Expanding the reactivity of Rosenthal's reagent to cyclopropenes and allenes

Abstract

The reactions of the Rosenthal reagent Cp₂Ti(η²-BTMSA) with cyclopropenes and allenes were systematically investigated. With cyclopropenes, selective 1,2-insertion into the Ti–C bond induces a ring expansion from three- to five-membered frameworks, affording titanacyclopentenes. These bicyclic complexes undergo rearrangement under mild conditions to generate highly reactive titanium carbene species, which subsequently participate in [2 + 2] cycloaddition reactions with alkynes (BTMSA and 1,2-diphenylacetylene), yielding titanacyclobutenes. In contrast, reactions with allenes proceed via ligand exchange of the alkyne ligand, giving titanacyclopentane derivatives. In addition to these transformations, the titanium complex was found to catalyze the isomerization of cyclotrideca-1,2-diene to cyclotrideca-1,3-diene. All isolated complexes were comprehensively characterized by NMR spectroscopy, and the solid-state structures of most species were identified by single-crystal X-ray diffraction.

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Introduction

Olefin complexes of early transition metals have been of considerable interest in organometallic chemistry for many years, largely due to their key roles in many transformations, including polymerization¹ as well as metathesis reactions.² Seminal work by Bercaw and co-workers established olefin complexes as crucial intermediates in such processes (Fig. 1, I), while the introduction of sterically demanding Cp* ligands (Cp* = pentamethylcyclopentadienyl) subsequently enabled the isolation of discrete and well-defined examples.³

Fig. 1 Simplified structures of titanium olefin complexes (I–III) and titanacyclopentane complexes (IV–VI).

Beyond simple olefins, allenes and cyclopropenes were also shown to coordinate in an η² fashion (Fig. 1, II and III). Binger and co-workers reported several pioneering examples of η²-cyclopropene and η²-allene titanium complexes.^4–6 The direct attempts to generate the corresponding titanacyclopentanes (IV) from ethylene were often unsuccessful, primarily due to rapid decomposition accompanied by the release of ethylene.⁷ Binger's group achieved the isolation of substituted titanacyclopentanes (Fig. 1, V and VI) from the reaction of Cp₂Ti(PMe₃)₂ with allenes and cyclopropenes, thereby establishing important precedents for controlled C–C bond formation at low-valent titanium centers.^4–6 Driven by the need for reliable low-valent group 4 metal sources, metallocene-based Ti^II and Zr^II complexes subsequently emerged as powerful tools in organometallic synthesis. Prominent among these are bis(trimethylsilyl)acetylene (BTMSA) adducts, commonly referred to as “Rosenthal complexes” (e.g. Cp₂Zr(L)(η²-BTMSA) or Cp₂Ti(η²-BTMSA); L = pyridine or THF). These compounds act as masked Ti^II or Zr^II sources, releasing highly reactive low-valent fragments under mild conditions through simple ligand dissociation (Scheme 1, top).^8,9

Scheme 1 Rosenthal's reagents (top) as Cp₂Ti^II and Cp₂Zr^II synthons in ligand exchange and 1,2-insertion reactions (bottom).

The resulting Cp₂M^II fragments represent versatile platforms for the straightforward construction of structurally diverse titanium and zirconium complexes and have been employed in various C–C bond-forming reactions, as well as catalytic transformations.^8,10 Notably, Cp₂Ti^II species may also undergo oxidation reactions to form Cp₂Ti^III complexes, either through comproportionation,¹¹ via reaction with metal salts such as AgOTf (with concomitant formation of Ag⁰), or through protonation accompanied by H₂ evolution.¹² The two fundamental elementary reactions of Rosenthal complexes are (i) ligand exchange involving the BTMSA ligand and (ii) 1,2-insertions of unsaturated substrates [S] into the three-membered titanium ring system (Scheme 1, bottom). The sequence of reactions may vary depending on the substrate and the transition metal, with 1,2-insertion sometimes occurring prior to ligand substitution.⁸ Cumulated π-systems have been investigated extensively, for example by the groups of Burlakov,¹³ Rosenthal,^14,15 Tonks,¹⁶ and Beweries.¹⁷ In particular, the synthesis of stable 1-metallacyclobuta-2,3-dienes has demonstrated how Cp₂M^II fragments can engage in cyclization reactions with cumulated systems.¹⁸ Likewise, carbodiimides have been studied in detail both experimentally and theoretically, providing pathways to Arduengo-type carbene complexes including a new type of heterocumulene coordination mode.^14–16

