A Germylene/Borane Lewis Pair and the Remarkable C=O Bond Cleavage Reaction toward Isocyanate and Ketone Molecules

Article abstract of DOI:10.1002/chem.201603544

A germylene/borane Lewis pair (2) was prepared from a 1,1-carboboration of amidinato phenylethynylgermylene (1) by B(C₆F₅)₃. Compound 2 reacted with iPrNCO and (4-MeOC₆H₄)C(O)Me, respectively, with cleavage of the C=O double bond. In the first instance, O and iPrNC insert separately into the Ge?B bond to yield a GeBC₂O-heterocycle (3) and a GeBC₃-heterocycle (4). In the second case (4-MeOC₆H₄)(Me)C inserts into the Ge?N bond of 2 while O is incorporated in the Ge?B bond to form a Ge-centered spiroheterocycle (5). The reaction of 2 with tBuNC to give 6, which has almost the same structure as 4, proved the formation of the isonitrile during transformation from 2 and iPrNCO to 3 and 4. The kinetic study of the reaction of 2 and iPrNCO gave evidence of proceeding through a GeBC₃O-heterocycle intermediate. In addition, a DFT study was performed to elucidate the reaction mechanism.

Full text of DOI:10.1002/chem.201603544

DOI: 10.1002/chem.201603544
Communication
&
Main Group Chemistry
A Germylene/Borane Lewis Pair and the Remarkable C=O Bond
Cleavage Reaction toward Isocyanate and Ketone Molecules
[a]
[a]
[a]
[a]
[a]
[c]
Jiancheng Li, Bin Li, Rui Liu, Liuyin Jiang, Hongping Zhu,* Herbert W. Roesky,*
[b]
[b]
[d]
[d]
Sayan Dutta, Debasis Koley,* Weiping Liu, and Qingsong Ye
product. The condensation reaction of aldehydes or ketones
with amines has been well-known to yield the imines. This pro-
cess exhibits a deoxygenation under elimination of water,
which is accompanied with the C=N bond formation. However,
Abstract: A germylene/borane Lewis pair (2) was pre-
pared from a 1,1-carboboration of amidinato phenylethy-
nylgermylene (1) by B(C F ) . Compound 2 reacted with
6
5 3
iPrNCO and (4-MeOC H )C(O)Me, respectively, with cleav-
[2]
6
4
the reaction mechanism is quite complex. The Fischer–
age of the C=O double bond. In the first instance, O and
iPrNC insert separately into the GeꢀB bond to yield
a GeBC O-heterocycle (3) and a GeBC -heterocycle (4). In
Tropsch synthesis converts CO and H to hydrocarbons, carry-
2
ing a hydrogenation deoxygenation reaction through elimina-
tion of water as well, but operates under catalytic conditions
2
3
the second case (4-MeOC H )(Me)C inserts into the GeꢀN
[3]
6
4
when using transition metals. Recently, Erker and Stephan in-
bond of 2 while O is incorporated in the GeꢀB bond to
form a Ge-centered spiroheterocycle (5). The reaction of 2
with tBuNC to give 6, which has almost the same struc-
ture as 4, proved the formation of the isonitrile during
transformation from 2 and iPrNCO to 3 and 4. The kinetic
study of the reaction of 2 and iPrNCO gave evidence of
dependently illustrated this reaction process using P/B frustrat-
ed Lewis pairs (FLPs) with or without an excess of the Lewis
[4]
acid as stoichiometric reactant. Also well-known is the cross-
[5a,b]
[5c,d]
coupling reaction of the Wittig
and Tebbe
reagents.
These reagents efficiently deoxygenate aldehydes, ketones,
and/or carboxylic acid derivatives, respectively, and couple
with the remaining fragment to yield a-olefins. The organo-
phosphorus and -titanium oxides are formed as the respective
byproducts.
proceeding through a GeBC O-heterocycle intermediate.
3
In addition, a DFT study was performed to elucidate the
reaction mechanism.
