Chemistry Reference
In-Depth Information
TABLE 8B.22. Decarboxylative Allylic Alkylation according to Burger and Tunge
O
O
O
R'
O
R'
R
R
R
R
[Pd
2
(dba)
3
](0.2mol %),
(
R
,
R
)-
DPPBA-1
(0.4 mol %),
C
6
H
6
,rt
or
or
()
n-4
R'
O
O
()
n-4
O
R'
O
Entry
R or n
R
′
Time (h)
Yield (%)
ee (%)
1
Me
Me
15
82
86
2
5
Me
15
85
86
3
6
Me
24
75
94
4
7
Me
45
81
98
5
6
Bn
27
71
90
6
6
18
a
94
80
i
- Pr
7
6
Ph
18
69
92
For the formula of the ligand, see Figure 8B.3.
a
Reaction temperature: 50 ° C.
before the nucleophile adds to the (allyl)Pd complex and (ii) the nucleophile preferen-
tially adds at the allylic face opposite to Pd. However, the reaction is not generally ste-
reospecifi c. It was postulated that Pd
II
is bound to the oxygen of the nonstabilized ketone
enolate in the (π-allyl)Pd complex. Crossover experiment showed complete as well as
incomplete scrambling depending on the substrates and the solvent. The experiments do
not allow to distinguish between C-C bond formation in a solvent-separated ion pair
and a bimolecular reaction of a Pd-O enolate with the (π - allyl)Pd complex.
The fi rst enantioselective transformation of an allylic enol carbonate into an α - allyl
ketone was described by Behenna and Stoltz (Scheme 8B.42,
A
) [154a] . They found
that the ligand
t
- Bu - PHOX
(
L31
) is particularly well suited for the promotion of these
transformations. The reaction allowed a variety of α-substituted cyclic ketones with a
quaternary chirality center to be prepared. The reaction conditions employed were also
suited for the transformation of racemic α - substituted 2 - carboxyallylcyclohexanones
into enantiomeric enriched α - allyl cyclohexanones (Scheme 8B.42 ,
B
) [177] . Crossover
experiments with deuterium-labeled substrates showed a high degree of scrambling
both between termini of the allylic moiety and between species derived from two
substrates.
Recently, Enquist and Stoltz used double asymmetric induction for the synthesis of
a diallylated cyclohexane-1,4-dione that was the starting material for a short synthesis
of the terpene (− ) - cyanthiwigin F (Scheme 8B.43 ) [178] . Typical for double asymmetric
induction, a very high degree of enantioselectivity is accompanied by a low degree of
diastereoselectivity [179] .
Trost and Xu used
DPPBA - 4
(
L1d
) (Fig. 8B.3) as ligand in the decarboxylative
allylic alkylation of cyclic (Table 8B.23) [180] as well as acyclic (Table 8B.24) [181] enol
