COPPER, SILICON, LEAD, BISMUTH, AND ZIRCONIUM COMPOUNDS
Other organometallic compounds have also been utilized for the coupling reaction with acyl halides.
The Suzuki protocol (see Sect. III.2.2) for organoboron compounds [R3B and RB(OR)2] cannot be applied for the cross-coupling reaction, since acyl halides react readily with NaOH to give the less reactive carboxylic acids. Instead of these organoboranes, organoborates (R4B) were used for the reaction. Actually, organoborates themselves are known to react with acyl chlorides smoothly in THF-hexane mixed solvent system to give the desired ketones in good yields (Scheme 10).[5],[46]Neither alkenylboranes nor alkenylborates undergo clean acylation even in the presence of palladium catalyst.[15] In contrast, when sodium tetraphenylborate was used, the coupling reaction with acyl chlorides did not proceed, but the desired ketones were produced in the presence of palladium catalyst (Scheme 10).[47]
NaBPh4 + Ph O
Cl Pd(PPh3)4 (1 mol %) THF, 25 °C, 20 h
[47]
94%
Ph O Ph B– n-Bu
Li+ +
Ph O
Cl n-Bu
O Ph 89%
THF-hexane, 25 °C [5]
“without catalyst”
Scheme 10
Organoaluminum compounds undergo the coupling reaction with acyl chlorides in the presence of palladium catalyst.[15],[48]Not only alkyl-, alkenyl-, and alkynylalanes but also alkenylalanates are used for the reaction. This reaction can also proceed in the absence of palladium catalyst, but the stereoselectivity is much better when the reaction is catalyzed by palladium (Scheme 11).[15]
n-Hex
AlMe2
Me
+ Me
O Cl
1,2-dichloroethane, −30 °C
“Pd(PPh3)2”[18] (cat.), THF, r.t.
n-Hex
AlMe2(n-Bu)Li Me
n-Hex
Me O
Me
Conditions
+ Me
O
Cl conditions B conditions A or B
>90%−
Yield Selectivity 66−93%
92%
[15]
[15]
n-Hex
Me O
Me
>90% 96%
Yield Selectivity
+ Ph
O
Cl Pd(PPh
3)4 (5 mol %) THF, 0−25 °C
[48]
n-Bu AlEt2
n-Bu
O
Ph 74%
A:
B:
Scheme 11
Organozirconium compounds also undergo the Pd-catalyzed cross-coupling reaction with acyl halides.[15] In the presence of ZnCl2, the reaction can be carried out more efficiently.[17]Acylzirconium species, which are produced via hydrozirconation of alkynes or alkenes by zirconocene hydrochloride followed by insertion of CO, couple with acyl chlorides to give -diketones (Scheme 12).[49]
n-Hex Cp2ZrHCl
CH2Cl2, r.t., 30 min
CO (1 atm) r.t., 2 h n-Hex
ZrCP2
Cl
n-Hex
O ZrCP2 Cl
Ph
O Cl
PdCl2(PPh3)2 (5 mol %) toluene, r.t., 48 h
[49]
n-Hex
O O
Ph 65%
Scheme 12
Tetraalkylsilanes are stable compounds. Therefore, it is necessary to activate the compounds for the coupling reaction. Since dialkylsilacyclobutanes have high strain energy, palladium complexes can easily insert into the carbon–silicon bond oxidatively, and the resultant complexes couple with acyl chlorides to give 1-sila-2-oxa-3-cyclohex- ene derivatives (Scheme 14).[52]
As mentioned before, the Pd-catalyzed coupling reaction of tetraalkyltins with acyl halides is troublesome: the reaction is slow and requires high temperature. In particular, when tetraalkyltin compounds bearing longer alkyl chains are used, the reaction becomes sluggish. In such cases, tetraalkylleads can be used instead of tetraalkyltins (Scheme 15).[53]It is noteworthy that two alkyl groups on the lead can be used for the coupling reaction, whereas only one alkyl group can couple with acyl chlorides when organotin compounds are used. The reason for the different reactivity is presumably
H
H + (n-Hept)2CuLi
n-Hept CuLi
2
+
Me Cl
O n-Hept O
Me Pd(PPh3)4 (3 mol %)
ZnBr2 (1.0 equiv) THF, r.t.