Despite this breadth of reactivity, systematic studies on the interaction of cyclopropenes and allenes with Ti^II centers remain scarce. Allenes, with their unique electronic distribution and potential for regio- and stereodivergent activation, and cyclopropenes, which combine significant ring strain with partial cumulated character, are particularly underexplored in the coordination sphere of Ti^II complexes. Notably, Binger and co-workers reported first investigations into the reactivity of both allenes and cyclopropenes with the related complex Cp₂Ti(PMe₃)₂.^4,6 Insights from related transformations, such as cumulene activation and metallacyclocumulene formation, suggest intriguing mechanistic pathways, including metallacyclopentadiene intermediates, ring-opening processes, and novel insertion modes. Herein, we report the reactivity of Cp₂Ti(η²-BTMSA) 1 with cyclopropenes and allenes, providing new insights into the interaction of Ti^II centers with strained and cumulated π-systems and enabling selective C–C bond formation through controlled substrate activation.

Results and discussion

Reaction of 1 with cyclopropenes

The cyclopropenes were prepared in accordance with published procedures,¹⁹ and after purification by chromatography or distillation, were degassed and stored under inert atmosphere at −30 °C. Reactions of Cp₂Ti(η²-BTMSA) 1 with cyclopropenes a–d in n-hexane at ambient temperature led to an immediate colour change of the solution from yellow to deep purple, accompanied by the precipitation of purple solids. After one hour of stirring, the supernatant was removed, and the residue was washed with small portions of n-hexane and dried under high vacuum. The resulting complexes 2a–d were obtained as purple solids in good isolated yields (67–87%, Scheme 2). They exhibit limited solubility in n-hexane but dissolve readily in aromatic solvents (toluene, benzene) or THF, and decompose in the solid state upon heating above 117 °C.

Scheme 2 Formation of bicyclic titanacyclopentene complexes 2a–d from the reaction of Cp₂Ti(η²-BTMSA) 1 with cyclopropenes a–d via 1,2-insertion.

Product formation in these reactions was found to be independent of the stoichiometry. Accordingly, substoichiometric amounts of cyclopropenes led to incomplete conversion with unreacted complex 1 remaining, whereas the use of an excess did not affect the outcome. However, Binger and co-workers reported on additional insertion reactions under related conditions, resulting in the formation of titanacyclopentanes.⁴ The selective formation of single products in the present case was confirmed by NMR spectroscopy. For example, the ¹H NMR spectrum of 2a exhibits two singlets for the SiMe₃ groups at δ = 0.06 and 0.10 ppm, and two singlets for the Cp ring protons at δ = 5.83 and 5.91 ppm, the latter consistent with restricted rotation at the titanium center, indicating an asymmetric coordination environment in solution. A singlet at δ = 1.65 ppm is assigned to the methyl substituent of the cyclopropane moiety. In addition, the methine protons of the cyclopropane ring appear as doublets at δ = 1.45 and 1.73 ppm (J = 8.4 Hz). The structures of 2a–d were further corroborated by single-crystal X-ray diffraction. Single crystals suitable for X-ray analysis were obtained by slow evaporation of concentrated n-hexane solutions at ambient temperature. The molecular structure of 2b is depicted in Fig. 2, while those of 2a, 2c, and 2d are provided in the SI. The solid-state data reveal the presence of two distinct molecular orientations within the unit cell, in which complexes 2a–d adopt either a supine or a prone conformation.²⁰ These two orientations correspond to inversely oriented arrangements of the ligand framework, indicating that the complexes crystallize in two well-defined but opposing conformers. Notably, in both conformations the methyl group and the hydrogen atoms consistently adopt a trans configuration (Fig. S99–S102). We attribute this high selectivity primarily to steric effects in the transition state of cyclopropene incorporation. The Cp ligands and the methyl groups of the trimethylsilyl substituents are creating a sterically congested environment on both faces of the metallacycle.

