Structural Characterization and Unique Catalytic Performance of Silyl

Article pubs.acs.org/JACS

Structural Characterization and Unique Catalytic Performance of Silyl-Group-Substituted Geminal Dichromiomethane Complexes Stabilized with a Diamine Ligand Masahito Murai,*,† Ryuji Taniguchi,† Naoki Hosokawa,† Yusuke Nishida,† Hiroko Mimachi,† Toshiyuki Oshiki,† and Kazuhiko Takai*,†,‡ †

Division of Applied Chemistry, Graduate School of Natural Science and Technology, and ‡Research Institute for Interdisciplinary Science, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan S Supporting Information *

ABSTRACT: Stabilization by a silyl group on the methylene carbon and a diamine ligand led to the isolation of gemdichromiomethane 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.

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INTRODUCTION Bench-stable gem-dimetalloalkanes (R2CM1M2), possessing two metal atoms on the same carbon, are useful synthons equivalent to dianions.1 A classic example is the Tebbe reagent (Ti−CH2− Al), which is a widely used organometallic reagent for methylenation of carbonyl compounds.2 In contrast to their synthetic potential, however, the variety and number of structurally characterized reactive gem-dimetalloalkanes are limited, probably due to their instability.3 The isolation and direct structural characterization of these compounds is challenging; however, our recent report indicated that the use of a sterically hindered bipyridyl ligand enabled isolation and structural characterization of a new gem-di(iodozincio)methane complex.4 Based on this successful result, an approach involving the use of bulky functional groups to kinetically stabilize the reactive methylene center was envisioned to be a useful strategy for identifying other gem-dimetalloalkane species for organic synthesis. Organochromium reagents are among the growing number of organometallic reagents with unique reactivity.5 They offer significant benefits for carbon−carbon bond-forming reactions, including allylation, nickel-catalyzed alkenylation, radical coupling reactions, and oligo- or polymerization of ethylene.5 Recently, CrCl2 was also shown act as a useful catalyst for crosscoupling of aryl halides with Grignard reagents by Knochel and co-workers.5j Another traditional but unique use of chromium reagents is in the alkylidenation of aldehydes.6,7 Takai reported © 2017 American Chemical Society

that the use of organochromium reagents prepared from CrCl2 and di- or trihalomethanes (RCHX2) converted aldehydes into functionalized alkenes in good yields (Scheme 1a).7 The Scheme 1. Impact of TMEDAa on Chromium-Promoted Iodoalkylidenation of Olefins and Iodocyclopropanation of Alkenes

a

TMEDA = N,N,N′,N′-tetramethylethylenediamine.

reaction proceeded under mild conditions with excellent stereoselectivity and functional group compatibility to provide synthetically useful alkenyl iodides, silanes, and boranes. Therefore, the method has been applied to the total synthesis of many natural products.8 Later studies showed a dramatically different reaction course, with cyclopropanes obtained from alkenes upon treatment of CrCl2 with a proper diamine ligand Received: July 18, 2017 Published: August 16, 2017 13184

DOI: 10.1021/jacs.7b07487 J. Am. Chem. Soc. 2017, 139, 13184−13192

Journal of the American Chemical Society

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(Scheme 1b).9 Compared with typical cyclopropanation protocols, including Simmons−Smith-type cyclopropanation with zinc carbenoids (RZnCH2X) and catalytic decomposition of diazo compounds with appropriate metal catalysts,10,11 this approach easily provides functionalized cyclopropanes containing iodo, silyl, and boryl groups in a single step from commercially available olefins. While gem-di(dichlorochromio)methanes, XCH(CrCl2)2 (X = I, SiR3, and Bpin), were proposed as reactive species in the previous studies,9a−c their isolation and direct structural characterization have remained elusive. The unique reactivity shown in Scheme 1 implies that gem-di(dichlorochromio)methane reagents themselves tend to react with polar carbon− heteroatom double bonds of aldehydes, while treatment of diamine additives changes reactivity preferentially toward the nonpolar carbon−carbon double bonds of alkenes. This finding indicated that isolation and direct structural characterization of chromium intermediates could provide insights into the origin of the reactivity and expand the application of organochromium reagents. The present study describes the synthesis and identification of a storable gem-di(dichlorochromio)methane complex kinetically stabilized with a diamine ligand and a bulky trimethylsilyl (SiMe3) group on the methylene carbon. The stability as well as the catalytic performance of the gem-dichromiomethane complex for both cyclopropanation and alkylidenation was investigated. The kinetic study provided useful insights into the effect of diamine ligands and the nature of the unique dinuclear chromium reactive intermediate for silylcyclopropanation.12 These studies also revealed unique reactivity control of the gemdichromiomethane species by a Lewis acid (ZnCl2) and Lewis base (TMEDA). Similar reactivity control of gem-dimetallomethanes was reported previously by Grubbs and co-workers,13 who stated that the Tebbe reagent promoted the methylenation of aldehydes, while titanacarbene species generated by addition of 4-(dimethylamino)pyridine (DMAP) promoted olefin metathesis polymerization (Scheme 2). In the current reaction, gemdichromiomethane itself promoted alkylidenation of aldehydes, while cyclopropanation of alkenes occurred via pretreatment of TMEDA. Similar reactivity control is very rare for gemdimetallomethanes.