The deoxygenation of the carbonyl compounds directing to
the free carbene-like species appears to be difficult and to
[
6]
The deoxygenation reaction of carbonyl compounds is an im-
portant process, which has been extensively utilized to pro-
duce a variety of organic molecules such as imines, hydrocar-
date only rare examples have been reported. Efficient and se-
lective reducing agents are indeed rare. In recent years, the
groups of Kira, Cui, and Roesky have reported on silylenes in-
[
1]
[7]
bons, carbene-like species, and others. It is essential that this
reaction proceeds with a multiple bond cleavage of the C=O
functionality, during which the deoxygenated remaining
moiety couples with the functional group to form the target
volved in the deoxygenation of aldehydes and ketones.
Nonetheless, such compounds are still limited. Moreover, the
related reaction mechanisms are not clear due to the compli-
cated process that is induced by these too reactive moieties.
[8]
Recently we worked on the synthesis of the germylenes and
IV
[9]
the Ge /P FLP. The reactions of some of these compounds
with aldehydes, ketones, or their derivatives have also been
[a] J. Li, B. Li, R. Liu, L. Jiang, Prof. Dr. H. Zhu
State Key Laboratory of Physical Chemistry of Solid Surfaces
National Engineering Laboratory for Green Chemical Productions of
Alcohols-Ethers-Esters, College of Chemistry and Chemical Engineering
Xiamen University, Xiamen, 361005 (P. R. China)
carried out. However, only the C=O bond activation or addition
[8d,e,9]
reactions were found.
We were then intrigued with the
deoxygenation of the carbonyls by germylenes in a reaction
similar to that of the silylenes. A new germylene that has an in-
tramolecular borane connectivity was synthesized (2, Figure 1),
which has been tested to function as a deoxygenation com-
E-mail: hpzhu@xmu.edu.cn
[
b] S. Dutta, Dr. D. Koley
Department of Chemical Sciences
Indian Institute of Science Education and Research Kolkata
Mohanpur 741246 (India)
pound for isocyanate by forming the respective GeC BO- and
2
E-mail: koley@iiserkol.ac.in
GeC B-heterocycles. Remarkably, during the reaction, the for-
3
[
c] Prof. Dr. H. W. Roesky
mation of the free isonitrile was detected. However, the deoxy-
genation reaction of 2 with a ketone resulted in a Ge-centered
spiroheterocycle as the single product.
Institut fꢀr Anorganische Chemie, Georg-August-Universitꢁt
Tammannstraße 4, 37077 Gçttingen (Germany)
E-mail: hroesky@gwdg.de
[
d] Prof. W. Liu, Q. Ye
Amidinato phenylethynyl germylene 1 was prepared (see Ex-
perimental Section in the Supporting Information) and then
State Key Laboratory for Platinum Group Metals
Kunming Institute of Precious Metals, Kunming
Yunnan 650106 (P. R. China)
treated with B(C F ) in toluene at 658C for 12 h to give com-
6
5 3
19
pound 2 with 92% isolated yield (Figure 1). The F NMR spec-
trum displayed two sets of C F resonances (d=ꢀ129.1 (o-F),
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201603544.
6
5
Chem. Eur. J. 2016, 22, 1 – 6
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&
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Communication
Figure 1. Synthesis and X-ray crystal structure of 2 (H-atoms are omitted for
o
clarity). Selected bond lengths [ꢁ] and angles [ ]: Ge(1)-C(2) 1.935(2), C(2)-
C(1)1.359(3), C(1)-B(1) 1.659(3), B(1)-Ge(1) 2.160(2), Ge(1)-N(1) 1.933(2), Ge(1)-
N(2) 1.939(2); Ge(1)-C(2)-C(1) 93.9(1),C(2)-C(1)-B(1) 111.9(2), C(1)-B(1)-Ge(1)
78.2(1), B(1)-Ge(1)-C(2) 75.2(1), N(1)-Ge(1)-N(2) 67.5(1).
Scheme 1. Reactions of 2 with the respective isocyanate and ketone mole-
cules.