Fig. 2 Molecular structure of the supine isomer of complex 2b. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms (except for H13 and H14) and the prone isomer are omitted for clarity.

To minimize steric repulsion, the smaller methyl substituent of the cyclopropene is preferentially oriented toward this hindered region, whereas the bulkier groups are directed away from it, resulting in the exclusive formation of a single observable isomer. The molecular structure of 2b reveals a distorted tetrahedral coordination geometry at the titanium center and shows cyclopropene insertion through formation of a new C12–C13 bond (1.5117(9) Å). The resulting titanacyclopentene deviates from planarity due to the sp³ hybridization of the former cyclopropene carbons C13 and C14. The Ti1–C11 (2.1920(7) Å) and Ti1–C14 (2.1495(7) Å) bond lengths are slightly elongated compared to the sum of the covalent radii (∑r_cov(Ti–C): 2.11 Å),²¹ and are consistent with reported Ti–C bonds (2.22 Å).²² Within the cyclopropane fragment, the C–C bond lengths (C13–C14: 1.5192(9) Å, C14–C15: 1.5349(9) Å, C13–C15: 1.5416(9) Å) are modestly elongated relative to typical C(sp³)–C(sp³) single bonds in unstrained cyclopropanes (1.51 Å).²³ Table 1 summarizes the bond lengths and angles of complexes 2a–d, which exhibit comparable structural features. With extended reaction times, a subsequent transformation of 2a–d was observed. At ambient temperature this process proceeds slowly and can be monitored by NMR spectroscopy; heating NMR samples (C₆D₆) to 60 °C for 24 h accelerated the conversion, accompanied by a distinct colour change from purple to orange (Scheme 3). The ¹H NMR spectra of complexes 2a–d display new characteristic resonances that are consistent with the structural reorganization. Taking complex 3a as a representative example, the former cyclopropane methine doublets at δ = 1.45 and 1.73 ppm are shifted downfield to δ = 5.91 and 6.07 ppm with a slightly increased coupling constant (J = 9.4 Hz), indicative of significant electronic changes within the ring system. Correspondingly, the ¹³C NMR spectrum shows resonances for the methine carbons at δ = 123.8 and 135.6 ppm, which are consistent with a change in hybridization from sp³ to sp². The quaternary carbons of the BTMSA moiety also shift from δ = 159.8 and 231.5 ppm to δ = 107.0 and 250.9 ppm. Additionally, the ²⁹Si NMR signals shift from δ = −20.3 and −14.4 ppm to δ = −13.8 and 1.1 ppm, as a result of the ring contraction. Additional changes in the ¹H NMR spectra include the methyl resonance (from δ = 1.29 to 1.75 ppm) and slight upfield shifts of the Cp resonances (from δ = 5.94, 5.97 to 5.27, 5.32 ppm). Related titanacyclopentene complexes have been shown to undergo facile ligand dissociation, thereby promoting C–C bond activation in the absence of steric shielding at the α-position.^5,24 We attribute the observed rearrangement to a ring-opening process that generates highly reactive titanium carbene intermediates in situ. While analogous processes are well established for ruthenium carbenes in ring-opening metathesis polymerization (ROMP),²⁵ direct observation of titanium carbene species remains challenging.⁴ Previously, we reported titanavinylidene complexes that readily undergo formal [2 + 2] cycloadditions with alkynes.²⁶ Similarly, we propose that the transient titanium carbene complexes react with the liberated BTMSA ligand to afford titanacyclobutene complexes 3a–d. In line with this hypothesis, heating 2c and 2d in the presence of 1,2-diphenylacetylene (60 °C, 16 h) resulted in a colour change of the solution from purple to green, and the corresponding cycloadducts 4c and 4d were isolated in 56–61% yield (Scheme 3).

Scheme 3 Ring-opening of 2a–d to titanium carbene intermediates and subsequent [2 + 2] cycloadditions with alkynes yielding titanacyclobutenes 3a–d and 4c,d.

