Synthesis of Linear Allylsilanes via Molybdenum-Catalyzed

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Synthesis of Linear Allylsilanes via Molybdenum-Catalyzed Regioselective Hydrosilylation of Allenes Sobi Asako,* Sae Ishikawa, and Kazuhiko Takai* Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan S Supporting Information *

ABSTRACT: A simple molybdenum-based catalytic system for hydrosilylation of allenes has been developed. The reactions of mono- and disubstituted allenes with secondary and tertiary silanes proceeded smoothly and selectively to afford linear allylsilanes. The origin of the unprecedented linear selectivity was investigated by density functional theory studies to reveal that the reaction consists of the following steps: (1) concerted hydromolybdation/Si−H oxidative addition from a Mo(CO)4/ allene/silane adduct to form (π-allyl)molybdenum, (2) allyl rotation from the initially formed (π-allyl)molybdenum to a thermodynamically more stable isomer, and (3) reductive elimination at the less-hindered allyl carbon to afford a linear allylsilane. KEYWORDS: molybdenum, hydrosilylation, allenes, linear allylsilanes, allyl rotation, density functional theory

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vinylsilanes under Ni,5a Pd,5b,c Al,5e or Au5f catalysis (Scheme 1b) have been reported. Among them, the Pd- and Ni-based catalytic systems developed by Montgomery et al. are particularly attractive and have recently been expanded to hydrosilylation of 1,3-disubstituted allenes.5d Subsequent density functional theory (DFT) studies on the mechanism of the Pd- and Ni-catalyzed regioselective hydrosilylations revealed that both the metals and the size of NHC ligands are crucial for the reactivity and regioselectivity of the reactions.6 Despite these recent advances, no general methods to access linear allylsilanes via allene hydrosilylation are available.7 We report here a molybdenum-catalyzed hydrosilylation of allenes that selectively produces linear allylsilanes with high Z selectivity (Scheme 1c). The development of simple and efficient syntheses of (Z)-allylsilanes is still a challenging task.8 We also performed DFT calculations and revealed the origin of the unique linear selectivity.

INTRODUCTION

The hydrosilylation of unsaturated molecules such as alkenes, alkynes, and dienes has matured into one of the most efficient synthetic methods for the preparation of organosilanes, which are versatile synthetic intermediates that participate in a wide range of transformations.1,2 The major difficulty of the reaction resides in the control of regio- and stereoselectivity. For instance, the regio- and stereoselective hydrosilylation of internal alkynes is still a nontrivial challenge,3 and it is only recently that the first 1,2-hydrosilylation of 1,3-dienes has been achieved.4 Hydrosilylation of allenes is no exception. Despite the potential utility for the selective synthesis of allylsilanes and alkenylsilanes, such reactions have been largely ignored, partially because of the difficulty in selectively obtaining a single isomer among others. To date, only a few examples of selective hydrosilylation reactions of allenes that afford branched allylsilanes under Pd5a−c catalysis (Scheme 1a) and

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RESULTS AND DISCUSSION Reaction Development. We commenced our investigation with the identification of an effective catalyst for hydrosilylation of 3-butyl-1,2-heptadiene with Ph2SiH2 (Table 1). We first examined the rhenium carbonyl complexes Re2(CO)10 and ReCl(CO)5 because of our recent interest in their use as catalysts for organic synthesis,9 to find that they showed catalytic activity for hydrosilylation, albeit in low yields (entries 1 and 2). Further screening of various metal carbonyl

Scheme 1. Regioselective Hydrosilylation of Allene

Received: March 1, 2016 Revised: April 8, 2016 Published: April 14, 2016 © 2016 American Chemical Society

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DOI: 10.1021/acscatal.6b00627 ACS Catal. 2016, 6, 3387−3395

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ACS Catalysis Table 1. Screening of Metal Carbonyl Complexesa

Table 3. Molybdenum-Catalyzed Hydrosilylation of Allenesa

entry

catalyst

product/%b

c

Re2(CO)10 ReCl(CO)5 Mn2(CO)10 Cr(CO)6 Mo(CO)6 W(CO)6 Fe(CO)5 Ru3(CO)12

3 22 10 42 88 5 2 11

1 2 3c 4 5 6 7 8d a

The reaction was performed on a 0.2 mmol scale in toluene (0.25 M) at 120 °C for 20 h. bDetermined by 1H NMR using 1,1,2,2tetrachloroethane as an internal standard. cCatalyst (5 mol %). d Catalyst (3.3 mol %).