[50] 80%
H Me
Me i-Pr
Cu MgBr2
Me
i-Pr O t-Bu
84%
THF
+ THF
+
t-Bu Cl
O
Pd(PPh3)4 (3 mol %) THF, r.t.
[50]
H
Ph + Me2N
O Cl
PdCl2(PPh3)2 (0.1 mol %) CuI (0.5 mol %), PPh3 (0.5 mol %)
Et3N, 90 °C, 6 h [51]
Ph
O NMe2
92%
(i-Pr)Cu MgBr2
Scheme 13
Organocopper compounds are known to react with acyl halides to produce ke- tones.[3] In the case of alkenyl- and alkynylcopper compounds, however, the conju- gated addition of the organocopper compounds to the resultant ketones has been ob- served. To carry out the reaction successfully, palladium–phosphine complexes are used as catalysts. Alkenylcopper compounds in association with magnesium salts cou- ple with acyl halides in the presence of Pd(PPh3)4to give the desired ketones in good yields (Scheme 13).[50]In the case of lithium dialkenylcuprate, MgCl2or ZnCl2is re- quired to perform the coupling reaction successfully. The coupling reaction of alkynes with acyl halides can be carried out in the presence of PdCl2(PPh3)2and CuI catalysts (Scheme 13).[51]
Ph Cl O
Pd(PPh3)4 (1 mol %) THF, 65 °C, 3 h
[53] 84%
Cl Me
O Me
n-Bu Me
O Me Ph
Et O
72%
+
+
Pd(PPh3)4 (1 mol %) THF, 65 °C, 4 h
[53]
Ph3Bi + Me O
Cl Pd(OAc)2 (5 mol %) HMPA, 65 °C, 5 h (excess) [54]
Me O Ph
92%
Et4Pb (0.6 equiv)
n-Bu4Pb (0.6 equiv)
Scheme 15 Scheme 14 SiMe2
Ph O
Cl +
Et3N (0.1 equiv), toluene, r.t., 2 h [52]
Me2ClSi Ph
O 86%
PdCl2(PhCN)2 (4 mol %) Et3N (2 equiv), toluene, 80 °C, 4 h
[52]
PdCl2(PhCN)2 (4 mol %)
Me2Si O Ph 97%
due to the ease of disproportionation of trialkylchloroleads to tetraalkylleads and dialkyldichloroleads.
Triphenylbismuth can also be utilized for the coupling reaction (Scheme 15).[54]All three phenyl groups on bismuth can be used, since disproportionation of the intermediary dialkylchloro- and alkyldichlorobismuth is fast.
Siloxycyclopropanes are considered as homoenolate equivalents and also undergo the coupling reaction with acyl halides to produce 1,4-dicarbonyl compounds (Scheme 16).[55]
OSiMe3 OEt
O Cl Me
Cl PdCl
2(PPh3)2
(5 mol %) CHCl3, 90−100 °C
[55] O
O Cl EtO
Me 93%
+
Scheme 16
F. SUMMARY
1. Palladium-catalyzed coupling reactions of organozinc compounds with acyl halides are the fastest and mildest among the known methods using a variety of organometallic compounds.
2. Since organotin compounds are stable to water and oxygen, the method has widely been utilized for the synthesis of various compounds although the reaction requires a long time and high temperature.
3. Organometallic compounds, such as organomercury, boron, aluminum, zirconium, copper, silane, lead, and bismuth, can also be utilized for the coupling reactions, although these methods have not been studied extensively.
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649
III.2.12.2 Palladium-Catalyzed Cross-Coupling with Other -Hetero-Substituted
Organic Electrophiles
TAKUMICHI SUGIHARA
A. INTRODUCTION
-Hetero-substituted organic electrophiles include heteroaromatics and -hetero-substi- tuted alkenes having some leaving group at the -position. Since the heteroaromatics are quite stable due to aromaticity, the Pd-catalyzed coupling reaction of organometallic compounds usually proceeds without decomposition of the aromatic ring. It means that the heteroaromatics having the leaving group at the -position just behave as aryl halides.
In contrast, -hetero-substituted alkenes having the leaving group at the -position have three possibilities to react with organometallic compounds giving cis-, trans-, or ,-disubstituted alkenes. In this section, the Pd-catalyzed coupling reaction with - hetero-substituted organic electrophiles is discussed.