Catalytic Formation of Five-Membered Zirconacycloallenoids and

Article pubs.acs.org/Organometallics

Catalytic Formation of Five-Membered Zirconacycloallenoids and Their Reaction with Carbon Dioxide Santhosh Kumar Podiyanachari, Georg Bender, Constantin G. Daniliuc, Gerald Kehr, and Gerhard Erker* Organisch-Chemisches Institut, Westfälische Wilhelms-Universität, Corrensstrasse 40, D-48149 Münster, Germany S Supporting Information *

ABSTRACT: Hydrozirconation of phenylacetylene followed by a reaction with ((trimethylsilyl)ethynyl)lithium gave the respective (σ-alkenyl)(σ-alkynyl)zirconocene complex 6a. Its treatment with a catalytic amount of B(C6F5)3 (2 mol %) at room temperature resulted in efficient σ-ligand coupling with the formation of the isomeric five-membered zirconacycloallenoid complex 9a. Two similar examples with different substituents R1 (SiMe3, tBu) were also investigated. All three new zirconacycloallenoids (9a−c) were characterized by X-ray diffraction. The addition reaction of carbon dioxide to the zirconacycloallenoid complex 9c and two related examples from the literature were studied. The carbon dioxide addition products were also characterized by X-ray crystal structure analysis.

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INTRODUCTION The s-cis conjugated diene group 4 metallocenes exhibit a pronounced σ-complex character.1,2 Their typical structural features can be described by a contribution of the metallacyclopentene 1 and (π-butadiene)metallocene 1′ resonance hybrids (see Scheme 1). Similarly, the conceptually related

contribution of both their (π-enyne)metallocene and metallacycloallene resonance structures 3 and 3′ (Scheme 1).7 These first five-membered group 4 metallacycloallenoids carry quite unique functional groups, which was a necessary requirement originating from their specific synthetic routes. Later both the Suzuki group8 and our group9 developed a much more general synthetic entry to zircona- and hafnacycloallenoids that bore just simple alkyl, aryl, or silyl substituents. Their synthesis used the Negishi−Takahashi procedure for zirconocene generation (see Scheme 3).10 A variety of subsequent reactions of the five-

Scheme 1

Scheme 3

“Rosenthal metallacyclocumulenes” (2, 2′)3 and “Suzuki metallacycloalkynes” (4, 4′)4 can similarly be described by a participation of the metallacyclic σ structures and their respective π complexes. Both the Suzuki group5 and our group6 had described the first examples of the five-membered group 4 metallacycloallenoids 3a,b (Scheme 2). These previously predicted compounds can best be described by a

membered group 4 metallacycloallenoids was described,9 including their unique reaction with dihydrogen11 or their coordination chemistry with the elusive n-butylmagnesium hydride or with MgH2.12 The synthetic scheme depicted in Scheme 3 has the slight disadvantage that it requires the use of a specifically substituted conjugated enyne as a reagent. Therefore, we have looked for an alternative synthesis of the zirconacycloallenoids 3 that involved the preparation of the respective enyne ligand in the course of the metallacyclic synthesis. This was achieved by catalytic B(C6F5)3-induced13 coupling of a pair of σ-alkenyl and σ-alkynyl ligands at the zirconocene template. Three respective examples will be described in this article.

Scheme 2

Received: April 15, 2014 Published: June 26, 2014 © 2014 American Chemical Society

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dx.doi.org/10.1021/om500399w | Organometallics 2014, 33, 3481−3488

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RESULTS AND DISCUSSION Catalytic Preparation of the Zirconacycloallenoids. We prepared a small series of alkenylzirconocene chlorides by the usual hydrozirconation route and then introduced the σalkynyl ligand at zirconium by salt metathesis with the respective alkynyl lithium reagent. The synthesis of compound 6a is a typical example. The reaction of phenylacetylene with the hydrozirconation reagent [Cp2Zr(H)Cl]n (Schwartz reagent)14 gave compound 5a (see Scheme 4). Its reaction

Compound 6a was then treated in toluene solution with a catalytic amount of the strong Lewis acid B(C6F5)3 (2 mol %). The isomerization reaction went to completion within 12 h at room temperature. Workup gave the zirconacycloallenoid product 9a as an orange solid in 65% yield (see Scheme 5). Scheme 5

Scheme 4

with ((trimethylsilyl)ethynyl)lithium furnished complex 6a. It was isolated as an amorphous solid in 63% yield. In solution complex 6a features the typical 1H NMR signals of the transstyryl σ-ligand (δ 7.78, 6.83, 3 J HH = 19.3 Hz, with corresponding olefinic 13C NMR signals at δ 183.2 and 138.6). The alkynyl ligand shows a pair of 13C NMR resonances at δ 162.0 and 128.7, respectively, and a 1H NMR signal of its SiMe3 substituent at δ 0.27 (29Si NMR: δ −24.2). Compound 6a shows a sharp 1H NMR Cp signal at δ 5.92 (13C NMR: δ 111.1). Single crystals suitable for the X-ray crystal structure analysis of complex 6a were obtained by crystallization from a saturated solution in n-pentane at low temperature (ca. −35 °C). The compound shows a typical bent-metallocene framework. The trans-styryl σ-ligand and the σ-alkynyl ligand are both attached at the front side of the bent-metallocene wedge (angle C1−Zr− C3 101.5(1)°). The trans-CHCH−Ph moiety is found rotated almost perpendicular to the metallocene σ-ligand plane (dihedral angle C1−Zr1−C3−C4 −102.8(3)°; see Figure 1).15