Table 2. Optimization of Reaction Conditionsa

entry

x

concn/M

time/h

temp/°C

product/%b

1 2 3 4 5 6 7 8 9

10 10 10 10 10 10 5 5 5

0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.5 1.0

20 20 20 20 20 12 12 12 12

80 90 100 110 120 110 110 110 110

13 57 87 91 88 81 75 83 91

a

The reaction was performed on a 0.2 mmol scale. bDetermined by 1H NMR using 1,1,2,2-tetrachloroethane as an internal standard.

complexes led us to identify Mo(CO)6 as a particularly efficient catalyst for the hydrosilylation of the allene that selectively afforded the linear allylsilane in 88% yield after performing the reaction in toluene at 120 °C for 20 h (entry 5). While molybdenum carbonyl complexes have been reported to be efficient catalysts for hydrosilylation of aldehydes, ketones, α,βunsaturated carbonyl compounds, norbornadiene, and 1,3dienes,10,11 to the best of our knowledge they have never been used for the hydrosilylation of allenes. With this initial discovery, further optimization of the reaction conditions was conducted using Mo(CO)6 as the catalyst (Table 2). First, the influence of reaction temperature was examined to find that 100 °C was necessary for high conversion, probably to remove carbonyl ligands and create open coordination sites for the allene and silane substrates (entries 1−5). The catalyst loading and reaction time could be reduced to 5 mol % and 12 h, respectively, without decreasing the reaction efficiency when the concentration was increased from 0.25 to 1.0 M, giving the allylsilane product in 91% yield (entry 9). Scope of the Allene Hydrosilylation. With the optimized conditions in hand, the scope of the molybdenum-catalyzed hydrosilylation reaction was investigated (Table 3). The reactions of 1,1-disubstituted allenes proceeded smoothly and selectively to afford linear allylsilanes (entries 1−6). When unsymmetrically substituted allenes were used, allylsilanes were obtained as a mixture of E and Z isomers, where the ratio of the

a Reaction conditions unless specified otherwise: allene (0.2 mmol), R3SiH (1.2 equiv), and Mo(CO)6 (5 mol %) in toluene (0.2 mL) at 110 °C for 12 h. bIsolated yields. cDetermined by 1H NMR and GLC. d Branched allylsilane was obtained in 3% yield. eLinear allylsilane (21%) was obtained as a mixture with branched allylsilane (45%). f Mo(CO)3(MeCN)3 was used instead of Mo(CO)6 at 60 °C. gLinear allylsilane (53%) was obtained as a mixture with branched allylsilane (3%) and alkenylsilane (5%).

Z product increased as the size difference between the two substituents increased (entries 4−6). A 1,3-disubstituted allene also took part in this reaction (entry 7). Monosubstituted allenes bearing an alkyl, silyl, or boryl substituent reacted equally well with secondary and tertiary silanes to give the corresponding linear allylsilanes with Z selectivity (entries 8− 16). Although monosubstituted allene with a primary alkyl chain produced branched allylsilane as the major product under the standard conditions (entry 13), the simple replacement of Mo(CO)6 catalyst with Mo(CO)3(MeCN)3 restored the linear selectivity and the linear allylsilane was selectively obtained together with a small amount of other isomers (entry 14). It should be noted that the silyl- and boryl-substituted allylsilanes serve as versatile bifunctional reagents for the synthesis of complex molecules (entries 15 and 16).12 Branched allylsilanes, vinylsilanes, and other isomers were either not observed or detected only in a trace amount except in entries 8, 13, and 14. 3388


