3.6 Cross-Coupling Reactions with Organometallic Compounds
3.6.6 Organosilicon Compounds (Hiyama Coupling)
3.6.6.1 Introduction and Preparative Methods of Organosilicon Compounds In contrast to organometallic reagents of Mg, B, and Sn, organosilicon com- pounds are inert for transmetallation under normal Pd-catalyzed coupling condi- tions. Hiyama has discovered that the C—Si bond can be activated by nucleophiles such as F−and OH−by forming pentacoordianted silicates1, and transmetallation of organosilicon compounds proceeds in the presence of a fluoride anion source as a promoter. Thus, smooth cross-coupling of organosilanes with halides is pos- sible by the addition of a stoichiometric amount of promoters, typically TASF [(Et2N)3S+(Me3SiF2−)] or TBAF [(n-Bu)4NF], and the reaction is called Hiyama coupling [75,76]. KF and CsF are effective only in some cases. Recent improve- ment in the coupling reaction has been reviewed by Denmark and Sweis [77].
Si R1 R3 R2
Y F
R1R2R3SiY + F Ar-Pd-R1 +R2R3SiYF
1 Ar-R1 + Pd(0)
Ar-PdX
−
−
An established synthetic method for arylsilanes is lithiation of aryl halides, fol- lowed by the reaction with R3SiCl. Alkenylsilanes are produced by hydrosilylation of 1-alkynes catalyzed by Rh or Pt complexes. Also aryl- and alkenylsilanes2are synthesized by Pd-catalyzed reaction of hexamethyldisilane with halides in the presence of TASF [78,79].
I
Pd(PPh3)4, TASF R3SiCl
H-SiR3
Br R
Li R
+
SiR3 R
8 8 R
HMPA, rt, 92%
R
SiR3 Pt or Rh cat
+
MeO2C MeO2C SiMe3
2 BuLi
Me3SiSiMe3
Masuda and co-workers have discovered a new synthetic method for aryltri- ethoxysilanes3. They found that3can be prepared in high yields by Pd-catalyzed silylation of aryl iodides such as 4-iodoanisole and bromides with triethoxysilane using P(o-Tol)3as a ligand in NMP [80]. Of course, the silylation is competitive with hydrogenolysis to form the reduced arene4. DeShong and co-workers have studied the scope and limitation of the reaction [81]. They found that biphenylyl(di- t-butyl)phosphineIV-1 is the most effective ligand. Although aryl iodides are the substrates of choice, an acceptable yield of arylsiloxanes may be obtained from the corresponding bromide. Interestingly, satisfactory results were obtained from electron-rich 4-iodoanisole and 4-bromoanisole (5). The steric effects on the reac- tion are serious and no reaction of 2-bromoanisole (6) occurs.
3 4
i-Pr2NEt, NMP, 68%
Pd2(dba)3, P(o-Tol)3
i-Pr2NEt, NMP, 92%
I Si(OEt)3
OMe
+
Pd2(dba)3,IV-1
i-Pr2NEt, NMP, 86% OMe
H
Pd2(dba)3, (IV-1) H-Si(OEt)3
OMe Br
OMe
Si(OEt)3
OMe 5
H-Si(OEt)3
+
i-Pr2NEt, NMP, 68%
H-Si(OEt)3
0 % 6
Pd2(dba)3, (IV-1) Br
OMe
Si(OEt)3
OMe +
One of the advantages of Hiyama coupling is that silicon is a comparatively environmentally benign element, since organosilicon compounds are oxidized ulti- mately to inactive silica gel. Better tolerance to functional groups is another advantage. A drawback, however, is consumption of more than stoichiometric amounts of expensive additives. In addition, functional groups protected by silyl groups can not be tolerated, because they are deprotected by a fluoride anion.
3.6.6.2 Couplings of Arylsilanes
Arylsilanes bearing heteroatoms such as F, Cl, OH, and OR can be used as coupling partners. Of these, OH appears to be most reactive. Easily prepared arylalkoxysi- lanes are good coupling partners [82,83]. The carbene ligand XVI-2 is a good ligand for the coupling of phenyltrimethoxysilane (7) with activated aryl chlorides.
