Me6TREN-Supported Alkali Metal Hydridotriphenylborates [(L)M

May 23, 2017 - ... Reagents for Organic Synthesis; John Wiley & Sons Ltd.: New York, ...... Ross McLellan , Alan R. Kennedy , Robert E. Mulvey , Saman...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/Organometallics

Me6TREN-Supported Alkali Metal Hydridotriphenylborates [(L)M][HBPh3] (M = Li, Na, K): Synthesis, Structure, and Reactivity Hassan Osseili, Debabrata Mukherjee, Klaus Beckerle, Thomas P. Spaniol, and Jun Okuda* Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, Aachen 52074, Germany S Supporting Information *

ABSTRACT: Triphenylborane (BPh3) abstracted the β-SiH in [(L)M{N(SiHMe2)2}] (L = Me6TREN; M = Li, Na, K) in THF to give the hydridotriphenylborates [(L)M][HBPh3]. Reactions in benzene favored silazide instead of hydride abstraction to give [(L)M][Ph3B−N(SiHMe2)2]. The hydridotriphenylborates [(L)M][HBPh3] catalyzed the chemoselective hydroboration of benzophenone by pinacolborane (HBpin), with the lithium derivative being the most active. The solution structure of [(L)Li][HBPh3] was qualitatively investigated in the context of its superior catalytic activity. Fluxional coordination of L in [(L)Li(THF)]+ in tandem with a THF solvent molecule was revealed by NMR spectroscopy. paraSubstituents in [HB(C6H4-p-X)3]− influenced the rate which is apparently determined by the addition of the B−H function to the isolable alkoxy borate intermediate [(L)Li][Ph2CHOBPh3].

■

Scheme 1. Solvent-Dependent β-SiH vs N(SiHMe2)2 Abstraction from [(L)M{N(SiHMe2)2}] (M = Li, Na, K) by BPh3

INTRODUCTION Light alkali metal hydridoborates [M(HBR3)] with variable substituents R (M = Li, Na, K; R = hydride, alkyl, aryl, amido, alkoxy) are versatile reagents for the stoichiometric reduction of polar functionalities in organic synthesis.1 [(L)M][HBPh3] was recently found to catalyze the hydroboration of carbonyls and CO2 chemoselectively, with the lithium derivative exhibiting exceptionally high activity.2 Tripodal tris{2-(dimethylamino)ethyl}amine (Me6TREN = L) is an effective chelating ligand that stabilizes reactive organoalkali metal compounds in monomeric form.3 The alkali metal hydridotriphenylborates [(L)M][HBPh3] [M = Li (1), Na (2), K (3)] were synthesized by BPh 3 -induced β-SiH abstraction from [(L)M{N(SiHMe2)2}] in THF.2 We report here on the details of synthesis, solution structure of the lithium catalyst, along with some details of the hydroboration catalysis.

■

RESULTS AND DISCUSSION Reactions of [(L)M{N(SiHMe2)2}] with BPh3. The tetramethyldisilazide adducts [(L)M{N(SiHMe2)2}], readily available by mixing L and [M{N(SiHMe2)2}]4 in hydrocarbon or ethereal solvents, acted as suitable precursors for [(L)M][HBPh3] by reacting with BPh3 under β-hydride abstraction in THF (Scheme 1). This synthetic route is clean and high yielding (86−97%). The byproduct cyclodisilazane (Me2HSiN−SiMe2)2 can be removed by washing with npentane, whereas the products can be recrystallized in pure form from THF at −35 °C. Hydridotriphenylborates 1−3 are colorless crystals and highly soluble in THF but insoluble in aliphatic and aromatic hydrocarbons. They were fully characterized by spectroscopic and analytical techniques including X-ray crystallography for 1 and 3.2 © XXXX American Chemical Society

Changing the solvent from THF to benzene altered the reaction pathway. BPh3 abstracted the silazide group N(SiHMe 2 ) 2 instead of the β-hydride from [(L)M{N(SiHMe2)2}] (Scheme 1). The resulting tetramethyldisilazidoborates [(L)M][Ph3B−N(SiHMe2)2] [M = Li (4), Na (5), K (6)] are also colorless crystals and show solubility similar to that of 1−3. Whereas the potassium compound was quantitatively converted, the lithium and sodium homologues were contaminated with their [HBPh3]− derivatives by 25 and 12%, respectively. Complexes 4−6 were characterized by 1H, 13 C{1H}, and 11B NMR spectroscopy. The 11B NMR spectra in Received: April 21, 2017