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hydrosilylation of a carbon−carbon double bond in the presence of an alkyne13 and provide a novel synthetic entry to alkynylallylsilanes, which serve as useful substrates for further transformations.14 To date, there have been few methods available for the synthesis of alkynylallylsilanes bearing a substituent at the allyl moiety.14,15 Reactions under UV Irradiation. The optimized conditions using Mo(CO)6 as catalyst require high temperature for smooth reaction (vide supra). To lower the reaction temperature and thereby improve the selectivity, we performed the reaction at room temperature under UV irradiation for the removal of carbonyl ligands (Table 4).16 As expected, the reaction proceeded much more quickly in toluene and reached completion within 1 h with improved Z selectivity, albeit in lower yield (entries 1−3). The reaction under UV irradiation promoted product isomerization, and prolonged reaction time resulted in lower Z selectivity (entry 3). The milder conditions at room temperature allowed us to examine the reactions in solvents possessing lower boiling points, among which Et2O and CH2Cl2 showed the best results in terms of yield and selectivity (entries 6 and 7). The new conditions under UV irradiation were applied to other substrates, all of which afforded the corresponding linear allylsilanes with improved Z selectivity (Figure 1). Computational Studies. To understand the origin of the unprecedented selectivity for linear allylsilanes, DFT calculations (B3LYP-D3 and M06) were performed using cyclohexylallene and Me3SiH as model substrates. The predicted energy trends were consistent between the two methods. Therefore, we discuss the free energies calculated at the B3LYP-D3/SDD:6-31G(d) level, unless otherwise noted. First, relevant molybdenum carbonyl complexes and their adducts with an allene and a silane at the early stage of the catalytic cycle were investigated (Table 5). The dissociation of one and two CO ligands is endergonic by 31.2 and 62.0 kcal/mol, respectively.17 The resulting Mo(CO)5 and Mo(CO)4 complexes can be stabilized by coordination of either an allene (1, 2) or a silane (3, 4) but only to a small extent, insufficient for compensating the loss of CO ligands. This would explain why high temperature is necessary for the thermal reaction, while room temperature is enough once CO ligands are removed by UV irradiation. We could not locate both an allene and a silane on Mo(CO)5 at the same time to form a seven-coordinate complex or find a transition state leading to a π-allyl intermediate such as Mo(CO)5(allyl)(SiMe3). Instead, a

Table 4. Molybdenum-Catalyzed Hydrosilylation under UV Irradiationa

entry

solvent

time

product/%b

E/Zc

1 2 3 4 5 6 7

toluene toluene toluene hexane THF Et2O CH2Cl2

10 min 1h 12 h 1h 1h 1h 1h

34 70 67 58 27 73 74

6/94 6/94 16/84 6/94 3/97 4/96 4/96

a

Reaction conditions: allene (0.2 mmol), PhMe2SiH (1.2 equiv), and Mo(CO)6 (5 mol %) in solvent (0.2 mL) were stirred at 25 °C for 1 h under UV irradiation (250−385 nm). bDetermined by 1H NMR in the presence of 1,1,2,2-tetrachloroethane as an internal standard. c Determined by GLC.

Figure 1. Examples of the molybdenum-catalyzed allene hydrosilylation under UV irradiation. Reactions were carried out in Et2O at 25 °C for 1 h under UV irradiation.

Allene Hydrosilylation with Alkynylsilane. The present reaction conditions enabled the unprecedented chemoselective hydrosilylation of allene with alkynylsilane (eqs 1 and 2). Thus,

the reactions of cyclohexylallene with mono- and bis(alkynyl)silanes proceeded equally efficiently to afford the alkynylallylsilane and bis(alkynyl)allylsilane, respectively, with linear and Z selectivity. These reactions constitute a rare example of

Table 5. Calculated Energies of Molybdenum Carbonyl Complexes and Their Adducts with Cyclohexylallene and Me3SiHa

a

Gibbs free energies and enthalpies (italicized) are given in kcal/mol, calculated at the B3LYP-D3/SDD:6-31G(d) level. 3389


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ACS Catalysis Scheme 2. Reaction Pathways of Molybdenum-Catalyzed Allene Hydrosilylation

Figure 2. Molybdenum-catalyzed hydrosilylation of cyclohexylallene and Me3SiH via path A (energies in kcal/mol).

