Reactions of an Isolable Dialkylgermylene with Acyl Chlorides

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Cite This: Organometallics XXXX, XXX, XXX−XXX

Reactions of an Isolable Dialkylgermylene with Acyl Chlorides Forming Acyl(chloro)germanes and Diacylgermanes Huaiyuan Zhu, Ningka Wei, Zhifang Li,* Qian Yang, Xu-Qiong Xiao, Guoqiao Lai, and Mitsuo Kira* Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou 311121, Zhejiang, People’s Republic of China

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S Supporting Information *

ABSTRACT: The reactions of isolable dialkylgermylene 1 with benzoyl and substituted benzoyl chlorides afford the corresponding aroyl(chloro)germanes in high yields. While 2,2-dimethylpropanoyl chloride reacts similarly, the reactions of more reactive alkanoyl chlorides such as acetyl, propanoyl, and butanoyl chlorides give rather unexpectedly the corresponding dialkanoylgermanes 3 together with alkanoyl(chloro)germanes 2 (2:3 = 4:1). Aroyl- and alkanoyl(chloro)germanes 2a−2g and dialkanoylgermanes 3e−3g obtained were fully characterized by multinuclear NMR spectroscopy, highresolution mass spectrometry (HRMS), and by single-crystal X-ray diffraction studies for 2a and 3f. UV−vis spectra of 3e−3g and a TDDFT study of the model diacylgermanes showed two separated n → π* absorption bands, suggesting significant electronic interaction between the two carbonyl groups in a molecule through the central germanium atom.

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INTRODUCTION Acylsilanes (aroyl- and alkanoyl-silanes) and their germanium and tin congeners have been known as an electronically unique class of group-14 element compounds1 with remarkably redshifted n → π* transition bands due to enhanced σ−n conjugation,2 being widely used as distinct reagents in organic synthesis. Polyacylgermanes have been of great recent interest, because of their promising advantages as photoinitiators of free-radical polymerization, for example, in the dental filling.3 As the first acylgermane, benzoyl(triphenyl)germane was synthesized by Brook and co-workers in 1960 via the hydrolysis of the corresponding α,α-dibromobenzylgermanes (method 1).4 As more widely applicable synthetic methods of acylgermanes, a variety of synthetic methods like the hydrolysis of germyldithianes (method 2),5 the reactions of acyl chlorides, esters, amides and thioesters with germyllithiums or other germylmetallic reagents (eq 1, method 3),6−12 the reactions of digermanes with acyl chlorides in the presence of palladium catalysts (eq 2, method 4),13 and insertion of germylenes into the C(O)−Cl bond of acyl chlorides (method 5)14 have been utilized. More recently, the synthesis of polyacylgermanes that are effective initiators of the photopolymerization using visible lights, have been developed by applying the above methods. Haas et al. have reported the synthesis of tetraaroylgermanes by the reaction of Ge(SiMe3)4 with aroyl chlorides in the presence of tBuOK and an excess amount of KF (eq 3).15 Ph3GeM + RCOCl → Ph3GeCOR + MCl M = Li,K,Cu·Me2S,etc.

BuOK

Ge(SiMe3)4 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ → KGe(SiMe3)3 t −Me3SiO Bu

excess PhCOCl/KF

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ (PhCO)4 Ge

(3)

During the course of our study of the reactions of isolable silylene A16 and stannylene B17 (Chart 1) with various reagents, we have found the facile insertion of these tetrylenes into C(O)−Cl bonds of aroyl chlorides giving the corresponding aroyl(chloro)silanes and -stannanes in good yields.18 On the other hand, the reactions of tetrylenes A and B with simple alkanoyl chlorides such as acetyl and propionyl chlorides gave Chart 1. Structural Formulae of Silylene A, Stannylene B, Germylenes C and D, Dialkylgermylene 1, Acyl(chloro)germanes 2, Diacylgermanes 3, and Dichlorogermane 4

(1)

Me3GeGeMe2Ph + PhCOCl Received: January 25, 2019

Pd catalyst

⎯⎯⎯⎯⎯⎯⎯⎯⎯→ PhMe2GeCOPh + Me3GeCl © XXXX American Chemical Society

(2) A

DOI: 10.1021/acs.organomet.9b00052 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