A

DOI: 10.1021/acs.organomet.7b00308 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics THF-d8 show a singlet at δ −2.7 ppm, which is attributed to the boron atom in the [Ph3B−N(SiHMe2)2]− anion.5 Complexes 5 and 6 were also characterized by X-ray crystallography. 5 has an ionic structure with a free [Ph3B− N(SiHMe2)2]− anion, whereas 6 is zwitterionic (Figure 1). In 6,

alkyls [M{C(SiHMe2)3}2(L′)] (M = Ca, Yb; L′ = 2THF, TMEDA) in benzene, where the SiH is activated through metal interactions.7 Hydride abstraction from [Ru(PMe3)4(SiMe3)H] 8 and the methylene group abstraction from Cp′2CeCH2OMe (Cp′ = 1,2,4-(Me3C)3C5H2)9 are other examples. It also interacts reversibly with the Ce−H in Cp′2CeH9 and reacts with tBuLi to give [Li(HBPh3)] that is contaminated with the BPh3−• radical anion.1c B-Methyl-9-BBN reacts with tBuLi in hydrocarbon solvents to a mixture of “ate” complexes including the hydridoborate.1c Reactions of [Li{N(SiHMe2)2}] with group 14 electrophiles such as Me3SiCl are also solvent-dependent, but in this case, cyclodisilazane is the major product in hexane, whereas THF favors substitution rather than elimination.10 A molecular zinc chloride [(ToM)ZnCl] (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) reacts with [Li{N(SiHMe2)2}] by substitution in benzene but hydride abstraction/elimination in THF.11 The tetramethyldisilazidoborates 4−6 were compared with the hydridoborates 1−3. Heating 4−6 in THF-d8 at 60 °C converted them into 1−3 within a variable amount of time (Scheme 2). Conversion for the potassium was exceptionally Scheme 2. Thermolysis of 4−6 to Give 1−3

sluggish. The cyclodisilazane [(Me2HSiN−SiMe2)2] byproduct was identified by 1H NMR spectroscopy. These reactions were accelerated by a further equivalent of BPh3. Thus, both abstractions are independent processes depending on the solvent, and 4−6 are not the intermediates during the synthesis of 1−3 in THF. The coordination of THF possibly prevents BPh3 from accessing the interior M−N bond. Solution Structure of [(L)Li][HBPh3] (1). The lithium catalyst (1, TOFPh2CO = 66.6 × 103 h−1) was >300 times more active than sodium and potassium (2 and 3, TOFPh2CO ≥ 0.2 × 103 h−1) in the hydroboration of benzophenone by HBpin (eq 1).2 Moreover, coordination of L appeared to be critical for

Figure 1. Molecular structures of 5 (top) and 6 (bottom). Displacement parameters are drawn at 50% probability. Hydrogen atoms except for the Si-H are omitted for clarity.

one of the three phenyl rings of the BPh3 moiety is η1-bonded to the potassium through the ipso-carbon [K···Cipso = 3.2798(14) Å]. Moreover, the orientation of L in 6 is slightly bent to accommodate an additional three-centered two-electron K···H−Si bonding interaction (K···H = 2.70 Å) from one of the two SiH that leads to a six-membered metallacycle. This is also corroborated by its solid-state IR (KBr) spectrum that exhibits two distinct νSiH absorptions at 2117 and 2072 cm−1 (see Supporting Information). [K{N(SiHMe2)2}] in the solid state has a one-dimensional polymeric structure with each potassium being involved in double β-agostic K···H−Si bonding interactions (K···H = 2.67 Å) from a single N(SiHMe2)2 group.6 In comparison, the [HBPh3]− in 3 was η3-bonded to the potassium through the ipso- and two ortho-carbons of one phenyl ring, but no K···H−B interaction was found.2 Similar K+···Cπ noncovalent interactions were reported for [(L)KCH2Ph],3b [(L)KCH2(3,5-Me2-C6H3)],3c and [(L)KSiPh3]3e complexes. The solvent-controlled abstraction reactions here are remarkable, as BPh3 is only moderately Lewis acidic and the [(L)M{N(SiHMe2)2}] are classical tetramethyldisilazides. Group abstractions by BPh3 generally are rare. Few examples include the β-SiH abstraction from the calcium and ytterbium

faster catalysis when compared to the ligand-free [Li(HBPh3)].2 That prompted us to analyze the solution structure of 1 in more details. 1 crystallized from THF as a solvent-separated ion pair with free [HBPh3]− anion (Figure 2).2 The lithium center adopts a distorted trigonal bipyramidal geometry with a κ4-coordination from L, where the apical nitrogen atom (Napical) and a THF molecule occupy the axial positions. The THF binding is apparently weak and occurs during the crystallization, whereas the NMR spectrum and elemental analysis of a rigorously dried sample correspond to a THF-free complex. B