ꢀ
158.4 (p-F), ꢀ164.2 ppm (m-F) vs. d=ꢀ139.0 (o-F), ꢀ157.8 (p-
F), ꢀ163.2 ppm (m-F)), in a 2:1 ratio. This is supportive of a 1,1-
The X-ray single-crystal diffraction study revealed that 3 is
a GeBC O-heterocycle, whereas 4 showed a GeBC -heterocyclic
arrangement (Figure 2). This result indicates a clear C=O bond
cleavage of the isopropyl isocyanate and the O and iPrNC
insert, respectively, into the GeꢀB bond of 2. These two new
heterocycles keep the planarity as well (D=0.0518 ꢁ for 3 and
carboboration reaction that involves a probable abstraction of
2
3
ꢀ
the PhCC anion by B(C F ) from the Ge atom to form
6
II
5 3
ꢀ
[PhCCB(C F ) ] and the Ge cation. Then one C F group from
6
5 3
6 5
the B center migrated to the adjacent acetylenic carbon atom,
following by a back-bonding of another acetylenic carbon
[
10]
atom to the Ge center. As a consequence, the respective = 0.0268 ꢁ for 4). Note that, within the GeBC O-heterocycle, the
CB(C F ) and =C(C F ) groups were formed. The B NMR reso-
2
11
Ge(1)ꢀO(1) bond length (1.721(2) ꢁ) is between those of the
6
5 2
6 5
[15]
nance was observed at d=ꢀ16.6 ppm, which is in the range
Ge=O double bond of germanones (1.646(2)–1.672(3) ꢁ) and
[11]
[16]
of germylene-borane adducts (d=2.8 to ꢀ42.0 ppm), but
significantly deviates from that found in three-coordinate
boron compound Ge[C(Ph)C(Ph)B(NDippCH) ][N(SiMe ) ] (d=
the single bond of germoxanes (1.765(4)–1.820(2) ꢁ). These
data suggest a probable Ge=O!B bonding and their reso-
+
ꢀ
nance form (Ge ꢀOꢀB ) is attributable to the polarity of the
2
3 2
[
12]
[15]
43.5 ppm). These data indicate the formation of a four-coor-
Ge=O bond intensively discussed in the literature. In con-
II
dinate B atom reminiscent of a Ge !B donor–acceptor interac-
tion present in 2. The X-ray crystal structure analysis unambig-
uously evidenced 2 as a 1,1-carboboration product that had
a cis-arrangement of the [tBuC(NCy) ]Ge and B(C F ) moieties
trast, within the GeBC -heterocycle the GeꢀC
=
bond length
NiPr
3
C
(2.019(5) ꢁ) is longer by about 0.10 ꢁ than that of the GeꢀC
=
C
C
(1.918(6) ꢁ) and much longer than those of the Ge=C double
[17]
bonds of known compounds (1.761(2)–1.840(4) ꢁ).
2
6 5 2
1
along the vinyl C(1)ꢀC(2) bond (1.359(3) ꢁ). A GeꢀB bond
2.160(2) ꢁ) is indicated by comparison with the germylene-
The H NMR data indicated that in the above-NMR-scale re-
action compound 3 appeared to form as the major product
when an excess of the isopropyl isocyanate was employed
(Figures S3 and S4 in the Supporting Information). To elucidate
this reaction process in detail, kinetic study of the reaction of 2
and 0.5 iPrNCO was conducted. As can be seen from the data
recorded in Figure S2 in the Supporting Information, the con-
sumption of 2 and iPrNCO is observed while products 3 and 4
are gradually generated. It was noted that this reaction process
was clearly accompanied with the formation of an intermedi-
ate and its further reaction to the new products. We have tried
to isolate the intermediate by controlling the reaction time
and temperature, but failed. Furthermore, we treated 2 directly
with tert-butyl isonitrile, which reacted very fast (within several
minutes) in toluene at room temperature and yielded com-
pound 6 (Figure 3). Compound 6 has almost the same struc-
ture as that of 4 in which only the iPr group of the iPrNC part
is replaced by the tBu group. Compound 6 was characterized
on the basis of NMR spectroscopy, X-ray crystallography, and
CHN element analysis. Similar reaction patterns have been ob-
served in the reaction of the P/B Lewis pair with isonitriles. The
(
[
11]
borane adducts (2.015(7) ꢀ2.186(3) ꢁ) and borylgermanium
[
13]
compounds (2.141(2) ꢁ). The four-membered GeBC -hetero-
2
cycle forms a distorted plane (D=0.0446 ꢁ). However, the
Ge(1)-C(2)-C(1) (93.94(13)8) and C(2)-C(1)-B(1) (111.88(17)8)
bond angles are deviated from the ideal 908 due to the ring
strain. To the best of our knowledge heterocycles containing
II
the Ge !B bond motif have not been reported so far. Com-
[
14]
pound 2 can be well ascribed as a Lewis pair.