In the coupling of 4-bromotoluene, PCy3 is more effective than the carbene lig- and [84]. Curiously, coupling of7with deactivated 4-bromoanisole (5) proceeded in DMF to afford 8 even when PPh3 was used. Furthermore, biphenylyldicyclo- hexylphosphine IV-2 is effective for the coupling of 4-chloroanisole with 7 to provide8 [85].
5
7 8
7 8
Si(OMe)3 Me
Br
Me
OMe
Br
OMe
Cl Si(OMe)3
Si(OMe)3
OMe OMe 7
Pd(OAc)2, PCy3
TBAF, dioxane, 100%
+
TBAF, DMF 85°C, 74%
Pd(OAc)2,IV-2 +
Pd(OAc)2, PPh3
TBAF, DMF 85°C, 71%
+
Lee and Fu reported that coupling of phenyltrimethoxysilane (7) with alkyl bromides is possible under selected conditions. 1-Phenyldodecane was obtained in 81 % yield by the reaction of 7 withn-dodecyl bromide at room temperature.
P(t-Bu)2Me as an effective ligand and Bu4NF (2.4 equiv.) as an activator were used [86].
PdBr2, P(t-Bu)2Me Bu4NF, rt, 81%
7
+ Si(OMe)3
C12H25Br C12H25Ph
Some halides on silicon are activating groups. Generally two fluorine atoms are required for aryl –aryl coupling. For example, coupling of ethyl(2-thienyl)difluoro- silane (9) with 3-iodothiophene10afforded the bisthiophene11using a ligandless Pd catalyst in the presence of KF [87].
10
11 + (h3-allyl-PdCl)2
9
DMF, KF, 100 °C 82%
S Si(Et)F2 S
I
MeO2C
MeO2C S S
Electron-deficient aryl chlorides can be coupled with organo di- or trichlorosi- lanes such as aryldichloroethylsilane and aryltrichlorosilane in the presence of KF.
In this case, thein situfluorination of chlorosilanes with KF occurs. Electron-rich alkylphosphines such as tri(isopropyl)phosphine or bis(dicyclohexylphosphino)- ethane are effective ligands. For example, coupling of the dichlorosilane12 with 4-chloroacetophenone at 120◦C in DMF gave 13in 62 % yield [88].
+
12
KF, DMF 120°C, 62%
13 Cl
SiEtCl2
MeO
O OMe
O PdCl2[P(i-Pr)3]2
In addition to dichloro groups, some other activating groups are known. Denmark and Wu found that arylchlorosiletanes [aryl(chloro or fluoro)silacyclobutanes] are reactive coupling partners and ascribed the high reactivity at first to ‘strain release Lewis acidity’ of the siletane, but later to the formation of ring-opened silanol derivatives. The anisylchlorosiletane14reacted smoothly with iodobenzene using P(t-Bu)3as a ligand in the presence of TBAF [89].
14
+ P(t-Bu)3, TBAF THF reflux, 91%
Si Cl
OMe
I
OMe 8
(h3-allyl-PdCl)2
3.6.6.3 Couplings of Alkenylsilanes
Trimethylsilanes are easily available. Coupling of commercially available trimethylvinylsilane (15) with the iodide 16 proceeds most satisfactorily using TASF and ligandless Pd in HMPA [90].
TASF, HMPA 50°C, 98%
15
16
(h3-allyl-PdCl)2, SiMe3
I +
Silanes couple under promoter-free conditions with good electrophiles like aryl- diazonium salts. Matsuda and co-workers have discovered that benzenediazonium tetrafluoroborate reacts withβ-trimethylsilylstyrene derivative17using ligandless
Pd(dba)2in MeCN at room temperature. Two products, namely the expected ipso product18 and thecine product19, were obtained [91]. On the other hand, reac- tion of α-trimethylsilylstyrene 20 with 21 afforded the cine product 22 regio- and stereoselectively [92]. An application of the coupling of aryldiazonium salt to α,α-bis(trimethylsilyl)-1,4-divinylbenzene derivative 23 afforded the (E, E)- bis(styryl)benzene derivative 24cleanly [93].
