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Rhodium-Catalyzed 2-Aryl Phenol-derived Six-membered Silacyclization: Straightforward Access towards Dibenzooxasilines and Silicon-containing Planar Chiral Metallocenes Wen-Tao Zhao, Zhuo-Qun Lu, Hanliang Zheng, Xiao-Song Xue, and Dongbing Zhao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01992 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018
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ACS Catalysis
Rhodium-Catalyzed 2-Aryl Phenol-derived Six-membered Silacyclization: Straightforward Access towards Dibenzooxasilines and Silicon-containing Planar Chiral Metallocenes Wen‐Tao Zhao, Zhuo‐Qun Lu, Hanliang Zheng, Xiao‐Song Xue, and Dongbing Zhao* State Key Laboratory and Institute of Elemento‐Organic Chemistry, College of Chemistry, Nankai University, Tianjin, 300071, China ABSTRACT: The
C/Si switch strategy has been regarded as a useful and efficient strategy for the discovery of drugs and materials. Thus, development of methodology to access diverse silacycles is of great significance and in great demand. Among these, C−H bond silylation provides a powerful and straightforward synthetic method to form diverse silacycles in an atom- and step-economical fashion. However, C−H bond silylation has not been used to access any 6-membered silicon-bridged π-conjugated scaffolds and enantioselective 6-membered C‒H silylation has never been presented. Herein, we successfully accessed diverse 6-membered π-conjugated dibenzooxasilines via C−H bond silylation and investigated their photophysical properties. Furthermore, we realized enantioselective 6-membered C−H siylation to directly afford the planar chiral metallocene oxasilolanes with high ee (up to 95% ee). We also demonstrated the synthetic usefulness of dibenzooxasilines and planar chiral metallocene-fused benzooxasilolines as valuable synthetic intermediates via diverse additional transformations. Moreover, the 6-membered silicon-bridged ladder π-conjugated systems were designed and rapidly constructed by using our methods. The “isomerization” and “silicon” effects on molecular geometries and photophysical properties were also evaluated detailedly. KEYWORDS: silicon, heterocycle, metallocenes, silylation, enantioselectivity, π‐conjugated systems
1. INTRODUCTION Silacycles have attracted increasing attention due to their many important applications in medicinal chemistry as biomedically relevant agents with low toxicity and favorable metabolic profiles (Figure 1a),1 materials chemistry as πconjugated functional materials with high electron-affinity, hole-blocking and solid-state luminescence (Figure 1b),2 as well as organic synthesis as valuable synthetic intermediates.3 In light of this fact, there is highly desired for developing efficient catalytic strategies to achieve silacycles from readily available precursors. Dibenzooxasilines as the conjugated 6membered silacycles would potentially be utilized as chemical feedstocks to access a number of important biaryl-skeletal motifs via further oxidations, reductions and/or silicon-based crosscoupling reactions and present modified optoelectronic properties compared with their
dibenzopyrans isostere because of the σ*-π* conjugation (Figure 1c). Furthermore, the planarchirality can also be created by replacement of the one aryl group to ferrocenyl-substituent on dibenzooxasilines. However, until now, only two catalytic methods including Pd-catalyzed direct arylation of C−H bond and Rh-catalyzed transmetalation were developed to yield dibenzooxasilines (Figure 1c).4 Both methods still suffer some disadvantages such as the difficulties of accessing starting materials and moderate atomeconomy. Furthermore, the photophysical properties of 6-membered silicon-bridged πconjugated scaffolds has never been investigated. The planar chiral 6-membered ferrocene oxasilolanes has also never been presented even the ferrocenes with planar-chirality have been widely utilized as important and privileged scaffolds for development of chiral ligands or catalysts5 and the enantioselective C−H bond functionalization to creating planar-chiral
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Figure 1. (a) Some biologically active silacycles. (b) Some well‐known π‐conjugated silacycles. (c) The presented two catalytic methods to yield dibenzooxasilines. (d) our work: 2‐Aryl phenol or 2‐ferrocenyl phenol‐derived six‐membered C–H silylation.