The results of the reactions of 1 with 5e−5g are sharply different from those of dialkylsilylene A and dialkylstannylene B under similar reaction conditions, which gave a complex mixture18a and the corresponding dichlorostannane, respectively (Scheme 1).18b X-ray Diffraction Study of 2a and 3f. Molecular structures of benzoyl(chloro)germane 2a and di(propanoyl)germane 3f were determined by X-ray single-crystal diffraction analysis. Yellow single crystals of 2a and pale yellow single crystals of 3f suitable for X-ray crystallography were obtained by slowly evaporating the solvent from their hexane and pentane solutions, respectively. The ORTEP drawing of 2a and 3f are depicted in Figures 1 and 2, respectively. Compound 2a was crystallized in space group P2(1)/c with two crystallographically independent molecules in an asymmetric unit. The structural parameters of the two molecules in a unit cell are similar, and hence, only the structural parameters of the molecule including Ge(1) are shown in the caption of Figure 1. The sum of bond angles around C(17) and C(40) are 360°, being in accord with the sp2 character of the carbonyl carbon atom. The distance of Ge(1)−C(17) (2.018(2) Å) and Ge(2)−C(40) bonds (2.014(2) Å), are significantly longer than those of Ge(1)−C(1) and Ge(1)−C(4) bonds (1.975(2) Å) and reported normal Ge−C bond length (1.92−1.97 Å). Similar long distance of the Ge−C(O) bond has been observed for the (ferrocenyl)acetylgermanes (2.016 Å),21 acetyltriphenylgermane (2.011 Å),22 and tetraacylgermane (2.011−2.050 Å).15 The origin may be majorly ascribed to the effective α-effect or π(CO)−σ*(GeC) mixing. Compound 3f crystallizes in the P2(1)/n space group. The two alkyl carbons and two propanoyl carbons around the germanium atom are arranged tetrahedrally. The Ge−C(O) bonds of 3f are even longer than those of 2a with 2.0250(19) Å (Ge1−C17) and 2.0428(19) Å (Ge1−C20). The CO distance (C17−O1 = 1.215(3) Å, C20−O2 = 1.200(2) Å) is not very different from those of usual ketones (1.21 Å). The dihedral angles of C(1)−Ge(1)−C(17)−O(1) and C(1)− Ge(1)−C(20)−O(2) are 43.43 and 63.67°. UV−Vis Spectral Analysis. The UV−vis absorption spectra of acyl(chloro)germanes 2a−2g and diacylgermanes 3e−3g in the region of 300−500 nm are depicted in Figures 3 and 4, respectively, and the spectral data are shown in Table 1. Alkanoyl(chloro)germanes 2d−2g show the absorption bands with vibrational fine structures with the maxima (λmax) of around 360 nm (εmax = ∼200). The maxima are comparable with those of reported alkanoylgermanes (360−460 nm)15,23,24 and assignable to their n(O) → π*(CO) transition, while they are significantly red-shifted from those of the normal ketones (λmax = ∼280 nm, εmax = ∼20). Similarly, the bands with the maxima at around 430 nm of benzoylgermanes 2a−2c are also assigned as their n(O) → π*(CO) transition, while the bands are ca. 50−80 nm red-shifted from those of 2e−2g, because of the lower-lying carbonyl π*orbitals of 2a−2c. It is noteworthy that the λmax values of 2,2-dimethylpropanoylgermane 2d and aroylgermane 2c with p-trifluoromethylphenyl substituent are ca. 10 nm red-shifted compared to those of 2e− 2g and those of 2a and 2b, respectively. Interestingly, 3e−3g show two separated bands with the maxima at around 350 and 400 nm in the n(O) → π*(CO) absorption region (Figure 4), suggesting significant interaction between two n(O) and/or two π*(CO) orbitals through Ge−C(O) σ bonds.

quite different results from each other. The former gave a complex mixture, while the latter the corresponding dichlorostannane; no insertion products were detected. Though Lappert et al.14a and Jutzi et al.14b have found the facile insertion of germylenes C and D, respectively, into acyl chlorides giving the corresponding acyl(chloro)germanes, knowledge of these types of reactions is still limited. We have studied herein the reactions of isolable dialkylgermylene 119 (Chart 1) with various acyl chlorides in detail. Germylene 1 inserts into the C(O)−Cl bonds of benzoyl, substituted benzoyl, and 2,2-dimethylpropanoyl chlorides providing exclusively the corresponding acyl(chloro)germanes 2, similarly to the corresponding dialkylsilylene A and dialkylstannylene B.18 In contrast, 1 reacts with simple alkanoyl chlorides giving rather unusually a significant amount of the corresponding diacylgermanes 3, in addition to the corresponding acyl(chloro)germanes 2 as major products. Significant interaction of the two carbonyl groups in the diacylgermanes through Ge−C(O) σ bonds is discussed on the basis of the results of their UV−vis spectra and TDDFT calculations.

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RESULTS AND DISCUSSION When dialkylgermylene 1 was treated with benzoyl and 4substituted benzoyl chlorides 5a−5c (1:1 mol ratio) in THF at room temperature, the corresponding benzoyl(chloro)germanes 2a−2c were obtained in high yields, indicating that the C−Cl bond is much more reactive than the carbonyl group toward the germylene (eq 4). No significant difference was

observed in the reactivity among benzoyl chlorides 5a−5c. Similarly, 2,2-dimethylpropanoyl chloride 5d readily furnished the corresponding 1:1 adduct 2d in 86% isolated yield (eq 5). In contrast to the reactions of germylene 1 with 5a−5d, 1 reacted with simple alkanoyl chlorides such as acetyl (5e), propanoyl (5f) and butanoyl chlorides (5g) (mole ratio of 1:5 = 1:5) to give the corresponding acyl(chloro)germanes 2e−2g, dialkyldiacylgermanes 3e−3g, and dialkyldichlorogermane 4 in a ratio of ca. 4:1:1 (eq 6).20 Products 2a−2g and 3e−3g were fully characterized by NMR and HRMS.

B


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Organometallics Scheme 1. Reactions of Dialkylsilylene A and Dialkylstannylene B with Simple Alkanoyl Chlorides

Figure 1. Molecular structure of 2a. (Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at the 30% probability level.) Selected bond lengths (Å), bond angles (deg), and a dihedral angle (deg): Ge(1)−C(1) 1.975(2), Ge(1)−C(4) 1.975(2), Ge(1)−C(17) 2.018(2), Ge(1)− Cl(1) 2.2102(8), O(1)−C(17) 1.211(3), C(17)−C(18) 1.484(3), C(1)−Ge(1)−C(4) 99.42(10), C(1)−Ge(1)−C(17) 125.99(9), C(4)− Ge(1)−C(17) 115.18(10), C(1)−Ge(1)−Cl(1) 110.41(7), C(4)−Ge(1)−Cl(1) 110.03(7), C(17)−Ge(1)−Cl(1) 95.66(8), C(18)−C(17)− Ge(1) 123.87(18), O(1)−C(17)−Ge(1) 114.83(18), O(1)−C(17)−C(18) 121.3(2), C(1)−Ge(1)−C(17)−O(1), 129.18.