The reaction of 2 with isopropyl isocyanate was carried out
by using toluene as solvent. Thus, at room temperature this re-
action proceeded smoothly. However, the product formation
depended on the molar ratio of the two precursors (see Sec-
tions IV and V in the Supporting Information). The reaction on
an NMR-scale in [D ]toluene solution showed that a strict
8
molar ratio (1:0.5) of the precursors was necessary to obtain
two clean products in an almost equal amount, as indicated by
the presence of two sets of the proton resonances after about
6
h (see Figures S1 and S2 in the Supporting Information). A
scaled-up reaction of 2 and half equivalent of isopropyl isocya-
nate was then accomplished, which expectedly gave com-
pounds 3 (colorless crystals) and 4 (yellow crystals; Scheme 1).
resulting PBC -heterocycles were formed by synergistic 1,1-ad-
3
[18]
dition of the P/B LP to the terminal C atom of the isonitriles.
In compound 2 the borane part serves as a ligand acceptor
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Communication
o
Figure 2. Crystal structures of 3 (left) and 4 (right). H-atoms are omitted for clarity. Selected bond lengths [ꢁ] and angles [ ]: for 3, Ge(1)-C(2) 1.914(3), C(2)-
C(1)1.347(4), C(1)-B(1) 1.653(4), Ge(1)-O(1) 1.721(2), B(1)-O(1) 1.512(4), Ge(1)-N(1) 1.894(2), Ge(1)-N(2) 1.905(2); Ge(1)-C(2)-C(1) 103.7(2), C(2)-C(1)-B(1) 119.1(2),
C(1)-B(1)-O(1) 106.8(2), Ge(1)-O(1)-B(1) 109.4(2), O(1)-Ge(1)-C(2) 99.3(1), N(1)-Ge(1)-N(2) 69.1(1); for 4, Ge(1)-C(2) 1.918(6), C(2)-C(1)1.363(8), C(1)-B(1) 1.659(8),
Ge(1)-C(9) 2.019(5), B(1)-C(9) 1.623(9), C(9)-N(1) 1.273(8), Ge(1)-N(2) 1.931(5), Ge(1)-N(3) 1.937(4); Ge(1)-C(2)-C(1) 108.6(4), C(2)-C(1)-B(1) 122.1(5), C(1)-B(1)-C(9)
108.6(5), Ge(1)-C(9)-B(1) 104.5(4), C(9)-Ge(1)-C(2) 96.0(2), N(2)-Ge(1)-N(3) 68.7(2).
ysis (NPA) charges on Ge and B atoms are +1.702 and +0.066,
respectively. The Wiberg bond index (WBI) calculated for the
GeꢀB bond is 0.824. The KS-HOMO and KS-LUMO of 2 com-
prise the GeꢀB s orbital and CꢀC p* orbital as pictorially repre-
sented in Figure S5.
The reaction of 2 with iPrNCO might involve a dipolar addi-
tion of 2 to the C=O bond of iPrNCO as a similar reaction pat-
tern has been reported for the P/B FLP toward the isocyan-
[20a]
ates.
Then an intermediate D was formed, which is calculat-
ed to be relatively more stable than 2 in energy by 9.6 kcal
ꢀ
1
mol (Figure 4). The calculated activation barrier for the step
Figure 3. Reaction of 2 with tert-butyl isonitrile and X-ray crystal structure of
product 6 (H-atoms are omitted for clarity). Selected bond lengths [ꢁ] and
angles [ ]: Ge(1)-C(2) 1.919(3), C(2)-C(1) 1.352(4), C(1)-B(1) 1.658(4), B(1)-C(9)
°
S
ꢀ1
is moderate (D G =20.1 kcalmol ). Nonetheless, a transition
L
o
°
state [2-D] is possible, which is characterized by a single
1
1
1
6
.655(4), Ge(1)-C(9) 2.031(3), C(9)-N(1) 1.244(4), Ge(1)-N(2) 1.924(2), Ge(1)-N(3)
.919(2); Ge(1)-C(2)-C(1) 109.6(2), C(2)-C(1)-B(1) 121.6(2), C(1)-B(1)-C(9)
08.9(2), Ge(1)-C(9)-B(1) 103.3(2), C(9)-Ge(1)-C(2) 95.8(1), N(2)-Ge(1)-N(3)
8.0(1).