ferrocenes have been widely investigated.6 Among the various existing reactions that form silacycles, transition metal-catalyzed C−H bond silylation provides a powerful and straightforward synthetic method in an atom- and step-economical fashion.7 We wondered if the simple 2-aryl phenol-derived hydridosilyl ether are possible to go through the intramolecular 6-membered silylation, which would constitute a new route to access the dibenzooxasiline scaffold. Furthermore, we are also full of curiosity whether the planar chiral 6-membered metallocene oxasilolanes can be produced via an enantioselective 6-membered C‒H silylation starting from simple 2-ferrocenyl phenols. These planar chiral 6-membered
ferrocene oxasilolanes might be very useful as new chiral π-building blocks to access a lot of planar chiral ferrocenyl-aryl skeletons via further transformation. To achieve this transformation, several formidable challenges need to be overcomed: 1) the reactivity. Even the intramolecular silylation of C−H bonds to form 5-membered π-conjugated silacycles has been extensive studied, until now C−H bond silylation has still not been used to access any 6-membered silicon-bridged πconjugated scaffolds.8 In fact, 2-aryl phenolsderived C−H silylation has never been described in the literature;9 2) the control of enantioselectivity. To date, there is still no
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ACS Catalysis
example on enantioselective silylation of C−H bonds to form 6-membered silacycles. Herein, we met these challenges and for the first time successfully accessed diverse dibenzooxasilines starting from simple 2-aryl phenols and investigated their photophysical properties (Figure 1d). Furthermore, we realized the first enantioselective 6-membered C−H siylation to directly afford the novel planar chiral metallocene oxasilolanes with high ee (up to 95% ee, Figure 1d). 2. RESULTS AND DISCUSSION Table 1. Optimization of the reaction condition[a]
Entry
[M] cat.
L
Sol.
Yield[%]b
1
[Ir(cod)OMe] 2
Phen
Diox.
only 3 (85)
2
Rh(PPh3)3Cl
‐‐‐
Diox.
56
3
Rh(PPh3)3Cl
Xphos
Diox.
40
4
Rh(PPh3)3Cl
dppe
Diox.
20
5
Rh(PPh3)3Cl
BINAP
Diox.
29
6
Rh(PPh3)3Cl
Xantphos
Diox.
62
7
Rh(PPh3)3Cl
DavePho s
Diox.
38
8
Rh(PPh3)3Cl
Xantphos
Hex.
31
9
Rh(PPh3)3Cl
Xantphos
THF
48
10
Rh(PPh3)3Cl
Xantphos
Tol.
68
11
Rh(PPh3)3Cl
Xantphos
DCE
n.r.
12
[Rh(COD)Cl]2
Xantphos
Tol.
87
13[c]
[Rh(COD)Cl]2
Xantphos
Tol.
40
14[d]
[Rh(COD)Cl]2
Xantphos
Tol.
85
15[e]
[Rh(COD)Cl]2
Xantphos
Tol.
59
[a] Reactions were carried out by using [M] cat. (2 mol%), lig‐ and (4 mol%), 3,3‐dimethylbutene (1.2 equiv.), and 1 (0.2 mmol) in solvent (1 mL) for 24 h at 120 °C under an nitrogen atmosphere. [b] Isolated yield. [c] without H2 scavenger; [d] 12 h; [e] 100 °C.