DOI: 10.1021/acs.organomet.7b00308 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Supporting Information) provided the activation parameters as ΔH⧧ = 13.3 ± 0.9 kcal mol−1, ΔS⧧ = 5.2 ± 0.3 cal mol−1 K−1, and ΔG⧧298 K = 11.8 ± 0.9 kcal mol−1. The low activation barrier underlines the facile exchange process. A positive ΔS⧧ is consistent with a dissociative mechanism. Coordination of L at Li is often labile in solution, and both κ43b−d and κ33a,f modes exist in the solid state. With larger-sized sodium and potassium, κ4 is the only mode identified so far.3b,c,e,g Solid [(L)Li{N(SiMe3)2}] shows κ3-bonding from L with a dangling CH2CH2NMe2 arm, whereas the NMR spectroscopic data in toluene-d8 over a wide range of temperatures suggest κ4 in solution by exhibiting one set of resonances for the three arms.3a [(L)Li(2-tBu-C5H5N)] also behaves in a similar way.3f Among other metals, [(L)MgMe2],13 [(L)Pd(NCS)][NCS],14 [(L)Ru(Tp)][BPh4] (Tp = trispyrazolylborate),15 and [(L)Cu(PPh3)][BPh4]16 all exhibit κ3bonding from L in solid state but κ4 in solution. Variabletemperature 1H NMR analysis of [(L)Cu(PPh3)][BPh4] in acetone-d6 showed resonance broadening upon cooling.16 L is bonded to zinc in a κ2-fashion in [(L)Zn(tBu)2] and bridges three gallium centers in [(L){Ga(tBu)3}3] with each metal bound to one arm.17 Without more evidence for 1, we presently refrain from specifying the dynamic coordination pattern of L and the THF solvent molecule at the lithium cation. Notably, unlike 1, [(L)Li(HBEt3)] and [(L)Li(BH4)] have fivecoordinate lithium centers with bridging hydrides, and both are soluble in aromatic hydrocarbons.3g Catalytic Hydroboration. As mentioned earlier, the lithium catalyst 1 was >300 times more active than sodium 2 and potassium 3 in the hydroboration of benzophenone by HBpin (eq 1).2 We have attempted to rationalize this superiority of 1 over 2 and 3, based on the solution behavior discussed above. Several control experiments were carried out since fast catalysis prevented rigorous kinetic measurements. 1 H and 11B NMR spectra in THF-d8 showed no reaction between 1 and HBpin or BPh3 and HBpin. However, 1 reacted rapidly with Ph 2 CO by insertion to give [(L)Li][Ph2CHOBPh3] (7) (Scheme 3) that is fully characterized

Figure 2. [(L)Li(THF)][HBPh3] as a solvent-separated ion pair.

The sodium (2) and potassium (3) derivatives also exhibit κ4-bonding of L in the solid state.2 NMR spectroscopic data in THF-d8 support a κ4-mode in solution as well for all three metals with a pseudo-C3v-symmetric ligand environment.2 A closer look at the 13C{1H} NMR spectra revealed an interesting feature. For 1, one of the methylene (CH2) resonances at δ 53.0 ppm is abnormally broad at room temperature (Figure 3),

Figure 3. Variable-temperature 13C{1H} NMR spectra of 1 in THF-d8. Only the L region is shown for clarity.

whereas 2 and 3 show no such peculiarity. It is also apparently absent in other [(L)Li] complexes known in the literature.3a−d,g,12 Two-dimensional 1H−13C HSQC and 1H−15N HMBC spectra of 1 in THF-d8 at room temperature attributed this broad resonance to the N(CH2CH2NMe2)3 carbons (Cb) (see Supporting Information). A variable-temperature 13C{1H} NMR spectroscopic analysis of 1 in THF-d8 was subsequently performed between 213 and 333 K (Figure 3). The broad resonance for Cb at δ 53.0 ppm (298 K) gradually became sharper at both temperature limits, shifting upfield to δ 50.6 ppm (213 K) upon cooling and slightly downfield to δ 53.1 ppm (333 K) upon heating. This gives an overall Δδ(Cb) = 2.5. In comparison, Δδ values for the other set of methylene carbons [Δδ(Cc) = 1.0] and the N-methyl carbons [Δδ(Ca) = 0.1] are marginal. The NMe2 (Ca) signal become broader in the low-temperature range. In the corresponding 1H NMR spectra under variable temperature, the methylene (CH2) resonances, particularly the CbH2 at δ 2.44 ppm, become broader upon cooling and shifted upfield (see Supporting Information). In addition, the 1H NMR spectrum of a recrystallized and mildly dried sample of 1 at 233 K showed distinct resonances of protio-THF separated from the residual solvent signal (see Supporting Information). This behavior indicates a fluxional coordination from L and the THF molecule, possibly to maintain a four-coordinate lithium center in solution at room temperature. At the lowtemperature limit, all donor sites appear to bind to give a fivecoordinate lithium, consistent with the solid state, whereas at higher temperature, a faster exchange results in a time average picture. An Eyring plot derived from a line-shape analysis (see

Scheme 3. Isolation of 7 from 1 and Ph2CO

including a crystal structure. Like 1, complex 7 also has a separate ion pair structure in the solid state with a cation [(L)Li(THF)] + with THF bonded and a free anion [Ph2CHOBPh3]− (Figure 4). The fluxional coordination pattern described for 1 is maintained in 7. The latter reacted further with HBpin in THF-d8 to give Ph2CHOBpin under immediate regeneration of 1. These two stoichiometric reactions support the two-step mechanism proposed earlier (Figure 5). The insertion step appeared to be equally fast for all three metals, at least qualitatively. While HBpin reacted with 7 immediately, the reaction with in situ prepared [(L)Na][Ph2CHOBPh3] required ca. 15 min to complete. Therefore, the group transfer is probably the rate-limiting step. Orbital limitations make it unlikely that hydride and alkoxide units are directly exchanged between the two boron centers by σ-bond metathesis. We rather propose that the cationic lithium center C

DOI: 10.1021/acs.organomet.7b00308 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

is similar to that observed for benzophenone hydroboration catalyzed by 1,3,2-diazaphospholene (KIE = 2.69)18 and a magnesium-catalyzed dihydroboration of alkyl nitriles (KIE = 2.79).19 An electronic effect on the catalysis rate was also noticed with two other modified lithium catalysts [(L)Li][HB(C6H4-p-Me)3] (8) and [(L)Li][HB(C6H4-p-CF3)3] (9). Both 8 and 9 were synthesized following β-SiH abstraction from the tetramethyldisilazide precursor [(L)M{N(SiHMe2)2}] by the respective boranes (Scheme 1). They have solubility and spectroscopic behavior similar to that of 1. Complex 8 was also characterized by X-ray crystallography that shows a similar ion pair structure as 1 (see Supporting Information). Whereas 0.01 mol % of 8 was sufficient to complete the hydroboration of benzophenone within 10 min, catalyst 9 gave only 35% conversion after 5 h under identical conditions.