18 19
MeCN, 25 °C 100%
Pd(dba)2
+ SiMe3
Me N2BF4
Me
Me
74 : 26 + 17
N2BF4
SiMe3
Me Me
+ Pd(dba)2
MeCN, 25 °C 97%
20
21 22
+
Pd(dba)2
MeCN, 80%
N2BF4
Br
C8H17O
OC8H17
Me3Si SiMe3
C8H17O
OC8H17 Br
Br 23
24
The reaction of17(or25) to give19(or31) can not be explained by the ‘oxida- tive addition–transmetallation–reductive elimination’ mechanism. In the reaction of 25, carbopalladation to form 26 and 27 is the first step. Desilylpalladation of 26 affords the expected ipso product 28. On the other hand, the intermediate 27 undergoes syn dehydropalladation to give 29, to which syn addition of H-PdX occurs to generate30. Then anti desilylpalladation provides the cine product 31.
This reaction is not completely fluoride-free, because the BF4−anion is present.
anti elimination +
29
31 syn addition
syn addition
syn elimination
syn addition
anti elimination 25
26 Ph
H H SiMe3
Ar Pd
H SiMe3 Ph H
28
27
30
Ph
Ar H
H
Pd Ar
SiMe3
Ph H Ph
Ar
SiMe3 H
H Pd
SiMe3 Ph Ar
Ph Ar
F ArN2BF4
Ar-Pd-X
H-PdX Ar-Pd-X
H-PdX Pd(0)
−
F−
Reactivity of alkenylsilanes changes by replacing one to three methyl groups in 32 with other groups. Introduction of F enhances reactivity as shown by the reaction of 1-silyl-1-octene 32 with 1-naphthyl iodide (33) to afford 34. Intro- duction of one F gives the highest reactivity possible owing to easy formation of the pentacoordinate silicate from the alkenylsilane without any electron-donating group. No reaction occurs with octenyltrimethylsilane under this condition [94].
(h3-allyl-PdCl)2, +
34 TASF, THF, 50 °C
n= 0 n= 1 n= 2 n= 3 SiMe3-nFn
24 h 10 h 48 h 24 h
0%
81%
74%
0%
33 32
C6H13
C6H13 I
Coupling of thetrans-styrylsilane35with iodobenzene gave rise mainly totrans- stilbene (36) as theipso product and only a small amount of thecine product37.
Under the same conditions, cine product 41 was obtained to some extent from theα-silylstyrene 38in addition toipsoproduct 40. Theipso : cine ratios change depending on substituents (R) of the aryl iodides39 [95].
Ph-I
(h3-allyl-PdCl)2 TBAF, THF 60°C, 93%
96 : 4 Ph
SiFMe2
Ph Ph Ph
Ph
Ph Me2FSi
Ph Ar
Ph
Ar I
R
93 : 7 75 : 25 60 : 40
35 36 37
41 40
39 38
72%
69%
63%
R = CF3 = H = OEt
+ +
+ +
(h3-allyl-PdCl)2, TBAF, THF 60°C, 93%
The 2-thienyl (Th) group in the alkenylsilane 42 also promotes the coupling, which occurs at room temperature in the presence of TBAF to provide 43 [96].
The 2-thienyl group is considered to facilitate the formation of pentacoordinate silicate or to be replaced readily by OH.
42 Th = 2-thienyl 2 43
+ THF, rt, 97%
Pd(OAc)2, TBAF C6H13
SiMe2Th
C6H13
OMe I
MeO
The alkenylsiletane44is very reactive and reacts with 4-iodoanisole (2) at room temperature with ligandless Pd catalyst in 10 min to give 46 in 94 % yield [97].
Here again 44is considered to be transformed to a silanol before coupling.
