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Palladium-Catalyzed Hydrometalation and Bismetalation of Biphenylene Takanori Matsuda* and Haruki Kirikae Department of Applied Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
bS Supporting Information ABSTRACT: 2-Silyl- and 2-borylbiphenyls have been synthesized by the palladium-catalyzed reaction of biphenylene with hydrosilanes and a hydroborane, respectively, via the cleavage of the four-membered ring of biphenylene. Under similar conditions, the CC bond of biphenylene undergoes formal σ-bond metathesis with various intermetallic linkages to afford 2,20 bis(metallo)biphenyls.
S
ignificant developments have been made in the area of catalytic cleavage of CC bonds by transition metals, which allow the utilization of the CC bonds as a functional group in organic synthesis.1 The known catalytic functionalization of biphenylene involving the cleavage of CC bonds2 includes dimerization,3 hydrogenolysis,4 carbonylation,5 alkyne insertion,5b,6 and coupling with alkenes, arylboronic acids, and ketones;6a,7 in such catalytic functionalization, CC or CH bonds are formed concomitantly with the cleavage of the CC bond of biphenylene.8 Thus far, the catalytic introduction of atoms other than carbon and hydrogen into biphenylene via CC bond cleavage has been limited to only one example, where biphenylene reacts with bis(trimethylsilyl)acetylene in the presence of a nickel catalyst to give alkynylsilylation products arising from a formal CC/CSi σ-bond metathesis.9 While hydrometalation10 and bismetalation reactions11 represent an attractive transformation enabling the introduction of metallic elements such as silicon and boron into unsaturated organic molecules, biphenylene has never been exploited for these addition reactions. In this paper, we describe reactions of biphenylene with hydrosilanes and a hydroborane in the presence of palladium catalysts to give the corresponding hydrometalation products via cleavage of the CC bond of biphenylene. Moreover, the formal σ-bond metathesis reaction of biphenylene is applicable to homo- and heterointermetallic linkages such as SiSi and SiB bonds, affording 2,20 -bis(metallo)biphenyls. Our initial attempt explored the reaction of biphenylene (1) with hydrosilanes. In the presence of palladium catalysts prepared in situ from 5 mol % of Pd(dba)2 (dba = dibenzylideneacetone) and 10 mol % of Buchwald-type biaryl phosphine ligands (Figure 1), 1 was reacted with triethylsilane (2a, 1.5 equiv) in toluene at 110 °C to afford 2-(triethylsilyl)biphenyl (3a) via hydrosilylative CC bond cleavage (Table 1).12 Among the ligands studied, DavePhos (L1) and MePhos (L3) exhibited superior activity for the reaction as compared to other biaryl phosphines such as t-BuDavePhos (L2) (entries 13).13 Dimethyl(phenyl)silane (2b) and triethoxysilane (2c) can be used as r 2011 American Chemical Society
Figure 1. Buchwald-type biaryl phosphine ligands used in this study.
hydrosilylating agents, forming the corresponding 2-silylbiphenyls 3b,c in 92% and 43% yields, respectively (entries 4 and 5). Analogously, 1 underwent hydroboration with pinacolborane (2d) but with lower efficiency under the conditions that were optimal for hydrosilylation. The reaction at 140 °C using L3 as the ligand gave the hydroboration product 3d in 56% yield (entry 8). Next, we examined the reaction of 1 with organometallic compounds possessing intermetallic linkages; the results are summarized in Table 2.14 The bis-silylation reaction of 1 with hexamethyldisilane (4a) carried out in the presence of Pd(dba)2 (5 mol %) and L1 or L2 (10 mol %) in toluene at 110 °C gave 2,20 -bis(trimethylsilyl)biphenyl (5a) in good yields (entries 1 and 2).15 The reaction of 1 with 1,1,2,2-tetramethyl-1,2-diphenyldisilane (4b) afforded bis(silyl)biphenyl 5b in 75% yield (entry 4). Digermane 4c also reacted in the presence of the Pd(dba)2L3 catalyst, affording bis(germyl)biphenyl 5c in excellent yield (entry 5). In contrast, the reaction with distannane 4d was sluggish and the corresponding product 5d was formed in fairly low yield, even with a higher catalyst loading and a longer reaction time (entry 6). In addition to the homointermetallic linkages, heterointermetallic linkages of group 13 and 14 elements could also be employed in the bismetalative CC bond cleavage of 1. The addition of silylstannane 4e across 1 similarly occurred to form 2-silyl-20 -stannylbiphenyl 5e in 57% yield (entry 7). 2-Boryl-20 -silylbiphenyls 5f,g were obtained in Received: May 24, 2011 Published: July 12, 2011 3923
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Organometallics
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Table 1. Palladium-Catalyzed Hydrosilylation and Hydroboration of Biphenylene (1)a
entry
2 (HE)
1
2a (HSiEt3)
Table 3. Reaction of 1 with Diboron 4h
ligand
3 (yield, %b)
L1
3a (95)
2
2a
L2
3a (77)
3
2a
L3
3a (92)
4
2b (HSiMe2Ph)
L1
3b (92)
5
2c (HSi(OEt)3)
L1
3c (43)
6
2d (HB(pin)c)
L1
3d (35)
7 8
2d 2d
L2 L3
3d (19) 3d (56d)
yield, %a
a
entry
cat. (amt, mol %)
conditions
5h
3d
1 2
Pd(dba)2/L1 (5/10) Pd(dba)2/P(n-Bu)3 (10/20)
toluene, 110 °C, 4 h xylene, 140 °C, 5 h
8 57
34
3
Pd(PPh3)4 (10)
toluene, 110 °C, 9 h
57
Isolated yield.
