Angewandte
Chemie
À
a Sn P donor–acceptor bond. A three-membered ring with
The tin and carbon atoms of the ring are centers of chirality,
these elements is to the best of our knowledge unknown.
Because of their interesting chemical properties, three-
membered rings containing tin atoms have been an attractive
synthetic goal.[12] The reason for the ring formation might be
the steric shielding of the m-terphenyl substituent together
with a combination of the Lewis acidity of the stannylene and
the Lewis basicity of the phosphine substituent. Geminal
donor–acceptor bonds between N and Si were studied
extensively by Mitzel.[13] Obviously the phosphine reacts as
a donor and forms an intramolecular bond with the tin atom,
but we found formation of only one pair of enantiomers (R,R
configuration shown in Figure 1). The reason for this diaste-
reoselectivity might be due to the sterical requirements of the
m-terphenyl ligand, which exhibits trans position to the
phenyl group at the ring carbon atom.
To better understand the bonding situation in the ring, we
studied the molecule by DFT calculations on revPBE-
D3(BJ)/TZP(all electrons) level of theory using the ADF
program.[23] Full computational details are given in the
Supporting Information. A geometry optimization starting
from the crystal structure and subsequently a natural bond
analysis (NBO) and an atoms-in-molecules analysis (AIM)
were calculated. The NBO analysis indicates a donor–
acceptor interaction of the phosphorus lone pair donating
into a tin p orbital. Additionally, in the topological analysis of
the electron density (AIM), a bond path and a bond critical
point between the phosphorus and the tin is found, confirming
a bonding interaction.
We started to investigate the chemistry of the cyclic
stannylene in reactions of the small ring with unsaturated
organic molecules. The addition of element–element bonds to
alkynes has previously been carried out by using radical
reaction conditions[24] or palladium catalysis.[25] Stannylene 3
reacts in a straightforward manner at room temperature with
phenylacetylene or trimethylsilylacetylene under addition of
À
which is 2.663(1) ꢀ long. The Sn P distance in 3 lies in the
À
range of published Sn P bond lengths and belongs to the
group of longer distances.[6a,14] The Sn C1 bond (2.312(3) ꢀ)
À
À
is longer than Sn C bond lengths in the Sn2C ring (2.209(12),
2.196(12) ꢀ) and comparable with bonds in benzyltin com-
pounds.[12d,15] The P C1 bond length inside the ring can be
À
[16]
À
compared with P C distances inside a phosphasilirane,
a phosphirane,[17] or known benzylphoshines.[18] The angles
inside the ring (at C 79.3(1)8, P 58.6(1), Sn 42.1(1)8) are larger
for carbon and phosphorus than comparable values in three-
membered rings.[12d] The angle at tin however is the smallest
angle at tin found in three-membered rings (Sn2C 50.7, 51.1;
Sn3 60.0; Sn2N 49.4, 49.2).[12a–d] The bond length of the
À
substituent at tin (Sn C3 2.2217(2) ꢀ) can be compared with
the published example, namely 2,6-Trip2(H3C6)Sn-
SnMe2C6H3-2,6-Trip2 (2.201(2 ꢀ).[19]
the Sn P bond to the triple bond to give phosphastannacy-
À
The 31P NMR spectrum recorded in the solid state exhibits
a signal for the cyclic molecule 3 at À50.3 ppm with a coupling
constant with the tin atom JSn-P of 364 Hz. In solution at room
temperature, the molecule shows a resonance in the 31P NMR
spectrum at À38.6 ppm with a coupling constant of JSn-P
82.7 Hz. This coupling constant is small in comparison to
clopentenes 4 and 5, respectively (Scheme 2).[26] In the case of
1
a variety of JSn-P coupling constants (1682, 1628 Hz for
Sn[P(SiiPr3)(SiFtBuTrip)]2,[20] 1274 Hz for Sn[PPh-2,6-C6H3-
(CH2NMe2)2]2,[14h]
1050 Hz
767 Hz
for
for
Sn[P(CH-
(SiMe3)2C6H4NMe2]2,[14d]
Ph3Sn[h2-P,P-
MeP3C3tBu3]).[14e] We have studied the temperature depend-
ence of the 31P NMR resonance (808C, À36.8 ppm; À908C,
À37.6 ppm) and the tin–phosphorus coupling in solution:
cooling the sample from 808C to À908C results in an increase
of the coupling constant from 14.5 Hz to 220 Æ 20 Hz. The
119Sn NMR resonance in solution at room temperature at
716 ppm lies between signals typical for triply coordinated
SnII ([SnPh3]À À98.4 ppm)[21] and signals for dialkylstanny-
lenes with tin having a coordination number of two ([Sn-
(tBu)(C6H3-2,6-Trip2)] 1904 ppm).[19] Owing to a half width of
290 Hz of the 119Sn NMR signal, the coupling with the
phosphorus atom was not detected. Attempts to obtain
Scheme 2. Formation of the phosphastannacyclopentenes 4 and 5.
Ar*=2,6-Trip2C6H3.
1-pentene, the reaction with an excess of the olefin takes one
day at room temperature to reach completion (Scheme 3).
After dissolving crystals of 6 in benzene, the formation of an
equilibrium mixture between the trinuclear cycle 3, olefin,
and cyclopentane 6 was detected. The new five-membered
ring molecules 4–6 were characterized by elemental analyses,
NMR spectroscopy, and in the case of 4 and 6 by X-ray crystal
À
structure analysis. The reaction of the P Sn bond with the
a
119Sn NMR spectrum of compound 3 in the solid state
were not successful. By evaluation of the spin–lattice
relaxation time in 119Sn solution NMR spectroscopy at
variable temperature, we are able to estimate the chemical
shift anisotropy to be ꢀ 2000 ppm.[10,22] This probably
accounts for the difficulties in observing the resonance in
the solid state. The ring formation reaction (Scheme 1) is
stereoselective, as we detect only one signal in solution in the
1H NMR spectrum (3.67 ppm) for the CH unit of the ring and
also only one signal in the 31P NMR spectrum (À38.6 ppm).
Scheme 3. Formation of the phosphastannacyclopentane 6. Ar*=2,6-
Trip2C6H3.
Angew. Chem. Int. Ed. 2013, 52, 5640 –5643
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5641