Table 1 Selected bond lengths (Å) for complexes 2a–d

Complex	Ti1–C11	Ti1–C14	C11–C12	C12–C13	C13–C14	C13–C15	C14–C15	C11–Ti1–C14
2a	2.2037(9)	2.1392(9)	1.3662(13)	1.5099(13)	1.5267(13)	1.5442(12)	1.5204(12)	83.72(3)
2b	2.1920(7)	2.1495(7)	1.3640(9)	1.5117(9)	1.5192(9)	1.5416(9)	1.5349(9)	86.49(3)
2c	2.1911(5)	2.1423(5)	1.3645(7)	1.5120(6)	1.5186(7)	1.5469(7)	1.5291(7)	87.048(18)
2d	2.1903(6)	2.1445(6)	1.3667(8)	1.5134(8)	1.5159(8)	1.5469(8)	1.5299(8)	86.58(2)

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The complexes exhibit good solubility in n-hexane, toluene, and benzene and decompose in the solid-state upon heating above 122 °C. NMR spectroscopy readily reveals the selective substitution of BTMSA by 1,2-diphenylacetylene. Although both alkynes are present in the reaction mixture, 1,2-diphenylacetylene is clearly preferred, as indicated by the ¹H NMR spectrum (see Fig. S39). New aromatic resonances corresponding to complex 4d appear at δ = 6.84–7.24 ppm. The quaternary carbons of the incorporated 1,2-diphenylacetylene are observed at δ = 103.2 and 213.1 ppm in the ¹³C{¹H} NMR spectrum. The methine proton doublets at δ¹H = 5.37 and 6.74 ppm (J = 9.7 Hz) are comparable to those of complex 3d (δ¹H = 6.06 and 6.61 ppm, J = 10.1 Hz), supporting structural similarity of the central metallacycle.

The formation of complexes 3 and 4 was further corroborated by single-crystal X-ray diffraction. Single crystals of 4c and 4d suitable for X-ray analysis were obtained from concentrated benzene-d₆ and n-hexane solutions by slow evaporation at ambient temperature. The solid-state structures unambiguously confirm the formation of titanacyclobutenes and the substitution of BTMSA by 1,2-diphenylacetylene, consistent with NMR spectroscopic data. The molecular structure of 4d is shown in Fig. 3, while the structure of 4c is provided in the SI. The solid-state data also reveal the presence of both supine and prone conformations within the unit cell (see Fig. S103 and S104).²⁰

Fig. 3 Molecular structure of the prone isomer of complex 4d. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms (except for H25 and H26) and the supine isomer are omitted for clarity.

The molecular structures of 4c and 4d reveal a distorted tetrahedral coordination environment at the titanium center, defined by two η⁵-cyclopentadienyl ligands and one bidentate ligand. The exocyclic C–C bonds are slightly shortened compared to typical C–C single bonds (4c: 1.4736(11) Å; 4d: 1.4729(18) Å).^21,23 The newly formed C–C bonds within the titanacyclobutene ring are elongated single bonds (4c: 1.5364(11) Å; 4d: 1.5371(18) Å), yet notably shorter than the corresponding bond in a related complex reported by Tebbe et al. (1.598(5) Å).²⁷ In that work, the bonding situation was described as a titanium–methylene–acetylene adduct with a substitution-labile BTMSA ligand, formed via [2 + 2] cycloaddition between in situ generated titanium carbene species and BTMSA. This mechanism of carbene formation and subsequent cycloaddition with olefines and alkynes has also been also widely explored by Grubbs et al.²⁸

The exocyclic C13–C14/C26–C27 bonds (4c: 1.3518(11) Å; 4d: 1.3502(19) Å) exhibit double bond character when compared to the sum of the covalent radii (∑r_cov(C–C): 1.34 Å)²⁹ and typical C=C double bonds (1.34 Å).²³ The resulting four-membered rings of 4c and 4d, composed of atoms Ti1, C11, C12, and C13/C25, are slightly puckered due to the sp³ hybridization of the carbon atom of the former cyclopropane moiety. Accordingly, these sp³-hybridized carbon atoms deviate from the mean plane by approximately 2.7° (4c) and 4.2° (4d). The Ti–C bond lengths (avg. 2.11 Å) are in line with expected values for titanium carbon single bonds (∑r_cov(Ti–C): 2.11 Å),²¹ and are consistent with those observed in complexes 2a–d (avg. 2.12 Å). Table 2 summarizes the bond lengths and angles of complexes 4c and 4d.