44
Pd2(dba)3, TBAF +
46 rt, 10 min, 94%
2 I
MeO Si
n-C5H11 Me
MeO
C5H11
3.6.6.4 Couplings of Aryl- and Alkenylsilanols, and Alkoxysilanes
Silanols are apparently the most effective coupling reagents, which react under mild conditions in the presence of promoters [98]. The coupling of alkenylsilanols with deactivated aryl iodides occurs with ligandless Pd(dba)2 [99]. Reaction of the silanol 47 with the triflate 48 proceeded smoothly in the presence of bulky biphenylylphosphine (IV-1). Curiously, PdBr2 was a more active catalyst than PdCl2 and use of TBAFã6H2O gave better results [100,101].
Coupling of the silanol47with electron-deficient 4-iodoacetophenone proceeded at room temperature in 10 min to afford 49 in 79 % yield using ligandless Pd catalyst. Also coupling with the less reactive aryl iodide 2 occurred efficiently with ligandless Pd catalyst [99].
49 +
Pd2(dba)3, TBAF rt, 10 min, 79%
Pd2(dba)3, TBAF rt, 10 min, 95%
TBAF-6H2O dioxane, 93%
47
47
46
n-C5H11
Si OH
Me Me
I
O
C5H11
O
n-C5H11 Si OH
Me Me
I
MeO MeO
C5H11
C5H11 Si
OMe
TfO Me
Me
OH MeO
C5H11
2 46
48 47
+
PdBr2,IV-1
+
Vinylation of aryl iodides can be carried out using the commercially available cyclooligodisiloxane50at room temperature [102].
Pd2(dba)3, TBAF
+ rt, 6 h, 63%
51 2
50
I
MeO OMe
Si O
Si O Si O O Si
Me
Me Me
Me
The coupling products are obtained by one-pot hydrosilylation/cross-coupling of alkynes. Pt-catalyzed hydrosilylation of alkyne with tetramethyldisiloxane (52) generates the alkenylsilane53, which is coupled with 4-iodoanisole (2) using lig- andless Pd catalyst and TBAF at room temperature to afford46in 84 % yield [103].
MeO
n-C5H11
I
MeO
2 +
52
2
46 Pd2(dba)3, TBAF
DVDS = 1,3-divinyl-1,1,3,3-tetramethyldisiloxane
rt, 10 min., 84%
Si O Si Me H
Me Me
Me H
n-C5H11
n-C5H11 Si Me
Me O 53
t-Bu3P-Pt(DVDS)
Coupling of reactive alkoxyalkenylsilanes has been applied to the synthesis of medium-sized rings. The cyclic silyl ether 54 was converted to cyclodeca- 3,5-dienol (55) at room temperature [104]. In many silane coupling reactions, it has been claimed that ligandlessπ-allylpalladium chloride is an effective catalyst precursor. Possibly, chloride ion is essential for transmetallation as compared with unreactive Pd(OAc)2.
I Si
O Me Me
OH (h3-allyl-PdCl)2,
TBAF, THF, rt, 63%
54
55
3.6.6.5 Fluoride-Free Procedures
The couplings of organosilanes discussed so far are carried out in the presence of more than equimolar amounts of promoters, typically TBAF, which is expensive.
The promoters preclude wide use of the reaction. Mori and Hiyama discovered that Ag2O can be used as a promoter for the reaction of silanols [105]. Reaction of the arylsilanol 56 with iodobenzene to give 8 proceeded smoothly in THF in the presence of an equimolar amount of Ag2O. Interestingly no reaction occurred when Pd(OAc)2, PPh3, and TBAF were used. Silanediols and silanetriols are more reactive than silanols. Coupling of the arylsilanediol57, alkenylsilanediol58, and the triol 60 with less reactive 4-iodoanisole (2) occurred smoothly to provide 8, 59 and61. These silanols are prepared easily by hydrolysis of the corresponding chlorosilanes and used without isolation.