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RESULTS AND DISCUSSION Synthesis and Structure Characterization of gemDi(dichlorochromio)methane Complex with TMEDA Ligand. Reaction of CrCl2 with TMEDA in THF at 25 °C resulted in the formation of (tmeda)CrIICl2 complex 1 as a blue precipitate (Scheme 3).14 (tmeda)CrIIICl3 was also prepared as Scheme 3. Synthesis of (tmeda)SiCH(CrCl2)2 Complex 2

a reference compound by reaction of CrCl3 with TMEDA in acetonitrile at 25 °C (see Supporting Information (SI) for details). The structures of these complexes were confirmed by X-ray crystallographic analysis (Figure 1; see also Figures S1

Figure 1. X-ray crystal structure of (tmeda)CrIICl2 complex 1 (left) and (tmeda)CrIIICl3 complex (right). Color code: green, Cr; blue, N; yellow, Cl. All hydrogen atoms omitted for clarity.

and S3 in SI). The single crystal obtained by slow evaporation of acetonitrile showed the chlorine-bridged dimeric structure of 1. (tmeda)CrCl3 exhibited a square-pyramidal configuration with an empty site trans to the Cr−Cl bond. A single crystal exhibiting a trigonal-bipyramidal geometry was not obtained. Next, complex 1 was treated with (diiodomethyl)trimethylsilane (here after referred to as “SiCHI2”)15 to yield (tmeda)SiCH(CrCl2)2 complex 2 along with the generation of (tmeda)CrCl2I as a byproduct via two consecutive singleelectron transfers of 1 (Scheme 3). Pure 2 was isolated in 57% yield as a red solid by extraction of the crude mixture with dichloromethane and hexane (v/v = 3/2), followed by removal of organic solvents under reduced pressure. Although the 1H NMR spectra for 1 and 2 displayed only broad resonances due to the paramagnetic nature of chromium compounds, elemental analysis supported the structural formula of these complexes (see Experimental Section for details). After an extensive trial, a single crystal of (tmeda)SiCH(CrCl2)2 complex 2 suitable for X-ray crystallographic analysis was obtained by recrystallization from a mixture of THF and hexane. The ORTEP drawing shown in Figure 2 shows the molecular structure of 2, which has two chromium centers with a TMEDA ligand bridged by a methine carbon. Selected interatomic distances and angles are listed in Table 1. The Cr1−C7−Cr1′ bond angle of 89.0° was larger than that previously reported for the Cr−C−Cr bond angle: 71.8° for CH2[Cp*Cr(μ-Me)]2, 84.1° for [Cp*Cr(μ-CHSiMe3)]2, and

Scheme 2. Reactivity Control of gem-Dimetallomethanes Using a Lewis Acid and Base

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treating 2 equiv of 2 with allyl benzyl ether (Scheme 4).12 The results clearly show that the real reactive species of the Scheme 4. Silylcyclopropanation of an Olefin with gemDi(dichlorochromio)methane Complex 2

Figure 2. X-ray crystal structure of the gem-di(dichlorochromio)methane complex 2. Left, front view; right, side view. Thermal ellipsoids are drawn at the 50% probability level. Color code: green, Cr; pink, Si; blue, N; yellow, Cl. All hydrogen atoms omitted for clarity.

chromium-mediated silylcyclopropanation was a gemdichromiomethane, not chromium carbenoid (Cr−CR2−I).9d,e Note that isolation of several gem-dichromiomethane complexes has been reported previously;16 however, none were utilized as gem-dimetallomethane reagents. To obtain further insights into the reactive nature of 2, its stability in the solid state was examined utilizing cyclopropanation of allyl benzyl ether as a reaction probe (see Scheme S3 in SI for details). The results indicated that 2 was relatively stable, even in air, but was unstable toward water. Moreover, the solid sample of 2 was stable, even at high temperatures, and no notable decrease in yield was observed during silylcyclopropanation after heating at 100 °C for 30 min. Although a major drawback of the di(dichlorochromio)methane species used previously was their instability, no obvious decrease in reactivity toward cyclopropanation was observed, even after 6 months, due to stabilization by TMEDA, as long as 2 was kept in the solid state under an argon atmosphere. Several main-group metal-based organometallic reagents having metal−halogen bonds were unstable in the solution state due to Schlenk equilibrium.17 For example, the gemdi(iodozincio)methane complexes, R2C(ZnX)2, underwent a change in chemical structure via Schlenk equilibrium, i.e., selftransmetalation to produce dimeric, trimeric, and oligomeric zinc species, as reported previously.4 In contrast, 2 did not undergo decomposition in THF at 25 °C for 24 h, and the corresponding silylcyclopropane 3a was obtained in 88% yield from reaction with allyl benzyl ether (Scheme 5a).18 Because SiCH(CrCl2)2 species without TMEDA ligand lost reactivity after being kept in THF for 24 h, TMEDA effectively stabilized the generated gem-dichromiomethane species (Scheme 5b). Effect of Ligands on Silylcyclopropanation of Olefins. The effect of ligands on silylcyclopropanation was investigated

Table 1. Selected Interatomic Distances (Å) and Angles (deg) for 2 Cr1−N1 Cr1−N2 Cr1−C7 Cr1−Cr1 Cr1−Cl2

2.179(3) 2.366(3) 2.082(3) 2.307(1) 2.422(1)

Cr1−Cl3 Cr1−Cr1′

2.411(1) 2.920(1)

Cr1−C7−Cr1′ N1−CM−N2

89.04(18) 81.46(11)

81.7° for MeCH[η5-Py′Cr(μ-Cl)]2 (Py′ = 2,5-di-tert-butylpyrrolide anion).16b,c,f In addition, the Cr1−C7 bond distance of 2.082 Å was longer than those reported for typical gemdichromiomethane complexes: 2.033 and 2.050 Å for CH2(Cp*Cr(μ-Me)) 2 , 1.993 and 2.017 Å for [Cp*Cr(μCHSiMe3)]2, and 2.048 Å for MeCH[η5-Py′Cr(μ-Cl)]2.16b,c,f The current Cr−C bond distance is even longer than those of alkylchromium complexes, although none of the gemdichromiomethane complexes reported previously possessed nitrogen ligands and thus are difficult to compare directly.16 The longer Cr−C bond distance may be attributable to the trans influence by the strong σ-donating nature of the diamine ligand. The two Cr−N bond distances of 2, which ranged from 2.179 to 2.366 Å, are longer than those of (tmeda)CrCl2 1 (2.149−2.187 Å) and (tmeda)CrCl3 (2.165−2.198 Å) but in the range typically observed for Cr−N dative bonds. The bridging chlorides (2.411−2.422 Å) have longer Cr−Cl distances than the terminal chloride (2.307 Å). The Cr−Cl distances of terminal chlorides of 1 (2.366−2.396 Å) were longer than those of 2, because the geometry around Cr of monomer part of 1 was roughly square planar, and the trans effect exerted by TMEDA affected it more directly. Although other gem-di(dichlorochromio)methane complexes having silyl, iodo, and boryl functionalities on the bridged methine carbons,7 stabilized with various ligands, including pyridine, bipyridine, and PPh3, could be synthesized in a similar procedure, all attempts to obtain suitable crystals for single-crystal X-ray crystallographic analysis by recrystallization failed. These results may imply that the bulky trimethylsilyl group effectively stabilized the unstable gem-dichromiomethane species, which was advantageous for isolation and structural characterization by X-ray crystallography. Reactivity and Stability of gem-Di(dichlorochromio)methane Complex with TMEDA. (tmeda)SiCH(CrCl2)2 complex 2 can be utilized as a silylmethylidene carbene equivalent for the cyclopropanation of olefins.9b,c Cyclopropylsilane 3a was obtained in 92% yield (trans/cis = 66/34) by

Scheme 5. Stability of gem-Dichromiomethane Species in THF

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advantageous for promoting the approach of alkenes to the active center of chromium reactive species.19 Catalytic Cyclopropanation of Alkenes with gemDi(dichlorochromio)methane Complexes. As shown in Scheme 3, 2 equiv of (tmeda)CrIIICl2I was formed as a byproduct via two consecutive single-electron transfers of (tmeda)CrIICl2 1 to (diiodomethyl)trimethylsilane (SiCHI2) during the generation of (tmeda)SiCH(CrCl2)2 complex 2. Given the reduction of (tmeda)CrIIICl2I back to 1, the reaction would proceed even with a catalytic amount of (tmeda)SiCH(CrCl2)2 complex 2 (Scheme 6). Although Fürstner et al. have

(Table 2). Without any ligands, the expected 3a was obtained, albeit in low yield (entry 1). Compared to reaction with Table 2. Effect of Ligands on Silylcyclopropanation of Alkenes

entry

ligand

recoverya/%

yield of 3aa/%

trans/cis

1 2 3 4b 5b 6b 7 8 9 10 11 12 13 14b 15

none TMEDA TEEDA Et3N DMAP 2,6-lutidine 2,2′-bipyridyl 9,10-phenanthroline L1 L2 L3 L4 L5 PPh3 dppe

66 0 59 57 78 66 62 73 20 88 81 70 80 0 0

26 92 22 11 0 5 0 0 70 0 0 4 trace 26 54

56/44 66/34 78/22 70/30 − 57/43 − − 67/33 − − 62/38 − 65/35 76/24

a

Scheme 6. Working Hypothesis for the Mechanism of Chromium-Catalyzed Silylcyclopropanation of Olefins

addressed this issue, development of catalytic redox processes based on a ligand-coordinated CrII and CrIII cycle remains limited.7f,20 Thorough investigation let to the realization that the inexpensive and less toxic manganese powder could promote the reduction efficiently. Using 6 equiv of manganese powder, catalytic silylcyclopropanation of allyl benzyl ether occurred with 10 mol% of 2.12 The yield was increased when 1 equiv of SiCHI2 and 3 equiv of manganese were treated at the beginning, with further addition of the remaining SiCHI2 and manganese in 8 h at 50 °C (Table 3, entry 1). No cyclopropanation occurred with manganese in the absence of 2, and 92% of the allyl benzyl ether was recovered intact (entry 2). Use of the manganese is important, because cyclopropanation did not proceed using other reductants, including magnesium, zinc, iron, or indium.21 Although the exact reason for this is not clear, it might be due to competition between

Determined by 1H NMR. bUsed 16 equiv of ligand.

TMEDA, the stereoselectivity was improved to trans/cis = 78/ 22 with sterically more hindered TEEDA (N,N,N′,N′tetraethylethylenediamine) as a ligand (entries 2 and 3). Monodentate nitrogen ligands, such as triethylamine, DMAP, and 2,6-lutidine, as well as pyridine-based bidentate ligands, including 2,2′-bipyridyl and 9,10-phenanthrene, nearly shut down the silylcyclopropanation of allyl benzyl ether (entries 4− 8). Although N,N,N′,N′-tetramethylcyclohexane-1,2-diamine (L1) promoted cyclopropanation, similar alkylamino-groupsubstituted bidentates L2, L3, and L4 did not provide the expected 3a (entries 9−12). Tridentate ligands, such as N,N,N′,N″,N″-pentamethyldiethylenetriamine (L5), were also ineffective for the current silylcyclopropanation (entry 13). In contrast, phosphine-based ligands, such as triphenylphosphine and 1,2-diphosphinoethane, gave silylcyclopropane 3a in 26% and 54% yields, respectively (entries 14 and 15). These results confirm the importance of a TMEDA ligand in silylcyclopropanation for reaction efficiency and stereoselectivity. The trans effect by the strongly σ-donating TMEDA ligand elongated the carbon−chromium bonds of gem-dichromiomethane complex 2, which was confirmed by the ORTEP drawing shown in Figure 2 (Cr−C bond distance of 2.082 Å was longer than in other typical alkylchromium complexes). Because the elongated C−Cr bond of TMEDA-ligated 2 can be cleaved easily, the formation of a chromium reactive species (see dinuclear chromocarbene species 8 in Figure 5) would be favored in the presence of TMEDA. The coordinating flexibility of TMEDA ligand compared with L1 and L2 was also

Table 3. Catalytic Silylcyclopropanation of Olefins with 2a

a

1 equiv of Me3SiCHI2 and 3 equiv of Mn were treated at the beginning, with further addition of remaining amount in 8 h. b Determined by 1H NMR of crude mixture. cWithout 2; 92% of allyl benzyl ether was recovered. d0.2 M in THF. e20 mol% of 2 was used. 13187