the same TS as the concerted path from 6. Therefore, in the following sections, we used Mo(CO)4/allene/Me3SiH adduct 6a and its analogues as a starting point of the reaction. Scheme 2 shows four major reaction pathways that we considered in this study. The first two paths (A and B) involve a hydromolybdation TS concerted with Si−H bond oxidative addition, whereas the last two (C and D) involve a

concerted Si−H oxidative addition/allene hydromolybdation transition state (TS) from the Mo(CO)4/allene/Me3SiH adduct 6 was found (vide infra), which has been similarly proposed in the Pd-catalyzed hydrosilylation of allenes.6 The oxidative addition of the Si−H bond in Mo(CO)4/Me3SiH complex 4 forms molybdenum hydride 5. Attempts to locate an allene hydromolybdation TS from 5 and an allene resulted in 3390


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Figure 3. Energy profiles for molybdenum-catalyzed allene hydrosilylation via path A (energies in kcal/mol).

intermediates 7a−d are interconnected via allyl rotation18 or anti−syn isomerization, 7a has a chance to isomerize to the more stable 7c via TS7a7c (40.4 kcal/mol), which then undergoes reductive elimination via the lower TS7cRE (34.3 kcal/mol), leading to an experimentally observed linear (Z)allylsilane, before producing a branched allylsilane via TS7aRE (44.1 kcal/mol). Thus, it is low-barrier rotational isomerization to the more stable π-allyl intermediate that plays a key role in determining the selectivity for linear allylsilane. In contrast to the linear selectivity observed in the hydrosilylation with cyclohexylallene, branched allylsilane was selectively obtained when an allene bearing a smaller substituent was used in the presence of Mo(CO)6 (Table 3, entry 13). To explain this selectivity switch, similar calculations were performed using methylallene as a substrate. Important parts of the energy profiles for hydrosilylation of cyclohexylallene and methylallene starting from 7a (R = Cy) and 7a′ (R = Me) are shown in Figure 3 (see the Supporting Information for the complete results). When the Cy substituent was replaced with Me, the barrier for reductive elimination leading to branched allylsilane decreased by 5.1 kcal/mol (ΔG⧧RE_branch_Cy = 14.6 kcal/mol from 7a and ΔG⧧RE_branch_Me = 9.5 kcal/mol from 7a′) because of the reduced steric repulsion, whereas the barrier for rotational isomerization leading to linear allylsilane increased by 1.5 kcal/mol (ΔG⧧rot_Cy = 10.9 kcal/mol and ΔG⧧rot_Me = 12.4 kcal/mol). Thus, the reaction with methylallene has a smaller barrier for branched-product-producing reductive elimination in comparison to that for allyl rotation and therefore affords branched allylsilane, which is consistent with the experiment. The two systems have similar geometrical parameters during the allyl

silamolybdation TS concerted with Si−H bond oxidative addition. Depending on the regioselectivity of the allene insertion, we further divided each of them into two parts: one leading to a (π-allyl)molybdenum intermediate and the other leading to an alkenylmolybdenum intermediate. It should be noted that each of the four paths still has several isomeric structures when the allene has a substituent. We found that the reaction takes place preferentially via path A and affords a linear allylsilane through reductive elimination from the thermodynamically more stable π-allyl intermediate, which is accessible by allyl rotation of the initially formed π-allyl intermediate. Path A: Hydromolybdation Concerted with Si−H Oxidative Addition ((π-Allyl)molybdenum). A summary of the reaction mechanism via path A and the energy profiles are shown in Figures 2 and 3, respectively. Four routes were taken into consideration starting from Mo(CO)4/allene/Me3SiH complexes 6a−d via TSA featuring concerted hydromolybdation/Si−H oxidative addition to the corresponding (π-allyl) molybdenum complexes 7a−d. Among them, the path from 6a to 7a was found to have the lowest transition state, TSA1 (52.5 kcal/mol), which is least sterically demanding with a cyclohexyl substituent pointing away from the metal center. The Si−H and C−H bond lengths in TSA1 (1.95 and 1.55 Å, respectively; Figure 4) indicate that the present system has an earlier transition state in comparison with the Pd system (2.48 and 1.63 Å, respectively).6 The π-allyl intermediates 7a−d undergo reductive elimination to afford branched allylsilane, linear (Z)allylsilane, and linear (E)-allylsilane, respectively, where the transition states leading to branched allylsilane from 7a,b are higher in energy by as much as 10 kcal/mol than those leading to linear allylsilane from 7c,d. Because (π-allyl)molybdenum 3391


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Figure 4. Selected optimized structures of molybdenum complexes involved in path A (R = Cy) calculated at the B3LYP-D3/SDD:6-31G(d) level. Color code: gray, carbon; white, hydrogen; red, oxygen; blue, silicon; green, molybdenum.

