Ethylene carbonate splitting and Claisen-type self-addition of γ-butyrolactone promoted by an oxygen-bridged Ga/P FLP

Abstract

The versatile reactivity of the geminal oxygen-bridged frustrated Lewis pair (FLP) Bis₂Ga–O–P^tBu₂ (Bis = CH(SiMe₃)₂; GaOP) is presented towards a series of C=O double bonds, strained oxygen-containing rings, alkynes plus alkenes and a C–Cl bond. By adding benzaldehyde, cyclopentanone (CP), γ-butyrolactone (GBL) or ethylene carbonate (EC), classical 1,2-adducts are formed. The reversibility of the GaOP·EC adduct stands out, enabling a splitting of EC after several days or when heated to 70 °C, resulting in FLP adducts of CO₂ and ethylene oxide (EO). The latter consists of a six-membered ring and is obtained via a ring-opening reaction, as verified by a further reaction involving the similar propylene oxide (PO). The GaOP·GBL adduct is further converted, with two adduct molecules combining into a 2 : 1 FLP adduct in a Claisen-type addition and ring-opening. Tests towards various acetylene containing species revealed that the ratio of deprotonation to ring-closure product can be adjusted. While PhC≡CH is almost exclusively bound as the deprotonation product, for Me₃SiC≡CH, the ring-closure product also forms. In the reaction with benzyl chloride, Bis₂(Cl)Ga–O–P(Bn)^tBu₂ is formed by a C–Cl bond activation.

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Introduction

Understanding the activation and transformation steps of small molecules is of crucial importance for many chemical applications. With the concept of frustrated Lewis pairs (FLPs), D. W. Stephan introduced a way to polarise substrates through the simultaneous action of Lewis acid (LA) and Lewis base (LB) functions.¹ Over the last two decades, this field of research has generated enormous interest, particularly within main-group chemistry and in the pursuit of transition metal-free catalysis.^2–5

While many transition-metal-based systems boast catalysts for a variety of transformations,^6–13 the investigations of main-group-based FLP-mediated conversions have focused primarily on hydrogenation reactions, C–C couplings and CO₂ reductions.^14–19 Although FLPs are capable of activating many types of small molecules,^20–25 the majority form irreversible adducts, making the development of catalytic reactions difficult. Only a few undergo further transformation due to a conditional reversibility, which is essential for the broader application of FLPs as catalysts.^26,27

Multiple reactivity studies have shed light on the following adjustment screws for tuning intramolecular FLP reactivity: the combination of LA and LB elements, their respective substituents and the linker unit. The literature predominantly features boron-containing FLP systems that have to bear strongly electron-withdrawing groups in order to be reactive.²⁸ Such groups are unnecessary for the heavier homologue aluminium, which is inherently more Lewis acidic.²⁹ However, its strong oxophilicity is a major drawback, particularly when it comes to the release of a transformed product that contains oxygen atoms.

Instead, the group 13 element gallium has a similar Lewis acidity level but prefers softer Lewis bases according to the concept of hard and soft acids and bases (HSAB) by R. G. Pearson.^30,31 W. Uhl et al. took advantage of this tendency with the benzylidene-bridged E/P FLPs (E = Al and Ga) in a reaction involving carbon dioxide: the aluminium system irreversibly captures CO₂, while the gallium analogue binds it reversibly.^29,32

Similarly, adjusting the linker unit, Y. Wang et al. have shown the reversibility of a CO₂ adduct with an oxygen-bridged B/P FLP.³³ Reducing the Lewis acidity using donating atoms like oxygen affects the reactivity. Recently, we demonstrated how to tune FLPs for specific reactivities with four chalcogen-bridged systems Bis₂E–X–P^tBu₂ (Bis = CH(SiMe₃)₂, E = Al and Ga, X = O and S).³⁴

As Bis₂Al–O–P^tBu₂ (AlOP) imposes with H₂ and CO₂ activation, resulting in a reduction of carbon dioxide to the formate stage,³⁵ Bis₂Ga–O–P^tBu₂ (GaOP) forms an unusual temperature-dependent H₂ equilibrium by cleavage into Bis₂GaH and ^tBu₂P(O)H.³⁶ In particular, this difference in oxophilicity prompted us to test reactivity towards molecules with C=O double bonds and small rings containing oxygen atoms.

In this work, we show impressive reactivities of Bis₂Ga–O–P^tBu₂: the cleavage of ethylene carbonate (EC) to ethylene oxide (EO) and CO₂, a Claisen addition of γ-butyrolactone (GBL), the ring opening of propylene oxide (PO) and deprotonation versus ring formation of phenyl- and trimethylsilylacetylene plus C–Cl bond activation of benzyl chloride.

Results and discussion

We probed the reactivity of the FLP Bis₂Ga–O–P^tBu₂ (GaOP) with a series of molecules containing C=O double bonds: benzil, benzophenone, benzaldehyde, cyclopentanone (CP), γ-butyrolactone (GBL) and ethylene carbonate (EC). Unlike the AlOP system, which forms a stable 1,2-addition product with benzil, the reaction of the GaOP system and benzil ends up in an unselective decomposition. As no reaction was observed with benzophenone, benzaldehyde with a less sterically demanding carbonyl unit was tested, affording the 1,2-adduct 1 (Scheme 1).

Scheme 1 Reactions of GaOP with benzaldehyde, cyclopentanone (CP), γ-butyrolactone (GBL) and ethylene carbonate (EC) to five-membered ring adducts 1–4. Tests with benzil and benzophenone show decomposition or no reaction.

In the ¹H NMR spectrum, the resonances of the Bis and tert-butyl groups are split due to the implemented stereocentre. The chemical shifts of the methine protons at the gallium atom with −0.26 and −0.48 ppm indicate a tetra-coordination. The aldehyde proton resonance at 5.85 ppm is significantly shifted towards higher field compared to “free” benzaldehyde (9.64 ppm). The presence of a five-membered ring is confirmed by ¹³C{¹H} NMR spectroscopy with the carbon stereocentre leading to a doublet at 75.5 ppm (¹J_P,C = 40.4 Hz) and by ³¹P{¹H} NMR with a signal at 70.9 ppm.

Tests towards CP, GBL and EC result in similar adducts 2–4 with five-membered ring motifs (Scheme 1). In all cases, the ¹H NMR signals of the adducts are broadened, indicating weak and potentially reversible adduct formation. In particular, the signals of the CH₂ groups show poor resolution and are low field-shifted in relation to their resonances in the free substrates.

The ¹H NMR shifts of the methine protons range from −0.20 to −0.62 ppm, characteristic of quadruple coordinated gallium atoms (Table 1). Likewise, the ¹³C{¹H} and ³¹P{¹H} NMR shifts as well as the ¹J_P,C coupling constants are consistent with normal 1,2-addition adducts.

Table 1 ¹H, ¹³C{¹H} and ³¹P{¹H} NMR spectroscopic data of 1, 2, 3, 4, 7 and 8

Compound	No.	δ(^1H) (GaCH)/ppm	δ(^1H) (^tBu)/ppm	δ(^13C{^1H}) (PCX3)/ppm	δ(^31P{^1H})/ppm
GaOP·PhCHO	1	−0.48, −0.26	0.84/1.15 (^3JP,H = 13.4/13.3 Hz)	75.5 (^1JP,C = 40.4 Hz)	70.9
GaOP·CP	2	−0.62, −0.39	0.99/1.11 (^3JP,H = 13.0/13.3 Hz)	87.0 (^1JP,C = 40.0 Hz)	77.2
GaOP·GBL	3	−0.38, −0.20 br.	1.10 br.	109.9 (^1JP,C = 73.2 Hz)	74.1
GaOP·EC	4	−0.30 br.	1.19 br.	122.7 (^1JP,C = 120.4 Hz)	70.6
GaOP·EO	7	−0.40 br.	0.87 (^3JP,H = 14.0 Hz)	23.7 (^1JP,C = 54.5 Hz)	75.1
GaOP·PO	8	−0.49, −0.39	0.85/0.90 (^3JP,H = 13.9/14.0 Hz)	30.0 (^1JP,C = 54.5 Hz)	73.0

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Further confirmation was provided by elemental analysis and the determination of the molecular structures in the crystalline state, which verified the proposed connectivity for 1, 2 and 3 (Fig. 1). Despite numerous crystallizations attempts, the ethylene carbonate adduct 4 could only be obtained in a resin-like form.

Fig. 1 Molecular structures of 1, 2 and 3 in the solid state. Hydrogen atoms (except H(1) in 1) and minor occupied disordered parts are omitted for clarity. Ellipsoids are set at the 50% probability level. Selected distances, bond lengths [Å] and angles [°]: 1: Ga(1)⋯P(1) 2.989(1), Ga(1)–O(1) 2.004(2), Ga(1)–O(2A) 1.834(13), O(1)–P(1) 1.524(2), P(1)–C(23A) 1.891(16), O(2)–C(23) 1.44(2); Ga(1)–O(1)–P(1) 115.2(2), C(1A)–Ga(1)–C(8A) 126.9(8), O(1)–Ga(1)–O(2A) 86.4(4), O(2A)–Ga(1)–P(1)–C(23A) 25.0(9); 2: Ga(1)⋯P(1) 2.990(1), Ga(1)–O(1) 1.986(1), Ga(1)–O(2) 1.894(1), O(1)–P(1) 1.538(1), P(1)–C(23) 1.897(2), O(2)–C(23) 1.404(2); Ga(1)–O(1)–P(1) 115.5(1), C(1)–Ga(1)–C(8) 119.1(1), O(1)–Ga(1)–O(2) 87.0(1), O(2)–Ga(1)–P(1)–C(23) 18.9(1); 3: Ga(1)⋯P(1) 2.986(1), Ga(1)–O(1) 1.993(1), Ga(1)–O(2) 1.907(1), O(1)–P(1) 1.533(1), P(1)–C(23) 1.906(1), O(2)–C(23) 1.360(1); Ga(1)–O(1)–P(1) 115.2(1), C(1)–Ga(1)–C(8) 122.4(1), O(1)–Ga(1)–O(2) 86.9(1), O(2)–Ga(1)–P(1)–C(23) 18.4(1).

A comparison of the molecular structures shows similar five-membered rings: the Ga(1)–O(1)–P(1) angles (115.2(2)–115.5(1)°) and the Ga(1)⋯P(1) distances (2.986(1)–2.990(1) Å) are identical. The gallium atoms are, as expected, in distorted tetrahedral environments, with the O(1)–Ga(1)–O(2) angles ranging from 86.4(4) to 87.0(1)°. Only the O(2)–C(23) bond length varies significantly with 1.44(2) (for 1), 1.404(2) (for 2) and 1.360(1) Å (for 3). However, these values are higher than those of the carbon dioxide adduct (6: 1.285(3) Å), evidencing an oxygen–carbon single bond.

Strikingly, the 1,2-addition adducts 3 and 4 undergo further conversions in solution within two days at ambient temperature (partially) or when heated to 70 °C for six hours (fully). The GBL adduct 3 is subject to a rearrangement, forming the adduct type 5 in a Claisen addition and ring-opening reaction (Scheme 2). In this reaction, two molecules of the adduct 3 react to a Claisen addition product, which then forms the α,β-unsaturated ketone resulting from a ring-opening. This molecule is bound to two FLP units. The reaction to 5 is presumably triggered by the deprotonation of the α-proton, resulting in the formation of the P–H unit (Scheme 2 or SI Scheme S1).

Scheme 2 The 1,2-adduct 3 is further converted in a Claisen-type addition, forming a 2 : 1 FLP Claisen addition adduct with subsequent ring-opening via the proposed mechanism and structure 5 (left). Heating 4 to 70 °C results in the splitting of EC, leading to the FLP CO₂ and EO adducts (6/7) and one half of an equivalent of EC (right). In a ring-opening reaction, propylene oxide forms a six-membered ring adduct 8. Also exchange reactions of 4 and 6 with propylene oxide (PO) result in adduct 8.

