard). Isolated yield is shown in parentheses. ci-Pr2NEt (0.5 mL) was used. d Pd(OAc)2 (10 mol %) was used. eThe iodide (0.050 mmol) was used. fNaI (0.025 mmol) was used. gNaI (0.10 mmol) was used. h6 days, 144 h.

The Pd(OAc)2-catalyzed reaction of 1a with 2a (5 equiv) in iPr2NEt at 30 °C gave 3aa in 17% yield (entry 1). The use of THF as a solvent with 1.5 equiv of i-Pr2NEt slightly improved the yield of 3aa (entry 2). Phosphines (PPh3, P(o-tol)3, P(2furyl)3, PCy3, and dppe), cyclooctadiene, or norbornadiene as an additive were not effective for this transformation (not shown). Screening of solvents (entries 2−5) revealed that DMF was the best solvent for this reaction (entry 5). An increased amount of Pd(OAc)2 did not improve the yield of 3aa (entry 6). Therefore, other Pd(II) sources were tested (entries 7−10). Only PdBr2 was more effective than Pd(OAc)2 (entry 9). For further improvement of the catalytic system, several metal salts were tested as additives (entries 11−17). The addition of 10 mol % of NaI improved the yield of 3aa up to a 68% isolated yield (entry 12). Other combinations, such as PdCl2−NaI and PdI2−NaI, had minimal effects on the reaction (entries 11 and 13). Other metal iodides and TBAI were not effective, and the yield of 3a did not exceed 68% (entries 12 vs 16−18). Although the role of NaI is not clear in this stage, it is possible that the NaI stabilizes the Pd species in situ15 during

entry

3

Ar

72 h (%)

6 d (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

3ba 3ca 3da 3ea 3fa 3ga 3ha 3ia 3ja 3ka 3la 3ma 3na 3oa 3pa 3qa 3ra 3sa 3ta 3ua 3va 3wa 3xa 3ya 3ab 3ac

1-naphthyl p-tolyl p-Ph-C6H4 o-Et-C6H4 p-(Me2N)-C6H4 p-(MeO)-C6H4 m-(MeO)-C6H4 o-(MeO)-C6H4 p-(MeO2C)-C6H4 m-(MeO2C)-C6H4 o-(MeO2C)-C6H4 p-F-C6H4 p-Cl-C6H4 o-Cl-C6H4 p-Br-C6H4 o-Br-C6H4 o-(F3C)-C6H4 p-(Me(O)C)-C6H4 o-(O2N)-C6H4 2-Br-4-Me-C6H3 9-anthracenyl 9-phenanthryl 2-furyl 1-tosyl-3-indolyl Ph Ph

75 67 41 29 77 96 63 37 94 57 37 52 75 35 65 43 11 54 20 47 72 90 83 41 66 75

83 74 92 41 83 − 69 39 − 81 51 80 73 77 64 42 32 84 24 42 74 97 − 46 − −

a All reactions were carried out with 1 (0.50 mmol), 2 (2.5 mmol), PdBr2 (0.025 mmol), NaI (0.050 mmol), and i-Pr2NEt (0.75 mmol) in DMF (0.5 mL) at 30 °C for 72 h or 6 days.

thalene (1b) for 72 h gave the corresponding fulvene 3ba as dark red crystals in 75% isolated yield. The structure of 3ba was clearly confirmed by X-ray crystal structure analysis.16 A prolonged reaction for 6 days improved the yield of 3ba up to 83%. Therefore, a 6-day reaction was also conducted, when the fulvenes 3 were obtained in low to moderate yields after 72 h of stirring. A variety of electronically and sterically diverse aryl iodides 1c to 1u were treated with 2a under the optimized reaction conditions. The following tendencies were recognized for the reaction. (1) The electronic nature of the substituents on the aromatic ring of aryl iodides did not affect the reactivity. B


Letter

Organic Letters (2) The steric effect of the substrates was extremely important for this reaction. The presence of an ortho-substituent in 1 severely decelerated the reaction (entries 4, 8, 11, 16, 17, 19, and 20). 9-Iodoanthracene (1v), 9-iodophenanthrene (1w), and 2-iodofuran (1x) were good reaction partners in this transformation (entries 21−23). The reaction with 3-iodo-1tosyl-1H-indole (1y) also gave the corresponding fulvene 3ya in moderate yield (entry 24). Other silyl groups on the alkyne terminus were examined by using 2b and 2c. The yields of fulvenes 3ab and 3ac were lower than that of 3aa (entries 25 and 26). The steric hindrance of the silyl moiety on the spcarbon also affected the yield of this transformation. A plausible mechanism of this transformation is outlined in Scheme 2. Arylpalladiums 5 generated from Pd(0) and 1 are

Scheme 3. Mechanistic Aspects

Scheme 2. Plausible Mechanism

As a result, 3ba was recovered intact, and no silyl scrambling of 3ba was observed. This fact is in contradiction with the intermolecular process. The reactions of 12 and 15 with 2a proceed probably via cyclopentadienylcarbinylpalladiums A1 (Scheme 4), correScheme 4. Rational Explanation for Formation of 13/14/16

