Phosphazene: Preparation, Reaction and Catalytic Role - Superbases for Organic Synthesis - page 173

Table 5.9 Synthesis of enamine using the Peterson reaction

R R

TMSCH 2 EWG

N N

t Bu-P4 base (10 mol%)

CHO EWG

THF, rt, time

X X

Run X R EWG Time (h) Yield (%)

1 H Me CO 2 Et 24 90

92 a

2 H Me CO 2 Et 24

78 a

3 H Me CN 24

4 Me Me CO 2 Et 48 74

5 OMe Me CO 2 Et 48 47

6 CO 2 Et Me CO 2 Et 24 85

7 CO 2 Et Me CN 48 87

8 CN Me CO 2 Et 20 80

9 CN Me CN 48 80

83 a 10 H CH 2 CH ¼ CH 2 CO 2 Et 24

42 a 11 H CH 2 CH ¼ CH 2 CN 24

99 a 12 H CH 2 Ph CO 2 Et 24

88 a 13 H CH 2 Ph CN 24

a Without solvent.

condensed with N-methylformanilide to give the enaminonitrile in 78% (run 3). Various
substituents on the aromatic ring of the formanilides were compatible and the tolerance of
alkokycarbonyl and cyano groups during the condensation is considered to be synthetically
important (runs 4-9). N-Allyl and N-benzyl formanilides were also reacted to give the
corresponding enamines [58] (runs 10-13).

5.3.2.5 Promotion of Halogen-Zinc Exchange Reaction
The strong affinity of a t Bu-P4 base for protons is regarded as synthetically useful. However,
the ability of a t Bu-P4 base to activate organometallic compounds is largely undocumented.
Control of the reactivities of organometallic compounds is the key to the success of selective
bond formation, and the catalytic promotion of organometallics by the t Bu-P4 base is an
important subject. In recent years, organozinc compounds have been widely used in organic
synthesis. One of themost powerful methods for the preparation of functionalized organozinc
derivatives is the halogen-zinc exchange reaction, and the promotion of the exchange
reaction by a catalytic t Bu-P4 base has been investigated [59] (Table 5.10).
When the reaction of 4-iodobenzoate and diethylzinc in THF in the presence of 30mol
% t Bu-P4 base was carried out at room temperature, the halogen-zinc exchange reaction
proceeded smoothly. In the absence of t Bu-P4 base, the exchange reaction was very slow
and only a trace of de-iodinated product was detected. Other phophazene bases with
weaker basicity, such as t Bu-P2 base and t Bu-P1, showed less or no effect on the halogen-
zinc exchange reaction. DBU also showed no promotive reactivity on the exchange
reaction.When the amount of t Bu-P4 basewas reduced to 10 mol%, the exchange reaction
became significantly slower in THF. In order to optimize the reaction conditions further,