The product was characterized by NMR spectroscopy (see Table 1) and by X-ray diffraction (see Figure 2 and Table 2). In Table 1. Selected NMR Spectroscopic Data of the Zirconacycloallenoid Compounds 9a−ca R1 R2

9a

9b

Ph SiMe3

SiMe3 SiMe3

t

129.3 135.2 87.9 54.6

132.1 129.6 84.6 85.9

3.96 0.11 5.34 5.01

4.04 1.14 5.34 5.00

13

C1 C2 C3 C4

135.0 132.2 83.0 66.6

C3−H C4−H CpA CpB

4.69 2.62 5.33 4.86

9c Bu SiMe3

C NMR

1

H NMR

a

Chemical shifts relative to TMS (1H, 13C); in C6D6 at 299 K.

its crystal structure complex 9a shows the newly formed disubstituted conjugated enyne ligand which was formed by carbon−carbon coupling from the alkenyl and alkynyl σ-ligands at zirconium. The trans stereochemistry of the olefinic moiety was retained in the course of the coupling reaction. All four carbon atoms of the newly formed enyne functionality are found to be bonded to zirconium. The bonds of the metal to the acetylene unit are somewhat shorter than those between the metal and the olefinic moiety. The C1−C2 bond is short, reminding us of its alkynyl origin; the adjacent C2−C3 bond is in the olefinic range, and the C3−C4 bond is markedly longer toward the single-bond range (see Table 2). The compound is a typical example of a five-membered zirconacycloallenoid.8,9 It is probably best described by means of a hybrid between the (π-

Figure 1. Molecular structure of compound 6a. Thermal ellipsoids are shown at the 30% probability level. 3482


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Scheme 6

9 by a similar pathway. We assume that the reaction sequence is initiated by alkynyl abstration by the boron Lewis acid from the σ-complex 6 to generate the reaction intermediate 7. Alkyne insertion into the remaining Zr−C(sp2) σ-bond at the (σalkenyl)zirconocene cation moiety would then give the zwitterionic C−C coupling product 8 (see Scheme 5). In the final step the B(C6F5)3 Lewis acid catalyst would be cleaved from the newly formed framework to yield complex 9. The B(C6F5)3-catalyzed alkenyl/alkynyl coupling reaction is not limited to the starting material 6a (R1 = Ph, R2 = SiMe3) but can be carried out with the other σ-alkenyl/σ-alkynyl zirconocene complexes prepared in this study (see Scheme 4) as well. Thus, the C−C coupling reaction of the doubly SiMe3 substituted starting material 6b was achieved by treatment with 2 mol % of the B(C6F5)3 catalyst in toluene solution under typical reaction conditions (room temperature, 12 h) to give the corresponding zirconacycloallenoid isomer 9b as a yellow solid in 81% yield. Complex 9b shows the typical NMR data of this class of compounds (see Table 1). The compound was also characterized by X-ray diffraction (see Table 2 and Figure 3). Complex 6c also undergoes the C−C coupling reaction catalyzed by B(C6F5)3 (1 mol %) to give the five-membered zirconacycloallenoid compound 9c (orange solid, isolated in 92% yield). Complex 9c was also characterized by X-ray diffraction (for details see Table 2; the structure is depicted in

Figure 2. Molecular structure of the zirconacycloallenoid 9a. Thermal ellipsoids are shown at the 30% probability level.

Table 2. Selected Structural Data of the Zirconacycloallenoids 9a−c R1 R2 Zr1−C1 Zr1−C2 Zr1−C3 Zr1−C4 C1−C2 C2−C3 C3−C4 C2−C1−Si1 C1−C2−C3 C2−C3−C4 Zr1−C1−Si1 Zr1−C4−X C1−C2−C3−C4 Si1−C1···C3−C4 a

9a

9bc

Ph SiMe3 2.351(3) 2.324(3) 2.443(3) 2.447(3) 1.282(5) 1.380(5) 1.424(5) 138.4(3) 152.9(3) 121.8(3) 148.2(2) 133.9(2)a 60.6(10) −128.2

SiMe3 SiMe3 2.362(3) 2.327(4) 2.365(2) 2.411(3) 1.254(5) 1.373(5) 1.455(5) 144.1(3) 152.8(4) 124.7(4) 141.6(2) 136.9(2)b 55.7(11) −120.3

9cc t

Bu SiMe3 2.350(4) 2.323(3) 2.427(4) 2.423(4) 1.278(5) 1.374(5) 1.417(5) 140.1(3) 152.6(4) 123.5(4) 145.1(2) 140.3(3)a 56.5(10) −117.1

X = C21. bX = Si2. cData for molecule A.