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Journal of the American Chemical Society halogen−metal exchange and the desired generation of 2 in the presence of metals other than manganese. The reaction was also promoted with catalytic amounts of 1 by using manganese as a reductant (see Table S2 in SI). However, the use of 2 as a catalyst is recommended, considering the instability of 1 toward air and moisture and the difficulty in obtaining 1 in a pure form.22 The generality of catalytic silylcyclopropanation using 2 as a catalyst was investigated briefly. The stereoselectivity was improved in the reaction of allylbenzylamine having coordinating functionality to furnish the corresponding cyclopropane 3b in 74% yield (trans/cis = 80/20) (Table 3, entry 3). Arylolefins as well as alkylolefins were applicable for the current cyclopropanation (entry 4). Myrcene reacted selectively at the terminal double bond to afford the corresponding 3d in 58% yield (entry 5). This terminal selective cyclopropanation cannot be achieved by traditional Simmons−Smith reaction, in which di- and trisubstituted double bonds react preferentially over monosubstituted alkenes.23 Mechanistic Investigation of Silylcyclopropanation of Olefins. As shown in Scheme 1, the unique change in reactivity of 2 occurred upon addition of TMEDA in the current study. The gem-dichromiomethane species, SiCH(CrCl2)2, itself prefers polar carbon−heteroatom double bonds of aldehydes, while the TMEDA-ligated species reacted with relatively nonpolar carbon−carbon double bonds of alkenes to provide cyclopropane derivatives. Similar reactivity control of gemdimetallomethane has been reported by Grubbs and coworkers, involving dissociation of the Tebbe reagent from titanacarbene species and Me2AlCl, promoted by DMAP (Scheme 7).13 While the Tebbe reagent promoted the

Figure 3. Kinetic expression of the rate for formation of 3a with chromocarbene species 2′.

cyclopropanation kinetics. A roughly linear relationship between yield of 3a and time was observed during the initial stage of the reaction (Figure 4a), and the initial rate of

Scheme 7. Generation of Titanacarbene Species from Tebbe Reagent and Their Reactivitya

a

As reported by Grubbs et al.13

methylenation of aldehydes, the titanacarbene species was reported to promote the polymerization of olefins by the metathesis mechanism via the formation of titanacyclobutane intermediates. Based on the Grubbs’s report, the generation of mononuclear chromocarbene complex 2′ was hypothesized to be promoted by the TMEDA ligand in the presence of olefins, and the formation of chromocyclobutanes with alkenes thought to occur in the current silylcyclopropanation. Since the reaction was promoted by mononuclear 2′, the rate for the formation of 3a (d[3a]/dt) can be expressed by the equation shown in Figure 3. Here, K is the equilibrium constant for the dissociation of 2 to 2′ and (tmeda)CrCl3 7, and k is the rate constant for the cyclopropanation. The rate expression could be reduced by the approximation of [2′] = [7]. Based on this expression, an inverse first-order dependence on the concentration of 2 was expected initially. However, this rate expression was not consistent with the experimental data obtained from

Figure 4. (a) Reaction profile for cyclopropanation of allyl benzyl ether in THF at 50 °C with 20 mol% (●), 30 mol% (■), 40 mol% (▲), and 50 mol% (×) of 2. (b) Plot of initial rate for the formation of 3a vs concentration of 2.

cyclopropanation was first-order-dependent on the amount of 2 (Figure 4b). These results indicate that reaction with allyl benzyl ether was promoted by the dinuclear chromium species without dissociation to mononuclear chromocarbene species 2′. Note that an induction period and sigmoidal reaction profile were observed in the early stage of reaction when catalyst loading was less than 30 mol%. This is consistent with the in situ formation of the reactive dinuclear chromocarbene species from 2, and above-discussed initial rate of cyclopropanation was determined excluding the data of the induction period. 13188


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Journal of the American Chemical Society The reaction mechanism for cyclopropanation is proposed as shown in Figure 5, in which the chlorine-bridged dinuclear

Table 4. Effect of a Lewis acid on Silylalkylidenation of an Aldehyde with gem-Di(dichlorochromio)methane Complex 2a

yield /%

Figure 5. Proposed reaction mechanism for chromium-catalyzed silylcyclopropanation.

entry

MXn

4a

5

1 2b 3c 4 5 6 7 8

− MnCl2 CoCl2 FeCl3 TiCl4 ZnCl2 ZnBr2 ZnI2

0 0 14 8 0 74 57 61

6 0 18 6 76 0 0 0

a

Trans/cis ratio of 4a was determined to be >99/1 by 1H NMR of crude mixture. b49% of Ph(CH2)2CHO was recovered c12% of Ph(CH2)2CHO was recovered.

chromium complex acts as a catalytic active species based on the kinetic study. First, (diiodomethyl)trimethylsilane (SiCHI2) is reduced by 4 equiv of (tmeda)CrCl2 1 to form (tmeda)SiCH(CrCl2)2 complex 2 along with 2 equiv of (tmeda)CrCl2I. The (tmeda)CrCl2I is reduced back to catalytically active species 1 by manganese powder. Based on the Grubbs’s report in Scheme 7,13 the resulting 2 is converted into chromocarbene species 8 in the presence of TMEDA and olefins, which then forms chromocyclobutane intermediate 9 via [2+2]cycloaddition with olefins. Here, coordination to another chromium center promotes the approach of olefins to chromocarbene moiety. Only the terminal double bond of olefins can participate in the catalytic cycle due to the steric hindrance around the chromium center; the trimethylsilyl group prefers to locate opposite of the substituent on alkenes in intermediate 8, which can explain the chemo- and stereoselectivity observed in Table 3, i.e., trans-silylcyclopropanes were formed as major products, and the terminal double bond of myrcene reacted selectively. Finally, reductive elimination from chromocyclobutane 9 afforded silylcyclopropane 3 along with regeneration of active catalytic species 1. Reactivity Control of gem-Di(dichlorochromio)methane Complex upon Treatment with ZnCl2. A previous our study revealed that alkylidenation of aldehydes occurred with functionalized diiodomethane derivatives and CrCl2.6 However, (tmeda)SiCH(CrCl2)2 complex 2 was inert toward silylalkylidenation, and the corresponding alkenylsilane was not obtained from reaction with 3-phenylpropanal in THF. Fortunately, it was found that treatment of 2 with ZnCl2 prior to addition of the aldehyde altered the reaction course to furnish the corresponding alkenylsilane 4a in 74% yield (E/Z = > 99/1) (Table 4, entry 6). At first, ZnCl2 was thought to activate 3-phenylpropanal as a Lewis acid to promote the nucleophilic attack of a gem-di(dichlorochromio)methane reagent. However, a similar acceleration effect of ZnCl2 was not observed in the silylalkylidenation using CrCl2 without TMEDA in THF (Table S1 in SI). Other metal salts, such as ZnBr2 and ZnI2, also promoted the reaction (entries 7 and 8), whereas TiCl4, MnCl2, FeCl3, CoCl2, and CuI were not effective, and 4a was formed in significantly lower yield, probably due to competitive decomposition of the aldehyde via aldol condensation leading to 5 (entries 1−5). Although