a
Reaction conditions for hydrosilylation: 1, 2ac (1.5 equiv), Pd(dba)2 (5 mol %), ligand (10 mol %), toluene, 110 °C, 23 h. Reaction conditions for hydroboration: 1, 2d (2.0 equiv), Pd(dba)2 (5 mol %), ligand (10 mol %), xylene, 140 °C, 8 h. b Isolated yield. c pin = pinacolato (OCMe2CMe2O). d 3d was obtained in 20% yield when the reaction was carried out in toluene at 110 °C for 13 h.
Scheme 1. Catalytic Cycle for the Hydrometalative and Bismetalative CC Bond Cleavage of 1a
Table 2. Bismetalative CC Bond Cleavage of 1a
amt of Pd, 4 (EE0 )
entry
time,
mol %
ligand
h
5 (yield, %b)
1 2
4a (Me3SiSiMe3) 4a
5 5
L1 L2
6 3.5
5a (75) 5a (77c)
3
4a
5
L3
3
5a (66)
4
4b (PhMe2SiSiMe2Ph)d
5
L1
3
5b (75)
5
4c (Me3GeGeMe3)
5
L3
4
5c (90)
6
4d (Me3SnSnMe3)
10
L1
24
5d (22)
7
4e (Bu3SnSiMe3)
10
L3
7
5e (57)
8
4f (PhMe2SiB(pin))e
9
4g (Et3SiB(pin))e
5
L2
5
5f (61)
10
L2
4
5g (41)
a Reaction conditions: 1, 4 (1.5 equiv), Pd(dba)2 (5 or 10 mol %), ligand (10 or 20 mol %), toluene, 110 °C, 39 h. b Isolated yield. c 80% yield with 10 mol % of Pd(dba)2 and 20 mol % of L2. d 1.2 equiv. e 2.0 equiv.
moderate yields by the silaborative CC bond cleavage of 1 using silylboranes 4f,g, respectively (entries 8 and 9). Diboration of 1 with bis(pinacolato)diboron (4h) using biaryl phosphines as the ligands resulted in the formation of a mixture of the desired diboration product 5h and monoboration product 3d, with the latter predominating (Table 3, entry 1).16 After several ligands were screened, tri-n-butylphosphine was found to be an optimal ligand for the selective diboration of 1 in xylene at 140 °C, affording the 2,20 bis(boryl)biphenyl 4h in 57% yield. Interestingly, selective
a
E stands for H, B, Si, etc.
monoboration with 4h occurred with Pd(PPh3)4 as the catalyst (entry 3). Scheme 1 depicts a plausible catalytic cycle for the hydrometalative and bismetalative CC bond cleavage of biphenylene, although we have no experimental evidence for the intermediate species.17,18 First, the oxidative addition of the EE bond of A to Pd(0) species B occurs to form palladium(II) intermediate C. The CC bond of biphenylene (1) then oxidatively adds to C to form the palladium(IV) intermediate D. Finally, a double reductive elimination process (D f E f B) yields product F and regenerates the catalyst.19 In summary, we have developed a method for synthesizing 2-metallo- and 2,20 -bis(metallo)biphenyls by hydrometalation and bismetalation of biphenylene. Palladium(0)biaryl phosphine complexes work efficiently as catalysts for the introduction of main-group metallic elements into biphenylene via the cleavage of the CC bond.
’ ASSOCIATED CONTENT
bS
Supporting Information. Figures, text, and tables giving experimental procedures and characterization data for new
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Organometallics compounds. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Fax: +81 3 5261 4631. E-mail:
[email protected].
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