enyne)metallocene and metallacyclic allene structures (see Scheme 5). In solution compound 9a exhibits the 1H/13C NMR signals of a pair of diastereotopic Cp groups at zirconium. The C4−H 13 C/1H NMR resonances are reminiscent of a typical metallacyclic σ-structure. All of the remaining 13C NMR framework signals (C3, C2, C1) are in the olefinic range, although the C(2) 13C NMR resonance is markedly shifted to lower values in comparison to those for typical allene compounds.16 Again, a description of the zirconacycloallenoid by the resonance structures 9a and 9a′ seems justified (see Scheme 5). We had previously found that bis(alkynyl)zirconocene complexes 10 can be very efficiently coupled by treatment with catalytic amounts of B(C6F5)3 to give the “Rosenthal zirconacyclocumulenes” 2.17 We were even able to isolate the alleged zwitterionic intermediate 12 from the respective stoichiometric reaction (see Scheme 6).18 Therefore, it was tempting to describe the formation of the zirconacycloallenoids

Figure 3. Molecular structure of complex 9b. Thermal ellipsoids are shown at the 30% probability level; only one molecule from two found in the asymmetric unit is shown. 3483


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the Supporting Information). For its NMR spectroscopic characterization see Table 1, the Experimental Section, and the Supporting Information. Reactions of Five-Membered Zirconacycloallenoids with Carbon Dioxide. The five-membered zirconacycloallenoid compounds react readily with carbon dioxide. However, the outcome of the reaction depends to some extent on the substitution pattern. The reaction of the zirconacycloallenoid 9c (R1 = tBu, R2 = SiMe3) provides a typical example. Compound 9c reacts readily with CO2 (2 bar) in toluene solution at room temperature (1 h reaction time). Workup gave the dimeric reaction product 15 in 62% yield (see Scheme 7). Scheme 7 Figure 4. View of the heterodimeric product 15, produced by CO2 addition to the zirconacycloallenoid 9c. Thermal ellipsoids are shown at the 30% probability level; hydrogens are omitted for clarity. Selected bond lengths (Å) and angles (deg): Zr1−O3 2.321(3), Zr1−C1 2.381(4), Zr1−O1 2.201(3), Zr2−O1 2.304(3), Zr2−C12 2.338(4), Zr2−O3 2.205(3), C1−C2 1.361(5), C2−C3 1.485(6), C2−C4 1.478(6), C4−C5 1.300(6), C3−O2 1.216(5), C11−C12 1.353(5), C13−C11 1.478(6), C12−C14 1.470(6), C13−O4 1.213(5), C14− C15 1.320(6); C1−Zr1−O1 71.5(1), C1−Zr1−O3 135.2(1), Zr1− C1−Si1 129.6(2), C1−C2−C3 117.8(4), C2−C4−C5 132.4(4), C12−Zr2−O3 71.2(1), Zr2−C12−C14 122.2(3), C12−C11−C13 115.3(4), Zr2−C12−C11 115.6(3).

for ca. 2 months: 16:1:7). This means that the carboxylation of 9c that is proceeding by means of the reactive (η2-alkyne)metallocene isomer 13c is nonregioselective. It must be assumed that it produces the two regioisomeric CO2 addition products 14a,b almost in equimolar amounts, but then their association reaction is remarkably selective. We find predominating adduct formation between the different five-membered regioisomers of 14 to give the “heterodimer” 15 as the major product (see Scheme 7). We assume that this remarkable set of subsequent steps of unselective CO2 addition followed by rather selective association is mostly sterically controlled. The major product of this reaction, the “heterodimer” 15, shows the 1 H/13C NMR signals of two pairs of Cp ligands (each of relative intensity 10) and of two different tBu groups. There are the NMR resonances of two different CHCH−R alkenyl groups; both show typical trans 3JHH coupling constants, and there are two markedly different 13C NMR Zr−C (δ 235.9, 224.5) and CO resonances (δ 180.0, 175.4). Compound 15 shows a slightly split IR carbonyl band at 1653 cm−1 (for further details see the Supporting Information). We also encountered a situation where a homodimeric CO2 addition product was predominantly formed. We had formed the five-membered zirconacycloallenoid 9d (R1 = Ph, R2 = tBu) by trapping in situ generated zirconocene (by the Negishi− Takahashi route, see above) with 1-phenyl-4-tert-butylbutenyne, as we had previously described.9 The reaction of 9d with CO2 gave practically a single product (shown by in situ NMR monitoring), which was identified as the homodimer of the fivemembered CO2 addition product 17 (see Scheme 8 and Figure 5). This means that it was again the reactive (η2-alkyne)zirconocene isomer (here 16) of the zirconacycloallenoid 9d that had preferentially reacted with the added carbon dioxide. However, this reaction proceeded with high regioselectivity, giving almost exclusively the CO2 coupling product from the apparently less hindered styryl-substituted side. The resulting