Figure 5. Selected optimized structures of molybdenum complexes involved in path A (R = Me) calculated at the B3LYP-D3/SDD:6-31G(d) level. Color code: gray, carbon; white, hydrogen; red, oxygen; blue, silicon; green, molybdenum. 3392


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Figure 6. Molybdenum-catalyzed hydrosilylation of cyclohexylallene and Me3SiH via path B (energies in kcal/mol).

Figure 7. Molybdenum-catalyzed hydrosilylation of cyclohexylallene and Me3SiH via paths C and D (energies in kcal/mol).

rotation (7a/TS7a7c/7c vs 7a′/TS7a′7c′/7c′), except that the Mo−Callyl_sub bond length is longer in 7a than in 7a′, reflecting more steric crowding in the former (2.54 and 2.48 Å, respectively; Figures 4 and 5). The larger driving force for the allyl rotation with a Cy substituent (ΔGrot_Cy = −2.5 kcal/ mol and ΔGrot_Me = −0.7 kcal/mol) gained by releasing the steric hindrance would explain the lower rotational barrier with the Cy substituent. Another experimental observation is that the use of Mo(CO)3(MeCN)3 instead of Mo(CO)6 resumed linear selectivity for the reaction using an allene with a small substituent (Table 3, entry 14). DFT calculations with the Mo(MeCN)4/methylallene/Me3SiH system predicted a higher activation barrier for branched-product-producing reductive elimination (TS7a″RE = 14.0 kcal/mol) in comparison to allyl rotation (TS7a″7c″ = 12.8 kcal/mol) leading to the linear product and thus well reproduced this phenomenon (see Supporting Information).19 Path B: Hydromolybdation Concerted with Si−H Oxidative Addition (Alkenylmolybdenum). Next, two reaction paths via path B were examined, the first of which has the substituent pointing away from the Mo center and hence is the most favorable among the others (Figure 6). Complexes 6e,f form alkenylmolybdenum complexes 8e,f via concerted hydromolybdation/Si−H oxidative addition, which ultimately affords the corresponding alkenylsilane and vinylsilane. The highest transition states lie in the alkenyl−Si reductive elimination step

(TS9eRE: 54.2 kcal/mol) and the initial step (TSB2: 56.7 kcal/ mol), respectively, both of which are higher in energy than TSA1 (52.5 kcal/mol) in path A. The computational result is consistent with the experimental observation that alkenylsilanes and vinylsilanes were not produced under the current reaction conditions with Mo(CO)6 catalyst. Paths C and D: Silamolybdation Concerted with Si−H Oxidative Addition. Finally, concerted silamolybdation/Si−H oxidative addition paths C and D were investigated (Figure 7). All four transition states considered are much higher in energy than TSA1, excluding the possibility of these mechanisms operating. Unlike the linear alignment of Si−H−C atoms in TSA (Figure 4) and TSB, H−Si−C bond angles are 73.7−89.2° in TSC and TSD because of the bulkiness of a SiMe3 group in comparison with a hydrogen atom. The steric effect is also reflected in the skewed relation between the Mo−Si bond and the CC double bond, which makes their interaction weak and hence renders silamolybdation unfavorable.

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CONCLUSION In conclusion, we have developed a simple molybdenum-based catalytic system for the hydrosilylation of allene with unprecedented selectivity for linear allylsilane. The chemoselective allene hydrosilylation in the presence of alkyne was demonstrated for the first time. The use of inexpensive Mo(CO)6 as a catalyst without any exogenous ligands is particularly attractive.20 A DFT study on the reaction 3393