Table 2 Selected bond lengths (Å) and angles (°) of complexes 4c and 4d

Complex	Ti1–C11	Ti1–C13/Ti1–C25	C11–C12	C12–C13/C12–C25	C13–C14/C25–C26	C14–C15/C26–C27	C11–Ti1–C13/C11–Ti1–C25
4c	2.1104(8)	2.1763(8)	1.3511(11)	1.5364(11)	1.4736(11)	1.3518(11)	69.97(3)
4d	2.1099(13)	2.1882(13)	1.3476(19)	1.5371(18)	1.4729(18)	1.3502(19)	69.32(5)

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Reaction of 1 with allenes

The allenes were either obtained from commercial suppliers (for b and j) or prepared according to literature procedures.³⁰ After purification by chromatography or distillation, the allenes were degassed and stored at −30 °C under inert conditions without decomposition. Reactions of Cp₂Ti(η²-BTMSA) 1 with allenes a–m were carried out in n-hexane at ambient temperature (Scheme 4). Upon addition, the reaction mixtures immediately changed colour from yellow to red, and red precipitates formed (except 5c and 5d, which were isolated as red oils). After stirring for 1 h, the supernatant was decanted, and the residue was washed with a small amount of n-hexane and dried under high vacuum. The resulting complexes 5a–m were obtained in yields of 28–96%. With the exception of 5c and 5d, the complexes exhibit limited solubility in n-hexane but dissolve readily in aromatic solvents (toluene, benzene) or in THF. In the solid state, they decompose upon heating above 117 °C.

Scheme 4 Formation of titanacyclopentane complexes 5a–m from Cp₂Ti(η²-BTMSA) 1 and allenes a–m via ligand substitution.

Unlike the results reported by Binger et al., the product formation was independent of stoichiometry and unaffected by the presence of PMe₃.⁶ In the reaction of 1 with the parent allene propa-1,2-diene a, stirring for 15 minutes at ambient temperature followed by work-up initially yielded traces of the 2,4-dimethylene-1-titanacyclopentane complex as a minor side product (see Fig. S51). In solution, this species gradually converts into the 2,5-dimethylene-1-titanacyclopentane complex 5a within one hour, indicating cleavage of the newly formed C_B–C_C bond and subsequent dissociation and reorganization within the titanium coordination sphere at ambient temperature. Formation of this side product has also been reported by Binger et al. for the corresponding zirconium complex; however, in that case no further transformation was observed.⁶ In the ¹H NMR spectrum of 5a, the product could be clearly identified as the titanacyclopentane complex. In addition to the singlet for the Cp protons at δ = 5.97 ppm, the CH₂ protons of the newly formed bridging C_B–C_C bonds in the ring system give a single signal at δ = 2.48 ppm, while the chemically inequivalent alkene protons at C_E and C_F resonate at δ = 3.45 and 5.38 ppm. These data indicate a symmetrical coordination environment at the titanium center. Single crystals of 5a were obtained from a concentrated n-hexane solution at ambient temperature by slow evaporation. Single-crystal X-ray analysis confirmed the formation of the titanacyclopentane complex, consistent with the spectroscopic data. The molecular structures of 5a and 5e are shown in Fig. 4, with the structures of 5b and 5i–l being provided in the SI.

Fig. 4 Molecular structures of complexes 5a (top) and 5e (bottom). Thermal ellipsoids are drawn at the 50% probability level. Most hydrogen atoms are omitted for clarity.

NMR spectroscopy and single-crystal X-ray diffraction demonstrate that the reaction of 1 with mono-substituted allenes b–h predominantly furnishes (E,E)-titanacyclopentane products (see Fig. 4, bottom). In the case of methoxyallene b, however, trace amounts of an additional 2,5-substituted (E/Z)-configured isomer 5b′ are observed, as evidenced by weak additional signals in the ¹H NMR spectrum (Fig. S55). This minor species does not undergo further isomerisation over time in solution, in contrast to complex 5a.