Condition Screening. In preliminary experiments, 2-phenyl phenol-derived silyl ether 1 was treated with [Ir(cod)OMe]2 (2 mol%), 1,10-phenanthroline (4 mol%, phen) and 3,3-dimethylbutene (1.2
equiv.) as H2 scavenger in dioxane (1 mL) at 120 °C for 24 h. Unfortunately, only bis-silylated side product 3 was produced by intermolecular cyclization (Table 1, entry 1). To our delight, employing Rh(PPh3)3Cl as the catalyst, we exclusively obtained the 6,6-diisopropyl-dibenzooxasiline 2 with 56% yield. After screening several parameters such as different RhI sources, ligands and solvents (For details, see Table 1), we found that the dehydrogenative silylation occurred smoothly to afford the desired product 2 in 85% yield in the presence of [Rh(COD)Cl]2 (2 mol%) and 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos, 4 mol%) in toluene at 120 °C for 12 h. Substrate Scope. With optimized conditions, the scope of 2-aryl substituted phenols was investigated (Figure 2). First, the effect of substituents on the silicon atom was evaluated by replacement of the i-propyl group to less sterically hindered ethyl group. The reaction temperature can be decreased to 100 °C with full conversion but lower yield (85% vs. 76%, 2 vs. 4), which might be a result of the weak stability of the corresponding (hydrido)silyl ether (chromatographically instable). Notably, the diethyl-(hydrido)silyl ether was prepared by Ir-catalyzed dehydrogenative coupling because of their instablity for column chromatography. In contrast, the bulky tert-butylphenyl on the silicon atom required a higher temperature (140 °C) to ensure the full conversion and good yield (5). Then, we studied the influence of the substitution on the Ar2 aromatic ring. From our results, we found that the reactivity changes only slightly with the steric and electronic properties of the substituents: Whenever the phenyl ring bears electron-neutral groups (6-10) electron-rich group (11) or electronwithdrawing groups (12-13) at any position the reaction proceeds smoothly at the standard condition. Furthermore, we proved that various substituents at the Ar1 aromatic rings were also tolerated in this transformation (14-28). It is important to stress that the reactions were preferred at the less sterically accessible position when a meta-substituent was attached to the Ar1 aromatic ring (Figure 2, 15, 17, and 21). For example, our catalytic system could be applied to regiospecifically prepare dibenzooxasiline 21 as a
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Figure 2. Scope of 2-aryl phenols in intramolecular dehydrogenative silylation. b140 °C.
single isomer, whereas the previously reported Gevorgyan’s system only afforded the mixture of the two isomers (2.9:1). This method was remarkably compatible with a variety of important functional groups such as halogens (12, 13, 24, and 25), dimethylamino (23), methoxy (11 and 22), trifluoromethyl (26), and ester (27) groups, which could be subjected to further synthetic transformations. In addition to simple dibenzooxasilines, heteroaromatic (28) product was also accessible by this procedure. To our delight, switching of the 2-aryl phenols to 2ferrocenyl phenol, the reaction also works well to yield the product 29. Enantioselective Six-membered Silacyclization. Having provided proof-of-principle for the 2-aryl phenol and 2-ferrocenyl phenol-derived C–H silylation to 6-membered silacycle and inspired by those examples on enantioselective C−H silylation for the synthesis of planar-chiral 5-membered ferrocene siloles,10 we wondered if the 2ferrocenyl phenol-derived dialkylhydridosilyl ethers are possible to go through the enantioselective dehydrogenative silylation to directly afford the novel planar-chiral ferrocenes bearing a 6-membered silacycle by employment of
the proper RhI species and chiral phospine ligand. After extensive survey of the reaction parameters, the optimized conditions were identified to be: [Rh(COD)OH]2 as the catalyst (2 mol%), Josiphos L1 as the ligand (6 mol%), cyclohexene as the hydrogen acceptor (1.1 eq.), mesitylene as the solvent (1 mL) and in the presence of 4 Å molecular sieve (500 mg), wherein the desired product 30 was afforded in 81% yield and 93% enantioselectivity at 120 °C. Control experiments were subsequently conducted to understand the role of each component. Switching of the Josiphos L1 to the other chiral phospine ligands such as Josiphos L2-L4, Segphos L5-L8, MeOBIPHEP L9-L11, BINAP L12, and Ph-SKP L13 would lead to the decrease in both the yield and the enantioselectivity (Table 2, entries 2-13). Changing the [Rh(COD)OH]2 to other RhI species also resulted in lower yields (see SI, table S1). The presence of 4 Å molecular sieves and a hydrogen acceptor is important for achieving both high reactivity and high enantioselectivity (see SI, table S1). Furthermore, the hydrogen acceptor also proved to be essential, as the switching of cyclohexene to NBE led to lower yield and
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Table 2. Optimization for the asymmetric intramolec‐ ular dehydrogenative silylationa
Entry
L.