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(e) Fleming, I.; Barbero, A.; Walter, D. Chem. Rev. 1997, 97, 2063− 2192. (f) Ojima, I.; Li, Z. Y.; Zhu, J. W. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, pp 1687−1792. (g) Curtis-Long, M. J.; Aye, Y. Chem. - Eur. J. 2009, 15, 5402−5416. (h) Denmark, S. E.; Liu, J. H.-C. Angew. Chem., Int. Ed. 2010, 49, 2978−2986. (3) (a) Molander, G. A.; Retsch, W. H. Organometallics 1995, 14, 4570−4575. (b) Trost, B. M.; Ball, Z. T. J. Am. Chem. Soc. 2001, 123, 12726−12727. (c) Trost, B. M.; Ball, Z. T.; Jöge, T. Angew. Chem., Int. Ed. 2003, 42, 3415−3418. (d) Trost, B. M.; Ball, Z. T. Synthesis 2005, 2005, 853−887. (e) Trost, B. M.; Ball, Z. T. J. Am. Chem. Soc. 2005, 127, 17644−17655. (f) Rooke, D. A.; Ferreira, E. M. J. Am. Chem. Soc. 2010, 132, 11926−11928. (g) Kawasaki, Y.; Ishikawa, Y.; Igawa, K.; Tomooka, K. J. Am. Chem. Soc. 2011, 133, 20712−20715. (h) Rooke, D. A.; Ferreira, E. M. Angew. Chem., Int. Ed. 2012, 51, 3225−3230. (i) Sumida, Y.; Kato, T.; Yoshida, S.; Hosoya, T. Org. Lett. 2012, 14, 1552−1555. (j) Ding, S.; Song, L.-J.; Chung, L. W.; Zhang, X.; Sun, J.; Wu, Y.-D. J. Am. Chem. Soc. 2013, 135, 13835−13842. (k) Rummelt, S. M.; Radkowski, K.; Roşca, D.-A.; Fürstner, A. J. Am. Chem. Soc. 2015, 137, 5506−5519. (4) Parker, S. E.; Börgel, J.; Ritter, T. J. Am. Chem. Soc. 2014, 136, 4857−4860. (5) (a) Miller, Z. D.; Li, W.; Belderrain, T. R.; Montgomery, J. J. Am. Chem. Soc. 2013, 135, 15282−15285. (b) Miller, Z. D.; Montgomery, J. Org. Lett. 2014, 16, 5486−5489. (c) Tafazolian, H.; Schmidt, J. A. R. Chem. Commun. 2015, 51, 5943−5946. (d) Miller, Z. D.; Dorel, R.; Montgomery, J. Angew. Chem., Int. Ed. 2015, 54, 9088−9091. (e) Sudo, T.; Asao, N.; Gevorgyan, V.; Yamamoto, Y. J. Org. Chem. 1999, 64, 2494−2499. (f) Kidonakis, M.; Stratakis, M. Org. Lett. 2015, 17, 4538−4541. (6) Xie, H.; Zhao, L.; Yang, L.; Lei, Q.; Fang, W.; Xiong, C. J. Org. Chem. 2014, 79, 4517−4527. (7) A reaction of 1,1-disubstituted allene under the Pd catalysis5a is the only example that showed selectivity for linear allylsilane. (8) (a) Wrighton, M. S.; Schroeder, M. A. J. Am. Chem. Soc. 1974, 96, 6235−6237. (b) Shimizu, N.; Imazu, S.; Shibata, F.; Tsuno, Y. Bull. Chem. Soc. Jpn. 1991, 64, 1122−1128. (c) Abdelqader, W.; Chmielewski, D.; Grevels, F.-W.; Ö zkar, S.; Peynircioglu, N. B. Organometallics 1996, 15, 604−614. (d) Desponds, O.; Franzini, L.; Schlosser, M. Synthesis 1997, 1997, 150−152. (e) Tietze, L. F.; Völkel, L.; Wulff, C.; Weigand, B.; Bittner, C.; McGrath, P.; Johnson, K.; Schäfer, M. Chem. - Eur. J. 2001, 7, 1304−1308. (f) Occhipinti, G.; Hansen, F. R.; Törnroos, K. W.; Jensen, V. R. J. Am. Chem. Soc. 2013, 135, 3331−3334. (g) Chen, C.; Dugan, T. R.; Brennessel, W. W.; Weix, D. J.; Holland, P. L. J. Am. Chem. Soc. 2014, 136, 945−955. (h) Parasram, M.; Iaroshenko, V. O.; Gevorgyan, V. J. Am. Chem. Soc. 2014, 136, 17926−17929. (i) Koh, M. J.; Khan, R. K. M.; Torker, S.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2014, 53, 1968−1972. (9) Kuninobu, Y.; Takai, K. Chem. Rev. 2011, 111, 1938−1953. (10) (a) Adams, K. P.; Joyce, J. A.; Nile, T. A.; Patel, A. I.; Reid, C. D.; Walters, J. M. J. Mol. Catal. 1985, 29, 201−208. (b) Keinan, E.; Perez, D. J. Org. Chem. 1987, 52, 2576−2580. (c) Fuchikami, T.; Ubukata, Y.; Tanaka, Y. Tetrahedron Lett. 1991, 32, 1199−1202. (d) Abdelqader, W.; Ö zkar, S.; Peynircioglu, N. B. Z. Naturforsch., B: J. Chem. Sci. 1993, 48, 539−540. (e) Schmidt, T. Tetrahedron Lett. 1994, 35, 3513−3516. (f) Dioumaev, V. K.; Bullock, R. M. Nature 2000, 424, 530−532. (g) Kayran, C.; Rouzi, P. Z. Naturforsch., B: J. Chem. Sci. 2001, 56, 1138−1142. (h) Stosur, M.; Szymańska-Buzar, T. J. Mol. Catal. A: Chem. 2008, 286, 98−105. (i) Arias-Ugarte, R.; Sharma, H. K.; Morris, A. L.C.; Pannell, K. H. J. Am. Chem. Soc. 2012, 134, 848− 851. (j) Sharma, H. K.; Arias-Ugarte, R.; Tomlinson, D.; Gappa, R.; Metta-Magaña, A. J.; Ito, H.; Pannell, K. H. Organometallics 2013, 32, 3788−3794. (k) Pal, R.; Groy, T. L.; Bowman, A. C.; Trovitch, R. J. Inorg. Chem. 2014, 53, 9357−9365. (11) Hydrosilylation with oxo, imido, and other molybdenum complexes: (a) Fernandes, A. C.; Fernandes, R.; Romão, C. C.; Royo, B. Chem. Commun. 2005, 213−214. (b) Reis, P. M.; Romão, C. C.; Royo, B. Dalton Trans. 2006, 1842−1846. (c) da Costa, A. P.; Reis, P. M.; Gamelas, C.; Romão, C. C.; Royo, B. Inorg. Chim. Acta 2008, 361,

