Handbook of Functionalized Organometallics Applications in S

48
3 Functionalized Organoborane Derivatives in Organic Synthesis
Functionalized boronic esters can be alternatively prepared from readily available
iodoaryl boronic esters such as 17 and 18. I/Mg-exchange with i-PrMgCl´LiCl
provides a bimetallic B/Mg-species such as 19 and 20 that react with a range of
electrophiles (aldehyde, allyl bromide, acid chloride, 3-iodo-2-cyclohexenone) providing
the products 21 in good yields (Scheme 3.5) [11].
The Pd-catalyzed reaction of such polyfunctional boronic esters like 21b with
various aryl halides provides the desired cross-coupling products like 22 in high
yields [10,11]. Interestingly, a one-pot reaction allowing the selective reaction with
two electrophiles is possible. Thus, the treatment of the meta-iodophenyl boronic
pinacol ester 18 with i-PrMgCl´LiCl followed by a transmetallation to the copper
derivative and subsequent reaction with 2-methyl-3-iodocyclohexenone provides
the intermediate boronic ester 23 that after a Suzuki±Miyaura cross-coupling, furnishes
the heterocyclic product 24 in 52% yield (Scheme 3.6) [11].
B O O
17: p-I
18: m-I
B O O
i-PrMgCl·LiCl B PhCHO
O O
THF, -78 ºC, 1 h
I MgCl
B O O
Ph
O
19: p-MgCl
20: m-MgCl
B O O
O
O
B O O
B O O
Ph
OH
21a: para: 83%
21b: meta: 71%
B O O
21c: 77% 21d: 72% 21e: 73% 21f: 68% 21g: 67%
Scheme 3.5 Preparation of polyfunctional boronic esters via B/Mg-bimetallic aromatics.
CO 2Et
Finally, this approach can be extended to various heterocyclic systems allowing
the preparation of polyfunctional boronic esters such as 25±27 (Scheme 3.) [11].
The boronic esters 26 and 27 have been converted to the polyfunctional heterocycles
like- 28 and 29 in excellent yields using the Suzuki±Miyaura cross-coupling
reaction (Scheme 3.7).