Multinuclear NMR spectroscopy analysis approves the formation of 5. The ³¹P NMR spectrum shows a doublet of multiplets at 63.2 ppm (¹J_P,H = 422.9 Hz), confirming the generation of phosphane oxide. The broadened signal indicates its interaction with the Lewis acid moiety. The ¹H NMR shifts of the methine protons at −0.30 and −0.48 ppm verify tetra-coordinated gallium atoms and suggest the presence of two different Bis₂Ga-groups, which are also confirmed by ²⁹Si NMR spectroscopy. The framework of the Claisen added and ring-opened product 5 is identified by ¹H and ¹³C{¹H} NMR spectroscopy, with five CH₂ units observable in the ¹³C DEPT-135 NMR spectrum. The remaining quaternary carbon atoms show shifts of 188.0, 178.7 and 90.7 ppm, respectively. The ¹H–¹H COSY NMR spectrum validates the correlation of the methylene protons at 3.68 and 2.49 ppm and of those at 4.07, 2.49 and 2.10 ppm. All these assignments are consistent within the corresponding 2D NMR spectra (¹H–¹³C HSQC and HMBC). As for the 1,2-adduct 4, multiple attempts to crystallize 5 failed and only yielded resin-like residues.

A different transformation is observed for 4. The added ethylene carbonate (EC) is cleaved, resulting in the formation of the FLP adducts of carbon dioxide (6) and of ethylene oxide (EO) (7) accompanied by the release of half an equivalent of EC (Scheme 2). This observation lends support to the assumed reversible nature of the adduct formation with EC.

There are two main pathways for the splitting of EC (see SI Scheme S2): the activation of the carbonyl unit followed by ring-opening (path A) or ring-opening, where the carbonate unit remains intact followed by an extrusion (retro-insertion) of CO₂ leading to the EO adduct 7 (path B). Both pathways have been reported in the literature,^37,38 but the experimental outcome (formation of more CO₂ adduct 6) matches only path A, as 7 is stable in the presence of CO₂.

In the ¹H NMR spectrum one set of signals corresponds to the literature values of 6.³⁴ Characteristic of the EO adduct 7 is the doublet of triplets of the oxygen-bound CH₂ group at 4.10 ppm (³J_P,H = 18.5 Hz, ³J_H,H = 5.6 Hz). Due to overlapping resonances, it was not possible to localise a resonance for the second CH₂ unit exactly in the ¹H NMR spectrum, even when using 2D NMR. In contrast, the ¹³C{¹H} NMR spectrum contains a resonance in this regard as a doublet at 23.7 ppm (¹J_P,C = 54.5 Hz).

To further verify the formation of a six-membered ring, we reacted GaOP with the similar propylene oxide (PO). In a ring-opening reaction the adduct 8 was formed. Both 7 and 8 have comparable chemical shifts. For example, the ³¹P{¹H} NMR shifts differ slightly (7: 75.1 ppm; 8: 73.0 ppm). In the ¹³C{¹H} NMR spectrum the carbon atom attached to the oxygen atom is detected at 60.5 (7) and 65.6 ppm (8), respectively. The propylene unit of 8 induces signals at 4.35, 1.63, 1.36 and 1.32 ppm in the ¹H NMR spectrum, which are similar to those of 7. Due to the diastereotopic nature of the P–CH₂ protons, both resonances are split into doublets of multiplets with a characteristic ³J_P,H coupling constant of 14.8 Hz (see SI Fig. S38).

Crystal structure determinations of both the EO and PO adducts 7 and 8, confirm the six-membered ring motif (Fig. 2). Since they differ only by an exocyclic methyl group, the structural parameters are nearly the same. The Ga(1)⋯P(1) distances of 3.191–3.198(1) Å are longer than in free FLP GaOP (3.082(1) Å).³⁶ The Ga(1)–O(1)–P(1) angles in 7 (129.1(1)°) or 8 (129.3(1)°) are slightly wider than that in GaOP of 126.2(1)°.

Fig. 2 Molecular structures of 7 and 8 in the solid state. Hydrogen atoms and minor occupied parts are omitted for clarity. Ellipsoids are set at the 50% probability level. Selected distances, bond lengths [Å] and angles [°]: 7: Ga(1)⋯P(1) 3.198(1), Ga(1)–O(1) 2.007(1), Ga(1)–O(2) 1.866(1), O(1)–P(1) 1.527(1), P(1)–C(23) 1.826(1), C(23)–C(24) 1.535(2), O(2)–C(24) 1.401(1); Ga(1)–O(1)–P(1) 129.1(1), O(1)–Ga(1)–O(2) 92.3(1), O(2)–Ga(1)–P(1)–C(23) 15.2(1); 8: Ga(1)⋯P(1) 3.191(1), Ga(1)–O(1) 2.001(2), Ga(1)–O(2) 1.854(3), O(1)–P(1) 1.523(2), P(1)–C(23) 1.847(4), C(23)–C(24) 1.520(6), O(2)–C(24) 1.393(5); Ga(1)–O(1)–P(1) 129.3(1), O(1)–Ga(1)–O(2) 93.1(1), O(2)–Ga(1)–P(1)–C(23) 14.3(3).

In an NMR spectroscopic study, we tested the exchange reaction of the ethylene carbonate adduct 4 and the CO₂ adduct 6 with an excess of propylene oxide (PO). When 4 is reacted with PO, the formation of adduct 8 and free EC is observed at ambient temperatures. For 6, heating to 70 °C is necessary to exchange the carbon dioxide with PO. These findings gave further insights and strengthened the proposed mechanism (path A) of the ethylene carbonate splitting (see SI Scheme S2).

Relating to previous investigations of slightly polarized alkynes with the AlOP system,³⁹ we tested the reactivity of phenyl- and trimethylsilylacetylene with the GaOP FLP. Interestingly, there is a clear difference in the product ratio of deprotonation vs. ring-closure. Whereas deprotonation predominates in the case of phenylacetylene (>96%), and crystallization gave the deprotonation product 9a in pure form, only mixtures with deprotonation/ring-closure ratios of approximately 2 : 1 (10a : 10b) were obtained for trimethylsilylacetylene (Scheme 3).

Scheme 3 Reactions of GaOP with phenylacetylene and trimethylsilylacetylene result in deprotonation (9a/10a) or ring-closure (9b/10b) products. Using benzyl chloride leads to Bis₂(Cl)Ga–O–P(H)^tBu₂ (11).