Figure 2. Molecular structure of 3f. (Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at the 30% probability level.) Selected bond lengths (Å), bond angles (deg), and dihedral angles (deg): Ge(1)−C(1) 2.0013(16), Ge(1)−C(4) 2.0135(16), Ge(1)− C(17) 2.0250(19), Ge(1)−C(20) 2.0428(19), O(1)−C(17) 1.215(3), O(2)−C(20) 1.200(2), C(17)−C(18) 1.489(3), C(20)− C(21) 1.508(3), C(1)−Ge(1)−C(4) 97.29(7), C(1)−Ge(1)−C(17) 113.94(8), C(4)−Ge(1)−C(17) 122.94(8), C(1)−Ge(1)−C(20) 113.27(7), C(4)−Ge(1)−C(20) 112.68(7), C(17)−Ge(1)−C(20) 97.60(8), C(18)−C(17)−Ge(1) 123.13(17), O(1)−C(17)−Ge(1) 115.00(16), O(1)−C(17)−C(18) 121.5(2), C(21)−C(20)−Ge(1) 120.30(14), O(2)−C(20)−Ge(1) 119.26(14), O(2)−C(20)−C(21) 120.44(18), C(1)−Ge(1)−C(17)−O(1) 43.43°, C(1)−Ge(1)− C(20)−O(2) 63.67°.

Figure 3. UV−vis absorption spectra of 2a−2g (DCM solution).

6-31+G(d,p) level.25 They include 2a′, 2e′, and 3e′ as models of 2a, 2e, and 3e, acetyl(methyl)germane 6 to compare the substituent effects at germanium atom on the band maxima with those of 2e′, and molecules 7, 8, and 9 to understand the features of the 1,3-dicarbonyl interaction through a heavy group-14 atom; in all the model compounds shown in Chart 2, trihydrosilyl groups are used instead of trimethylsilyl groups in the experimentally studied compounds. Structural parameters around the germanium atom of 2a′ and 3e′ at the ground state, are good in accord with those of 2a and 3f determined by the X-ray analysis (Figures 1 and 2); lengths of Ge−C(O), Ge−C(ring), and C−O bonds of 2a′ are 2.002, 1.969 (av), and 1.226 Å, respectively, and those of 3e′ are 2.030, 1.986, and 1.219 Å. The dihedral angle C1−Ge−

TDDFT Study. To elucidate the nature of the split n(O) → π*(CO) transition of diacylgermanes 2e−2g, the DFT and the time-dependent DFT (TDDFT) calculations of the molecules shown in Chart 2 were performed at the B3LYP/ C


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type transitions, respectively (Table S2). The calculated band separation (Δν) between the two bands of 3e′ is 0.18 eV (1450 cm−1), which is comparable to that of experimentally observed Δν of 3e (2510 cm−1). The corresponding splitting values [Δν] for 7 and 8 are 1270 and 3780 cm−1; the Δν values are in the order E = Si > Ge > C. While the Δν values for the dialkanoylgermane and -silane is large enough to observe the separated n → π* bands, the separation for dibenzoylgermane 9 (Δν = 81 cm−1) is too small to observe the two separated bands as shown in Table S4; actually, polybenzoylgermanes are reported to show broad overlapped n → π* bands at >430 nm.23,24 The lowest n → π* transition energies of 3e′ and 8 are significantly smaller than that of compound 7, but the energies are not monotonously decrease from carbon to silicon to germanium; the transition energy of germane 3e′ is even higher than that of silane 8. Kohn−Sham FMOs (frontier molecular orbitals) of 3e′ and energy levels of the FMOs of 3e′, 7, and 8 are shown in Figure 3. The FMO levels of 3e′, 7, and 8 are in the order nS (HOMO−1) < nA(HOMO) < π*S(LUMO) < π*A(LUMO− 1) regardless of the central atom (Figure 5 (b)). However, not only the separation between nA (HOMO) and π*S (LUMO) (ΔεHL) but those between nS and nA (Δεn) and between πS* and πA* orbitals (Δεπ*) depend strongly on the central atom. The ΔεHL values of 5.38, 4.30, and 4.57 eV for 7, 8, and 3e′ (Table 1) are parallel to the calculated lowest transition energies of 7 (4.08 eV), 8 (3.04 eV), and 3e (3.31 eV) (Table S4). Pertinent σ and σ* orbitals of the E−C(O) and E− C(ring) bonds may interact with the n and π* orbitals. Thus, nonbonding nS and nA orbitals may be raised possibly by the interaction with the σS[Ge−C(O)] and σA[Ge−C(O)] orbital, respectively and lowered by less important nS−σ*S and nA−σ*A interaction; the n−σ interaction is the same with that proposed first by Ramsey, Brook, Bassindale, and Bock.2 The nA orbital is higher than the nS orbital, indicating that the nA−σA interaction is more effective than the nS−σS interaction. The nA energies and Δεn values increase in the order 7 < 8 < 3e′ would mean that the n−σ interaction is more effective in the order Ge > Si > C. On the other hand, π*S and π*A orbital levels would be lowered by the π*−σ* (E−C) interaction and raised by the less important π*−σ (E−C) interaction. The fact that the πS* level is lowered and the Δεπ* values increase in the order 8 > 7 > 3e′ would be compatible with the well-known π*−σ*(E−C) interaction or the α-effect, which is known to be larger in the order Si > Ge ∼ C.26 Two types of interactions, n−σ and π*−σ* may explain the unusual features of the calculated FMO levels and the n−π* transition bands of dialkanoylmethanes, -silanes, and -germanes. Reaction Mechanisms. Because the reactions of 1 with 5a−5d smoothly proceed and give single insertion products 2a−2d almost exclusively, the reaction mechanism will be simple and probably the concerted insertion of the germylene toward the C(O)−Cl bonds of the acyl chlorides shown in Scheme 2, as suggested for the insertion reactions of the related silylene and stannylene with aroyl chlorides.18 Alternatively, 2a−2d may be formed by the chlorineabstraction of 1 from acyl chlorides giving germyl radical-acyl radical pair followed by facile coupling in cage. Occurrence of the two mechanisms has been well discussed by Kocher, Lehnig, and Neumann.27 The reactions of 1 with alkanoyl chlorides 5e−5f give not only the insertion product 2 but also a significant amount of diacylgermane 3 and dichlorogermane 4, indicating no

Figure 4. UV−vis absorption spectra of 3e−3g (DCM solution).

Table 1. Absorption Maxima and Their Molar Absorptivities of Acyl(chloro)germanes 2a−2g and Diacylgermanes 3e−3g n → π* band 1 compound

λmax (nm)

2a 2b 2c 2d 2e 2f 2g 3e 3f 3g

406 402 416 368 358 358 362 356 354 356

−1

ε (M

n → π* band 2 −1

cm )

190 200 200 210 250 230 220 300 320 350

λmax (nm)

ε (M−1 cm−1)

− − − − − − − 391 388 392

300 340 370

Chart 2. Theoretically Investigated Acylgermanes and Related Compounds and Their Atom Labelling Scheme

C3−O4 of 2a′ is 107.1, which is comparable to those of 2a (125° (av)). The optimized structures of 3e′, 7, and 8 have the C2 symmetry around the central group-14 atom with the C1− E−C3−O4 dihedral angles of 115.5, 103.5, and 136.4°, respectively, while 3f in the crystal is unsymmetrical. Theoretical n → π* transition maxima of 2a′ (406 nm) and 2e′ (356 nm) are in excellent agreement with those of 2a (406 nm) and 2e (358 nm) shown in Table 1, suggesting that the n → π* transition energies theoretically calculated are reliable and not sensitive to the dihedral angle C1−Ge−C3−O4. Comparison of the n → π* transition band maxima between 2e (X = Cl) and 6 (X = Me) suggests no significant effects of the substituent at the germanium atom on the bands. Two band maxima of 3e′ (356 and 372 nm) are roughly in accord with those of 3e (356 and 391 nm; Table 1), indicating that the observed two bands are assignable to the two n → π* transition bands with A and B irreducible representations, i.e., nA → π*S (and nS → π*A) type and nA → π*A (and nS → π*S) D


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Scheme 3. Possible Radical Pathways for the Reactions of 1 with Alkanoyl Chlorides 5e−5g

radical may either couple with acyl radical in cage giving 2a− 2g or escape to encounter with acyl chloride out of cage, giving dichlorogermane 4 and the acyl radical. (d) The acyl radical may add to germylene 1 affording an acylgermyl radical, which further reacts with another acyl radical giving 3e−3g. The reaction pathways may be feasible if the coupling reaction (b) is comparable or slower than the reaction (c). The above mechanism seems acceptable, though no direct evidence for the generation of acyl or germyl radicals during the reactions has been obtained to date.30 Because M−Cl (M = Si, Ge, Sn) bond dissociation energies (BDE) are larger than the BDE of C−Cl bond,32 the reactions of silylene A and stannylene B (Chart 1) with alkanoyl chlorides18 could also proceed in a similar manner shown in Scheme 3. Formation of a complex mixture from silylene A (Scheme 1) suggests the intermediary chlorosilyl radical should undergo the predominant addition to carbonyl oxygens due to its higher oxophilicity than that of the germanium and tin congeners. Exclusive dichlorostannane formation from stannylene B (Scheme 1) may occur if a type (c) reaction is much faster than a type (b) reaction in the stannylene case. While the apparent differences of the reaction profile among the silylene, germylene, and stannylene are interesting, further works will be required to discuss the origin. In conclusion, the reactions of isolable dialkylgermylene 1 with 2,2-dimethylpropanoyl, benzoyl, and substituted benzoyl chlorides afford the corresponding acyl(chloro)germanes in good yields. Similar reactions of 1 with simple alkanoyl chlorides give rather unusually a significant amount of the corresponding dialkanoylgermanes 3 together with the formal C(O)−Cl insertion products 2. The mechanisms for the

Figure 5. (a) Kohn−Sham frontier MOs of 3e′. (b) Comparison of the energy levels of the FMOs of 7, 8, and 3e′.

concerted mechanism is applicable. Though the nature of the reactions is not straightforward, the following radical pathways are tentatively proposed (Scheme 3), in line with the consideration of Neumann et al. for the reactions of dimethylgermylene with alkyl halides:27 (a) Germylene 1 abstracts chlorine atom from an acyl chloride to give a chlorogermyl-acyl radical pair. (b, c) Resulting chlorogermyl

Scheme 2. Possible Mechanisms for the Reactions of 1 with Aroyl and 2,2-Dimethylpropanoyl Chlorides

E


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237.9 (CO), 50.1 (CH3Cq), 33.4 (ring Cq), 27.2 (CqCH3), 19.5 (ring CH2), 4.1 (SiCH3), 3.3 (SiCH3); 29Si NMR (99 MHz, CDCl3) δ 5.8, 3.0. HRMS m/z [M + Na]+ calcd for C21H49ClGeNaOSi4 561.1658, found 561.1639. Reactions of 1 with Alkanoyl Chlorides (5e−5g). A THF solution of a HCl-free acyl chloride (1.20 mmol) was added to a solution of dialkylgermylene 1 (0.1 g, 0.24 mmol) in THF at room temperature. The color of the solution changed from deep orange to pale yellow quickly, then turned to colorless. The reaction mixture was allowed to stir for 2 h, and then the solvent was removed under vacuum. The resulting residue was purified by preparative silica-gel column using petroleum ether and ethyl acetate (20:1) as the eluent, giving the corresponding acyl(chloro)germane (2), diacylgermane (3), and dichlorogermane as white solids. 2e, White solid (59 mg, 47%); mp 69−70 °C; 1H NMR (400 MHz, CDCl3) δ 2.65 (s, 3H), 2.03−2.18 (m, 4H), 0.23 (s, 18H), 0.18 (s, 18H); 13C NMR (101 MHz, CDCl3) δ 232.5 (CO), 33.3 (ring Cq), 29.7 (C(O)CH3), 19.6 (ring CH2), 3.8 (SiCH3), 3.5 (SiCH3); 29Si NMR (99 MHz, CDCl3) δ 5.72, 3.28. HRMS m/z [M + Na]+ calcd for C18H43ClGeNaOSi4 519.1189, found 519.1170. 3e, Pale yellow solid (17 mg, 13%); mp 67−68 °C; 1H NMR (400 MHz, CDCl3) δ 2.45 (s, 6H), 2.00 (s, 4H), 0.19 (s, 36H); 13C NMR (101 MHz, CDCl3) δ 241.7 (CO), 38.3 (C(O)CH3), 34.6 (ring Cq), 16.8 (ring CH2), 3.9 (SiCH3); 29 Si NMR (99 MHz, CDCl3) δ 4.47. HRMS m/z [M + Na]+ calcd for C20H46GeNaO2Si4 527.1684, found 527.1682. 2f, White solid (64 mg, 50%); mp 90−92 °C; 1H NMR (400 MHz, CDCl3) δ 3.11−3.17 (q, J = 7.2 Hz, 2H), 2.06−2.15 (m, 4H), 1.08 (t, J = 7.2 Hz, 3H), 0.23 (s, 18H), 0.17 (s, 18H); 13 C NMR (101 MHz, CDCl3) δ 234.5 (CO), 41.3 (C(O) CH2), 33.3 (ring Cq), 19.7 (ring CH2), 7.3 (CCH3), 3.7 (SiCH3), 3.6 (SiCH3); 29Si NMR (99 MHz, CDCl3) δ 5.68, 3.29. HRMS m/z [M + Na]+ calcd for C19H45ClGeNaOSi4 533.1345, found 533.1335. 3f, Pale yellow solid (18 mg, 14%); mp 97−99 °C; 1H NMR (400 MHz, CDCl3) δ 2.76−2.82 (q, J = 6.8 Hz, 4H), 1.99 (s, 4H), 0.99 (t, J = 7.2 Hz, 6H), 0.18 (s, 36H); 13C NMR (101 MHz, CDCl3) δ 242.8 (CO), 44.4 (C(O)CH2), 34.7 (ring Cq), 16.4 (ring CH2), 7.3 (CCH3), 3.8 (SiCH3); 29Si NMR (99 MHz, CDCl3) δ 4.46. HRMS m/z [M + Na]+ calcd for C22H50GeNaO2Si4 555.1997, found 555.2019. 2g, White solid (60 mg, 46%); mp 78−80 °C; 1H NMR (400 MHz, CDCl3) δ 3.10 (t, J = 7.2 Hz, 2H), 2.03−2.17 (m, 4H), 1.63 (sext, J = 7.2 Hz, 2H), 0.95 (t, J = 7.2 Hz, 3H), 0.23 (s, 18H), 0.17 (s, 18H); 13C NMR (101 MHz, CDCl3) δ 232.6 (CO), 48.7 (C(O)CH2), 32.0 (ring Cq), 18.6 (ring CH2), 15.3 (CH3CH2), 12.4 (CCH3), 2.5 (SiCH3), 2.3 (SiCH3); 29Si NMR (99 MHz, CDCl3) δ 5.64, 3.28. HRMS m/z [M + Na]+ calcd forC20H47ClGeNaOSi4 547.1502, found 547.1490. 3g, Pale yellow solid (10 mg, 12%); mp 84−85 °C; 1H NMR (400 MHz, CDCl3) δ 2.76 (t, J = 7.2 Hz, 4H), 1.98 (s, 4H), 1.54 (sext, J = 7.2 Hz, 4H), 0.88 (t, J = 7.2 Hz, 6H), 0.18 (s, 36H); 13C NMR (101 MHz, CDCl3) δ 242.3 (CO), 53.1 (C(O)CH2), 34.7 (ring Cq), 30.9 (CH3CH2), 16.2 (ring CH2), 13.5 (CCH3), 3.9 (SiCH3); 29Si NMR (99 MHz, CDCl3) δ 4.5. HRMS m/z [M + Na]+ calcd for C24H54GeNaO2Si4 583.2310, found 583.2341. X-ray Structure Determination. Single crystals of 2a and 3f suitable for X-ray analysis were obtained by the recrystallization from hexane and dichloromethane. The Xray diffraction data were collected on a Bruker Smart Apex

formation of dialkanoylgermanes are difficult to explain, but a free-radical mechanism is proposed. Significant electronic interaction between two carbonyl groups in the dialkanoylgermanes is evidenced by their UV−vis spectra and the TDDFT calculations. The theoretical calculations suggest that the separation of the n−π* transition bands in the dialkanoylsilanes are much larger than that of the corresponding dialkanoylgermane.

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EXPERIMENTAL PROCEDURES Air-sensitive compounds were manipulated in a controlled dry argon atmosphere using standard Schlenk techniques. THF was distilled from sodium−benzophenone. All the other reagents were obtained from commercial suppliers and used without further purification. Dialkylgermylene 1 was prepared according to literature procedures.19 Germylene 1 and other air-sensitive materials were handled in an MBraun glovebox. 1 H (400 MHz), 13C (101 MHz), and 29Si (99 MHz), NMR spectra were recorded on a BRUKER AV-400 MHz instrument using Me4Si (1H, 13C, and 29Si) as external standards. Highresolution MS was measured on a Thermo Scientific LTQ Orbitrap XL spectrometer. UV−vis absorption spectra were recorded on a Shimadzu UV-2550 UV−vis Spectrophotometer. The purity of all new compounds (2a−2g, 3e−3g) was confirmed by their 1H, 13C, and 29Si NMR spectra, which are provided in the Supporting Information (SI). Reactions of 1 with Aroyl (5a−5c) and 2,2-Dimethylpropanoyl Chlorides (5d). An acyl chloride (0.28 mmol) was added to a THF solution of dialkylgermylene 1 (0.1 g, 0.24 mmol) at room temperature. The color of the solution changed from deep orange to pale yellow quickly. The reaction mixture was allowed to stir for 2−3 h, and then the solvent was removed under vacuum. Recrystallization from hexane at −20 °C gave the corresponding acyl(chloro)germane as crystals. 2a, Yellow solid (116 mg, 85%); mp 153−155 °C; 1H NMR (400 MHz, CDCl3) δ 8.08 (d, J = 7.2 Hz, 2H), 7.60 (t, J = 7.2 Hz, 1H), 7.52 (t, J = 7.6 Hz, 2H), 2.11−2.22 (m, 4H), 0.30 (s, 18H), 0.17 (s, 18H); 13C NMR (101 MHz, CDCl3) δ 221.7 (CO), 137.4 (Ar), 133.9 (Ar), 130.1 (Ar), 128.5 (Ar), 33.6 (ring Cq), 18.6 (ring CH2), 3.9 (SiCH3), 3.1 (SiCH3); 29Si NMR (99 MHz, CDCl3) δ 5.8, 3.4. HRMS m/z [M + Na]+ calcd for C23H45ClGeNaOSi4 581.1345, found 581.1343. 2b, Yellow solid (115 mg, 83%); mp 151−153 °C; 1H NMR (400 MHz, CDCl3) δ 7.97 (d, J = 8.4 Hz, 2H), 7.32 (d, J = 8.0 Hz, 2H), 2.44 (s, 3H), 2.09−2.23 (m, 4H), 0.30 (s, 18H), 0.17 (s, 18H); 13C NMR (101 MHz, CDCl3) δ 220.8 (CO), 144.9 (Ar), 135.0 (Ar), 130.3 (Ar), 129.2 (Ar), 33.6 (ring Cq), 21.8 (PhCH3), 18.5 (ring CH2), 3.9 (SiCH3), 3.1 (SiCH3); 29Si NMR (99 MHz, CDCl3) δ 5.9, 3.4. HRMS m/z [M + H]+ calcd for C24H48ClGeOSi4 573.1683, found 573.1687. 2c, Yellow solid (137 mg, 88%); mp 156−157 °C; 1H NMR (400 MHz, CDCl3) δ 8.19 (d, J = 8.4 Hz, 2H), 7.79 (d, J = 8.4 Hz, 2H), 2.13−2.23 (m, 4H), 0.30 (s, 18H), 0.17 (s, 18H); 13 C NMR(101 MHz, CDCl3) δ 221.7 (CO), 139.6 (Ar), 135.0 (Ar), 130.2 (Ar), 125.7 (Ar), 122.1 (CF3), 33.6 (ring Cq), 18.9 (ring CH2), 3.9 (SiCH3), 3.1 (SiCH3); 29Si NMR (99 MHz, CDCl3) δ 6.0, 3.5. HRMS m/z [M + Na]+ calcd for C24H44ClF3GeNaOSi4 649.1219, found 649.1235. 2d, Pale yellow (116 mg, 86%); mp 139−140 °C; 1H NMR (400 MHz, CDCl3) δ 2.08 (d, J = 1.6 Hz,4H), 1.38 (s, 9H), 0.22 (s, 18H), 0.20 (s, 18H); 13C NMR (101 MHz, CDCl3) δ F


Article

Organometallics

-germanes by photoelectron spectroscopy. J. Organomet. Chem. 1974, 74, C41−C45. (3) (a) Neshchadin, D.; Rosspeintner, A. Acylgermanes: Photoinitiators and sources for Ge-centered radicals. insights into their reactivity. J. Am. Chem. Soc. 2013, 135, 17314−17321. (b) Lalevée, J.; Allonas, X.; Fouassier, J. P. Acylgermanes: Excited state processes and reactivity. Chem. Phys. Lett. 2009, 469, 298−303. (c) Ganster, B.; Fischer, U. K.; Noszner, N.; Liska, R. New photocleavable structures. Diacylgermane-based photoinitiators for visible light curing. Macromolecules 2008, 41, 2394−2400. (d) Moszner, N.; Zeuner, F.; Lamparth, I.; Fischer, U. K. Benzoylgermanium derivatives as novel visible-light photoinitiators for dental composites. Macromol. Mater. Eng. 2009, 294, 877−886. (e) Barner-Kowollik, C. A priori prediction of mass spectrometric product patterns of photoinitiated polymerizatioons. ACS Macro Lett. 2018, 7, 132−136. (4) Brook, A. G.; Quigley, M. A.; Peddle, G. J. D.; Schwartz, N. V.; Warnre, C. M. The spectral and chemical properties of α -silyl ketones. J. Am. Chem. Soc. 1960, 82, 5102−5106. (5) (a) Brook, A. G.; Duff, J. M.; Jones, P. F.; Davis, N. R. Synthesis of silyl and germyl ketones. J. Am. Chem. Soc. 1967, 89, 431−434. (b) Corey, E. J.; Seebach, D. Synthesis of 1, n-dicarbonyl derivates using carbanions from 1,3-dithianes. Angew. Chem., Int. Ed. Engl. 1965, 4, 1077−1078. (6) Iserloh, U.; Curran, D. P. Radical cyclizations of acylgermane oxime ethers and hydrazones: direct routes to cyclic hydrazones and oximes. J. Org. Chem. 1998, 63, 4711−4716. (7) Piers, E.; Lemieux, R. Reaction of (trimethylgermyl)copper (I)dimethyl sulfide with acyl chlorides: efficient syntheses of functionalized acyltrimethylgermanes. Organometallics 1995, 14, 5011−5012. (8) Nanjo, M.; Matsudo, K.; Mochida, K. Reactivities of triethylgermylborate in methanol. Chem. Lett. 2001, 30, 1086−1087. (9) Bravo-Zhivotovskii, D. A.; Pigarev, S. D.; Kalikhman, I. D.; Vyazankina, O. A.; Vyazankin, N. S. Reactions of triethylgermyllithium with N, N-dialkylatedcarboxamides. J. Organomet. Chem. 1983, 248, 51−60. (10) Kiyooka, S.; Miyauchi, A. Facile synthesis of acylgemanes. Chem. Lett. 1985, 14, 1829−1830. (11) Castel, A.; Rivière, P.; Satgé, J.; Ko, H. Y. New (diarylgermyl)lithiums. Organometallics 1990, 9, 205−210. (12) Castel, A.; Rivière, P.; Satgé, J.; Desor, D. J. Nouveaux aryldihydrogermyllithium. J. Organomet. Chem. 1992, 433, 49−61. (13) Yamamoto, K.; Hayashi, A.; Suzuki, S.; Tsuji, J. Preparation of substituted benzoyltrimethylsilanes and -germanes by the reaction of benzoyl chlorides with hexamethyldisilane or -germane in the presence of palladium complexes as catalysts. Organometallics 1987, 6, 974−979. (14) (a) Lappert, M. F.; Misra, M. C. Subvalent group 14 metal compounds XI *. Oxidative addition reactions of organic halides or acid anhydrides (including CH4-nCln, PhBr, BrN(SiMe3)2, ButCOCl, or (CF3CO)2O) to some bivalent group 14 metal amides or alkyls. J. Organomet. Chem. 1987, 330, 31−46. (b) Jutzi, P.; Hampel, B. Electrophilic attack at pentamethylcyclopentadienyl-substituted germylenes. Organometallics 1986, 5, 730−734. (15) (a) Radebner, J.; Eibel, A.; Leypold, M.; Gorsche, C.; Schuh, L.; Fisher, R.; Torvisco, A.; Neshchdin, D.; Geier, R.; Moszner, N.; Lisja, R.; Gescheidt, G.; Hass, M.; Stueger, H. Tetraacylgermanes: highly efficient photoinitiators for visible-light-induced free-radical polymerization. Angew. Chem., Int. Ed. 2017, 56, 3103−3107. (b) Radebner, J.; Leypold, M.; Eibel, A.; Maier, J.; Schuh, L.; Torvisco, A.; Fischer, R.; Moszner, N.; Gescheidt, G.; Stueger, H.; Hass, M. Synthesis, spectroscopic behavior, and photoinduced reactivity of tetraacylgermanes. Organometallics 2017, 36, 3624−3632. (16) (a) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. The first isolable dialkylsilylene. J. Am. Chem. Soc. 1999, 121, 9722−9723. (b) Kira, M. Isolable silylene, disilenes, trisilaallene, and related compounds. J. Organomet. Chem. 2004, 689, 4475−4488. (c) Kira, M.; Ishida, S.; Iwamoto, T. Comparative chemistry of isolable divalent compounds of silicon, germanium, and tin. Chem. Rec. 2004, 4, 243− 253. (d) Kira, M.; Iwamoto, T.; Ishida, S. A Helmeted dialkylsilylene.

CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) using the ω-2θ scan mode. The structures were solved by direct methods and refined on F2 by full-matrix least-squares methods using SHELX-2000.35 Crystal and refinement data for 2a and 3f are described in the SI. The supplementary crystallographic data for 2a and 3f are deposited with CCDC; the nos. are 1870548 and 1870547, respectively. TDDFT Calculations. All DFT and TDDFT calculations were performed using the Gaussian 16 package. A B3LYP functional with 6-31+G(d,p) basis sets was used for optimization of the ground state. The vertical excitation energies into excited singlet states were calculated via TDDFT. See the Supporting Information for the calculation details and full reference of Gaussian 16.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00052. 1

H, 13C and 29Si NMR spectra of 2a−2g and 3e−3g, Xray crystallography of compounds 2a and 3f, and details of the theoretical calculations (PDF) Supporting data (XYZ) Accession Codes

CCDC 1870547−1870548 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

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

Zhifang Li: 0000-0002-9548-2158 Xu-Qiong Xiao: 0000-0001-8424-9898 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Zhejiang Province (Grant No. LY19B020007, and LY17B010002), the National Natural Science Foundation of China (Grant No. 21771048), and Collaborative Innovation Center for the Manufacture of Fluorine and Silicone Fine Chemicals and Materials (FSi2018A018).

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REFERENCES

(1) (a) Brook, A. G. In Adv. Organomet. Chem.; Stone, F. G. A., Robert, W., Eds.; Academic Press: Cambridge, MA, 1969; Vol. 7, pp 95−155. (b) Page, P. C. B.; Mckenzie, M. J.; Klair, S. S.; Rosenthal, S. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; John Wiley & Sons: Hoboken, NJ, 1998; pp 1599− 1665. (c) Page, P. C. B.; Klair, S. S.; Rosenthal, S. Synthesis and chemistry of acyl silanes. Chem. Soc. Rev. 1990, 19, 147−195. (2) Ramsey, B. G.; Brook, A.; Bassindale, A. R.; Bock, H. σ→π*, a reassignment of the long wavelength UV transition in acyl-silanes and G


