951
Table 1. Pd-Catalyzed Suzuki-Miyaura allenylation reactions
of aryl iodides with B-allenyl-9-BBNa
triflate could not be allenylated in preparatively meaningful
yields under these conditions.
The new method allowed an assortment of aromatic and
heteroaromatic iodides to be allenylated in moderate to good
yields, independent of whether they carry electron-withdrawing
or -donating substituents (Table 1). As expected, various func-
tional groups remained intact. Surprisingly, however, ethyl
ortho-iodobenzoate could not be cross coupled (Entry 3),
whereas its para- and meta-congeners reacted smoothly (Entries
1 and 2). Since other ortho-substituted iodides were found
well suited (Entries 4, 10-12, 14, and 15), this failure is
tentatively ascribed to a ligation of the intermediate arylpalla-
dium species by the adjacent carbonyl group, which seems to be
sufficiently strong at ambient temperature to prevent trans-
metallation by the allenyl donor. When dihalobenzene deriva-
tives are used as the substrates, the allenylation occurs
exclusively at the iodide group, leaving a bromide, activated
chloride or fluoride substituent untouched (Entries 8, 9, 10, and
14). In the heterocyclic series, N,N-dibenzyl-5-iodouracil was
inert (Entry 13),20 whereas its O,O-dimethylated analog shown
in Entry 15 was allenylated in respectable yield.
Entry
Substrate
Product
Yield/%
COOEt
COOEt
COOEt
1
70
I
I
2
3
95
nr
COOEt
I
⎯
COOEt
I
4
5
6
7
89
87
91
51
OMe
OMe
I
MeO
F3C
MeO
F3C
I
I
Overall, we conclude that borane 7 in combination with
NaOMe is a suitable donor for Suzuki-Miyaura type allenyla-
tion reactions. Although the scope of the method still needs to be
extended beyond aryl iodides, it compares well to cross coupling
reactions using toxic allenyltin reagents or nonmetallic three-
carbon donors as the currently most widely practiced alterna-
tives.8,11
NC
NC
I
I
8
9
69
88
93
Br
Br
Br
Br
Preparation of B-allenyl-9-BBN: A first portion of propar-
gyl bromide (ca. 3 mL) is quickly added to a suspension of Al
grit (>97%, 3.33 g, 123 mmol) and HgCl2 (100 mg) in an-
hydrous Et2O (100 mL) and the resulting mixture is stirred at
ambient temperature for 15 min under Ar. After that time, a
mixture of the remaining 2-propynyl bromide (27.18 g total,
184 mmol) and 9-MeO-9-BBN (18.6 g, 122 mmol) is added over
45 min at such a rate as to maintain gentle reflux. Once the
addition is complete, the mixture is refluxed for 3.5 h before
insoluble residues are filtered off under Ar. The solvent is
carefully distilled off at ambient pressure and the residue
purified by distillation under vacuum to give B-allenyl-9-BBN
(bp 29-31 °C, 10¹3 mbar) as a colorless liquid (10.3 g, 52%).
1H NMR (400 MHz, CDCl3): ¤ 5.63 (t, J = 6.4 Hz, 1H), 4.64 (d,
J = 6.4 Hz, 2H), 2.00-1.84 (m, 6H), 1.84-1.68 (m, 6H), 1.38-
1.21 (m, 2H); 13C NMR (100 MHz, CDCl3): ¤ 220.8, 89.0 (br),
68.2, 34.2, 31.0 (br), 23.9; 11B NMR (128 MHz, CDCl3): ¤ 80.9.
Representative procedure: B-Allenyl-9-BBN 7 (394 mg,
2.46 mmol), NaOMe (133 mg, 2.46 mmol), and [Pd(PPh3)4]
(79 mg, 0.068 mmol) are successively added to a solution of
ethyl 4-iodobenzoate (9) (378 mg, 1.37 mmol) in degassed DMF
(5.5 mL) and the resulting mixture is stirred at ambient temper-
ature for 3 h under Ar. For work up, the mixture is diluted with
EtOAc (10 mL) and the reaction quenched with sat. aq. NH4Cl
(20 mL). The aqueous phase is extracted with EtOAc
(4 © 15 mL), the combined organic layers are washed with
brine, dried over Na2SO4, and evaporated, and the residue is
purified by flash chromatography (pentane/Et2O, 10:1) to give
allene 10 as a pale yellow liquid (180 mg, 70%). 1H NMR
(400 MHz, CDCl3): ¤ 7.98 (m, 2H), 7.34 (m, 2H), 6.20 (t,
J = 6.8 Hz, 1H), 5.20 (d, J = 6.8 Hz, 2H), 4.37 (q, J = 7.1 Hz,
2H), 1.39 (t, J = 7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3): ¤
I
10
F
F
MeOOC
MeO
I
MeOOC
MeO
11
12
94
74
OMe
OMe
OMe
I
OMe
I
O
BnN
13
⎯
nr
O
N
Bn
MeO
MeO
I
N
N
14
15
50
73
Cl
N
Cl
N
MeO
MeO
I
N
N
N
MeO
MeO
N
aAll reactions were performed using 7 (1.8 equiv), NaOMe
(1.8 equiv), and [Pd(PPh3)4] (5 mol %) in DMF at ambient
temperature; nr: no reaction. For characteristic spectral data of
the compounds, see the Supporting Information.21
210.6, 166.4, 138.9, 129.9, 128.8, 126.5, 93.6, 79.2, 60.8, 14.3;
~
IR (neat): ¯ = 2982, 2901, 1939, 1710, 1606, 1436, 1268, 1174,
1100, 1018, 872, 852, 773, 704 cm¹1; MS (EI) m/z (%): 188
(80) [M+], 160 (16), 143 (100), 115 (46); HRMS (ESI) m/z:
calcd for [C12H12O2 + Na]+: 211.07300; found: 211.07295.
Chem. Lett. 2011, 40, 950-952
© 2011 The Chemical Society of Japan