Heating the reaction to 70 °C for 24 h did not result in formation of the ring-closure products 9b and 10b. The same behaviour, along with a similar deprotonation/ring-closure ratio, was previously observed for AlOP·PhC≡CH (85 : 15).³⁹ Although the experimental outcome is consistent, quantum chemical calculations suggest that the ring-closure products 9b/10b are thermodynamically favoured (see SI). To explain these contradictions, we assume that the deprotonation products 9a/10a are kinetically preferred and the activation barriers of these meta-stable states to the ring-closure products 9b/10b are too high. Furthermore, a partial separation of 9a and 10a into di-tert-butylphosphane oxide and Bis₂GaC≡CR (R = Ph and SiMe₃) at higher temperatures, as we reported it for the H₂ adduct of the GaOP system,³⁶ would prevent a rearrangement to ring-closure.

While the ring-closure product 10b was identified by ¹H NMR spectroscopy by its characteristic doublet at 9.25 ppm (³J_P,H = 56.0 Hz), the deprotonation product 10a has a typical doublet at 5.97 ppm with a ¹J_P,H coupling constant of 442.2 Hz. The ³¹P NMR spectrum displays a doublet of multiplets with the same large coupling constant at 66.9 ppm (10a) and a single multiplet at 84.8 ppm (10b), confirming the identity of these two species. However, the ¹H NMR integral ratios of the deprotonation product 10b do not match exactly; there is a surplus of phosphane oxide present. Besides hydrolysis, the aforementioned partial separation into the phosphane oxide species and Bis₂GaC≡CSiMe₃ is probably another plausible explanation for these mismatching integral ratios. The excess of phosphane oxide undergoes a dynamic exchange with the deprotonation product 10a, meaning no “free” phosphane oxide being detected in the ³¹P NMR spectrum. Elemental analyses confirm this hypothesis and the compositions of 9 and 10.

We were able to determine the solid-state molecule structures of the deprotonation products 9a and 10a (Fig. 3). Contrary to the structure of AlOP·PhC≡CH in both forms,³⁹ and although the formation of the ring-closure product 10b is evident by NMR spectroscopy, the crystalline sample showed no evidence for different types of crystals. The determined bond lengths of C(23)–C(24) in the deprotonation products fall over the expected range for triple bonds (9a: 1.186(3) Å; 10a: 1.206(2) Å). The angles ∠(Ga–C≡C) (9a: 175.3(2)°; 10a: 177.5(2)°) and ∠(C≡C–R) (9a: 178.5(2)°; 10a: 178.0(2)°) in the acetylide units are similar, nearly matching an idealized bond angle of 180°. In contrast, both structures differ in their Ga(1)–O(1)–P(1) angles (9a: 155.3(1)°; 10a: 142.4(1)°) and their torsion angles τ(C(23)–Ga(1)–P(1)–H(1)) with 57.0(8)° for 9a and 41.1(9)° for 10a. These values for the heavier gallium homologue 9a are significantly smaller than those of its aluminium analogue AlOP·PhC≡CH (168.8(2)°; 95.9(5)°).³⁹

Fig. 3 Molecular structures of 9a, 10a and 11 in the solid state. Hydrogen atoms except H(1) and minor occupied disordered parts are omitted for clarity. Ellipsoids are set at the 50% probability level. Selected distances, bond lengths [Å] and angles [°]: 9a: Ga(1)⋯P(1) 3.469(1), Ga(1)–O(1) 2.037(1), O(1)–P(1) 1.512(1), Ga(1)–C(23) 2.004(1), C(23)–C(24) 1.187(2); Ga(1)–O(1)–P(1) 155.3(1), C(1)–Ga(1)–C(8) 117.9(1), C(23)–Ga(1)–P(1)–H(1) 57.0(8); 10a: Ga(1)⋯P(1) 3.338(1), Ga(1)–O(1) 2.008(1), O(1)–P(1) 1.514(1), Ga(1)–C(23) 1.978(2), C(23)–C(24) 1.206(2); Ga(1)–O(1)–P(1) 142.4(1), C(1)–Ga(1)–C(8) 121.6(1), C(23)–Ga(1)–P(1)–H(1) 41.1(9). 11: Ga(1)⋯P(1) 3.371(1), Ga(1)–O(1) 2.014(2), O(1)–P(1) 1.528(2), Ga(1)–Cl(1A) 2.257(3), P(1)–C(23) 1.827(3); Ga(1)–O(1)–P(1) 143.8(1), C(1)–Ga(1)–C(8) 122.8(1), Cl(1A)–Ga(1)–P(1)–C(23) 45.9(1).

Activation attempts with the non-polar diphenylacetylene and the alkene species styrene, stilbene, 1,2-diphenylethene and 1-phenyl-1-(trimethylsiloxyl)ethylene remained unsuccessful. However, we demonstrated the versatility of the GaOP FLP by reacting it with benzyl chloride. After heating to 70 °C, the adduct Bis₂(Cl)Ga–O–P(Bn)^tBu₂ (11) was isolated and fully characterized by means of NMR spectroscopy, elemental analysis and single crystal X-ray diffraction experiments (Fig. 3). Cleavage of the C–Cl bond resulted in the chloride ion bound to the Lewis acidic gallium and a benzyl group attached to the phosphorus atom.

In the ¹H NMR spectrum the formation of 11 is proven by a doublet of the CH₂ group at 3.40 ppm (³J_P,H = 14.0 Hz). The broad methine proton singlet at −0.06 ppm verifies the tetra-coordinated gallium atom. Besides the signal at 63.6 ppm, another species containing a P–H unit was detected by a doublet ³¹P NMR resonance at 69.7 ppm (¹J_P,H = 451.5 Hz). This minor impurity was identified as the formal hydrogen chloride adduct Bis₂(Cl)Ga–O–P(H)^tBu₂, likely originating from the partial hydrolysis of benzyl chloride. The chemical shifts are comparable with those of the recently published HBr adduct, GaOP·HBr.³⁶

The molecular structure of 11 shows a tetrahedral environment at the gallium atom and the Ga(1)–Cl(1A) bond length of 2.257(3) Å is slightly longer than in the free Bis₂GaCl (2.193(1) Å).⁴⁰ Compared with GaOP·HBr, the Ga(1)–O(1)–P(1) angles are almost identical (GaOP·HBr: 143.1(1)°; 11: 143.8(1)°). As the value of τ(C(23)–Ga(1)–P(1)–H(1)) = 45.9(1)° indicates, the chloride ion and benzyl group are twisted, in a similar manner to the proton and acetylide unit in the deprotonation adducts 9a and 10a.