converted into trienylpalladiums 8 by sequential intermolecular carbopalladation via vinyl- and dienylpalladiums 6 and 7. Then, intramolecular carbopalladation, that is syn addition, of 8 forms cyclopentadienylcarbinylpalladiums 9 as a single diastereomer.9 Reductive elimination of benzyl iodides 10 occurs from benzyl palladium iodide complexes 9 to generate the active Pd(0) species. Finally, elimination of TMS-I from 10 provides only one double bond isomer, (E)-isomers of 3. To check the presence of the vinylpalladium intermediate 6 in the catalytic cycle forming 3, the reaction of (Z)-βiodoalkenylsilane 11, a precursor of 6 (Ar = Ph), with 2a was conducted under the same conditions (Scheme 3a). Predictably, 11 was successfully converted into the corresponding fulvene 3aa in 85% yield, which clearly indicates the participation of 6 with the formation of 3 from 1 and 2a. In contrast, the reaction of 12 with 2a gave fulvenes 13 and 14 bearing the diphenylmethylsilyl moiety on the fulvene rings (Scheme 3b). In addition, (Z)-β-iodoalkenylsilane 15 underwent the cyclization with 2a to form only fulvene 16, bearing the tert-butyldimethylsilyl group coming from 15 at C3, as orange crystals (Scheme 3c). The structure of 16 was successfully confirmed by X-ray crystal structure analysis,16 and its C−C double bond geometry was assigned to the (E)configuration. The silyl moieties in 12 and 15 were thought to be removed by the sequential elimination of Pd and the silyl moiety according to our plausible mechanism. However, the original silyl groups on the sp2-carbons were found on the ring carbons in 13, 14, and 16. There is a possibility that the silyl scrambling observed in Scheme 3b and 3c is caused by the secondary reaction of the initially formed fulvene 3aa. To ascertain the possibility, the Pd-catalyzed reaction of 12 with 2a was carried out in the presence of fulvene 3ba (Scheme 3d).

sponding to 9. The silyl scrambling can be rationalized by metallotropic rearrangement of A1 around the cyclopentadienyl ring. Since cyclopenta-2,4-dienylsilanes are known to easily undergo [1,2]-metallotropic rearrangement,17 the silyl group can transfer to any carbon of the cyclopentadienyl ring. Therefore, A1 should be easily isomerized to A2 through A5 by migration of the silyl groups (Si2) coming from the starting alkenylsilanes. It is also conceivable that other isomers A6 and A7/A8 are generated from A2 and A4, respectively, by migration of the trimethylsilyl group (Si1) attached to the sp3carbon of the cyclopentadienyl ring. The fulvene products are formed from these intermediates A6 and A8 with elimination of the labile Si1 group on the sp3-carbon. The preferred formation of the scrambling products 13/16 and 14 is attributable to fast elimination of the Si1 group from A8 and A6, respectively, because the sterically less hindered silyl group is more subject to nucleophile-induced elimination than the bulky silyl group (Si2). The site-selective scrambling at C3 is likely due to avoiding 1,3-allylic strain caused by the Si2 group at C1. Consequently, the silyl scrambling strongly supports the proposed mechanism involving intramolecular carbopalladation of trienylpalladiums 8, and it clearly denies a possibility C


Organic Letters

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that the cyclization of 8 to fulvenes 3 proceeds by a common transmetalation between the alkenylpalladium and the alkenyl silane in 8 and reductive elimination sequence, such as the intramolecular Hiyama-type coupling reaction. We demonstrated the synthetic utility of 1,3-bissilylfulvenes 3 and 16 by further modification via halodesilylation. The reaction of 16 with NIS gave 17 in good yield, as dark-red crystals, via site-selective iododesilylation (Scheme 5). The

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00144. Experimental procedures and characterization data; Xray crystallographic data for 3ba (CCDC 1885560), 16 (CCDC 1885568), 17 (CCDC 1885569), and 18 (CCDC 1885570) (PDF)

Scheme 5. Regioselective Halodesilylation and Derivatization

Accession Codes

CCDC 1885560 and 1885568−1885570 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hidenori Kinoshita: 0000-0002-9761-9015 Katsukiyo Miura: 0000-0002-4535-8677 Notes

The authors declare no competing financial interest.

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only trimethylsilyl group at position 1 was replaced with iodide, and the regioisomer could not be detected. The Stille coupling of 17 gave 18 as dark-red crystals in reasonable yield. The structures of 17 and 18 were unambiguously established by X-ray crystal structure analyses.16 The site-selective iododesilylation of 3aa with NIS also provided 19 in excellent yield. These observations show that the iododesilylation occurs depending on not the type of silyl group but the position of the fulvene ring. It is known that fulvenes react with electrophiles on position 1 or 4,1 and this fact corresponds to our experimental observations. Prolonged reaction with an increased amount of NIS gave 1,3-diiodofulvene 20 in 78% yield with a trace amount of 19. However, 20 was highly unstable and decomposed in a couple of minutes under an ambient atmosphere. The Stille coupling of 19 gave 21 in moderate yield. Regioselective bromodesilylation of 3ba also proceeded successfully to form 22. Unfortunately, further modification of 18 and 21 cannot be achieved in this stage because 3-halogenated fulvenes are very unstable when generated from 21 by halodesilylation. In conclusion, we have developed a novel Pd-catalyzed cycloaddition for the synthesis of (E)-6-aryl-1,3-bissilylfulvenes with complete regio- and stereoselectivity in moderate to excellent yields. This transformation represents a straightforward method to introduce a 1,3-bissilylfulvenyl moiety into aromatic rings from readily available substrates. The proposed mechanism involves inter- and intramolecular carbopalladation followed by desilylative reductive elimination. The bissilylfulvenes obtained could be converted into halofulvenes by regioselective halodesilylation, which served as platforms for further modification to π-extended fulvenes. Further synthetic utility of the fulvene derivatives is under investigation in this laboratory.

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Palladium-Catalyzed Regio- and Stereoselective Synthesis of (E)-1,3

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