Reactions with disubstituted allenes i–m likewise furnished titanacyclopentane complexes, including those derived from sterically demanding substrates. The solid-state structures of selected products corroborate these findings and reveal that bulkier substituents adopt conformations oriented away from the titanium center. Significant bond lengths and angles of complexes 5a–5k are summarized in Table 3, highlighting the influence of sterically demanding substituents on the bonding parameters. Collectively, these results demonstrate the generality of the titanium-mediated allene cyclization and provide insight into the stereochemical preferences governing the formation of the resulting metallacycles.

Table 3 Selected bond lengths (Å) and angles (°) of complexes 5a, 5b, 5e, 5f, and 5i–k

Complex	Ti–CA	Ti–CD	CA–CB	CA–CF	CB–CC	CC–CD	CD–CE	CA–Ti–CD
5a	2.1787(7)	2.1787(7)	1.5128(11)	1.3386(10)	1.4736(11)	1.5128(11)	1.3386(10)	81.74(4)
5b	2.1687(16)	2.1573(15)	1.524(2)	1.329(2)	1.541(3)	1.516(2)	1.336(2)	81.51(6)
5e	2.1813(7)	2.1813(7)	1.5158(10)	1.3490(10)	1.5380(14)	1.5158(10)	1.3490(10)	81.04(4)
5f	2.173(3)	2.166(3)	1.519(4)	1.339(4)	1.536(5)	1.522(4)	1.343(4)	82.14(11)
5i	2.1824(6)	2.1857(6)	1.5250(9)	1.3471(9)	1.5323(10)	1.5251(9)	1.3466(9)	84.87(2)
5j	2.1790(12)	2.1780(12)	1.5256(17)	1.3427(16)	1.5329(18)	1.5261(17)	1.3439(17)	86.79(4)
5k	2.2112(11)	2.2006(10)	1.546(2)	1.3492(14)	1.517(3)	1.525(2)	1.3465(14)	87.09(4)

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We next investigated the reactivity of 1 toward cyclic allenes. Owing to the inherent ring strain and constrained geometry of these substrates, particularly in the case of cyclotrideca-1,2-diene, the enforced distortion of the cumulated π-system significantly destabilizes the allene framework. Consequently, simple coordination to the metal center is expected to provide a favourable interaction that partially compensates for this destabilization, whereas additional C_B–C_C bond formation would further increase structural distortion and should therefore be disfavoured. The reaction of 1 with allenes n–s proceeded smoothly under mild conditions to afford the corresponding titanacyclopentane complexes 5n–s in good to excellent yields (Scheme 5). As observed previously, product formation remained independent of stoichiometry. NMR spectroscopy revealed selective formation of single products, with no evidence of side reactions or isomerization. Single crystals suitable for X-ray analysis were obtained for several complexes, and the resulting structures confirmed the spectroscopic assignments. For instance, the molecular structure of 5r is shown in Fig. 5 (top), while those of 5n, 5o and 5p are provided in the SI. These results demonstrate that even strained cyclic allenes n–s undergo efficient titanium-mediated cyclization, underscoring the generality of this transformation and extending its scope to conformationally restricted systems.

Scheme 5 Reaction of 1 with allenes n–t to afford titanacyclopentane complexes 5n–t through ligand exchange and C–C bond formation.

Fig. 5 Molecular structures of complex 5r (top) and 5t (bottom). Thermal ellipsoids are drawn at the 50% probability level. Only one of six crystallographically independent molecular structures of 5r and only one of two crystallographically independent molecular structures of 5t are shown. Most hydrogen atoms are omitted for clarity.

A comparison of the bond lengths and angles (Table 4) reveals only a slight increase in the Ti–C_A and Ti–C_D bond distances, from 2.19 to 2.21 Å, consistent with the increased steric demand of the allene substituents. This is accompanied by a notable decrease in the C_A–Ti–C_D bond angles, which narrows from 86.8 to 78.4°. The C_B–C_C bond lengths fall within the range of elongated C–C single bonds compared to the sum of covalent radii, reaching up to 1.54 Å. The C_A–C_F and C_D–C_E double bonds are slightly elongated across the series, with complex 5p displaying the longest values (1.35 and 1.36 Å) and 5t the shortest (1.34 and 1.32 Å). In the molecular structures of complexes 5r and 5t, the corresponding ring systems of the former allenes adopt a trans configuration (Fig. 5).