Conclusions

Bis₂Ga–O–P^tBu₂ (GaOP) exhibits a remarkably broad and tuneable reactivity profile towards a wide variety of substrates, ranging from carbonyl compounds to strained oxygen heterocycles, alkynes and polar σ-bonds. In classical 1,2-additions to the C=O double bond of benzaldehyde, cyclopentanone (CP), γ-butyrolactone (GBL) and ethylene carbonate (EC), FLP adducts with five-membered rings are observed. Beyond simple adduct formation, the system reveals unexpected secondary transformations such as the Claisen-type reaction of two GBL adducts and subsequent trapping of the resulting product as a 2 : 1 FLP adduct. The reversible activation of EC is particularly notable, as it enables a fragmentation into CO₂ and ethylene oxide (EO), both of which were detected as the corresponding FLP adducts. This ring-opening reaction to a six-membered ring was proven using propylene oxide (PO), which results in the formation of a similar adduct. Reactions involving phenyl- and trimethylsilylacetylene further demonstrate that subtle changes in the substrate structure allow deprotonation and ring-closure to be deliberately steered, providing rare insight into selectivity control in FLP chemistry. Activating the C–Cl bond in benzyl chloride broadens the scope of activation to include polar σ-bonds. Taken together, these findings establish the GaOP system as a highly versatile and adaptable FLP, capable of reversible activation, bond cleavage and multistep substrate transformation.

Experimental section

General methods

All reactions and manipulations with air and moisture sensitive compounds were carried out under conventional Schlenk techniques or in a glove box using argon as an inert gas. Volatile compounds were handled in a vacuum line. The solvents n-hexane, toluene, toluene-d₈ and benzene-d₆ were dried over a Na/K alloy, distilled and degassed prior to use. Bis₂Ga–O–P^tBu₂ (GaOP) was prepared according to a literature procedure.³⁶ Benzaldehyde, cyclopentanone (CP), γ-butyrolactone (GBL), propylene oxide (PO), phenylacetylene, trimethylsilyl acetylene and benzyl chloride were degassed, dried over molecular sieves (4 Å) and distilled prior to use. Ethylene carbonate (EC) was used without further purification. NMR spectra were recorded using a Bruker Avance III 500, Avance III 500 HD, Ascend 500 neo2K or Ascend 500 neo3K spectrometer at ambient temperature unless otherwise stated. Chemical shifts were referenced to the residual proton or carbon signal of the solvent (benzene-d₆: ¹H: 7.16 ppm, ¹³C: 128.1 ppm; toluene-d₈: ¹H: 2.09 ppm) or externally (²⁹Si: SiMe₄, ³¹P: 85% H₃PO₄ in H₂O). Elemental analyses were carried out using a HEKATECH EURO elemental analyzer.

Synthetic procedure

Bis₂Ga–O–P^tBu₂ (GaOP) was dissolved in toluene (2 mL), the substrate was added and the reaction was stirred for 24 h at room temperature, unless otherwise stated. All volatiles were removed under reduced pressure and the residue was dried in vacuo.

Bis₂Ga–O–P^tBu₂·PhCHO (1). Using benzaldehyde (5 μL, 4.8 mg, 45 μmol, 1.2 equiv.) and after an additional washing step with n-hexane, 1 was obtained as a colourless solid (24 mg, 37 μmol, 97%). ¹H NMR (500 MHz, C₆D₆): δ [ppm] = −0.48 (s, 1H, GaCH), −0.26 (s, 1H, GaCH), 0.36–0.58 (m, 36H, Si(CH₃)₃), 0.84 (d, ³J_P,H = 13.4 Hz, 9H, C(CH₃)₃), 1.15 (d, ³J_P,H = 13.3 Hz, 9H, C(CH₃)₃), 5.85 (s, CHO), 7.02 (t, ³J_H,H = 7.2 Hz, 1H, para-H), 7.14 (t, ³J_H,H = 7.3 Hz, 2H, meta-H), 7.72 (d, ³J_H,H = 7.6 Hz, 2H, ortho-H). ¹³C{¹H} NMR (126 MHz, C₆D₆): δ [ppm] = 3.9 (s, Si(CH₃)₃), 4.6(s, Si(CH₃)₃), 4.6 (s, Si(CH₃)₃), 4.8 (s, Si(CH₃)₃), 6.6 (s, GaCH), 27.1 (s, C(CH₃)₃), 28.2 (s, C(CH₃)₃), 35.3 (s, C(CH₃)₃), 37.0 (s, C(CH₃)₃), 75.5 (d, ¹J_P,C = 40.4 Hz, CHO), 126.3 (s, ortho-C), 127.2 (s, para-C), 128.3 (s, meta-C), 141.6 (s, ipso-C). ²⁹Si{¹H} NMR (99 MHz, C₆D₆): δ [ppm] = −1.1 (s). ³¹P{¹H} NMR (202 MHz, C₆D₆): δ [ppm] = 70.9 (s). Elemental analysis calcd (%) for C₂₉H₆₂GaO₂PSi₄ (M_r = 655.85): C 53.11, H 9.53; found C 53.57, H 9.54.