Figure 4. Molecular structure of 7. Displacement parameters are drawn at 50% probability. Hydrogen atoms are omitted for clarity.

■

CONCLUSION Moderately Lewis acidic BPh3 mediates β-SiH and N(SiHMe2)2 group abstractions from a series of Me6TREN-coordinated group 1 tetramethyldisilazides. The two abstraction pathways are independent and solvent-controlled, with polar THF favoring the former and nonpolar benzene promoting the latter. The hydridotriphenylborates 1−3 catalyze the hydroboration of carbonyls and CO2; the order of activity being Li ≫ Na ∼ K. Solution behavior of the most active lithium catalyst (1) was studied in some detail, which divulged a dynamic coordination from Me6TREN to the cationic lithium center in tandem with a labile THF molecule. Based on that, the superior performance of the lithium catalyst was elucidated. The actual role of Me6TREN is quite intriguing due to its rate-enhancing ability when 1 is compared to the ligand-free [Li(HBPh3)] catalyst. In principle, chelation from Me6TREN is expected to coordinatively saturate the relatively small lithium cation and decrease its Lewis acidity significantly. Most likely, it renders lithium mononuclear3g,20 that is otherwise prone to form aggregates.21 At the same time, lithium probably retains sufficient Lewis acidity to become a powerful catalyst. Effects of other chelating ligands in comparison to Me6TREN are currently under investigation.

Figure 5. Proposed catalytic cycle and group transfer step in the hydroboration of Ph2CO using 1.

mediates this group exchange by abstracting the hydride from HBpin (Figure 4). Concurrent alkoxide transfer to “Bpin” generates the product Ph2CHOBpin along with an incipient lithium hydride and BPh3. The last two species should recombine instantly to regenerate the catalyst. BPh3 has a pronounced hydride affinity as it can easily break down the ionic lattice of LiH. Likewise, the electrophilic “Bpin” moiety would prefer to bind to the alkoxy (OCHPh2) group to form the boronic ester, adding to the driving force of the reaction. A tighter dative coordination from THF in the case of sodium (2) or arene coordination from [HBPh3]− in the case of potassium (3), in addition to a nonlabile κ4-bonding of L, would inhibit a metal participation. This probably explains why the higher Lewis acidity and better accessibility of lithium make 1 a far better catalyst than 2 and 3. A direct hydride transfer from HBpin to Ph2CO induced by the Lewis acidic metal center is another possibility. We ruled this out as using [(L)M][BPh4] (M = Li, 0.001 mol %; M = Na, 0.1 mol %) as catalysts were inactive. Initial carbonyl insertion into the B−H bond is therefore important. The hydroboration catalysis was further probed with a deuterium labeling study. A deuterated lithium catalyst [(L)Li][DBPh3] (1-d) was synthesized by mixing L with [Li(DBPh3)] in THF. 1-d was found to be equally active as 1 in the hydroboration of benzophenone. Catalytic hydroboration of benzophenone by HBpin using 0.001 mol % of 1 was reported earlier to be accomplished within 1.5 h at room temperature. The same catalysis with DBpin as the reducing agent took 4 h to complete under the same condition. Thus, the kinetic isotope effect (KIE), qualitatively estimated as 2.67, indicates that the B−H bond breaking is involved in the ratedetermining step (see Supporting Information). This KIE value

■

EXPERIMENTAL SECTION

All reactions were performed under a dry argon atmosphere using standard Schlenk techniques or under argon atmosphere in a glovebox. Prior to use, glassware were dried overnight at 130 °C and solvents were dried, distilled, and degassed using standard methods. [(L)M{N(SiHMe2)2}],2 complexes 1−3,2 B(C6H4-p-Me)3,22 and B(C6H4-pCF3)322 were prepared following literature procedure. LiD was purchased from Sigma-Aldrich. BPh3 (95% pure) was purchased from abcr and purified by sublimation. 1H, 13C{1H}, 11B, and 29Si{1H} NMR spectra were recorded on a Bruker Avance-III spectrometer at ambient temperature unless otherwise mentioned. Chemical shifts (δ ppm) in the 1H and 13C{1H} NMR spectra were referenced to the residual signals of the deuterated solvents. Abbreviations for NMR spectra are as follows: s (singlet), d (doublet), t (triplet), q (quartet), sep (septet), br (broad). IR spectra were recorded on KBr pellets using an AVATAR 360 FT-IR spectrometer. Abbreviations for IR spectra are as follows: w (weak), m (medium), s (strong), br (broad). Elemental analyses were performed on an elementar vario EL machine. X-ray diffraction data for 5−7 were collected on a Bruker APEX II diffractometer and reported in a crystallographic information file. [(L)Li][Ph3B−N(SiHMe2)2] (4). A solution of BPh3 (0.034 g, 0.140 mmol) in 2 mL of benzene was added to a solution of [(L)Li{N(SiHMe2)2}] (0.051 g, 0.138 mmol) in 3 mL of benzene. A colorless crystalline solid precipitated immediately upon mixing. The suspension was stirred for a further 30 min. The precipitate was D