characterization of the reactive species upon addition of ZnCl2 was attempted, obtaining information directly was difficult due to the instability of the resulting chromium complexes. After an extensive trial, the corresponding gem-dichromiomethane complex 6 stabilized with N,N,N′,N′-tetramethyl-1,3-propanediamine ligand was obtained from reaction of 2 with subsequent treatment of ZnCl2 followed by N,N,N′,N′tetramethyl-1,3-propanediamine (Scheme 8). The red single Scheme 8. Removal and Exchange of TMEDA Ligand of gem-Di(dichlorochromio)methane Complex 2

crystal of 6 was obtained by slow diffusion of its THF solution, and its structure was determined unambiguously by singlecrystal X-ray crystallography (Figure 6, left, see also Figure S4

Figure 6. X-ray crystal structure of the gem-dichromiomethane complex 6 stabilized with N,N,N′,N′-tetramethyl-1,3-propanediamine (left) and (tmeda)ZnCl2 complex (right). Color code: green, Cr; pink, Si; blue, N; yellow, Cl; red, Zn. All hydrogen atoms are omitted for clarity. 13189


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Journal of the American Chemical Society in SI). These results clearly suggest that the TMEDA ligand of 2 was removed by ZnCl2, which was further supported by precipitation of (tmeda)ZnCl2 as colorless crystals (Figure 6, right, and Figure S5 in SI). The efficiency of ligand exchange from TMEDA to N,N,N′,N′-tetramethyl-1,3-propanediamine was estimated by comparing the yields of cyclopropanation. The corresponding cyclopropane 3a was obtained in 56% yield with a trans/cis = 80/20 after ligand exchange to N,N,N′,N′tetramethyl-1,3-propanediamine. The yield and stereoselectivity were similar to those obtained with gem-dichromiomethane complex 6 (51% yield with trans/cis = 77/23), which indicated that the removal of TMEDA occurred almost quantitatively (see Scheme S1 and S2 in SI for comparison studies). The (tmeda)SiCH(CrCl2)2 complex 2 could also be utilized as a catalyst for silylalkylidenation of aldehydes by pretreatment with ZnCl2. 3-Phenylpropanal was converted to 4a in a similar yield compared to that obtained from the stoichiometric silylalkylidenation shown in Table 4, entry 6 (Table 5, entry 1).

Table 6. Effect of Ligand on Silylalkylidenation of Aldehydes

Table 5. Catalytic Silylalkylidenation of Aldehydes with 2a

entry

ligand

recoverya/%

yield of 3aa/%

trans/cisa

1 2b 3 4 5 6 7 8

TMEDA Et3N DMAP 2,2′-bipyridyl L1 L2 L3 dppe

0 4 0 8 0 0 0 0

74 5 54 31 58 56 47 7

>99/1 >99/1 >99/1 >99/1 >99/1 >99/1 >99/1 >99/1

a Determined by 1H NMR. bUsed 8 equiv of ligand. See Table 3 for the structures of L1−L3.

of 3-phenylpropanal with the pretreatment of ZnCl2, none of them gave 4a in superior yield (entries 2−8). This difference in yields may be a result of the efficiency of ligand dissociation from the chromium center. TMEDA gave the best result due to the high stability of the resulting (tmeda)ZnCl2 complex, which prevented rebound of TMEDA to ligand-free gem-dichromiomethane species. These results imply that the proper choice of ligands is key for the control of reactivity to attain both silylcyclopropanation and silylalkylidenation.

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CONCLUSIONS Covering the reactive methylene center with a bulky trimethylsilyl group and TMEDA ligand led to the isolation and direct Xray crystallographic characterization of reactive gem-dichromiomethane complexes. The isolated gem-dichromiomethane complex can serve as a silylmethylidene carbene equivalent for cyclopropanation of olefins, capable of being stored in the solid state without degradation for at least half a year. Furthermore, this complex itself preferred the nonpolar carbon−carbon double bond of alkenes to provide cyclopropane derivatives, while pretreatment with ZnCl2 changed the reactive nature of the complex, resulting in alkylidenation with polar aldehydes. Similar reactivity control upon addition of a Lewis acid (ZnCl2) or a Lewis base (TMEDA) is rare for gemdimetalloalkanes. 13 The catalytic performance of gemdichromiomethane complexes in both alkylidenation and cyclopropanation was also disclosed. Although the scope and limitations of the reactions over a range of aldehydes and olefins have not been demonstrated, we believe the current preliminary results sufficiently prove the usefulness of the present new findings. A kinetic study of the silylcyclopropanation clarified the role of TMEDA and provided insight into the unique reaction mechanism with a chlorine-bridged dinuclear chromium complex. The steric effect of a chromocarbene intermediate led to selective cyclopropanation of the terminal double bond of myrcene. This terminal selectivity is unique because di- and trisubstituted alkenes preferentially react in a traditional Simmons−Smith cyclopropanation.23,26 Overall, the current work provides invaluable information for the design of efficient and storable organochromium reagents.1 Application

a

(tmeda)SiCH(CrCI2)2 2 was treated with ZnCI2, before the addition of other reagents (see SI for detailed procedure). bDetermined by 1H NMR of crude mixture.