[M] cat.
Ee [%]b
Yield [%]c
1
L1
[Rh(COD)OH]2
93
83(81)d
2
L2
[Rh(COD)OH]2
69
71
3
L3
[Rh(COD)OH]2
71
72
4
L4
[Rh(COD)OH]2
79
46
5
L5
[Rh(COD)OH]2
23
16
6
L6
[Rh(COD)OH]2
63
48
7
L7
[Rh(COD)OH]2
63
62
8
L8
[Rh(COD)OH]2
72
65
9
L9
[Rh(COD)OH]2
77
61
10
L10
[Rh(COD)OH]2
63
60
11
L11
[Rh(COD)OH]2
2
69
12
L12
[Rh(COD)OH]2
29
47
13
L13
[Rh(COD)OH]2
64
24
14e
L1
[Rh(COD)OH]2
n.d
trace
a General conditions: [M] catalyst (2 mol%), phospine lig‐
and (6 mol%), H2 scavenger, substrate (0.1 mmol), and 4 Å molecular sieves (500 mg) were stirred at room tempera‐ ture in solvent (1 mL) for 2 h, then the dehydrogentive si‐ lylation was performed at 120 °C for 24 h under N2 atmos‐ phere. b ee was determined by HPLC on a chiral stationary phase. c Yields of 3a were determined by NMR using DMAP as the internal standard. d Yield of isolated product is given in parenthesis. e Change of the ethyl group on silicon to iso‐ propyl group. O R'2P
MeO MeO
Fe
Josiphos ligands: PR2 L1, R = Cy, R' = Cy; L2, R = Cy, R' = Ph; L3, R = tBu, R' = Ph; L4, R = tBu, R' = Cy;
O O O
MeOBIPHEP ligands: PAr 2 L9, Ar = 3,4,5-MeOPh; PAr 2 L10, Ar = 3,5-diMePh; L11, Ar = 3,5-di-iPr-4-Me2NPh
Segphos ligands: PAr2 L5, Ar = Ph; PAr2 L6, Ar = 3,5-diMePh; L7, Ar = 3,5-di-tBu-4-OMePh; L8, Ar = 3,5-diTMSPh;
PPh2 PPh2
L12, (R)-BINAP
PPh2 PPh2 L13, (S)-Spirophos
enantioselectivity (see SI, table S1). Increasing the amount of the hydrogen acceptor is not advantage for improving the yield (see SI, table S1). The effect of different solvents was also investigated (see SI, table S1). Mesitylene was proved to be the
best choice. We also evaluated the influence of different substituents on the silicon atom. Instead of the ethyl group to iso-propyl group would almost totally shut down the reactivity (Table 2, entry 14). X-Ray structural analysis of single crystal 30 was obtained to confirm the absolute configuration of our product, which could be assigned to be Rp (Figure 3, 30).11 With optimized conditions in hand, we tested a variety of 2ferrocenyl phenols-derived silyl ethers bearing diethyl group on the silicon atom to gain insight into the versatility of our catalytic system as summarized in Figure 3. Substituents of different sizes at the para-position of the hydroxyl group of phenyl ring, that is, methyl, ethyl, isopropyl, tertbutyl and cyclohexyl groups, reacted smoothly in good yields (72-82%) and excellent enantioselectivities (>92% ee) (Figure 3, 31-35). Whenever the para-position of the hydroxyl group of phenyl ring bears electron-withdrawing groups or electron-rich groups, such as methoxy, fluorine, chlorine, trifluoromethyl, and ester, the reaction proceeded smoothly under the optimized conditions, with comparable yields (56-76%) and excellent enantioselectivities (91-94% ee) (Figure 3, 36-40). Furthermore, we studied the influence of the substitution on the meta-position of the hydroxyl group of phenyl ring. In these cases, electron-neutral, electron-donating, and electronwithdrawing groups did not significantly affect the performance. Good yields and high enantiomeric ratios were observed, as well (Figure 3, 41-46). Surprisingly, our method could also be applied in silylation of sterically hindered substrate without any drop in enantioselectivity and yield (Figure 3, 47). Furthermore, the 3,4-disubstituted substrates bearing electron-neutral, electron-rich or electronwithdrawing group also underwent the reaction smoothly and the yields and enantioselectivities were comparable to those of the monosubstituted substrates (Figure 3, 48-50). Additionally, ruthenocene was also capable substrate (51, 93% ee). Finally, we proved that the reaction could easily be employed on 1 mmol scale without a significant decrease in enantioselectivity (30, 95% ee).