The compound was characterized by X-ray diffraction (see Figure 4). It revealed that the carboxylation reaction had actually taken place at the stage of the η2-alkyne isomer 13c of the metallacycloallenoid. This reactive intermediate can in principle form two different five-membered metallacyclic CO2 addition products, which are formed by addition occurring from the side of the SiMe3 substituent or from the CH CH−tBu substituent side. We actually found both substructures in the crystalline isolated product. The crystal contained the “heterodimer”. It features a nearly coplanar tricyclic central framework that was constructed by combining the two regioisomeric lactone-like five-membered rings by means of a pair of connecting oxygen−zirconium linkagesa structural motif that has often been encountered in five-membered group 4 alkoxy containing ring systems.19−21 Using the available lateral acceptor orbital in the central σ-ligand plane at the front side of the bent metallocene wedge22−24 creates the typical rhomboid-shaped central four-membered Zr2O2 unit that connects the pair of five-membered zirconacycles (see Figure 4). NMR spectroscopy in solution (CD2Cl2) of the freshly prepared compound 15 showed that the heterodimeric complex 15 is the major reaction product, but it is accompanied by a pair of homodimeric complexes in a ratio of ca. 8:2:1 (after storage 3484


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Eventually, we reacted the 1,3-disubstituted metallacycloallenoid compound 9e (see Scheme 9) with carbon dioxide. The

Scheme 8

Scheme 9

starting material was prepared as previously described by us via zirconocene trapping by 3-methyl-1-tert-butylbutenyne.9 The reaction of 9e with carbon dioxide is complicated; it produced a sizable amount of an insoluble white product, the composition of which so far has not been elucidated. However, we were able to isolate a crystalline fraction from the n-pentane extract (8% yield). This was identified as the seven-membered metallacyclic product 19, produced by CO2 insertion into the Zr−CH2 moiety of the five-membered zirconacycloallenoid starting material 9e. The X-ray crystal structure analysis of compound 19 (see Figure 6) shows that one oxygen atom of the inserted CO2

Figure 5. Molecular structure of the homodimeric CO2 addition product 18. Thermal ellipsoids are shown at the 30% probability level; hydrogens are omitted for clarity. Selected bond lengths (Å) and angles (deg): Zr1−O1 2.184(2), Zr1−O1* 2.352(2), Zr1−C1 2.433(3), C1−C2 1.358(4), C2−C3 1.475(4), C2−C4 1.490(4), C4−C5 1.306(5), C3−O2 1.212(4), C3−O1 1.347(4); O1−Zr1− O1* 62.8(1), C1−Zr1−O1 72.1(1), C1−C2−C3−O1 0.6(5), C2− C4−C5−C21 175.3(3).

Figure 6. A projection of the molecular structure of the CO2 insertion product 19 (thermal ellipsoids are shown with 10% probability). Selected bond lengths (Å) and angles (deg). Zr1−O1 2.157(4), Zr1− C1 2.313(5), Zr1−C2 2.451(5), C1−C6 1.516(8), C1−C2 1.278(8), C2−C3 1.385(11), C3−C4 1.467(12), C4−C5 1.490(13), C5−O1 1.398(15), C5−O2 1.162(15), O1−Zr1−C1 119.5(2), O1−Zr1−C2 88.6(2), Zr1−O1−C5 127.7(6), C6−C1−C2 128.7(5), C1−C2−C3 157.7(7), ΣC5OOC 359.4, C6−C1···C3−C10 −71.0, C6−C1···C3−C4 121.9.

five-membered lactone-like product 17 then has only the option to form the homodimeric complex 18 (see Scheme 8). We have isolated the homodimer 18 and characterized it by X-ray diffraction (see Figure 5). The centrosymmetric dimer 18 contains a pair of symmetry-equivalent five-membered metallacyclic substituents. They are connected in the usual way by a pair of oxygen−zirconocene linkages that form two sides of the central diamond-shaped Zr2O2 unit. In solution the pair of symmetry-equivalent Cp2Zr units in the homodimer 18 exhibit a single sharp 1H NMR resonance at δ 6.35 (corresponding 13C NMR signal at δ 113.4). The carbonyl carbon 13C NMR resonance occurs at δ 173.1 and the Zr−C carbon atom at δ 230.3. Compound 18 shows a strong IR (CO) band at 1677 cm−1.

molecule is bonded to zirconium. There is a conformational disorder around the carbonyl group which involves the carbon atoms C2−C5, C10, and the oxygen atom O2. The zirconium atom features a short bond to the allenoid carbon atom C1 and a ca. 0.14 Å longer bond to the adjacent central allenoid carbon center C2. The C1−C2 bond is short, as is typically observed for many zirconacycloallenoids,8,9 and the adjacent C2−C3 bond is longer by ca. 0.11 Å, which brings it into a long CC double-bond range. The C3−C4 bond is in the typical C(sp2)− C(sp3) σ-bond range, as is the adjacent C4−C5 bond. The 3485


