Chemistry Reference
In-Depth Information
O
O
PMP
PMP
PIFA
N
N
H
TFE, -20°C
R
R
CF
3
CO
2
559
560
Ar
Ar
Ar
BH
3
•Et
2
O
O
O
N
N
N
THF, rt
RO
HO
PMP
PMP
PMP
CF
3
CO
2
562
(R = TFA)
563
(H = H)
561
564
(67-72%; dr 1.8:1)
Na
2
CO
3
aq.
O
OMe
MeO
Ar
N
-2
Ar
N
-1
N
O
N
OMe
N
O
O
R
CF
3
CO
2
OMe
OMe
566
(71%)
567
(93%)
565
SCHEME 10.93.
Rather than invoking the involvement of an aziridinium ion intermediate, in their
most recent mechanistic proposal, Dom
ınguez and Tellitu have suggested that
amidotrifluoroacetoxylation proceeds through the mechanism outlined in Scheme
10.94. After generation of
by the action of PIFA, this singlet nitrenium ion was
suggested to undergo nonconcerted addition to the proximal position of the pendant
olefin to form primary cation
573
574
, which, upon interception by the neighboring arene,
would give rise to spiroazetidine
was then proposed to
arise either directly from this strained intermediate, through ring opening, or via a
more circuitous route involving ring expansion to form
575
. Formation of product
577
576
and subsequent ring
opening through C
C bond cleavage. Questions regarding the precise mechanistic
details of this useful transformation aside, support for the involvement of aryl
nitrenium ion intermediates was proffered from the conversion of substrate
578
to
phenol
580
: unable to undergo 4-
exo
-trig cyclization, delocalized cation
579
suffers
ion pair collapse at the
ortho
position of the aromatic ring.
Interestingly, attempts by Dom
ınguez
et al.
to extend their aminohydroxylation-
reduction protocol to 4-aryl-substituted pent-4-enamide derivatives
581
failed to
render oxamidation products but
582
(Scheme 10.95). The formation of these products was attributed to a 1,2-aryl
shift in carbocation
instead generated amino alcohols
583
585
584
.Ring
opening of this masked ketone during workup and subsequent borane reduction
would then give rise to the observed products.
, which generates
by way of phenonium ion