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Figure 3. The scope of rhodium-catalyzed C−H silylation for the synthesis of planar-chiral 6-membered metallocene-fused benzooxasilolines. General condition: [Rh(COD)OH]2 (2 mol%), phospine ligand L1 (6 mol%), the corresponding substrate, cyclohexene (1.1 equiv.) as the H2 scavenger, and 4 Å molecular sieves (500 mg) were stirred at room temperature in mesitylene (1 mL) for 2 h. Afterwards, the dehydrogentive silylation was performed at 120 °C for 48 h under an N2 atmosphere. Yield of isolated product is given. Yield of isolated product is given. Ee value was determined by HPLC on a chiral stationary phase. a for 24 h.
Mechanism Investigation. To elucidate the reaction mechanism, the kinetic isotope effect experiment was carried out to independently assess the rate of reaction for C−H vs. C−D activation. The value of kH/kD = 3.2 from two parallel reactions as well as the value of KIE = 2.3 from intermolecular competition indicated that the C−H bond cleavage process is likely involved in the rate-determining step (Figure 4). Additionally, during the investigation of substrate scope, it was observed that the reactivities of (hydrido)silyl ethers with an electron-donating substituent are obviously higher than those of an electronwithdrawing substituent, which are consistent with an electrophilic C−H activation mechanism.
Figure 4. The kinetic isotope effect experiment.
Synthetic Usefulness. Since our 2-aryl phenolderived six-membered C–H silylation proved easily scalable, we demonstrated the synthetic usefulness of the dibenzooxasilines and the planar chiral 6-membered ferrocene oxasilolanes obtained in our transformation as valuable synthetic intermediates. First, we conducted six additional transformations with our product 2
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(Figure 5a). Treatment of 2 with anhydrous CsF in DMF/D2O led to mono-deuterated product 52
Figure 5. The synthetic usefulness of the dibenzooxasiline 2 and the planar chiral 6-membered ferrocene oxasilolane (Rp)-30.
in 90% yield. Ortho-iodination also smoothly proceeded to give 53 in 90% yield by using iodine monochloride as iodination reagent. We proved that the Si–O bond of the dibenzooxasiline can be easily cleaved to afford silyl phenol 54 (76% yield) involving nucleophilic ring-opening reaction with nBuLi. Notably, these silyl phenols can also undergo further synthetic transformations of organosilicon reagents to approach diverse building blocks in organic synthesis. We could also submit compound 2 to the modified Woerpel’s oxidation condition to produce biphenol 55 in good yield. This strategy provides a general method for the preparation of biphenol skeletons, which have been widely utilized as the key synthetic intermediates to approach numerous
privileged ligands in catalysis. In addition to oxidation, we for the first time show that boroinduced desilylation of dibenzooxasiline 2 also occurred smoothly to yield the dibenzoxaborin 56 in good yield, which serves as a useful building block for constructing versatile π-conjugated systems. Finally, we investigated whether dibenzooxasiline can be served as a type of general organosilicon reagents to smoothly couple with aryl halides. We found that the Hiyama−Denmark cross-coupling of dibenzooxasiline and iodobenzene worked smoothly to afford the hydroxyl-substituted orthoterphenyl product 57 in 67% yield. Furthermore, we wondered if these transformations are enantiospecific by use of the planar chiral 6-membered ferrocene oxasilolanes as the substrate. Herein, we proved that the Si–O bond of the planar chiral 6-membered ferrocene oxasilolane 30 can be easily cleaved by carbon nucleophile (nBuLi) via nucleophilic ring opening reaction with retention of stereochemistry configuration, affording planar chiral silylsubstituted ferrocenyl-phenol 58 in 98% yield with 100% enantiospecificity (Figure 5b). Predictably, these planar chiral silyl-substituted ferrocenyl-phenols 58 is capable of