+
59
61 2
2
2
+ Pd(PPh3)4, Ag2O THF, 60 °C, 80%
+
+ 56
SiMe2OH I
OMe
60
SiEt(OH)2 I
OMe MeO
MeO
Ph
SiMe(OH)2
Ph I
MeO
Ph Ph
OMe
Ph
Si(OH)3
I
MeO
Ph
OMe 8
8
57
58
Pd(PPh3)4, Ag2O THF, 60 °C, 80%
Pd(PPh3)4, Ag2O THF, 60 °C, 71%
Pd(PPh3)4, Ag2O THF, 60 °C, 76%
Arylsilanols, although less reactive than alkenylsilanols, can be coupled in the presence of Cs2CO3 (2 equiv.) and AsPh3 in toluene. Under these fluoride-free conditions, coupling of p-methoxyphenylsilanol 56 with phenyl iodide afforded the coupling product 8 in high yield. Homocoupling occurred to a small extent.
Coupling of less reactive phenyl bromide proceeded selectively using DPPB as a ligand. Control of water content in Cs2CO3 is important for maximum activ- ity [106].
8
+ Cs2CO3+ 3H2O (h3-allyl-PdCl)2, AsPh3
90°C, toluene, 91%
56
+ 93.7 : 6.3 MeO
Si
I Me
OH Me
MeO OMe
8
+ Cs2CO3+ 3H2O (h3-allyl-PdCl)2, DPPB
90°C, toluene, 85%
56
+ 100 : 0 MeO
Si
Br Me
OH Me
MeO OMe
A straightforward route to diarylmethanes is double cross-coupling of (2-pyridyl)- silylmethylstannane 62with aryl halides. First, chemoselective coupling of alkyl- stannane in 62with 3-iodoanisole produced 63using P(C6F5)3 as a ligand. Then reaction of 4-iodoanisole (2) with the pyridylsilane63took place in the presence of Ag2O to give the diarylmethane64. The pyridyl group in63seems to activate the silane group for the coupling by coordination to Pd to assist transmetallation [107].
N Si Me2 Bu3Sn
N Si Me2
I OMe
I
OMe MeO
MeO
OMe THF, 65%
PdCl2(MeCN)2, P(C6F5)3
Pd(PPh3)4, Ag2O 62
64 63
2 THF, 44%
As a more useful fluoride-free process, the Hiyama group reported that coupling of ArSiRCl2 occurs smoothly by the addition of powdered NaOH (6 equiv.) as an activator, instead of fluorides. Coupling of activated aryl bromide 66 with65 proceeded in refluxing benzene using Pd(OAc)2 and PPh3 [108]. LiOH, K2CO3, and Na2CO3are less effective. Similarly, coupling of alkenyldichlorosilane68with 3-chlorotoluene (69) took place at 80◦C to give70. P(i-Pr)3is an effective ligand.
65
66
67
+ NaOH, benzene
80°C, 91%
SiEtCl2 Br
Me
Cl Me
n-Bu
SiMeCl2 O
O
n-Bu
68
69 70
Pd(OAc)2, PPh3 NaOH, benzene 80°C, 89%
+
PdCl2[P(i-Pr)3]2
Another good solution for the desirable fluoride- and silver-free procedure has been demonstrated by the Denmark group. They found that the reaction of the alkenylsilanol 47 with 4-iodoanisole (2) to afford 46 proceeded smoothly in the presence of 2 equivalents of KOSiMe3 as a base without other promoters using ligandless Pd(dba)2 as a catalyst in DME at room temperature [109]. Coupling of TBS-protected 2-iodophenol (71) with47 by this method proceeded to afford 72 without cleaving the silyl protection.
Further development of the fluorine-free procedure will certainly promote silane coupling chemistry.
47 71
Pd(dba)2, KOSiMe3
+
Pd(dba)2, KOSiMe3
2
DME, rt, 9 h, 80%
72 C5H11 Si
OH
Me Me I
MeO C5H11
OMe
+ DME, rt, 1 h, 88%
C5H11
Si OH Me Me
I C5H11
OMe
TBSO OTBS
47 46
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