mechanism revealed that the reaction proceeds through the following three steps: (1) concerted hydromolybdation/Si−H oxidative addition from a Mo(CO)4/allene/silane adduct to form (π-allyl)molybdenum via path A, (2) allyl rotation from the initially formed (π-allyl)molybdenum to a thermodynamically more stable isomer, and (3) reductive elimination at the less-hindered allyl carbon to afford a linear allylsilane. Further studies on molybdenum-catalyzed selective hydrosilylation of unsaturated molecules are ongoing.

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COMPUTATIONAL METHODS All calculations were performed with the Gaussian 09 program package.21 The density functional theory (DFT) method was employed using the B3LYP22 and M0623 hybrid functionals. Structures were optimized with a basis set consisting of the Stuttgart−Dresden (SDD) basis set and effective core potential (ECP) for Mo24 and 6-31G(d)25 for the rest. To take into account the dispersion contributions, we performed B3LYP calculations with Grimme’s D3 parameter set as implemented in Gaussian 09.26 Each stationary point was adequately characterized by normal-coordinate analysis (no imaginary frequencies for an equilibrium structure and one imaginary frequency for a transition structure) and thermal corrections were calculated under standard conditions (298.15 K, 1 atm). Cartesian coordinates and energies of all the reported structures are given in the Supporting Information. Visualizations of the complexes in Figures 4 and 5 were obtained using CYLView.27

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00627. Experimental procedures and physical properties of the compounds and details of the computational studies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for S.A.: [email protected]. *E-mail for K.T.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the MEXT for financial support (KAKENHI Grantin-Aid for Scientific Research (A) No. 26248030 to K.T., Grant-in-Aid for Young Scientists (B) No. 15K21180 to S.A.). The generous amount of computational time from the Research Center for Computational Science, Okazaki National Research Institute, is gratefully acknowledged.

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REFERENCES

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Synthesis of Linear Allylsilanes via Molybdenum-Catalyzed

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