Bis₂Ga–O–P^tBu₂·CP (2). Using cyclopentanone (10 μL, 9.5 mg, 113 μmol, 1.2 equiv.) and after an additional washing step with n-hexane, 2 was obtained as a colourless solid (58 mg, 91 μmol, 93%). ¹H NMR (500 MHz, C₆D₆): δ [ppm] = −0.62 (s, 1H, GaCH), −0.39 (s, 1H, GaCH), 0.42–0.50 (s, 36H, Si(CH₃)₃), 0.99 (d, ³J_P,H = 13.0 Hz, 9H, C(CH₃)₃), 1.11 (d, ³J_P,H = 13.3 Hz, 9H, C(CH₃)₃), 1.48(m, 2H, CH₂), 1.66 (m, 2H, CH₂), 2.03 (m, 4H, CH₂). ¹³C{¹H} NMR (126 MHz, C₆D₆): δ [ppm] = 5.3 (s, Si(CH₃)₃), 6.8 (s, GaCH), 9.9 (s, GaCH), 23.5 (d, ²J_P,C = 11.9 Hz, CH₂), 23.8 (d, ²J_P,C = 12.4 Hz, CH₂), 28.2 (s, C(CH₃)₃), 28.8 (s, C(CH₃)₃), 35.8 (d, ¹J_P,C = 41.4 Hz, C(CH₃)₃), 37.5 (d, ¹J_P,C = 41.8 Hz, C(CH₃)₃), 41.1 (s, CH₂), 42.5 (s, CH₂), 87.0 (d, ¹J_P,C = 40.0 Hz, PCO). ²⁹Si{¹H} NMR (99 MHz, C₆D₆): δ [ppm] = −1.2 (s). ³¹P{¹H} NMR (202 MHz, C₆D₆): δ [ppm] = 77.2 (s). Elemental analysis calcd (%) for C₂₇H₆₄GaO₂PSi₄ (M_r = 633.84): C 51.16, H 10.18; found C 50.78, H 10.14.

Bis₂Ga–O–P^tBu₂·GBL (3). Using γ-butyrolactone (10 μL, 11.3 mg, 131 μmol, 1.0 equiv.) and after an additional washing step with n-hexane, 3 was obtained as a colourless solid (70 mg, 110 μmol, 87%). ¹H NMR (500 MHz, C₆D₆): δ [ppm] = −0.38 (br. s, 1H, GaCH), −0.20 (br. s, 1H, GaCH), 0.45 (s, 36H, Si(CH₃)₃), 1.10 (m, 18H, C(CH₃)₃), 1.44 (m, 1H, CH₂), 1.60 (br. m, 1H, CH₂), 1.93 (br. s, 1H, CH₂), 2.11 (br. s, 1H, CH₂), 3.32 (br. s, 1H, CH₂), 4.00 (br. s, 1H, CH₂). ¹³C{¹H} NMR (126 MHz, C₆D₆): δ [ppm] = 5.1 (s, Si(CH₃)₃), 9.2 (s, GaCH), 24.7 (s, CH₂), 27.8 (s, C(CH₃)₃), 35.6 (d, ¹J_P,C = 47.5 Hz, C(CH₃)₃), 36.4 (d, ¹J_P,C = 48.5 Hz, C(CH₃)₃), 39.6 (s, CH₂), 67.1 (s, OCH₂), 109.9 (d, ¹J_P,C = 73.2 Hz, PCO₂). ²⁹Si{¹H} NMR (99 MHz, C₆D₆): δ [ppm] = −1.5 (s). ³¹P{¹H} NMR (202 MHz, C₆D₆): δ [ppm] = 74.1 (s). Elemental analysis calcd (%) for C₂₆H₆₂GaO₃PSi₄ (M_r = 635.82): C 49.12, H 9.83; found C 48.61, H 10.23.

Bis₂Ga–O–P^tBu₂·EC (4). Using ethylene carbonate (9.1 mg, 103 μmol, 1.1 equiv.) and after an additional washing step with n-hexane, 4 was obtained in a colourless, resin-like form (52 mg, 82 μmol, 90%). ¹H NMR (500 MHz, C₆D₆): δ [ppm] = −0.30 (br. s, 2H, GaCH), 0.45 (s, 36H, Si(CH₃)₃), 1.19 (m, 18H, C(CH₃)₃), 3.28 (br. s, 2H, CH₂), 3.80 (br. s, 2H, CH₂). ¹³C{¹H} NMR (126 MHz, C₆D₆): δ [ppm] = 4.9 (s, Si(CH₃)₃), 9.0 (s, GaCH), 27.5 (s, C(CH₃)₃), 35.8 (br. m, C(CH₃)₃), 63.4 (s, OCH₂), 122.7 (d, ¹J_P,C = 120.4 Hz, PCO₃). ²⁹Si{¹H} NMR (99 MHz, C₆D₆): δ [ppm] = −1.3 (s). ³¹P{¹H} NMR (202 MHz, C₆D₆): δ [ppm] = 70.6 (s). Elemental analysis calcd (%) for C₂₅H₆₀GaO₄PSi₄ (M_r = 637.79): C 47.08, H 9.48; found C 46.99, H 9.56.

Claisen addition ring-opening FLP adduct 5. Heating a solution of GaOP (55.7 mg, 101 μmol) and GBL (7.9 μL, 8.9 mg, 104 μmol, 1.0 equiv.) in toluene (3 mL) to 70 °C for 6 h, full conversion of the FLP to 5 was observed. After removing all volatiles in vacuo, and an additional washing step with n-hexane, 5 was obtained in a resin-like form (59.4 mg, 47 μmol, 92%). ¹H NMR (500 MHz, C₆D₆): δ [ppm] = −0.48 (s, 2H, GaCH), −0.30 (s, 2H, GaCH), 0.34 (s, 36H, Si(CH₃)₃), 0.34 (s, 18H, Si(CH₃)₃), 0.36 (s, 18H, Si(CH₃)₃), 0.99 (d, ³J_P,H = 15.0 Hz, 18H, C(CH₃)₃), 2.10 (tt, J = 11.9, 5.8 Hz, 2H, CH), 2.49 (m, 2H, CH), 2.49 (t, J = 8.3 Hz, 2H, CH), 3.68 (t, J = 8.2 Hz, 2H, CH), 4.07 (t, J = 6.0 Hz, 2H, CH₂). ¹³C{¹H} NMR (126 MHz, C₆D₆): δ [ppm] = 3.9/4.1/4.5 (s, Si(CH₃)₃), 7.5/10.3 (s, GaCH), 25.8 (s, C(CH₃)₃), 26.0 (s, CH₂), 30.9 (s, CH₂), 33.6 (d, ¹J_P,C = 59.0 Hz, C(CH₃)₃), 36.1 (s, CH₂), 65.7 (s, OCH₂), 67.8 (s, OCH₂), 90.7 (s, CCO₂Ga), 178.7 (s, CO₂Ga), 188.0 (s, C=O). ²⁹Si{¹H} NMR (99 MHz, C₆D₆): δ [ppm] = −1.4 (s), −0.5 (s), −0.3 (s). ³¹P{¹H} NMR (202 MHz, C₆D₆): δ [ppm] = 63.2 (s).