Table 4 Selected bond lengths (Å) and angles (°) of complexes 5n–p, 5r, and 5t

Complex	Ti–CA	Ti–CD	CA–CB	CA–CF	CB–CC	CC–CD	CD–CE	CA–Ti–CD
5n	2.1890(7)	2.1888(6)	1.5230(9)	1.3494(9)	1.5327(11)	1.5252(9)	1.3488(9)	86.79(2)
5o	2.1715(14)	2.1875(13)	1.5252(19)	1.348(2)	1.536(2)	1.523(2)	1.3439(19)	86.12(5)
5p	2.2047(6)	2.1792(5)	1.5282(8)	1.3552(8)	1.5314(9)	1.5243(8)	1.3535(8)	85.07(2)
5r	2.2063(5)	2.2047(5)	1.5236(6)	1.3483(6)	1.5449(6)	1.5228(6)	1.3480(6)	79.110(18)
5t	2.205(8)	2.205(7)	1.507(10)	1.337(12)	1.528(10)	1.526(10)	1.317(11)	78.4(3)

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The reaction of 1 with allene t was only successful when the cyclic allene was added directly to complex 1. In this case, the formation of 5t proceeded significantly more slowly than with the other allenes and did not afford a single, selective product. The formation of complex 5t′ as a side product was observed in the ¹H NMR spectra. In contrast to 5t, which displays two distinct Cp resonances indicative of a sterically constrained environment that renders the Cp rings chemically inequivalent, complex 5t′ exhibits only a single Cp signal. Notably, in 5t one CH₂ resonance of the Cp₂Ti-alkyl fragment is shifted markedly upfield (δ = −0.10 ppm). This pronounced high-field shift indicates an unusual magnetic environment in close proximity to the titanium center. In addition, one of the alkene protons in 5t is shifted significantly downfield, appearing at 5.47 ppm (compared to approx. 3.5 ppm in complexes 5a–5r), while the second alkene proton resonates between 1.20–2.20 ppm. These distinct chemical shift differences indicate a highly anisotropic proton environment. Examination of the single-crystal structure of 5t (Fig. 5, bottom) reveals that the steric demand of the tridecene ring forces the alkene protons into unequal spatial orientations relative to the titanium center (E and Z configuration). The proton H26 oriented toward the metal center may experience pronounced deshielding, accounting for the downfield shift (δ = 5.47 ppm), whereas the unusual upfield shift of the CH₂ resonance from one of the cyclotridecene units (C14, δ¹H = −0.10 ppm) could arise from shielding effects associated with the anisotropic environment of the Cp ligands (see Fig. S92). As previously noted for complexes 5r and 5t, the two cyclotridecene systems adopt a trans configuration.

Performing the reaction in n-hexane resulted in the formation of complexes 5t and 5t′, while the isomerization of the 1,2-allene t to the corresponding 1,3-diene t′ proceeded to a significant extent (Fig. S91). To assess the catalytic performance, the isomerization was monitored by NMR spectroscopy under catalytic conditions. While Cp₂Ti(η²-BTMSA) tended to form stable complexes with allene t, the sterically more demanding complex Cp*₂Ti(η²-BTMSA) enabled a smooth and selective conversion to the 1,3-diene t′, with no detectable titanium-containing byproducts or liberation of the BTMSA ligand (Fig. S96). The reaction was subsequently performed using 5 mol% of the Cp*₂Ti(η²-BTMSA) catalyst (Scheme 6), demonstrating efficient catalytic turnover under these conditions. This behaviour suggests that the increased steric bulk of the Cp* ligands restricts prolonged coordination of the allene, allowing only transient interaction sufficient to promote the isomerization.

Scheme 6 Titanium-catalyzed isomerization of cyclotrideca-1,2-diene t by Cp*₂Ti(η²-BTMSA) yields cyclotrideca-1,3-diene t′.