Reaction with sterically congested 2-phenylpropanal provided 4b in 74% yield with excellent stereoselectivity (entry 2). The bromo group in 4c, which can be further used in cross-coupling reactions, was tolerated under the current conditions (entry 3). Ethoxycarbonyl and ethynyl groups remained intact to provide 1,6-enyne 4d, which was applicable as a precursor for further cycloaddition reactions (entry 4).24 These results demonstrated the potential utility of the present chromium-catalyzed transformation in various organic syntheses.25 Impact of TMEDA and ZnCl2 on Control of Reactivity of gem-Dichromiomethane Complexes. To confirm the importance of the combination of TMEDA and ZnCl2 on reactivity control of gem-dichromiomethane species, the behavior of other gem-(dichromio)silylmethane species having different ligands was investigated upon addition of ZnCl2 (Table 6). The most drastic reactivity change upon addition of ZnCl2 was observed for the gem-dichromiomethane complex having the TMEDA ligand (entry 1). Although other gemdichromiomethane complexes coordinated by nitrogen- and phosphine-based ligands also promoted the silylalkylidenation 13190


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Ishihara (Okayama University) for HRMS measurements. The authors gratefully thank Division of Instrumental Analysis, Okayama University, for the X-ray single crystal structural analyses.

of the strategy for the preparation of novel early transitionmetal-based gem-dimetalloalkanes is ongoing in our laboratory.

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EXPERIMENTAL SECTION

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Synthesis of (tmeda)SiCH(CrCl2)2 Complex 2. A flame-dried two-necked round-bottom flask was charged with CrCl2 (197 mg, 1.6 mmol) and THF (4.0 mL), and TMEDA (196 mg, 1.6 mmol) was added dropwise at 25 °C. After stirring for 20 min, (diiodomethyl)trimethylsilane (136 mg, 0.40 mmol) was added, and the resulting mixture was stirred at 25 °C for 1 h. The reaction mixture was passed through a cotton plug under an argon atmosphere, and the filtrate was concentrated under reduced pressure. The resulting solid was extracted with CH2Cl2/hexane (v/v = 3/2) several times, and the solvent was removed under reduced pressure to afford (tmeda)SiCH(CrCl2)2 complex 2 (128 mg, 0.23 mmol, 57% yield) as a red solid. 1H NMR and 13C NMR were not applicable due to the paramagnetic nature of chromium compounds. However, the structure was unambiguously determined by the X-ray single crystallographic analysis of a red crystal of 2, obtained by recrystallization from THF. IR (KBr/cm−1): 3364, 2976, 2889, 2843, 2641, 1473, 1441, 1283, 1240, 1121, 1065, 1016, 955, 853, 799, 770. Anal. Calcd for C16H42Cl4Cr2N4 Si: C, 34.05; H, 7.50; N, 9.93. Found: C, 33.71; H, 7.21; N, 9.54. General Procedure for Chromium-Catalyzed Silylcyclopropanation of Alkenes (Table 3). To a suspension of (tmeda)SiCH(CrCl2)2 complex 2 (11.3 mg, 0.020 mmol for 3a and 3b, or 22.6 mg, 0.040 mmol for 3c and 3d) in THF (2.0 mL for 3a, 3c, and 3d, or 1.0 mL for 3b) were added (diiodomethyl)trimethylsilane (68.0 mg, 0.20 mmol), an olefin (0.20 mmol), and manganese powder (33.0 mg, 0.60 mmol), followed by stirring at 50 °C for 8 h. (Diiodomethyl)trimethylsilane (68.0 mg, 0.20 mmol) and manganese powder (33.0 mg, 0.60 mmol) were added and stirred at 50 °C for an additional 16 h. The reaction mixture was poured into water (10 mL), followed by extraction with Et2O (10 mL) three times. The combined organic extracts were dried over anhydrous MgSO4 and filtered, and the solvent was removed under reduced pressure. The residue was purified by flash column chromatography on silica gel to afford the corresponding cyclopropane derivatives 3.