Bis₂Ga–O–P^tBu₂·EO (7). After heating a solution of GaOP (60.5 mg, 110 μmol) and EC (9.6 mg, 110 μmol, 1.0 equiv.) in toluene (3 mL) to 70 °C for 6 h, full conversion of the FLP to a mixture of Bis₂Ga–O–P^tBu₂·CO₂ (6) and 7 was observed. The chemical shifts of 6 are in accordance with the literature values.³⁴ Crystals of 7, suitable for X-ray diffraction experiments, were obtained by the slow evaporation of a solution in C₆D₆. ¹H NMR (500 MHz, C₆D₆): δ [ppm] = −0.40 (br. s, 2H, GaCH), 0.49 (s, 36H, Si(CH₃)₃), 0.87 (d, ³J_P,H = 14.0 Hz, 18H, C(CH₃)₃), 4.10 (dt, ³J_P,H = 18.5 Hz, ³J_H,H = 5.6 Hz, 2H, OCH₂); due to overlapping resonances, as well in 2D spectra, signals for PCH₂ could not be localized exactly. ¹³C{¹H} NMR (126 MHz, C₆D₆): δ [ppm] = 5.1 (s, Si(CH₃)₃), 10.5 (s, GaCH), 23.7 (d, ¹J_P,C = 54.5 Hz, PCH₂), 25.9 (s, C(CH₃)₃), 26.2 (s, C(CH₃)₃), 33.6 (d, ¹J_P,C = 58.9 Hz, C(CH₃)₃), 35.4 (d, ¹J_P,C = 58.3 Hz, C(CH₃)₃), 60.5 (d, ²J_P,C = 7.6 Hz, OCH₂). ²⁹Si{¹H} NMR (99 MHz, C₆D₆): δ [ppm] = −0.5 (s). ³¹P{¹H} NMR (202 MHz, C₆D₆): δ [ppm] = 75.1 (s).

Bis₂Ga–O–P^tBu₂·PO (8). Using propylene oxide (10 μL, 8.3 mg, 143 μmol, 1.5 equiv.), the ring-opening adduct 8 crystallized from n-hexane at −18 °C, giving colourless needles (32 mg, 53 μmol, 55%). ¹H NMR (500 MHz, C₆D₆): δ [ppm] = −0.49 (s, 1H, GaCH), −0.39 (s, 1H, GaCH), 0.42–0.56 (m, 36H, Si(CH₃)₃), 0.85 (d, ³J_P,H = 13.9 Hz, 9H, C(CH₃)₃), 0.90 (d, ³J_P,H = 14.0 Hz, 9H, C(CH₃)₃), 1.32 (ddd, J = 14.8, 3.7, 1.8 Hz, PCH₂), 1.36 (dd, J = 5.7, 2.7 Hz, CH₃), 1.63 (ddd, J = 14.8, 10.8, 8.8 Hz, PCH₂), 4.35 (m, 1H, OCH). ¹³C{¹H} NMR (126 MHz, C₆D₆): δ [ppm] = 4.8 (s, overlapped, from ¹H–¹³C HSQC experiment, GaCH), 4.9 (s, Si(CH₃)₃), 5.2 (s, Si(CH₃)₃), 5.6 (s, Si(CH₃)₃), 5.6 (s, overlapped, from ¹H–¹³C HSQC experiment, GaCH), 26.2 (s, C(CH₃)₃), 26.5 (s, C(CH₃)₃), 29.4 (d, ³J_P,C = 13.6 Hz, CH₃), 30.0 (d, ¹J_P,C = 54.5 Hz, PCH₂), 35.1 (d, ¹J_P,C = 57.7 Hz, C(CH₃)₃), 35.7 (d, ¹J_P,C = 59.0 Hz, C(CH₃)₃), 65.6 (d, ²J_P,C = 5.9 Hz, OCH). ²⁹Si{¹H} NMR (99 MHz, C₆D₆): δ [ppm] = −1.7 (s), −1.1 (s), −0.7 (s), −0.2 (s). ³¹P{¹H} NMR (202 MHz, C₆D₆): δ [ppm] = 73.0 (s). Elemental analysis calcd (%) for C₂₅H₆₂GaO₂PSi₄ (M_r = 607.81): C 49.40, H 10.28; found C 49.14, H 10.53.

Bis₂Ga–O–P^tBu₂·PhC≡CH (9). Using phenylacetylene (10 μL, 9.3 mg, 91 μmol, 1.0 equiv.) and stirring at 70 °C, 9a was recrystallized from n-hexane at −18 °C, giving colourless needles of the deprotonation product (38 mg, 59 μmol, 64%). ¹H NMR (500 MHz, C₆D₆): δ [ppm] = −0.31 (s, 2H, GaCH), 0.46 (s, 36H, Si(CH₃)₃), 0.95 (d, ³J_P,H = 15.7 Hz, 18H, C(CH₃)₃), 5.98 (d, ¹J_P,H = 440.1 Hz, 1H, PH), 6.98 (t, ³J_H,H = 7.4 Hz, 1H, para-H), 7.05 (t, ³J_H,H = 7.3 Hz, 2H, meta-H), 7.62 (d, ³J_H,H = 7.8 Hz, 2H, ortho-H). ¹³C{¹H} NMR (126 MHz, C₆D₆): δ [ppm] = 4.6 (s, Si(CH₃)₃), 10.0 (s, GaCH), 26.0 (s, C(CH₃)₃), 33.8 (d, ¹J_P,C = 58.4 Hz, C(CH₃)₃), 109.7 (s, PhC≡CGa), 126.1 (s, PhC≡CGa), 127.6 (s, para-C), 128.4 (s, ipso-C), 128.6 (s, meta-C), 131.6 (s, ortho-C). ²⁹Si{¹H} NMR (99 MHz, C₆D₆): δ [ppm] = −0.9 (s). ³¹P{¹H} NMR (202 MHz, C₆D₆): δ [ppm] = 67.3 (s). Elemental analysis calcd (%) for C₃₀H₆₂GaOPSi₄ (M_r = 651.86): C 55.28, H 9.59; found C 54.61, H 9.88.