undergoing further synthetic transformations of organosilicon reagents to approach diverse planar chiral building blocks in organic synthesis such as the reported aryl-MOPF ligands, which were synthesized by chiral auxiliary-directed ortho-metalation.12 We also found that the Hiyama−Denmark crosscoupling of (Rp)-30 and iodobenzene worked successfully to afford the desired enantiomerically enriched phenyl-substituted ferrocene-phenol product 59 in 68% yield with 90% enantiospecificity (Figure 5b). We have proved that the drop of ee results from the competitive reaction of the protonation of the product 30 and sequential directed C−H bond arylation by conducting two control experiments (Figure 5c). In general, our method provides a efficient method for the preparation of chiral ferrocene-phenol skeletons, which could be utilized as the key chiral synthons to approach new privileged ligands bearing planar chirality. Further studies focused on the application of these new planar chiral ferrocene-phenol products are ongoing.
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Figure 6. Preparation of ladder π-conjugated systems 61, 63, 64 and 65.
Having established a structural library of dibenzooxasilines, the absorption spectra, the photoluminescence (PL) emission and the absolute quantum yields of compounds 2, 4, 5, 7, 12, 17, 19, 21 and 27 in both the solid state and CH2Cl2 solution are measured (Figure S1 and Table S2 in SI). Actually, all of them show blueshifted absorptions as well as the emissions in contrast to their analogues reported before.13 After the evaluation on the photophysical properties of these dibenzooxasilines, we turned our eyes to synthesize and character ladder π-conjugated systems bearing 6-membered dibenzooxasiline units by using our catalytic methods. Construction of New 6-membered Siliconbridged Ladder π-Conjugated Systems. Polycyclic ladder π-conjugated systems are an important class of scaffolds for organic electronics. In the past decades, a large number of fascinating ladder π -conjugated frameworks embedding various elements as the bridging moiety have been developed.14 Among these, silicon-bridged ladder π-conjugated systems are well known. The introduction of silicon atom is capable of bringing several characteristics into the ladder πconjugated systems, such as lower HOMO level, improved packing ability, and higher charge
mobility.2 However, even significant progress has been made in the synthesis of 5-membered siliconbridged ladder π-conjugated systems containing silole or 9-silafluorene units,15 6-membered silicon-bridged ladder π-conjugated systems have never been designed and characterized so far. Herein, we for the first time successfully created two isomers of ladder π-conjugated systems bearing 6-membered dibenzooxasiline units 61 and 63 by our Rh-catalyzed C−H bond silylation and their isosteres 64 and 65 to evaluate the “isomerization” and “silicon” effects on molecular geometries and photophysical properties (Figure 6). Molecular Geometries and Photophysical Properties. Initially, to evaluate the “isomerization” and “silicon” effects on molecular geometries, energy level and the distribution of molecular orbital, density functional theory (DFT) calculations were performed using Gaussian 09 at B3LYP/6-31G(d) level. As shown in Figure 7a, the optimized geometries of these four compounds suggest that the silicon-bridged structures significantly increases the dihedral angle between two phenyl subunits from 10-13° for 64 & 65 to 20° for 61 & 63. Meanwhile, the DFT calculations
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Table 3. Photophysical properties of 61, 63, 64 & 65.a in solution λabs (nm)
λem (nm)
in solid state
F
λem (nm)
in film
F
λem (nm)
DFT
F
HOMO (eV)
LUMO (eV)
61
361
413
0.30
429
0.28
419
0.28
-5.23
-1.27
63
333
385
0.35
401
0.27
424
0.30
-5.44
-1.30
64
397
447
0.75
464
0.34
447
0.16
-5.03
-1.27
65
371
398
0.79
537
0.49
539
0.49
-5.32
-1.33
a Absolute quantum yield determined with a integrating sphere system.