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(1) Representative reviews, see: (a) Marek, I.; Normant, J.-F. Chem. Rev. 1996, 96, 3241. (b) Matsubara, S.; Oshima, K.; Utimoto, K. J. Organomet. Chem. 2001, 617-618, 39. (c) Marek, I.; Normant, J. F. In Organozinc ReagentsA Practical Approach; Knochel, P., Jones, P., Eds.; Oxford University Press: Oxford, 1999; pp 119−137. (d) Normant, J. F. Acc. Chem. Res. 2001, 34, 640. (e) Endo, K. Bull. Chem. Soc. Jpn. 2017, 90, 649. (2) (a) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. J. Am. Chem. Soc. 1978, 100, 3611. For reviews on the methylenation of carbonyl compounds, see: (b) Matsubara, S.; Oshima, K. In Modern Carbonyl Olefination; Takeda, T., Ed.; Wiley-VCH: Weinheim, 2004; p 200. (3) X-ray crystallographic studies on gem-dimetallomethanes, see: (a) Hogenbirk, M.; Schat, G.; Akkerman, O. S.; Bickelhaupt, F.; Smeets, W. J. J.; Spek, A. L. J. Am. Chem. Soc. 1992, 114, 7302. (b) Vestergren, M.; Eriksson, J.; Hakansson, M. J. Organomet. Chem. 2003, 681, 215. (c) Layfield, R. A.; Bullock, T. H.; Garcia, F.; Humphrey, S. M.; Schuler, P. Chem. Commun. 2006, 2039. (d) Uhl, W.; Layh, M. J. Organomet. Chem. 1991, 415, 181. (e) Stasch, A.; Ferbinteanu, M.; Prust, J.; Zheng, W.; Cimpoesu, F.; Roesky, H. W.; Magull, J.; Schmidt, H.-G.; Noltemeyer, M. J. Am. Chem. Soc. 2002, 124, 5441. (f) Stasch, A.; Roesky, H. W.; Vidovic, D.; Magull, J.; Schmidt, H.-G.; Noltemeyer, M. Inorg. Chem. 2004, 43, 3625. (4) Nishida, Y.; Hosokawa, N.; Murai, M.; Takai, K. J. Am. Chem. Soc. 2015, 137, 114. (5) For reviews on chromium-promoted reactions, see: (a) Hodgson, D. M. J. Organomet. Chem. 1994, 476, 1. (b) Fürstner, A. Chem. Rev. 1999, 99, 991. (c) Wessjohann, L. A.; Scheid, G. Synthesis 1999, 1999, 1. (d) Takai, K. Org. React 2004, 64, 253. (e) Smith, K. M. Coord. Chem. Rev. 2006, 250, 1023. (f) Yamamoto, H.; Xia, G. Chem. Lett. 2007, 36, 1082. For oligo- and polymerization, see: (g) Wass, D. F. Dalton Trans. 2007, 8, 816. (h) Smith, K. M. Curr. Org. Chem. 2006, 10, 955. (i) Takai, K. In Comprehensive Organic Synthesis, 2nd ed.; Molander, G. A., Knochel, P., Eds.; Elsevier: Oxford, 2014; Vol. 1, pp 159−203. For recent reports, see: (j) Steib, A. K.; Kuzmina, O. M.; Fernandez, S.; Flubacher, D.; Knochel, P. J. Am. Chem. Soc. 2013, 135, 15346. (k) Zeng, X.; Cong, X. Org. Chem. Front. 2015, 2, 69 and references therein. (6) For pioneering works on the generation and utilization of gemdichromiomethane species for alkylidenation of aldehydes, see: (a) Takai, K.; Nitta, K.; Utimoto, K. J. Am. Chem. Soc. 1986, 108, 7408. (b) Okazoe, T.; Takai, K.; Utimoto, K. J. Am. Chem. Soc. 1987, 109, 951. (7) For chromium-mediated silyl-, boryl-, stannyl-, and iodoalkylidenation, see: (a) Takai, K.; Kataoka, Y.; Okazoe, T.; Utimoto, K. Tetrahedron Lett. 1987, 28, 1443. (b) Hodgson, D. M. Tetrahedron Lett. 1992, 33, 5603. (c) Hodgson, D. M.; Foley, A. M.; Lovell, P. J. Tetrahedron Lett. 1998, 39, 6419. (d) Takai, K.; Ichiguchi, T.; Hikasa, S. Synlett 1999, 1999, 1268. (e) Takai, K.; Hikasa, S.; Ichiguchi, T.; Sumino, N. Synlett 1999, 1999, 1769. For catalytic version of reaction, see: (f) Takai, K.; Kunisada, Y.; Tachibana, Y.; Yamaji, N.; Nakatani, E. Bull. Chem. Soc. Jpn. 2004, 77, 1581. For alkylidenation of aldehydes leading to acrylate derivatives, see: (g) Falck, J. R.; Bejot, R.; Barma, D. K.; Bandyopadhyay, A.; Joseph, S.; Mioskowski, C. J. Org. Chem. 2006, 71, 8178. (h) Baati, R.; Mioskowski, C.; Kashinath, D.; Kodepelly, S.; Lu, B.; Falck, J. R. Tetrahedron Lett. 2009, 50, 402. (8) For selected examples, see: (a) Evans, D. A.; Black, W. C. J. Am. Chem. Soc. 1993, 115, 4497. (b) Wender, P. A.; Schrier, A. J. J. Am. Chem. Soc. 2011, 133, 9228. (c) Dieckmann, M.; Kretschmer, M.; Li, P.; Rudolph, S.; Herkommer, D.; Menche, D. Angew. Chem., Int. Ed. 2012, 51, 5667. (d) Willwacher, J.; Fürstner, A. Angew. Chem., Int. Ed. 2014, 53, 4217. (e) Tsuzaki, S.; Usui, S.; Oishi, H.; Yasushima, D.; Fukuyasu, T.; Oishi, T.; Sato, T.; Chida, N. Org. Lett. 2015, 17, 1704.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b07487. Experimental procedures and characterization data (PDF) X-ray crystallographic data for 1 (CIF) X-ray crystallographic data for (tmeda)CrCl3 (CIF) X-ray crystallographic data for 2 (CIF) X-ray crystallographic data for 6 (CIF) X-ray crystallographic data for (tmeda)ZnCl2 (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Masahito Murai: 0000-0002-9694-123X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by a Grant-in-Aid (No. 26248030) from MEXT, Japan. The authors gratefully thank Dr. Hiromi Ota and Dr. Sobi Asako (Okayama University) for the valuable discussion, and Mr. Masato Kodera and Ms. Seina 13191