Bis₂Ga–O–P^tBu₂·Me₃SiC≡CH (10). Using trimethylsilylacetylene (10 μL, 7.1 mg, 72 μmol, 1.0 equiv.), stirring at 70 °C and after an additional washing step with n-hexane, 10 was obtained in a colourless resin-like form (41 mg, 63 μmol, 92%). Deprotonation product (10a): ¹H NMR (500 MHz, C₆D₆): δ [ppm] = −0.50 (s, 2H, GaCH), 0.27 (s, 9H, Si(CH₃)₃), 0.46 (s, 36H, CH(Si(CH₃)₃)₂), 0.96 (d, ³J_P,H = 15.6 Hz, 18H, C(CH₃)₃), 5.97 (d, ¹J_P,H = 442.2 Hz, 1H, PH). ¹³C{¹H} NMR (126 MHz, C₆D₆): δ [ppm] = 0.3 (s, Si(CH₃)₃), 4.8 (s, Si(CH₃)₃), 8.2 (s, GaCH), 26.2 (s, C(CH₃)₃), 33.8 (d, ¹J_P,C = 58.3 Hz, C(CH₃)₃), 115.7 (s, Me₃SiC≡CGa), 134.8 (s, Me₃SiC≡CGa). ²⁹Si{¹H} NMR (99 MHz, C₆D₆): δ [ppm] = −22.9 (s), −0.7 (s). ³¹P{¹H} NMR (202 MHz, C₆D₆): δ [ppm] = 66.9 (s). Elemental analysis calcd (%) for C₂₇H₆₆GaOPSi₅ (M_r = 647.95): C 50.05, H 10.27; found C 49.81, H 10.37. Ring-closure product (10b): ¹H NMR (500 MHz, C₆D₆): δ [ppm] = −0.50 (br. s, 2H, GaCH), 0.18 (s, 9H, Si(CH₃)₃), 0.38–0.43 (s, 36H, CH(Si(CH₃)₃)₂), 1.02 (d, ³J_P,H = 14.0 Hz, 18H, C(CH₃)₃), 9.25 (d, ³J_P,H = 56.0 Hz, 1H, PCH). ¹³C{¹H} NMR (126 MHz, C₆D₆): δ [ppm] = 2.0 (s, Si(CH₃)₃), 5.1/5.4 (s, Si(CH₃)₃), 27.5 (s, C(CH₃)₃), 35.1 (d, ¹J_P,C = 56.3 Hz, C(CH₃)₃), 139.9 (d, ¹J_P,C = 41.4 Hz, GaC=CP), 206.7 (d, ²J_P,C = 5.9 Hz, GaC=CP); a signal for GaCH could not be observed. ²⁹Si{¹H} NMR (99 MHz, C₆D₆): δ [ppm] = −8.2 (s), −1.6 (s). ³¹P{¹H} NMR (202 MHz, C₆D₆): δ [ppm] = 84.8 (s).

Bis₂(Cl)Ga–O–P(Bn)^tBu₂ (11). Using benzyl chloride (10 μL, 9.1 mg, 72 μmol, 1.0 equiv.), stirring at 70 °C and after an additional washing step with n-hexane, 11 was obtained as a light yellow solid (44 mg, 65 μmol, 94%). ¹H NMR (500 MHz, C₆D₆): δ [ppm] = −0.06 (s, 2H, GaCH), 0.36 (s, 36H, Si(CH₃)₃), 1.06 (d, ³J_P,H = 13.6 Hz, 18H, C(CH₃)₃), 3.40 (d, ³J_P,H = 14.0 Hz, 2H, CH₂), 7.00 (t, ³J_H,H = 7.3 Hz, 1H, para-H), 7.05 (t, ³J_H,H = 7.6 Hz, 2H, meta-H), 7.39 (d, ³J_H,H = 7.9 Hz, 2H, ortho-H). ¹³C{¹H} NMR (126 MHz, C₆D₆): δ [ppm] = 4.3 (s, Si(CH₃)₃), 15.8 (s, GaCH), 27.5 (s, C(CH₃)₃), 31.2 (d, ¹J_P,C = 48.4 Hz, CH₂), 36.7 (d, ¹J_P,C = 56.3 Hz, C(CH₃)₃), 127.0 (d, ⁴J_P,C = 2.5 Hz, para-C), 128.7 (s, meta-C), 130.8 (d, ³J_P,C = 5.0 Hz, ortho-C), 134.3 (d, ²J_P,C = 6.0 Hz, ipso-C). ²⁹Si{¹H} NMR (99 MHz, C₆D₆): δ [ppm] = −1.3 (s). ³¹P{¹H} NMR (202 MHz, C₆D₆): δ [ppm] = 63.6 (s). Elemental analysis calcd (%) for C₂₉H₆₃GaOPSi₄ (M_r = 676.31): C 51.50, H 9.39; found C 51.67, H 9.59.

Author contributions

J. Buth: investigation, methodology, validation, visualization, and writing (original draft); B. Neumann, J.-H. Lamm and H.-G. Stammler: investigation (SCXRD); Y. V. Vishnevskiy (quantum chemical calculations) and N. W. Mitzel: funding acquisition, project administration, supervision, writing, reviewing and editing.

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 (SI): experimental details, NMR spectra, crystallographic details, quantum chemical calculations (coordinates provided in a separate file), and references. See DOI: https://doi.org/10.1039/d6qi00380j.

CCDC 2528994–2529001 (for compounds 1, 2, 7, 8, 9a, 10a and 11) contain the supplementary crystallographic data for this paper.^41a–h

Acknowledgements

The authors thank Marco Wißbrock and Dr Andreas Mix for recording NMR spectra, Barbara Teichner for performing elemental analyses and Hannah Koch for providing support with syntheses.

This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation, grant MI 477/44-1, project no. 461833739 and grant VI 713/3-1, project no. 243500032). We acknowledge support by the Paderborn Center for Parallel Computing (PC2, HPC system Noctua 2) and by the Regional Computing Centre of the University of Cologne (RRZK, HPC system RAMSES) for providing computing time.

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