Figure 7. (a) The optimal molecular geometries using DFT of 61, 63, 64 and 65. (b) Fluorescence emission spectra in the solid state. (c) Fluorescence images of the four ladder π-conjugated systems in both the solid state and CH2Cl2 solution under UV light.
predict the HOMO/LUMO of 61, 63, 64, and 65 to be -1.27/-5.23, -1.30/-5.44, -1.27/-5.02eV, and 1.33/-5.32eV, respectively (Table 3). Generally, isomer 61 shows the same planarity with 63, but higher-lying HOMO. Compared to 64 and 65, the silicon-bridged structures of 63 and 61 exhibit larger band gaps with lower-lying HOMO and weaker planarity. To further understand the “isomerization” and “silicon” effects on the photophysical properties, we characterized all of the four ladder πconjugated systems in solution at RT, in the solid state (powders) and in the thin film. The absorption maxima, the photoluminescence (PL) maxima and quantum yields (F) are listed in
Table 3, whereas the corresponding absorption and emission spectra are depicted in Figure 6b-c and SI. Generally, incorporation of “silicon” into the ladder π-conjugated systems led to significant blue-shifts in the absorption maxima and the photoluminescence (PL) maxima, as well as slightly reduced light-emission efficiency in both the solid state and CH2Cl2 solution, which is in line with their planarity of molecular conformation. For the two silicon-bridged isomers of 61 and 63, the same Фf values and the photoluminescence (PL) maxima were detected in both the solid state and CH2Cl2 solution, which indicate the inactive rotation and the lack of close packing. However, their isosteres 64 and 65 show
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the distinct emissive behaviors. 64 exhibited the same photoluminescence (PL) maxima in all three states, but lower quantum efficiency in the solid state (powders) and the thin film, which might due to the nonradiative rotation or potential π-π stacking. The significant red-shift emission and relatively high Фf value of 65 in the solid state indicated an aggregation emission phenomenon. 3. CONCLUSION In conclusion, we have successfully developed a RhI-catalyzed dehydrogenative silylation, which constitutes a highly efficient catalytic method to directly access diverse dibenzooxasilines with a broad substituent scope. Furthermore, the enantioenriched planar-chiral ferrocenes bearing a 6membered silacycle have been for the first time synthesized via an asymmetric dehydrogenative C−H silylation of 2-ferrocenyl substituted phenolic silyl ethers. By using the resulting dibenzooxasilines and as valuable synthetic intermediates, we demonstrated several important downstream transformations to smoothly approach six important skeletal motifs via one step. Additionally, the high enantiospecificity for retention of configuration was attained if using these planar-chiral 6-membered ferrocene oxasilolanes as synthetic intermediates in nucleophilic ring opening reaction and Hiyama−Denmark cross-coupling. Finally, the “isomerization” and “silicon” effects of the 6membered silicon-bridged ladder π-conjugated systems on molecular geometries and photophysical properties were evaluated by creating two isomers of ladder π-conjugated systems bearing dibenzooxasiline units 61 and 63 and their isosteres 64 and 65. AUTHOR INFORMATION Corresponding Author * E‐mail:
[email protected].
Author Contributions W.‐T. Z. and Z.‐Q. L. contributed equally to this work. W.‐T. Z. and Z.‐Q. L. performed the experiments. H. Z. & X.‐S. X. performed the DFT calculations. D.Z. conceived the concept, directed the project and wrote the paper.
Notes The authors declare no competing financial interest.
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Supporting Information. Supporting Information includes experimental procedures, character of new starting materials and products, DFT Calculation, character of photophysical properties, NMR spectra and HPLC trace. This material is available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGMENT We are grateful for the financial support from the National Natural Science Foundation of China (21602115), 1000‐Talent Youth Program (020/BF180181), the Natural Science Founda‐ tion of Tianjin (18JCYBJC20400), the Fundamental Research Funds for the Central Universities and Nankai University.
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