DOI: 10.1021/acs.organomet.7b00308 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics isolated and washed with n-pentane (3 × 5 mL). Drying the precipitate under vacuum afforded 0.068 g of colorless crystals. 1H NMR of this solid in THF-d8 indicates a mixture of 4 and 1 in ∼3:1 ratio. Attempts to isolate pure 4 from this mixture by recrystallization were not successful, and analytical data were not collected. 4 is characterized only by NMR spectroscopy in solution. 1H NMR (400 MHz, THFd8): δ 7.48 (m, 6 H, o-Ph), 6.88 (m, 6 H, m-Ph), 6.73 (m, 3 H, p-Ph), 4.40 (sep, 2 H, 3JHH = 3.2 Hz, SiHMe2), 2.51 (m, 6 H, CH2), 2.36 (m, 6 H, CH2) 2.22 (s, 18 H, NMe2), −0.26 (d, 3JHH = 3.2 Hz, 12 H, SiHMe2). 13C{1H} NMR (100 MHz,THF-d8): δ 166.4 (ipso-Ph), 137.5 (o-Ph), 125.6 (m-Ph), 122.6 (p-aryl), 58.5 (CH2), 52.6 (CH2), 46.3 (NMe2), 3.4 (SiHMe2). 11B NMR (128 MHz, THF-d8): δ −2.7. 29 Si{1H} NMR (80 MHz, THF-d8): δ −19.1. 7Li{1H} NMR (156 MHz, THF-d8): δ −2.1. [(L)Na][Ph3B−N(SiHMe2)2] (5). A solution of BPh3 (0.051 g, 0.211 mmol) in 2 of mL of benzene was added to a solution of [(L)Na{N(SiHMe2)2}] (0.080 g, 0.207 mmol) in 3 mL of benzene. A colorless crystalline solid precipitated immediately upon mixing. The suspension was stirred for a further 30 min. The precipitate was isolated and washed with n-pentane (3 × 5 mL). Drying the precipitate under vacuum afforded 0.011 g of colorless crystals. 1H NMR of this solid in THF-d8 indicates a mixture of 5 and 2 in ∼88:12 ratio. X-rayquality single crystals were obtained from slow n-pentane diffusion into a concentrated THF solution at −35 °C. 1H NMR (400 MHz, THF-d8): δ 7.48 (m, 6 H, o-Ph), 6.87 (m, 6 H, m-Ph), 6.73 (m, 3 H, pPh), 4.40 (sep, 2 H, 3JHH = 3.2 Hz, SiHMe2), 2.51 (m, 6 H, CH2), 2.34 (m, 6 H, CH2), 2.19 (s, 18H, NMe2), −0.26 (d, 3JHH = 3.2 Hz, 12 H, SiHMe2). 13C{1H} NMR (100 MHz, THF-d8): δ 164.5 (ipso-Ph), 137.5 (o-Ph), 125.6 (m-Ph), 122.5 (p-Ph), 58.7 (CH2), 53.1 (CH2), 46.2 (NMe2), 3.4 (SiHMe2). 11B NMR (128 MHz, THF-d8): δ −2.7. 29 Si{1H} NMR (79.5 MHz, THF-d8): δ −17.3. Satisfactory CHN analysis data could not be obtained for 5. [(L)K][Ph3B−N(SiHMe2)2] (6). A solution of BPh3 (0.031 g, 0.128 mmol) in 2 mL of benzene was added to a solution of [(L)K{N(SiHMe2)2}] (0.050 g, 0.124 mmol) in 3 mL of benzene. A colorless crystalline solid precipitated immediately after mixing. The suspension was then stirred for a further 30 min. The precipitate was isolated and washed with n-pentane (3 × 5 mL). Drying the solid under vacuum afforded analytically pure 6 (0.073 g, 0.113 mmol, 91%) as a colorless powder. X-ray-quality single crystals were obtained from slow n-pentane diffusion into a concentrated THF solution at −35 °C. 1 H NMR (400 MHz, THF-d8): δ 7.51 (m, 6 H, o-Ph), 6.91 (m, 6 H, m-Ph), 6.77 (m, 3 H, p-Ph), 4.36 (sep, 2 H, 3JHH = 3.2 Hz, SiHMe2), 2.55 (m, 6 H, CH2), 2.30 (m, 6 H, CH2), 2.15 (s, 18 H, NMe2), −0.24 (d, 3JHH = 3.2 Hz, 12 H, SiHMe2). 13C{1H} NMR (100 MHz, THFd8): δ 163.9 (ipso-Ph), 137.3 (o-Ph), 125.9 (m-Ph), 122.8 (p-Ph), 59.4 (CH2), 54.6 (CH2), 46.4 (NMe2), 3.4 (SiHMe2). 11B NMR (128 MHz, THF-d8): δ −2.7. 29Si{1H} NMR (79.5 MHz, THF-d8): δ −17.1. IR (KBr, cm−1): 3058 (m), 3041 (m), 2970 (s), 2862 (s), 2828 (m), 2777 (s), 2117 (s, νSiH), 2072 (s, νSiH), 1578 (w), 1462 (s), 1427 (m), 1357 (m), 1303 (s), 1281 (m), 1237 (s), 1161 (m), 1131 (m), 1115 (s), 1069 (m), 1036 (m), 1019 (m), 987 (s), 922 (s), 885 (s), 833 (s), 789 (m), 764 (s), 744 (m), 732 (m), 706 (s), 612 (s). Anal. Calcd for C34H59BN5Si2K: C, 63.42; H, 9.24; N, 10.88. Found: C, 63.40; H, 9.18; N, 11.01. [(L)Li][Ph3B−OCHPh2] (7). A 2 mL THF solution of 1 (0.100 g, 0.208 mmol) and Ph2CO (0.038 g, 0.208 mmol) was stirred for 10 min at room temperature. Volatiles were removed under reduced pressure to obtain a white solid. The solid was washed with n-pentane (3 × 5 mL) and dried under vacuum to give analytically pure 7 (0.131 g, 0.201 mmol, 97%) as a colorless powder. X-ray-quality single crystals were obtained from slow n-pentane diffusion into a concentrated THF solution at −35 °C. 1H NMR (400 MHz, THFd8): δ 7.40 (m, 6 H, o-aryl), 7.29 (m, 4 H, o-aryl), 6.92 (m, 4 H, m-Ph), 6.81 (m, 2 H, p-Ph), 6.78 (m, 6 H, m-Ph), 6.67 (m, 3 H, p-H), 5.53 (s, 1 H, OCHPh2), 2.46 (m, 6 H, CH2), 2.29 (m, 6 H, CH2), 2.15 (s, 18 H, NMe2). 13C{1H} NMR (100 MHz, THF-d8): δ 164.0 (ipso-Ph), 152.1 (ipso-Ph), 135.9 (o-Ph), 128.2 (o-Ph), 127.6 (m-Ph), 126.0 (pPh), 125.1 (m-Ph), 122.9 (p-Ph), 78.6 (OCHPh2), 58.8 (CH2), 53.1 (CH2), 46.3 (NMe2). 11B NMR (128 MHz, THF-d8): δ 2.1 (s).