The isomerisation of allenes to 1,3-dienes has been achieved through a variety of strategies, including thermal, acidic, and metal-mediated methods. Early work in the 1960s and 1970s demonstrated that cyclopropyl-substituted allenes can undergo thermal rearrangement, while subsequent studies in the late 1980s reported that allenic systems are capable of forming cyclopentyl dienes at elevated temperatures.³¹ Acid-catalysed protocols, employing both mineral acids and organic Brønsted acids, have also been explored; however, these approaches generally show a restricted substrate scope and tend to favour electron-rich allenes.³² More recently, transition-metal complexes, particularly those of palladium and gold, have been employed to mediate allene rearrangements to 1,3-dienes, highlighting the potential of metal coordination to facilitate these transformations.³³ Given the well-established ability of titanium complexes to form metallacyclic intermediates with unsaturated substrates, we considered that cyclic allenes might undergo analogous activation pathways. Despite this conceptual precedent, to the best of our information, no examples of titanium-mediated isomerization of cyclic allenes to 1,3-dienes have been reported. We propose that allene t initially displaces the BTMSA ligand, resulting in a formal oxidative addition at the in situ generated Cp*₂Ti(II) fragment to give olefin complex I (Scheme 6). This complex may then undergo a formal 1,3-hydrogen shift, yielding olefin complex II. Subsequent reductive elimination is expected to furnish the conjugated 1,3-diene t′, while regenerating the Cp*₂Ti(η²-BTMSA) catalyst and completing the catalytic cycle. The transformation is likely driven, at least in part, by relief of ring strain in the cyclic allene. Recoordination of the alkyne ligand BTMSA to the titanium center is facilitated by its favorable π-acceptor properties relative to olefins. Mechanistic studies on 1,3-hydrogen shifts in allenic systems suggest that such rearrangements are electronically feasible, although the precise role of the titanium center in this process remains to be established.³⁴ Further investigations will also assess the applicability of this catalytic system to a broader range of (cyclic) allenes and provide deeper mechanistic insight into the titanium-mediated 1,3-hydrogen shift.

Conclusions

In summary, this work expands the reactivity profile of Rosenthal's reagent Cp₂Ti(η²-BTMSA) 1 toward cyclopropenes and allenes, enabling selective carbon–carbon bond formation under mild conditions. Direct and selective insertion of cyclopropenes afforded bicyclic titanacyclopentenes 2a–d, whose structures were confirmed by NMR spectroscopy and X-ray diffraction. Controlled rearrangement of 2a–d afforded the titanacyclobutenes 3a–d via in situ generated titanium carbene intermediates. These intermediates undergo either a [2 + 2] cycloaddition with the BTMSA ligand or react with 1,2-diphenylacetylene to give 4c and 4d. The methodology was extended to a broad range of allenes, furnishing diverse titanacyclopentanes 5a–t through substitution of the BTMSA ligand. For cyclotrideca-1,2-diene t, partial isomerization to the corresponding 1,3-diene t′ was observed, which was further promoted catalytically using Cp*₂Ti(η²-BTMSA). These results highlight a new example of titanium(II)-mediated catalytic isomerization of a strained cyclic allene, illustrating the capacity of Rosenthal-type complexes to enable both stoichiometric and catalytic transformations. They further underscore the versatility of Cp₂Ti(η²-BTMSA) in activating both cumulated and strained π-systems, expanding the known reactivity space of low-valent titanium complexes and providing a foundation for further exploration of Rosenthal-type complexes with unconventional unsaturated substrates.

Author contributions

ME was responsible for the conceptualization, methodology, synthesis, characterization, and preparation of the manuscript. DG was responsible for the synthesis of cyclopropenes and allenes. MS performed the diffraction work. SD and RB supervised the project, acquired the funding, and contributed to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: additional experimental details, further spectral and crystallographic data. See DOI: https://doi.org/10.1039/d6dt00059b.

CCDC 2463910–2463915 and 2470638–2470649 contain the supplementary crystallographic data for this paper.^35a–r

Acknowledgements

Financial support from the DFG Research Training Group 2226 is gratefully acknowledged. Jessica Reimer is thanked for her assistance in synthesizing cyclopropenes and allenes.

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