imaginary frequency with respect to the simultaneous forma-
tion of GeꢀC and BꢀO bonds. In the next step, the oxygen
atom approaches Ge, resulting in the formation of 3 and
iPrNC. This step, corresponding to an activation barrier of
ꢀ
1
2
9.4 kcalmol , is referred to as the rate-determining step. The
II
°
while the Ge atom behaves as a donor coordinating at the
terminal C atom of the isonitriles. To the best of our knowl-
edge, the formation of such a GeꢀC bond is unprecedented
single imaginary frequency in [D-3] depicts the GeꢀO bond
formation and concomitant cleavage of the GeꢀC and CꢀO
bonds. The BꢀO bond (1.518 ꢁ) in 3 with occupancy of 1.957 e
is polarized toward the more electronegative oxygen atom (O:
79%). The NPA analysis reveals +2.261 and +0.536 charges on
Ge and B atoms, respectively. Furthermore, approach of the
isopropyl isonitrile carbon towards the electron-deficient
boron in 2 leads to 4A, which shows the typical features of
a strong Lewis acid adduct (BꢀC3=1.604 ꢁ) with a shorter C3ꢀ
[
8,19]
among known organogermanium compounds.
Even with these experimental results in hand it was not pos-
sible to understand the reaction mechanism in detail with re-
spect to the C=O double bond cleavage of the isopropyl isocy-
anate when interacting with 2. Moreover, both compounds 3
and 4 were formed in different yields when the molar ratio of
the precursors changed. The Ge-based Lewis pair reaction
[
18,20b]
N bond length (1.173 ꢁ).
This process occurs through the
[
9]
°
S
ꢀ1
chemistry has been rarely reported. Therefore we performed
DFT calculations (see Section VI in the Supporting Information)
to illustrate the bonding scenario of 2 and also to predict
a probable reaction pathway of 2 and isopropyl isocyanate.
The formation of 2 from the initial precursor 1 is exergonic
relatively lower energy (D G =16.2 kcalmol ) transition state
L
°
[2-4A] . The imaginary mode of the transition-state vector
portrays the formation of the BꢀC bond. The final step is a bar-
rierless process, where the isopropyl isonitrile carbon atom in
4A approaches the Ge to furnish product 4, as has been sub-
stantiated by the experimental findings from the reaction of 2
with tert-butyl isonitrile to produce 6.
S
L
ꢀ1
S
L
(
DG =ꢀ22.0 kcalmol ) and also highly exothermic (DH =
ꢀ
1
ꢀ
38.9 kcalmol ), suggesting a facile conversion. The GeꢀB
bond (2.199 ꢁ) in 2 with occupancy of 1.872 e is almost equally
shared between the bonding partners (B: 44%) because of
nearly the same electronegativity. The natural population anal-
It appears that the calculated activation barriers for the reac-
°
S
ꢀ1
tions between 2+iPrNCO (D G =20.1 kcalmol ) and 2+
L
°
S
ꢀ1
iPrNC (D G =16.2 kcalmol ) are somewhat comparable.
L
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Figure 4. Energy profile for the reaction of 2 with isopropyl isocyanate at R-M06-2X(SMD)/def2-TZVP//ONIOM{R-BP86/def2-SVP:HF/STO-3G} level of theory.
Moreover, compound 3 is only a little more stable than 4 in
a pure product of colorless crystals (Scheme 1). No other com-
pound was formed even when an excess of 4-methoxyaceto-
phenone was employed. Compound 5 is a Ge-centered spiro-
heterocycle with two five-membered rings (Figure 5). The
ꢀ
1
energy by 6.8 kcalmol . Accordingly, when isopropyl isocya-
nate was excessively added the reaction appeared to not stop
at the formation of 3 and 4 as the final products in equal
amounts. As further reaction of 4 with the excess isopropyl iso-
cyanate continued, 3 accumulated to become a major part
under release of the isopropyl isonitrile as a free compound
while 4 decreased to become the minor product (Scheme 2).
This result is in line with the experiment by the reaction of 2
with three equivalent iPrNCO to give iPrNC, 3 (major), and 4
GeBC O-heterocycle is comparable in structure to that of 3.