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((trimethylsilyl)ethynyl)lithium (460 mg, 4.50 mmol, 1.1 equiv) at −78 °C, and then the reaction mixture was stirred for 6 h at the same temperature. Subsequently the reaction mixture was slowly warmed to room temperature and stirred for another 6 h. After this time, the lithium chloride was filtered through Celite under an argon atmosphere. The light brown filtrate was concentrated in vacuo, and the obtained material was washed with cold n-pentane (6 mL). Then the residue was dried in vacuo and compound 6a was isolated as a light brown amorphous solid (1.1 g, 2.60 mmol, 63%). Crystals suitable for an X-ray crystal structure analysis were obtained by crystallization of a saturated n-pentane solution of compound 6a at −35 °C. Anal. Calcd for C23H26SiZr: C, 65.50; H, 6.21; Found: C, 65.10; H, 6.29. 1H NMR (500 MHz, C6D6, 299 K): δ 7.78 (d, 3JHH = 19.3 Hz, 1H, CHZr), 7.36 (m, 2H, o-Ph), 7.20 (m, 2H, m-Ph), 7.07 (m, 1H, p-Ph), 6.83 (d, 3 JHH = 19.3 Hz, 1H, CHPh), 5.92 (s, 10H, Cp), 0.27 (s, 2JSiH = 7.0 Hz, 9H, SiMe3). 13C{1H} NMR (126 MHz, C6D6, 299 K): δ 183.2 ( CHZr), 162.0 (CZr), 139.4 (i-Ph), 138.6 (CHPh), 128.8 (m-Ph), 128.7 (CSi), 127.0 (p-Ph), 126.5 (o-Ph), 111.1 (Cp), 0.8 (1JSiC = 55.3 Hz, SiMe3). 29Si{1H} NMR (99 MHz, C6D6, 299 K): δ −24.2 (ν1/2 ≈ 1 Hz). Preparation of Complex 6b. Compound 6b was prepared by the same procedure as described for the preparation of compound 6a. (η5C5H5)2Zr(CHCHSiMe3)Cl (1.1 g, 3.11 mmol) reacted with ((trimethylsilyl)ethynyl)lithium (356 mg, 3.42 mmol, 1.1 equiv) to give compound 6b as a light yellow solid (1.1 g, 2.63 mmol, 84%). Anal. Calcd for C20H30Si2Zr: C, 57.49; H, 7.24; Found: C, 56.77; H, 7.28. 1H NMR (500 MHz, C6D6, 299 K): δ 9.38 (d, 3JHH = 22.4 Hz, 1H, CHZr), 7.14 (d, 3JHH = 22.4 Hz, 1H, CHSi), 5.63 (s, 10H, Cp), 0.34 (s, 2JSiH = 6.9 Hz, 9H, CSiMe3), 0.16 (s, 2JSiH = 6.6 Hz, 9H, CH SiMe3). 13C{1H} NMR (126 MHz, C6D6, 299 K): δ 214.5 ( Zr CH ), 154.5 (CZr), 128.7 (CSi), 108.7 (Cp), 106.1 (CHSi), 1.3 (1JSiC = 55.0 Hz, CSiMe3), −0.7 (1JSiC = 52.0 Hz, =CHSiMe3). 29 Si{DEPT} NMR (99 MHz, C6D6, 299 K): δ −5.8 (ν1/2 ≈ 25 Hz, =C SiMe3), −25.9 (ν1/2 ≈ 30 Hz, CSiMe3). Preparation of Complex 9a. A toluene (1 mL) solution of tris(pentafluorophenyl)borane (4.4 mg, 8.6 × 10−6 mol, 2 mol %) was added to a light brown solution of compound 6a (185 mg, 0.43 mmol) in the same solvent (5 mL). The reaction mixture was stirred at room temperature for 12 h. During this time the light brown reaction solution turned orange-red. A half volume of toluene was removed from the reaction solution in vacuo, and n-pentane (5 mL) was added to the reaction mixture. Then the insoluble material was filtered off through a short pad of Celite and the solvent was removed from the filtrate in vacuo. Compound 9a was isolated as an orange solid (120 mg, 0.28 mmol, 65%). Crystals suitable for an X-ray crystal structure analysis were obtained by crystallization of a saturated n-pentane solution of compound 9a at room temperature. Anal. Calcd for C23H26SiZr: C, 65.50; H, 6.21. Found: C, 63.92; H, 6.29. 1H NMR (600 MHz, C6D6, 299 K): δ 7.26 (m, 2H, m-Ph), 7.17 (m, 2H, o-Ph), 6.97 (m, 1H, p-Ph), 5.33 (s, 5H, CpA), 4.86 (s, 5H, CpB), 4.69 (d, 3JHH = 13.4 Hz, 1H, =CH), 2.62 (d, 3JHH = 13.4 Hz, 1H, ZrCH), 0.36 (s, 2 JSiH = 6.7 Hz, 9H, SiMe3). 13C{1H} NMR (151 MHz, C6D6, 299 K): δ 147.1 (i-Ph), 135.0 (1JSiC = 69.9 Hz, =CSi), 132.2 (C), 128.8 (mPh), 123.5 (o-Ph), 122.7 (p-Ph), 104.9 (CpA), 102.7 (CpB), 83.0 ( CH), 66.6 (ZrCH), 1.5 (1JSiC = 54.0 Hz, SiMe3). 29Si{DEPT} NMR (119 MHz, C6D6, 299 K): δ −8.2 (ν1/2 ≈ 7 Hz). Preparation of Complex 9b. Compound 9b was prepared by the same procedure as described for the preparation of compound 9a. Compound 6b (100 mg, 0.23 mmol) reacted with a catalytic amount of tris(pentafluorophenyl)borane (2.3 mg, 4.6 × 10−6 mol, 2 mol %) to give compound 9b as a light yellow solid (81 mg, 0.19 mmol, 81%). Crystals suitable for an X-ray crystal structure analysis were obtained by crystallization of a saturated n-pentane solution of compound 9b at −35 °C. Anal. Calcd for C20H30Si2Zr: C, 57.49; H, 7.24; Found: C, 56.79; H, 7.34. 1H NMR (600 MHz, C6D6, 299 K): δ 5.34 (s, 5H, CpA), 5.01 (s, 5H, CpB), 3.96 (d, 3JHH = 15.5 Hz, 1H, =CH), 0.36 (s, 2 JSiH = 6.7 Hz, 9H, CSiMe3), 0.30 (s, 2JSiH = 6.4 Hz, 9H, HCSiMe3), 0.11 (d, 3JHH = 15.5 Hz, 1H, ZrCH). 13C{1H} NMR (151 MHz, C6D6, 299 K): δ 135.2 (C), 129.3 (1JSiC = 70.3 Hz, =CSi), 104.3 (CpA),