Li{1H} NMR (156 MHz, THF-d8): δ −0.4. Anal. Calcd for C43H56BN4OLi: C, 77.93; H, 8.52; N, 8.45. Found: C, 77.62; H, 8.35; N, 8.40. [(L)Li][DBPh3] (1-d). A suspension of LiD (0.026 g, 2.905 mmol) and BPh3 (0.350 g, 1.445 mmol) in 5 mL of THF was heated to reflux for 24 h. The suspension was filtered, and the filtrate was evaporated under reduced pressure to give a pale-yellow solid. The solid was washed with n-pentane (3 × 5 mL) and dried under vacuum to obtain [Li(DBPh3)] (0.250 g, 0.996 mmol, 69%) as an off-white powder. L (0.230 g, 0.998 mmol) and [Li(DBPh3)] (0.200 g, 0.797 mmol) were mixed in 2 mL of THF to give a clear solution. Slow n-pentane diffusion into this solution at −35 °C precipitated colorless crystals within 24 h. The solid was isolated, washed with n-pentane (3 × 5 mL), and dried under vacuum to afford analytically pure [(L)Li][DBPh3] (0.100 g, 0.208 mmol, 26%) as a white powder. 1H NMR (400 MHz, THF-d8): δ 7.25 (m, 6 H, o-Ph), 6.83 (m, 6 H, m-Ph), 6.66 (m, 3 H, p-Ph), 2.47 (m, 6 H, CH2), 2.31 (m, 6 H, CH2) 2.16 (s, 18 H, NMe2). 13C{1H} NMR (100 MHz,THF-d8): δ 136.7 (o-Ph), 126.1 (mPh), 121.7 (p-Ph), 58.8 (CH2), 52.9 (CH2), 46.4 (NMe2). 11B NMR (128 MHz, THF-d8): δ −10.1 (s). 7Li{1H} NMR (156 MHz, THFd8): δ −2.3. Anal. Calcd for C30H45DBN4Li: C, 74.84; H, 9.84; N, 11.64. Found: C, 74.40; H, 9.55; N, 11.42. [(L)Li][HB(C6H4-p-Me)3] (8). A solution of B(C6H4-p-Me)3 (0.200 g, 0.704 mmol) in 2 mL of THF was added dropwise to a solution of [(L)Li{N(SiHMe2)2}] (0.260 g, 0.703 mmol) in 1 mL of THF. The resulting mixture was stirred for 2 h at room temperature. The solution was then concentrated under reduced pressure. Addition of n-pentane (5 mL) precipitated a colorless solid, which was washed with npentane (3 × 5 mL) and dried under vacuum to give analytically pure [(L)Li][HB(C6H4-p-Me)3] (8, 0.261 g, 0.499 mmol, 71%) as a white powder. X-ray-quality single crystals were obtained from slow npentane diffusion into a concentrated THF solution at −35 °C. 1H NMR (400 MHz, THF-d8): δ 6.74 (m, 6 H, o-aryl), 6.60 (m, 6 H, maryl), 3.68 (br, q, 1JBH = 79 Hz, 1 H, HB), 2.52 (m, 6 H, CH2), 2.35 (m, 6 H, CH2) 2.19 (s, 18 H, NMe2), 2.13 (s, 9 H, p-Me). 13C{1H} NMR (100 MHz,THF-d8): δ 164.2 (ipso-aryl), 136.0 (o-aryl), 127.9 (m-aryl), 123.7 (p-aryl), 58.8 (CH2), 52.9 (CH2), 46.4 (NMe2), 23.9 (p-Me). 11B NMR (128 MHz, THF-d8): δ −15.7 (d, 1JBH = 79 Hz). 7 Li{1H} NMR (156 MHz, THF-d8): δ −2.3. Anal. Calcd for C33H52BN4Li: C, 75.85; H, 10.03; N, 10.72. Found: C, 75.30; H, 9.73; N, 10.66. [(L)Li][HB(C6H4−p-CF3)3] (9). A solution of B(C6H4-p-CF3)3 (0.223 g, 0.500 mmol) in 2 mL of THF was added dropwise to an 1 mL of THF solution of [(L)Li{N(SiHMe2)2}] (0.185 g, 0.500 mmol). The resulting mixture was stirred for 2 h at room temperature. The solution was then concentrated under reduced pressure. Addition of n-pentane (5 mL) precipitated a light yellow solid, which was washed with n-pentane (3 × 5 mL) and dried under vacuum to give analytically pure [(L)Li][HB(C6H4-p-CF3)3] (9, 0.216 g, 0.316 mmol, 63%) as a light yellow powder. 1H NMR (400 MHz, THF-d8): δ 7.36 (m, 6 H, o-aryl), 7.18 (m, 6 H, m-aryl), 3.65 (br, q, 1JBH = 79 Hz, 1 H, HB), 2.53 (m, 6 H, CH2), 2.36 (m, 6 H, CH2), 2.20 (s, 18 H, NMe2). 13 C{1H} NMR (100 MHz,THF-d8): δ 170.29 (ipso-aryl), 136.4 (oaryl), 135.0 (m-aryl), 124.8 (p-aryl), 123.0 (p-CF3), 58.8 (CH2), 52.9 (CH2), 46.3 (NMe2). 11B NMR (128 MHz, THF-d8): δ −8.7 (d, 1JBH = 83 Hz). 19F NMR (377 MHz, THF-d8): δ −61.9. 7Li{1H} NMR (156 MHz, THF-d8): δ −0.4. Anal. Calcd for C33H43BF9N4Li: C, 57.91; H, 6.33; N, 8.19. Found: C, 57.50; H, 6.42; N, 8.44. 7

