Handbook of Functionalized Organometallics Applications in S

R H2O R
R
B(OH) 2
B(OH) 2
3.3 Preparation and Reactions of Functionalized Alkenyl Boranes
H 2O 2 (30 %) (1 equiv.)
1) H 2O 2 (30 %) (0.25 equiv.)
2) 4 Å MS, Cu(OAc) 2/NEt 3
Scheme 3.56 Oxidation of boronic esters.
3.3
Preparation and Reactions of Functionalized Alkenyl Boranes
R
OH
60 - 88 %
O
55 - 90 %
3.3.1
Synthesis of Alkenyl Boronic Acids by Transmetallation of Alkenyl Grignard Reagents
with Boronate Esters
Jiang has described the successful preparation of a-(trifluoromethyl)ethenyl boronic
acid by reaction of readily available 2-bromotrifluoropropene, magnesium and
an alkyl borate in an one-pot process [89]. The trifluoromethyl boronic acid 111,
thus obtained, was found to be stable for several months even in the presence of
air and moisture (Scheme 3.57).
F 3C Br
Mg B(OMe) 3
THF
25 ºC, 4 h
Scheme 3.57 Synthesis of functionalized alkenyl boronic
esters by transmetallation of an alkenyl Grignard reagent
with (MeO) 3B.
F 3C B(OMe) 2
H +
R
F 3C B(OH) 2
111: 90 %
3.3.2
Synthesis of Functionalized Alkenyl Boronic Acids by Hydroboration of Functionalized
Alkynes and their Suzuki Cross-coupling Reactions
Witulski has reported the first hydroboration of 1-alkynylamides. Thus, the hydroboration
of ynamide 112 with catechol borane in THF proceeded chemo- and
regioselectively yielding only the monohydroboration product, alkenyl boronic
ester 113 [90]. However, the isolation of the boronic ester 113 was complicated due
to its instability and difficulties of storage and purification. Therefore, it was
directly subjected to Suzuki±Miyaura cross-coupling yielding (E)-b-arylenamide
and 3-(2¢-amidovinyl)indoles such as 114 (Scheme 3.58).
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