C1−C2−C3 angle at the endocyclic allenoid unit in 19 is larger than that observed for the five-membered zirconacycloallenoids, and the allenic dihedral angle C6−C1···C3−C10 amounts to −71.0° (C6−C1···C3−C4 121.9°) in compound 19. In solution compound 19 shows one set of 13C NMR signals of the metallacyclic framework ranging from δ 151.3 (C1) through δ 146.5 and 108.9 (C2,3) to δ 46.8 (of the C4 CH2 group) all the way to the carbonyl carbon atom C5 at δ 183.0. Compound 19 features a 1H NMR signal at δ 1.41 for the 3CH3 substituent and the 1-tBu resonance at δ 1.09. Due to the chirality of the metallacycloallenoid framework we have observed a 1H NMR AB pattern of the endocyclic CH2 group adjacent to the carbonyl group (δ 2.98, 2.21) with a geminal coupling constant of 2JHH = 14.0 Hz. Likewise, we have monitored the 1H NMR resonance of a pair of diasteriotopic Cp ligands at zirconium at δ 5.66 and 5.56, respectively (corresponding 13C NMR signals at δ 110.8 and 110.3). Complex 19 shows a strong carbonyl band in the IR spectrum at ν̅ 1661 cm−1.

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CONCLUSIONS Thiele et al. had shown that bis(alkenyl)zirconocenes undergo a rapid σ-ligand coupling to give the conjugated diene zirconocenes 1.25 Similarly it was previously shown that “Rosenthal zirconacyclocumulenes” 2 can be formed by photochemically induced acetylide coupling of bis(alkynyl)zirconocene complexes 10.17a,20 Later we had shown that the isomerization of 10 to 2 can be very efficiently catalyzed by the boron Lewis acid B(C6F5)3, and the mechanistic pathway of that reaction was elucidated (see Scheme 6).17,18 We have now found that the analogous pathway can obviously be followed and utilized for the B(C6F5)3-catalyzed C−C coupling of the σalkenyl and σ-alkynyl pair of ligands at the zirconocene complexes 6 to yield the corresponding zirconacycloallenoids 9. For the small series of examples investigated in this initial study this catalytic pathway proceeded very efficiently and gave fivemembered metallacyclic products in good yields. We regard this new method as a useful addition to the existing small variety of synthetic methods leading to this unique class of compounds. It may become especially valuable because of its simple and straightforward preparation and assembly of the pair of σligands at the zirconocene framework. The zirconacycloallenoids are reactive organometallic compounds. Their preferred reaction mode with carbon dioxide seems to strongly depend on their specific substitution pattern. In our hands only compound 9e, which features the small CH2 group σ-coordinated to zirconium, is able to just simply insert CO2 into the Zr−C bond (although this might potentially represent only a minor reaction pathway in that specific system). The 1,4-substituted zirconacycloallenoid examples investigated in this study reacted with CO2 via their (η2alkyne)zirconocene isomers. It seems that in these cases the genuine σ-alkyl metallocene reactivity is less favored because of the presence of a bulky α substituent. This delicate balance of reactivity pattern seems to be a typical feature of the class of the zirconacycloallenoid systems that have now become more and more readily available by rather simple and easy to perform synthetic routes.