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Organometallics Bull. Chem. Soc. Jpn. 2007, 80, 258−275. (e) Kira, M. An isolable dialkylsilylene and its derivatives. A step toward comprehension of heavy unsaturated bonds. Chem. Commun. 2010, 46, 2893−2903. (f) Kira, M. Reactions of a stable dialkylsilylene and their mechanisms. J. Chem. Sci. 2012, 124, 1205−1215. (17) Kira, M.; Yauchibara, R.; Hirano, R.; Kabuto, C.; Sakurai, H. Synthesis and X-ray structure of the first dicoordinate dialkylstannylene that is monomeric in the solid state. J. Am. Chem. Soc. 1991, 113, 7785−7787. (18) (a) Xiao, X.-Q.; Liu, X. P.; Lu, Q.; Li, Z. F.; Lai, G. Q.; Kira, M. Reactions of an isolable dialkylsilylene with aroyl chlorides. A new route to aroylsilanes. Molecules 2016, 21, 1376. (b) Lu, Q.; Yan, C. T.; Xiao, X.-Q.; Li, Z. F.; Kira, M. Insertion of an isolable dialkylstannylene into C-Cl bonds of acyl chlorides giving acyl(chloro)stannanes. Organometallics 2017, 36, 3633−3637. (19) (a) Kira, M.; Ishida, S.; Iwamoto, T.; Ichinohe, M.; Kabuto, C.; Ignatovich, L.; Sakurai, H. Synthesis and structure of a stable cyclic dialkylgermylene. Chem. Lett. 1999, 28, 263−264. (b) Iwamoto, T.; Masuda, H.; Ishida, S.; Kabuto, C.; Kira, M. Diverse reactions of nitroxide-radical adducts of silylene, germylene, and stannylene. J. Organomet. Chem. 2004, 689, 1337−1341. (20) The reactions of 1 with 5e in the 1:1 mol ratio gave 2e, 3e, and 4 also in a ratio of 4:1:1 as determined by 1H NMR spectroscopy, but the reaction was not completed or clean. Hence, for the preparative reactions of 1 with 5e−5g, 5 times excess amounts of 5 were used and the results are shown in eq 6. (21) Sharma, H. K.; Cerantes-Lee, F.; Pannell, K. H. Organometalloidal derivatives of the transition metals XXVII*. Chemical and structural investigations on (ferrocenylacyl) germanes. J. Organomet. Chem. 1991, 409, 321−330. (22) Harrison, W. R.; Trotter, J. The structure of acetyltriphenylgermane. J. Chem. Soc. A 1968, 258−266. (23) (a) Feuerstein, W.; Höfener, S.; Klopper, W.; Lamparth, I.; Moszner, N.; Barner-Kowollik, C.; Unterreiner, A. N. Photophysical properties of benzoylgermane and para-substituted derivatives: Substituent effects on electronic transitions. ChemPhysChem 2016, 17, 3460−3469. (b) Jöckle, P.; Schweigert, C.; Lamparth, I.; Moszner, N.; Unterreiner, A.-N.; Barner-Kowollik, C. An in-depth mechanistic investigation of the radical initiation behavior of monoacylgermanes. Macromolecules 2017, 50 (22), 8894−8906. (c) Jöckle, P.; Kamm, P. W.; Lamparth, I.; Moszner, N.; Unterreiner, A.-N.; Barner-Kowollik, C. More than expected: Overall initiation efficiencies of mono-, bis-, andtetraacylgermane radical initiators. Macromolecules 2019, 52, 281− 291. (24) (a) Moszner, N.; Zeuner, F.; Lamparth, I.; Fisher, U. K. Benzoylgermanium derivatives as novel visible-light photoinitiators for dental composites. Macromol. Mater. Eng. 2009, 294, 877−886. (b) Hass, M.; Leypold, M.; Schnalzer, D.; Torvisco, A.; Stueger, H. Stable germenolates and germenes with exocyclic structures. Organometallics 2015, 34, 5291−5297. (25) Unterreiner have recently reported TDDFT calculations of some aroylgermanes including diaroyl- and tetraaroylgermanes using B3LYP/def2-TZVpp level of theory to elucidate the electronic features of the acylgermanes as the free-radical polymerizations.23 We have confirmed our calculations using the 6-31+G(d,p) basis sets for benzoyl- and p-substituted benzoyltrimethylgermans afford almost the same absorption maxima and oscillator strengths with those calculated by Unterreiner. See SI. (26) It is known that the electron-withdrawing ability of trimethylgermyl group is smaller than that of trimethylsilyl group: Gerson, F.; Heinzer, J.; Bock, H.; Alt, H.; Seidl, H. ESR.-messungen an radikal-anionen trimethylsilyl-substituierter π-elektronensysteme. Helv. Chim. Acta 1968, 51 (4), 707−718. (27) Neumann have shown by the minor-product analysis and a CIDNP study that thermally generated transient dimethylgermylene reacts with RCl via either concerted C−Cl bond insertion or Me2GeCl·/R· radical pair depending on the characteristics of the C− Cl bond; CCl4 reacts via the two-step pathway, while allyl chloride concertedly.28 While they have shown that the dimethylgermylene

does not react with chloroalkanes having larger C−Cl BDE than that of CCl4 (73 kcal/mol), this loadstar may depend on the lifetime of the germylene. Isolable stable germylene could react with C−Cl bonds with much higher BDE. See also ref 29. (28) Köcher, J.; Lehnig, M.; Neumann, W. P. Chemistry of heavy carbene analogs R2M (M= Si, Ge, Sn). 12. Concerted and nonconcerted insertion reactions of dimethylgermylene into the carbonhalogen bond. Organometallics 1988, 7, 1201−1207. (29) Ishida, S.; Iwamoto, T.; Kabuto, C.; Kira, M. Unexpected reactions of an isolable dialkylsilylene with haloalkanes. Chem. Lett. 2001, 30 (11), 1102−1103. (30) While acyl radicals are able to form the corresponding aldehydes via the hydrogen abstraction, the biacyl via the recombination, and the corresponding alkanes and their dimers via the decarbonylation as minor products,31 these products were not detected in our reactions. While a reviewer suggested that 3 may be formed by the reactions of 1 with the biacyl, the reaction of 1 with biacetyl gave the corresponding 1,4-adduct, a 1,3-dioxa-2-germa-4cyclopentene, in 85% isolated yields; the results will be reported elsewhere. No reaction occurred between 2f and 5f. (31) Chatgillialoglu, C.; Crich, D.; Komatsu, M.; Ryu, I. Chemistry of Acyl Radicals. Chem. Rev. 1999, 99, 1992−2069. (32) The BDE values of M−Cl bonds are reported to be 76.5, 99.2, 89.4, 87.6 kcal/mol for M = C, Si, Ge, and Sn, respectively.33 The C− Cl BDE of CH3Cl (83.5 kcal/mol) is reported to be similar to that of CH3COCl (83.5 kcal/mol).34 (33) Karni, M.; Kapp, J.; Schleyer, P. v R.; Apeloig, Y. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; John Wiley & Sons: Chichester, England, 2001; pp 1−163. (34) Lowry, T. H.; Richardson, K. S. In Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper Collins Publishers: New York, 1987; pp 161−162. (35) (a) Picou, C. L.; Stevens, E. D.; Shah, M.; Boyer, J. H. Structure of 4,4-difluoro-1,3,5,7,8-pentamethyl-3a,4a-diaza-4-s-indance. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1990, 46, 1148−1150. (b) SMART, SAINT, SADABS, and SHELXTL; Bruker AXS Inc.: Madison, WI, 2000.

H


Reactions of an Isolable Dialkylgermylene with Acyl Chlorides

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