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Journal of the American Chemical Society (9) (a) Takai, K.; Toshikawa, S.; Inoue, A.; Kokumai, R. J. Am. Chem. Soc. 2003, 125, 12990. (b) Takai, K.; Hirano, M.; Toshikawa, S. Synlett 2004, 1347. (c) Takai, K.; Toshikawa, S.; Inoue, A.; Kokumai, R.; Hirano, M. J. Organomet. Chem. 2007, 692, 520. Utilizing our pioneering study, Concellón et al. reported chromium-mediated cyclopropanation of α,β-unsaturated amides. Chromium carbenoids (Cl2Cr−CR2−X) are proposed as intermediates in their reports. See: (d) Concellón, J. M.; Rodríguez-Solla, H.; Méjica, C.; Blanco, E. G. Org. Lett. 2007, 9, 2981. (e) Concellón, J. M.; Rodríguez-Solla, H.; Méjica, C.; Blanco, E. G.; García-Granda, S.; Díaz, M. R. Org. Lett. 2008, 10, 349. (10) For reviews on the synthesis of cyclopropanes, see: (a) Lebel, H.; Marcoux, J.-F.; Molinaro, C.; Charette, A. B. Chem. Rev. 2003, 103, 977. (b) Pellissier, H. Tetrahedron 2008, 64, 7041. (11) For representative pioneering works, see: (a) Simmons, H. E.; Smith, R. D. J. Am. Chem. Soc. 1958, 80, 5323. (b) Simmons, H. E.; Smith, R. D. J. Am. Chem. Soc. 1959, 81, 4256. (c) Furukawa, J.; Kawabata, N.; Nishimura, J. Tetrahedron Lett. 1966, 7, 3353. (12) Silylcyclopropanes are useful synthetic building blocks. For the synthesis and transformation of silylcyclopropanes, see: (a) Paquette, L. A. Chem. Rev. 1986, 86, 733. (b) Mata, S.; López, L. A.; Vicente, R. Synlett 2015, 26, 2685 and the references therein. (13) (a) Straus, D. A.; Grubbs, R. H. Organometallics 1982, 1, 1658. (b) Meinhart, J. D.; Anslyn, E. V.; Grubbs, R. H. Organometallics 1989, 8, 583. For a review, see: (c) Beckhaus, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 686. (14) Bachmann, B.; Heck, J.; Hahn, F.; Massa, W.; Pebler, J. Z. Anorg. Allg. Chem. 1995, 621, 2061. (15) For the synthesis of (diiodomethyl)trialkylsilanes SiCHI2, see: (a) Matsubara, S.; Otake, Y.; Morikawa, T.; Utimoto, K. Synlett 1998, 1998, 1315. (b) Bull, J. A.; Charette, A. B. J. Org. Chem. 2008, 73, 8097. (c) Lim, D. S. W.; Anderson, E. A. Org. Lett. 2011, 13, 4806. See also ref 7f. (16) Isolation and structural characterization of gem-dichromium complexes have been reported. All of them possess η5-Cp, Cp*, or pyrrolide anion ligands, and were not used as gem-dimetallomethane reagents. See: (a) Köhler, F. H.; Krüger, C.; Zeh, H. J. Organomet. Chem. 1990, 386, C13. (b) Noh, S. K.; Heintz, R. A.; Janiak, C.; Sendlinger, S. C.; Theopold, K. H. Angew. Chem., Int. Ed. Engl. 1990, 29, 775. (c) Heintz, R. A.; Leelasubcharoen, S.; Liable-Sands, L. M.; Rheingold, A. L.; Theopold, K. H. Organometallics 1998, 17, 5477. (d) Wei, P.; Stephan, D. W. Organometallics 2003, 22, 1712. (e) Vidyaratne, I.; Nikiforov, G. B.; Gorelsky, S. I.; Gambarotta, S.; Duchateau, R.; Korobkov, I. Angew. Chem., Int. Ed. 2009, 48, 6552. (f) Licciulli, S.; Albahily, K.; Fomitcheva, V.; Korobkov, I.; Gambarotta, S.; Duchateau, R. Angew. Chem., Int. Ed. 2011, 50, 2346. (17) (a) Schlenk, W.; Schlenk, W. Ber. Dtsch. Chem. Ges. B 1929, 62, 920. (b) Hirai, A.; Nakamura, M.; Nakamura, E. J. Am. Chem. Soc. 1999, 121, 8665. (c) Goldsmith, P. J.; Teat, S. J.; Woodward, S. Angew. Chem., Int. Ed. 2005, 44, 2235. (d) Blake, A. J.; Shannon, J.; Stephens, J. C.; Woodward, S. Chem. - Eur. J. 2007, 13, 2462. (18) Yield of 4a in Scheme 3 decreased to 32% (trans/cis = 66/34), with 42% recovery of allyl benzyl ether. (19) Under the conditions shown in Table 2, regioselectivity of silylcyclopropanation was improved to trans/cis = 83/17 when (diiodomethyl)triisopropylsilane and TMEDA were used as precursor and ligand, respectively, although yield decreased to 27%. (20) (a) Hu, C.-M.; Chen, J. J. Chem. Soc., Chem. Commun. 1993, 72. (b) Fürstner, A.; Shi, N. J. Am. Chem. Soc. 1996, 118, 2533. (c) Fürstner, A.; Shi, N. J. Am. Chem. Soc. 1996, 118, 12349. (21) Addition of MnCl2, which could accumulate during the catalytic process, did not promote the reaction. (22) The solubility and stability of (tmeda)CrIICl2 1 were much lower than those of (tmeda)SiCH(CrIIICl2)2 2. Furthermore, recrystallization by slow evaporation of an acetonitrile solution was required to obtain 1 in pure form because of its low solubility in other typical organic solvents, including MeOH, THF, EtOAc, Et2O, acetone, CH2Cl2, and toluene. However, even a small amount of acetonitrile strongly inhibited the silylcyclopropanation of alkenes.

Thus, 2 was used as a robust and storable catalyst and promoter in the current work. (23) Due to electronic and steric effects, the reactivity of alkenes in the Simmons−Smith reaction is known to diminish in the order gemdisubstituted > trisubstituted > tetrasubstituted > cis-disubstituted > trans-disubstituted > monosubstituted alkenes. (24) (a) Trost, B. M.; Pfrengle, W.; Urabe, H.; Dumas, J. J. Am. Chem. Soc. 1992, 114, 1923. (b) Trost, B. M.; Hashmi, A. S. K. J. Am. Chem. Soc. 1994, 116, 2183. (c) Asao, N.; Shimada, T.; Yamamoto, Y. J. Am. Chem. Soc. 1999, 121, 3797. (25) The corresponding alkenylsilane was obtained from the catalytic alkylidenation of 4-dimethylaminobenzaldehyde in 43% yield (trans/cis = >99/1) under the reaction conditions shown in Table 5. However, the similar catalytic alkylidenation of 4-nitro- and 4-cyanobenzaldehydes did not proceed at all, which indicates that strongly coordinating functional groups retard the current alkylidenation. (26) For the utility of cyclopropane derivatives, see: (a) de Meijere, A.; Kozhushkov, S. I. Chem. Rev. 2000, 100, 93. (b) Salaün, J. Top. Curr. Chem. 2000, 207, 1. (c) Donaldson, W. A. Tetrahedron 2001, 57, 8589. (d) Faust, R. Angew. Chem., Int. Ed. 2001, 40, 2251. (e) Gnad, F.; Reiser, O. Chem. Rev. 2003, 103, 1603. (f) Reissig, H.-U.; Zimmer, R. Chem. Rev. 2003, 103, 1151. (g) Yu, M.; Pagenkopf, B. L. Tetrahedron 2005, 61, 321. (h) Tang, P.; Qin, Y. Synthesis 2012, 44, 2969. (i) Chen, D. Y.-K.; Pouwer, R. H.; Richard, J.-A. Chem. Soc. Rev. 2012, 41, 4631.

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Structural Characterization and Unique Catalytic Performance of Silyl

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