Chemistry Reference
In-Depth Information
O
OMe
O
OMe
Si(OEt)
3
Pd(OAc)
2
, PPh
3
,
TBAF
+
MeO
OMe
OMe
OMe
Br
2.317
2.318
2.319
OMe
MeO
Scheme 2.102
Pd(OAc)
2
, PPh
3
,
TBAF
OTf
O
O
+
Si
OMe
O
O
OMe
2.320
2.321
2.322
Scheme 2.103
MeO
OMe
MeO
OMe
(EtO)Me
2
Si
2.324
O
O
(Ph
3
P)
2
PdCl
2
, Et
3
N
O
O
I
(EtO)Me
2
Si
2.323
2.325
MeO
OMe
MeO
I
2.326
O
CO
2
Me
[(allyl)PdCl]
2
, (EtO)
3
P, T B A F
MeO
MeO
O
2.327
MeO
CO
2
Me
O
MeO
O
NMe
MeO
2.328
Scheme 2.104
Consideration of the mechanism led to the development of a fluoride-free system, activated by a mild
Brønsted base, such as Me
3
SiOK or hydrated Cs
2
CO
3
, rather than a Lewis base (Scheme 2.106).
118
The base
functions by deprotonating the silanol
2.335
(or generating a silanol by cleaving the disiloxane). The siloxide
2.337
changes the game. Rather than be converted into an “ate” complex, ligand exchange occurs at palladium,
forming a siloxide-palladium complex
2.338
, followed by facile intramolecular transmetallation.
119
Given that a different mechanism operates in the case of silanols, selective coupling of a silanol in the
presence of a silane derivative can be achieved by judicious choice of activator (Scheme 2.107).
120
In the