2
The GeC N -heterocycle deviates a little from the planarity
2
2
(DGeC2N2 =0.0835 ꢁ) when compared with the GeCN -heterocy-
2
cles found in 2–4 (DGeCN2 =0.0032–0.0176 ꢁ). The GeꢀC bond
length within the GeC N -heterocycle is 1.975(3) ꢁ, typical of
2
2
a single bond feature, which is a little longer than the GeꢀC
=
C
C
(minor; see Figures S3 and S4 in the Supporting Information).
distance (1.930(3) ꢁ) in the adjacent GeBC O-heterocycle. The
2
It is worth mentioning that this reaction reveals indeed a coop-
formation of 5 can be considered as a result of the C=O bond
II
erative interaction of the Ge /B Lewis pair with the isocyanate
cleavage
of
4-methoxyacetophenone,
but
the
(4-
and isonitrile. We have investigated the reaction using either
MeOC H )(Me)C reacts differently and inserts into the GeꢀN
6
4
[
21]
1
or [tBuC(NCy) ]GeCl in the absence or presence of B(C F ) .
bond while the O is simultaneously located in the GeꢀB bond.
2
6 5 3
However, no reaction occurred, while the reactions of the relat-
Computational results showed that the reaction proceeds
ed amidinato silylenes with ketones and 3,5-di-tert-butyl-o-ben-
through a GeBC O-heterocycle E as the intermediate, similar to
D in the C=O bond insertion (Figure S9 in the Supporting In-
formation).
3
[
22]
zoquinone yielded the SiCO- and SiC O -heterocycles.
2
2
Finally, we used 2 for the reaction with the ketone molecule.
The reaction of 2 and one equivalent 4-methoxyacetophenone
was accomplished in toluene at 658C and resulted in 5 as
In conclusion, we have developed a germylene/borane
Lewis pair 2 that features the GeBC -heterocycle having the
2
GeꢀB donor–acceptor bond. Compound 2 has been tested to
be capable of cleaving the C=O double bond of the isocyanate
(
iPrNCO) and ketone ((4-MeOC H )C(O)Me) molecules. The reac-
6 4
tion mechanism of 2 and isopropyl isocyanate was studied in
detail both experimentally and theoretically. The reaction of 2
with an excess of isopropyl isocyanate to yield isopropyl isoni-
trile may find application in the deoxygenation of the isocya-
nates in the synthesis of the free isonitriles by using 2 as a pre-
cursor. We are working on the progress of this study.
Scheme 2. Proposed reaction mechanism of 2 with excess isopropyl isocya-
nate.
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Communication
[
[
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Figure 5. Crystal structure of 5 (H-atoms are omitted for clarity). Selected
bond lengths [ꢁ] and angles [ ]: Ge(1)-C(2) 1.930(3), C(2)-C(1)1.342(4), C(1)-
B(1) 1.676(5), Ge(1)-O(1) 1.737(2), B(1)-O(1) 1.490(4), Ge(1)-N(1) 1.899(3),
Ge(1)-C(9) 1.975(3), C(9)-N(2) 1.514(4); Ge(1)-C(2)-C(1) 106.8(2), C(2)-C(1)-B(1)
o
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Keywords: C=O bond cleavage · coordination chemistry ·
density functional calculations
· germylene/borane Lewis
pairs · isonitrile
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Received: July 27, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
Give it a break: Germylene/borane
Lewis pair 1 reacts with iPrNCO and (4-
MeOC H )C(O)Me, respectively, with
&
Main Group Chemistry
J. Li, B. Li, R. Liu, L. Jiang, H. Zhu,*
H. W. Roesky,* S. Dutta, D. Koley,* W. Liu,
Q. Ye
6
4
cleavage of the C=O double bond (see
scheme). Fragments O and iPrNC insert
separately into the GeꢀB bond to yield
&
& – &&
2
and 3, and O and (4-MeOC H )(Me)C
6 4
A Germylene/Borane Lewis Pair and
the Remarkable C=O Bond Cleavage
Reaction toward Isocyanate and
Ketone Molecules
into the respective GeꢀB and GeꢀN
bonds to form 4. The reaction of 1 with
an excess of iPrNCO leads to iPrNC, 2
(major) and 3 (minor).
&
&
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!