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EXPERIMENTAL SECTION

Preparation of Complex 6a. Toluene (20 mL) was added to a mixture of (η5-C5H5)2Zr(CHCHPh)Cl (1.4 g, 4.09 mmol) and 3486


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100.8 (CpB), 87.9 (CH), 54.6 (ZrCH), 1.6 (CSiMe3), 1.4 (HCSiMe3). 29Si{DEPT} NMR (119 MHz, C6D6, 299 K): δ −1.1 (ν1/2 ∼≈25 Hz, HCSiMe3), −8.2 (ν1/2 ≈ 25 Hz, =CSiMe3). Preparation of Complex 9c. Compound 9c was prepared by the same procedure as described for the preparation of compound 9a. Compound 6c (501 mg, 1.24 mmol) reacted with catalytic amount of tris(pentafluorophenyl)borane (6.3 mg, 1.24 × 10−5 mol, 1 mol %) to give compound 9c as an orange solid (463 mg, 1.15 mmol, 92%). Crystals suitable for the X-ray crystal structure analysis were obtained by crystallization of a saturated n-pentane solution of compound 9c at −35 °C. Anal. Calcd for C21H30SiZr: C, 62.78; H, 7.53; Found: C, 62.03; H, 7.48. 1H NMR (500 MHz, C6D6, 299 K): δ 5.34 (s, 5H, CpA), 5.00 (s, 5H, CpB), 4.04 (d, 3JHH = 14.6 Hz, 1H, =CH), 1.28 (s, 9H, tBu), 1.14 (d, 3JHH = 14.6 Hz, 1H, ZrCH), 0.37 (s, 2JSiH = 6.7 Hz, 9H, SiMe3). 13C{1H} NMR (126 MHz, C6D6, 299 K): δ 132.1 (1JSiC = 71.5 Hz, =CSi), 129.6 (C), 103.7 (CpA), 100.2 (CpB), 85.9 (ZrCH), 84.6 (CH), 35.1 (tBu), 33.2 (tBu), 1.5 (1JSiC = 53.7 Hz, SiMe3). 29Si{DEPT} NMR (99 MHz, C6D6, 299 K): δ −8.2 (ν1/2 ≈ 3 Hz). Preparation of Complex 15. A toluene (5 mL) solution of complex 9c (144 mg, 0.35 mmol) was evacuated and exposed to carbon dioxide (2 bar) for 1 h at room temperature. During this time, the orange reaction solution turned light yellow. Then all volatiles were removed in vacuo, and the obtained residue was washed with npentane (4 mL). Finally a pale yellow solid (99 mg, 0.22 mmol, 62%) was isolated. Crystals suitable for an X-ray crystal structure analysis were obtained by crystallization of a saturated n-pentane solution of compound 15 at −35 °C. Anal. Calcd for C22H30O2SiZr: C, 59.27; H, 6.78. Found: C, 60.84; H, 6.73. The NMR experiments of the CD2Cl2 solution of the obtained yellow solid showed three isomers (ratio 8:2:1 [1H]). The major isomer was assigned to the heterodimeric complex 15, the minor isomers were assignted to the respective homodimeric complexes. Data for the major isomer are as follows. 1H NMR (600 MHz, CD2Cl2, 299 K): δ 6.47 (d, 3JHH = 15.7 Hz, 1H, 3′-H), 6.24 (d, 3JHH = 16.2 Hz, 1H, 4-H), 6.14 (d, 3JHH = 16.2 Hz, 1H, 3-H), 6.07, 6.00 (each s, each 10H, Cp), 5.29 (d, 3JHH = 15.7 Hz, 1H, 4′-H), 1.20 (s, 9H, t Bu′), 1.15 (s, 9H, tBu), 0.41 (s, 9H, SiMe3), 0.26 (s, 9H, SiMe3′). 13 C{1H} NMR (151 MHz, CD2Cl2, 299 K): δ 235.9 (C-1′), 224.5 (C1), 180.0 (CO′), 175.4 (CO), 141.8 (C-4), 139.8 (C-2), 134.9 (C-4′), 134.63 (C-2′), 134.55 (C-3′), 125.4 (C-3), 112.7, 112.3 (Cp), 33.84 (tBu), 33.79 (tBu′), 30.2 (tBu′), 29.9 (tBu), 3.7 (SiMe3), 2.2 (SiMe3′). 29Si{DEPT} NMR (119 MHz, CD2Cl2, 299 K): δ −8.5, −9.3. Data for the minor isomer (18%) are as follows. 1H NMR (600 MHz, CD2Cl2, 299 K): δ 6.48 (d, 3JHH = 15.6 Hz, 1H, H-3′), 5.29 (d, 3 JHH = 15.6 Hz, 1H, H-4′), 5.99 (s, 10H, Cp), 1.19 (s, 9H, tBu), 0.25 (s, 9H, SiMe3). 13C{1H} NMR (151 MHz, CD2Cl2, 299 K): δ 236.0 (C-1′), 180.3 (CO′), 139.4 (C-2′), 135.2 (C-4′), 134.8 (C-3′), 112.7 (Cp), 33.80 (tBu′), 30.2 (tBu′), 2.3 (SiMe3). 29Si{DEPT} NMR (119 MHz, CD2Cl2, 299 K): δ −9.2. Data for the minor isomer (9%) are as follows. 1H NMR (600 MHz, CD2Cl2, 299 K): δ 6.25 (d, 3JHH = 16.0 Hz, 1H, H-4), 6.14 (d, 3JHH = 16.0 Hz, 1H, H-3), 6.09 (s, 5H, Cp), 1.15 (s, 9H, tBu), 0.41 (s, 9H, SiMe3). 13C{1H} NMR (151 MHz, CD2Cl2, 299 K): δ 223.5 (C-1), 174.8 (CO), 140.2 (C-2), 142.0 (C4), 125.4 (C-3), 112.3 (Cp), 33.83 (tBu), 29.9 (tBu), 3.7 (SiMe3). 29 Si{DEPT} NMR (119 MHz, CD2Cl2, 299 K): δ −8.5. Preparation of Complex 18. n-Butylmagnesium chloride (0.34 mL, 2 M diethyl ether solution, 0.68 mmol, 2 equiv) was added to a solution of (η5-C5H5)2ZrCl2 (100 mg, 0.34 mmol) and (5,5dimethylhex-1-en-3-yn-1-yl)benzene9 (83 mg, 0.45 mmol, 1.3 equiv) in Et2O (8 mL) at −78 °C. After the removal of the dry ice bath, the reaction mixture was warmed to room temperature and stirred for 1 h. Then the red solution was heated at 60 °C for an additional 1 h. After this time all volatiles were removed in vacuo and the remaining residue was suspended in n-pentane (3 × 10 mL). Insoluble materials were filtered off through a short pad of Celite, and the orange filtrate was concentrated in vacuo to a volume of ca. 10 mL. Subsequently the evacuated flask was filled with carbon dioxide (2 bar) and the reaction solution was stirred overnight at room temperature. The resulting