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00308. Spectroscopic characterization of compounds 4−9, B(C6H4-p-Me)3, and B(C6H4-p-CF3)3, and crystallographic details for compounds 5−8 (PDF) E

DOI: 10.1021/acs.organomet.7b00308 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Accession Codes

(7) Yan, K.; Schoendorff, G.; Upton, B. M.; Ellern, A.; Windus, T. L.; Sadow, A. D. Organometallics 2013, 32, 1300−1316. (8) Dioumaev, V. K.; Plössl, K.; Carroll, P. J.; Berry, D. H. Organometallics 2000, 19, 3374−3378. (9) Werkema, E. L.; Andersen, R. A.; Yahia, A.; Maron, L.; Eisenstein, O. Organometallics 2009, 28, 3173−3185. (10) Wiseman, G. H.; Wheeler, D. R.; Seyferth, D. Organometallics 1986, 5, 146−152. (11) Mukherjee, D.; Ellern, A.; Sadow, A. D. J. Am. Chem. Soc. 2010, 132, 7582−7583. (12) Kennedy, A. R.; Mulvey, R. E.; O’Hara, C. T.; Robertson, G. M.; Robertson, S. D. Angew. Chem., Int. Ed. 2011, 50, 8375−8378. (13) Guard, L. M.; Hazari, N. Organometallics 2013, 32, 2787−2794. (14) Ferguson, G.; Parvez, M. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1979, 35, 2207−2210. (15) Slugovc, C.; Gemel, C.; Shen, J.-Y.; Doberer, D.; Schmid, R.; Kirchner, K.; Mereiter, K. Monatsh. Chem. 1999, 130, 363−375. (16) Eckenhoff, W. T.; Pintauer, T. Dalton Trans. 2011, 40, 4909− 4917. (17) Cadenbach, T.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E.; Pickrell, J.-A.; Robertson, S. D. Dalton Trans. 2012, 41, 10141−10144. (18) Chong, C. C.; Hirao, H.; Kinjo, R. Angew. Chem., Int. Ed. 2015, 54, 190−194. (19) Weetman, C.; Anker, M. D.; Arrowsmith, M.; Hill, M. S.; Kociok-Kohn, G.; Liptrot, D. J.; Mahon, M. F. Chem. Sci. 2016, 7, 628−641. (20) (a) Reger, D. L.; Collins, J. E.; Matthews, M. A.; Rheingold, A. L.; Liable-Sands, L. M.; Guzei, I. A. Inorg. Chem. 1997, 36, 6266−6269. (b) Giese, H.-H.; Habereder, T.; Nöth, H.; Ponikwar, W.; Thomas, S.; Warchhold, M. Inorg. Chem. 1999, 38, 4188−4196. (21) (a) Heine, A.; Stalke, D. J. Organomet. Chem. 1997, 542, 25−28. (b) Giese, H.-H.; Nöth, H.; Schwenk, H.; Thomas, S. Eur. J. Inorg. Chem. 1998, 1998, 941−949. (c) Knizek, J.; Nöth, H. J. Organomet. Chem. 2000, 614−615, 168−187. (d) Giese, H. H.; Habereder, T.; Knizek, J.; Nöth, H.; Warchhold, M. Eur. J. Inorg. Chem. 2001, 2001, 1195−1205. (e) Gálvez Ruiz, J. C.; Nöth, H.; Warchhold, M. Eur. J. Inorg. Chem. 2008, 2008, 251−266. (22) Crevier, T. J.; Bennett, B. K.; Soper, J. D.; Bowman, J. A.; Dehestani, A.; Hrovat, D. A.; Lovell, S.; Kaminsky, W.; Mayer, J. M. J. Am. Chem. Soc. 2001, 123, 1059−1071.

