Communications
Table 4: Ir-catalyzed selective reaction of w-arylalkanols 1 to b-methylhydroxy-a,w-
diarylalkanes 5 (Scheme 1, route B, step 1).[a]
successfully converted into the desired a,w-diary-
lalkanes 2 in good to excellent yields with high
selectivity (Table 6).
Entry w-Arylalkanol 1
Product 5
Yield
[%][b]
Although a detailed reaction mechanism for the
present coupling reaction has not been fully con-
firmed at this stage, the above Ir-catalyzed trans-
formation, exemplified by the conversion of 2-
phenylethanol (1a) to 1,3-diphenylpropane (2a),
can be rationally explained by the following sequen-
tial pathway (Scheme 2). First, the Ir catalyst serves
as a hydrogen acceptor from substrate 2a to give the
aldehyde A and an Ir-hydride species. Then alde-
hyde A reacts by a base-catalyzed aldol condensa-
tion to give the unsaturated aldehyde B and water.
Subsequently, B undergoes hydrogenation by the Ir-
hydride complex to give the intermediate 5a (step
1). Hydrogen transfer from alcohol 5a then gives
aldehyde C and an Ir-hydride complex. Then the
1
2
3
1h
1i
5h 81
5i 67
5j 66
5k 51
1j
4
5
6
1k
1l
5l 61
5m 55
1m
À
C(O) H bond of aldehyde C undergoes oxidative
addition to the Ir complex, followed by extrusion of
[a] The reaction was performed under the reaction conditions shown in Table 2
entry 9. [b] Yields of the isolated product after purification.
CO and b-hydrogen elimination, thus leading to the
alkyl chains. This preparation uses the sequential two-step
method, which involves the isolation of b-methylhydroxy-
a,w-diarylalkanes 5 as intermediates (Scheme 1, route B).
Therefore, compound 5 was initially prepared from 1 under
the reaction conditions shown in Table 3, entry 9 (Table 4).
Various w-arylalkanols (1h–k) were smoothly converted
under these reaction conditions and we successfully isolated
the corresponding b-methylhydroxy-a,w-diarylalkanes (5h–
5m) selectively in pure form.
Table 5: Ir-catalyzed reaction of b-(phenylmethyl)benzenepentanol (5h)
under various conditions (Scheme 1, route B, step 2).[a]
Entry Ir-catalyst [mol%]
Ligand
[mol%]
Yield of 2h [%][b]
1[c]
2
3
[(Cp*IrCl2)2](6)
[{IrCl(cod)}2](6)
none
35
Next, optimization of the reaction conditions for the
dehydrogenation/decarbonylation step (Scheme 1, route B,
step 2), from 5 to the a,w-diarylalkanes 2, was performed
using 5h as a model substrate (Table 5). Initially, the reaction
of 5h was carried out in the presence of [(Cp*IrCl2)2] catalyst
(6 mol%) combined with a base (K2CO3, 20 mol%) at 1608C.
This resulted in the formation of the desired 1,5-diphenyl-
pentane (2h) in low yield (35%; Table 5, entry 1).
To date, intense attention has been paid to the Ir- and Rh-
catalyzed decarbonylation of aldehydes.[15] Recently, Tsuji
and co-workers reported that [{IrCl(cod)}2] combined with
diphosphine ligands provides an active catalyst for the
decarbonylation of aldehydes.[15a] Therefore, the [{IrCl-
(cod)}2]/dppe catalytic system was employed in the reaction
of 5h. Selective formation of 2h was observed, however, the
yield was still not sufficiently high (Table 5, entry 2). After
further investigations we attained the optimized reaction
conditions, in which a combined [(Cp*IrCl2)2]/[{IrCl(cod)}2]/
dppe catalyst system was used, and under these reaction
conditions 2h was obtained in excellent yield (Table 5,
entry 3). The choice of the phosphine ligand was also
important and the best result was obtained when dppe was
used as the ligand; other phosphine ligands such as PPh3 and
binap gave lower yields of 2h.
dppe (12) 70
[(Cp*IrCl2)2](2)/[{IrCl(cod)}2](4) dppe (8) 96(93)
4
5
[(Cp*IrCl2)2](2)/[{IrCl(cod)}2](4) PPh3 (16) 54
[(Cp*IrCl2)2](2)/[{IrCl(cod)}2](4) binap (8) 66
6[d]
7[e]
[(Cp*IrCl2)2](2)/[{IrCl(cod)}2](4) dppe (8)
[(Cp*IrCl2)2](2)/[{IrCl(cod)}2](4) dppe (8)
94
89
[a] Reaction conditions : 5h (1 mmol) was treated with Ir catalyst, ligand,
and K2CO3 (20 mol%) in mesitylene (1.5 mL) at 1608C for 24 h.
[b] Yields were determined by GC using hexadecane as the internal
standard. The number in parenthesis shows the yield of the isolated
product. [c] In addition, 1,5-diphenyl-pentane and 2-benzyl-5-phenyl-
pentanal were obtained in 9% and 4%, respectively. [d] tBuOK
(20 mol%) was used as a base. [e] Cs2CO3 (20 mol%) was used as a
base. binap=rac-2,2’-bis(diphenylphosphino)-1,1’-binaphthyl.
intermediate 1,3-diphenyl-1-propene (3a). Subsequent
hydrogenation of 3a by the Ir-hydride would lead to the
desired product 2a. Recently, Madsen and co-workers
reported a mechanism, obtained by DFT studies, for the
Rh-catalyzed decarbonylation of aldehydes.[16] They reported
that the decarbonylation of aldehydes involves a rapid
À
oxidative addition into the C(O) H, bond, followed by a
rate-limiting extrusion of CO. In this case, the reaction using
2-arylethanols such as 1a would produce thermodynamically
stable alkenes 3a, in which the double bond is conjugated
with aryl groups,[15a] thus resulting in the direct one-step
Using the [(Cp*IrCl2)2]/[{IrCl(cod)}2]/dppe combined cat-
alyst system under the reaction conditions shown in Table 5,
entry 3, various b-methylhydroxy-a,w-diarylalkanes 5 were
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Angew. Chem. Int. Ed. 2011, 50, 8618 –8622