orange suspension was filtered through a frit, and the precipitate was dried in vacuo. Compound 18 was isolated as a light pink solid (50 mg, 0.11 mmol, 33%). Crystals suitable for the X-ray crystal structure analysis were obtained by layering a solution of compound 18 in dichloromethane with n-pentane at −30 °C. Anal. Calcd for C25H26O2Zr·CH2Cl2: C, 58.41; H, 5.28. Found: C, 57.86; H, 5.22. 1 H NMR (500 MHz, 299 K, CD2Cl2): δ 7.48 (m, 2H, o-Ph), 7.36 (m, 2H, m-Ph), 7.23 (m, 1H, p-Ph), 7.07 (d, 3JHH = 16.4 Hz, 1H, 3-H), 6.84 (d, 3JHH = 16.4 Hz, 1H, 4-H), 6.35 (s, 10H, Cp), 1.41 (s, 9H, t Bu). 13C{1H} NMR (126 MHz, 299 K, CD2Cl2): δ 230.3 (C-1), 173.1 (CO), 139.3 (i-Ph), 136.3 (C-2), 130.5 (C-4), 129.3 (C-3), 129.0 (m-Ph), 127.3 (p-Ph), 126.4 (o-Ph), 113.4 (Cp), 42.1 (tBu), 33.5 (tBu). Preparation of Complex 19. n-Butylmagnesium chloride (0.34 mL, 2 M diethyl ether solution, 0.68 mmol, 2 equiv) was added to a solution of (η5-C5H5)2ZrCl2 (100 mg, 0.34 mmol) and 2,5,5-trimethyl1-hexen-3-yne (42 mg, 0.34 mmol) in THF (5 mL) at −78 °C. After the removal of the dry ice bath, the mixture was warmed to room temperature and then stirred for 1 h. Subsequently the yellow solution was heated at 60 °C for an additional 1 h. After this time all volatiles were removed in vacuo and the remaining residue was suspended in npentane (3 × 5 mL). Insoluble materials were filtered off through a short pad of Celite, and the yellow-orange filtrate was concentrated in vacuo to a volume of 10 mL. Then the evacuated flask was filled with carbon dioxide (2 bar) and the reaction mixture was stirred overnight at room temperature. At this point the resulting suspension was filtered over Celite, and the filtrate was cooled to −30 °C to crystallize compound 19 as yellow crystals (10 mg, 8%), which were suitable for an X-ray crystal structure analysis. Anal. Calcd for C20H24O2Zr: C, 61.97; H, 6.24. Found: C, 61.04; H, 6.36. 1H NMR (600 MHz, 299 K, C6D6): δ 5.66 (s, 5H, CpA), 5.56 (s, 5H, CpB), 2.98, 2.21 (each d, 2JHH = 14.0 Hz, 1H, 4-H), 1.41 (s, 3H, Me), 1.09 (s, 9H, tBu). 13C{1H} NMR (151 MHz, 299 K, C6D6): δ 183.0 (CO), 151.3 (C-1), 146.5 (C-2)t, 110.8 (CpA), 110.3 (CpB), 108.9 (C-3)t, 46.8 (C-4), 37.0 (tBu), 32.8 (tBu), 21.7 (Me).

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ASSOCIATED CONTENT

* Supporting Information S

Experimental details and physical characterization of the new compounds, crystallographic data, and CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for G.E.: [email protected]. Author Contributions

C.G.D. performed the X-ray crystal structure analysis. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Deutsche Forschungsgemeinschaft is gratefully acknowledged. REFERENCES

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Catalytic Formation of Five-Membered Zirconacycloallenoids and

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