CCDC 1498800 and 1545138−1545140 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.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hassan Osseili: 0000-0002-2255-9230 Jun Okuda: 0000-0002-1636-5464 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft through the International Research Training Group “Selectivity in Chemoand Biocatalysis” for financial support and the Alexander von Humboldt Foundation for a fellowship to D.M. We thank KhaiNghi Truong for collecting crystallographic data, Dr. Gerhard Fink for NMR spectroscopic, and Wataru Hato and Takafumi Higuchi for experimental assistance.

■

REFERENCES

(1) (a) Walker, E. R. H. Chem. Soc. Rev. 1976, 5, 23−50. (b) Brown, C. A.; Hubbard, J. L. J. Am. Chem. Soc. 1979, 101, 3964−3966. (c) Brown, H. C.; Kramer, G. W.; Hubbard, J. L.; Krishnamurthy, S. J. Organomet. Chem. 1980, 188, 1−10. (d) Brown, H. C.; Narasimhan, S. J. Org. Chem. 1982, 47, 1604−1606. (e) Singaram, B.; Cole, T. E.; Brown, H. C. Organometallics 1984, 3, 774−777. (f) Brown, C. A.; Krishnamurthy, S. J. Org. Chem. 1986, 51, 238−240. (g) Yoon, N. M.; Kim, K. E.; Kang, J. J. Org. Chem. 1986, 51, 226−229. (h) Yoon, N. M.; Kim, K. E. J. Org. Chem. 1987, 52, 5564−5570. (i) Kim, K. E.; Park, S. B.; Yoon, N. M. Synth. Commun. 1988, 18, 89−96. (j) Herbert, C. B.; Ramachandran, P. V. Sixty Years of Hydride Reductions. In Reductions in Organic Synthesis; American Chemical Society: Washington DC, 1996; Vol. 641, pp 1−30. (k) Singaram, B.; Jackson, T. C.; Harrison, J. Synlett 1996, 1996, 383−384. (l) Zaidlewicz, M.; Brown, H. C. Lithium Triethylborohydride. In Encyclopedia of Reagents for Organic Synthesis; John Wiley & Sons Ltd.: New York, 2001. (2) Mukherjee, D.; Osseili, H.; Spaniol, T. P.; Okuda, J. J. Am. Chem. Soc. 2016, 138, 10790−10793. (3) (a) Cousins, D. M.; Davidson, M. G.; Frankis, C. J.; Garcia-Vivo, D.; Mahon, M. F. Dalton Trans. 2010, 39, 8278−8280. (b) Davidson, M. G.; Garcia-Vivo, D.; Kennedy, A. R.; Mulvey, R. E.; Robertson, S. D. Chem. - Eur. J. 2011, 17, 3364−3369. (c) Armstrong, D. R.; Davidson, M. G.; Garcia-Vivo, D.; Kennedy, A. R.; Mulvey, R. E.; Robertson, S. D. Inorg. Chem. 2013, 52, 12023−12032. (d) Kennedy, A. R.; Mulvey, R. E.; Urquhart, R. I.; Robertson, S. D. Dalton Trans. 2014, 43, 14265−14274. (e) Leich, V.; Spaniol, T. P.; Okuda, J. Chem. Commun. 2015, 51, 14772−14774. (f) Robertson, S. D.; Kennedy, A. R.; Liggat, J. J.; Mulvey, R. E. Chem. Commun. 2015, 51, 5452−5455. (g) Kennedy, A. R.; McLellan, R.; McNeil, G. J.; Mulvey, R. E.; Robertson, S. D. Polyhedron 2016, 103, 94−99. (4) Eppinger, J.; Herdtweck, E.; Anwander, R. Polyhedron 1998, 17, 1195−1201. (5) Layton, W. J.; Niedenzu, K.; Niedenzu, P. M.; Trofimenko, S. Inorg. Chem. 1985, 24, 1454−1457. (6) Sarazin, Y.; Roşca, D.; Poirier, V.; Roisnel, T.; Silvestru, A.; Maron, L.; Carpentier, J.-F. Organometallics 2010, 29, 6569−6577. F

DOI: 10.1021/acs.organomet.7b00308 Organometallics XXXX, XXX, XXX−XXX