reaction. Using the known spiro derivative 4[2d] the reaction
proceeded at a comparably low rate, but gave a product with
32% ee (Table 1, entry 4). This result demonstrated the
feasibility of the enantioselective diamination of styrene
using a chiral iodine(III) reagent. An increase in the ee value
to 50% was obtained with the iodine(III) reagent 5a, which
bears two lactate groups (Table 1, entry 5). This compound
was recently introduced for enantioselective oxygenation
reactions.[2b,4d,e,5] Changing the nitrogen source to bismesyl-
imide initially resulted in a significantly reduced conversion
(Table 1, entry 6). By introduction of the new reagent 6 a
significant improvement was achieved and diamination of
styrene produced 2a in 77% yield upon isolation (Table 1,
entry 7). The reaction with the corresponding chiral reagent
5a gave significantly enhanced enantiomeric excess (Table 1,
entry 8). Notably, product 2a could be obtained with 85% ee
at 08C on a 5 mmol scale (Table 1, entry 9), while the
sterically more congested reagent 5b led to a significantly
diminished reaction rate (Table 1, entry 10). Enantiomeri-
cally pure diamine 2a was obtained after a single crystalliza-
tion. The absolute S configuration of diamine 2a was
established by X-ray crystal analysis (Table 1).[13,14]
Direct intermolecular diamination reactions of nonfunc-
tionalized alkenes are still rare[9k–m,10] and enantioselective
versions constitute an entirely unexplored field. The present
procedure is particularly important as it solves both these
quests with the additional benefit that it requires no metal
catalyst.
We were able to isolate new hypervalent iodine(III)
reagents 7 and 8, which are formed under ligand-exchange
conditions (Scheme 2). The solid-state structure of compound
7 is depicted in Figure 1 and unambiguously proves the
from the IIII N bond. The irreversible formation of this bond
may be due to the acidic nature of the bissulfonylimide, which
À
could also contribute to the alkene oxidation.[17]
A series of different styrenes were converted into the
corresponding diamines using iodine(III) reagent 5a as the
oxidant[18] and bismesylimide as the nitrogen source (Table 2).
All of these reactions proceeded with good yields and high
asymmetric induction. Representative examples are depicted
in Table 2 and include compounds with para substitution
(entries 1–9), meta substitution (entries 10–12), and ortho
substitution (entries 13,14) as well as symmetric and non-
symmetric higher substitution patterns (entries 15–18). In all
these cases, very good enantioselection, with up to 95% ee,
was observed. A single crystallization led to enantiopure
material in almost all cases. The S configuration for products 2
was further confirmed with additional X-ray crystal structures
of compounds 2g, 2o, and 2p.[14] All products 2a–s display
spectroscopic data that are in agreement with the expected
vicinal diamine motif. These reactions represent the first
examples of enantioselective intermolecular carbon–nitrogen
bond-forming reactions using the chiral hypervalent IIII
reagent 5a. Importantly, the productsꢀ high crystallinity,
resulting from the two bissulfonyl groups, usually enabled
enantiomerically pure diamines to be obtained after a single
crystallization.
The present diamination conditions work well with a
series of other alkenes as well. However, styrenes represent a
privileged substrate class with respect to enantioselective
induction. Scheme 3 shows three additional reactions. First,
[15,16]
existence of an IIII N covalent bond.
A 1:1 mixture of 7
À
and Ts2NH promoted the diamination of styrene with an
outcome that was identical to those reactions that employed
the in situ reaction conditions from Table 1, entry 3. This
result demonstrates an unprecedented reactivity originating
À
Scheme 2. Synthesis of iodine(III) compounds 7 and 8 with an I N
bond.
Scheme 3. Diamination reactions of alkenes 1t–v.
when 1-octene (1t) was subjected to the standard reaction
conditions the corresponding diamination product 2t was
obtained in 60% yield but with a low ee value of 5%. Next,
the diamination of (E)-b-methyl styrene (1u) led to a single
diastereomer 2u, which was isolated in 87% yield and 90% ee
(99%ee after recrystallization). X-ray crystal analysis of this
compound revealed an unexpected 1S,2R configuration.[14]
The same stereochemical outcome was observed for a
diamination reaction of 5-decene, thus giving meso diamine
2v as a single product.[14] The observed stereochemistry for 2u
and 2v does not agree with the generally accepted mechanism
for the related 1,2-dioxygenation of alkenes.[3,4d,19]
Figure 1. Structure of 7 in the solid state (thermal elipsoids drawn at
the 50% probability level).
Angew. Chem. Int. Ed. 2011, 50, 9478 –9482
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim