σ-π Interactions in End-Capped Oligo(cyclohexylidenes)
J . Org. Chem., Vol. 65, No. 15, 2000 4591
belong to the appropriate irreducible representation23 for
interaction with the outer ring Hax-C-C-Hax PCMO.
Hence, the Lpπ(S) NBO’s will perturb the MO topologies
of 5 and 6.
(C2h, Cs, or C2v) of the studied end-functionalized bi- and
tercyclohexylidenes.
Exp er im en ta l Section
The NBO analyses for 1-4 reveal different basis
orbitals for the localized π-unit and the Lpπ(S) NBO’s,
which is shown for 2 and 4 on the left side of Figure 10.
TS interactions between the Lpπ(S) and π-NBO’s is
insignificant in all cases (NBO/TS, Figure 10). In the case
of 2 and 4 no sizable splitting between the Lpπ(S) or the
π-orbitals in 4 is observed. This is in line with the TS
separation of the sulfur- and nearest π-bond carbon
atoms [d(S1‚‚‚C4) ) 3.2 Å, 2].32 The situation changes if
TB interaction occur (TB1 and TB2). In the case of 2 the
interaction of a PCMO comprising of four Hax-C-C-Hax
NBO’s of the σ-skeleton with either the Lpπ(S) NBO’s
or the π-NBO’s causes a considerable destabilization of
the 3p-sulfur lone pairs and the π-orbital (TB1 Figure
10). Subsequently, in the TB2 step, simultaneous inter-
action of the aforementioned units causes the Lpπ(S)
NBO’s to split. In the case of 4 interaction of either
the π- or Lpπ(S) NBO’s with a PCMO comprising of six
Hax-C-C-Hax NBO’s not only results in a destabilization
of the π- and 3p-sulfur lone pairs, but also in a splitting
of the symmetry adapted π-orbitals [ag(π+) and bu(π-)]
by 0.55 eV (Figure 10). In the next step (TB2) the
interaction between the lone pairs at sulfur and the
π-orbital(s) occurs. The splittings obtained after the TB2
step in 2 and 4 are about the same as found for the
canonical MOs (right side of Figure 10).
Gen er a l Meth od s. Melting points are uncorrected. NMR
spectra (1H 300.133 MHz and 13C 75.47 MHz) are recorded in
CDCl3 unless otherwise stated; chemical shifts (ppm) are
reported downfield from TMS. Raman spectra were recorded
for neat solids. Elemental analyses were performed by Dornis
U. Kolbe, Microanalytisches Laboratorium, Mu¨lheim a.d.
Ruhr, Germany. Dry THF and CH2Cl2 were prepared by
distillation from Na and CaCl2, respectively. CH3CN was dried
by storage on 3 Å molecular sieves.
Syn th eses. 4-(Cyclohexylidene)tetrahydro-4H-thiopyran (1)
and 4-(4-(cyclohexylidenecyclohexylidene)tetrahydro-4H-thio-
pyran (3)4a as well as anti- (anti-7) and syn-4,4′-tert-butyl-1,1′-
bicyclohexylidene (syn-7)11b were synthesized according to
literature procedures.
4,4′-Bis(tetr a h yd r o-4H-th iop yr a n ylid en e) (2). A solu-
tion of 15 (3.39 g, 13.0 mmol) and triethyl phosphite (12.0 mL,
70 mmol) in toluene (200 mL) was heated at reflux tempera-
ture for 24 h. After cooling of the reaction mixture, the solvent
was removed in vacuo and the solid residue triturated with
CH3OH (25 mL). The solid was filtered off and, subsequently,
sublimed (80 °C, 0.1 Torr) to give pure 2 (1.93 g, 0.63 mmol,
74%): mp 144.1 °C; 1H NMR δ 2.55-2.59 (m, 8H), 2.63-2.67
(m, 8H) ppm; 13C NMR δ 30.6, 31.9, 129.6 ppm; Raman ν 2958,
2945, 2891, 2838, 1661, 1652, 1643, 1453, 1424, 1420 cm-1
.
Anal. Calcd for C10H16S2: C, 59.98; H, 8.06; S, 31.96. Found:
C, 60.11; H, 8.01.
4-(Te t r a h yd r o-4H -t h iop yr a n -4-cycloh e xylid e n e -4′-
ylid en e)t et r a h yd r o-4H-t h iop yr a n (4). A suspension of
â-hydroxy acid 17 (0.280 g, 0.63 mmol) and N,N-dimethylfor-
mamide dineopentyl acetal (0.58 g, 2.5 mmol, 0.7 mL) in dry
CH3CN (10 mL) was heated at reflux temperature under an
inert N2 atmosphere overnight. The resulting white precipitate
was filtered off and recrystallized form hot CHCl3 (40 mL) to
afford pure 4 (0.150 g, 0.54 mmol, 62%): mp 213.9 °C; 1H NMR
The NBO analyses for the less symmetric cases 1 and
3 yield analogous results (not shown). The TB interaction
is mainly due to the PCMO’s shown in Figure 11. After
interaction of the semilocalized π- and Lpπ(S) orbitals the
energy differences are close to those calculated for the
canonical MO’s.
δ 2.25 (s, 8H), 2.53-2.58 (m, 8H), 2.62, 2.67 (m, 8H) ppm; 13
C
NMR δ 28.9, 30.6, 32.0, 127.7, 130.6 ppm. Anal. Calcd for
C
16H24S2: C, 68.52; H, 8.62; S, 22.86. Found: C, 68.40; H, 8.70.
Tetr a h yd r o-4H-th iop yr a n -4-on a zin e (13). To a solution
Con clu sion s
of tetrahydro-4H-thiopyran-4-one (12, 5.00 g, 43.0 mmol) in
CH3CH2OH (125 mL) heated at reflux temperature was added
dropwise to a solution of H2NNH2‚H2O (>99%, 105 mL, 21.6
mmol) in CH3CH2OH (50 mL). The reaction mixture was
heated at reflux temperature overnight. After cooling to room
temperature and removal of solvent in vacuo pure 13 was
obtained (4.90 g, 21.5 mmol, 100%): 1H NMR δ 2.66-2.87 (m,
16H) ppm; 13C NMR δ 29.1, 30.2, 30.3, 37.5, 163.5 ppm.
3,7,11-Tr it h ia -14,15-d ia za d isp ir o[5.1.5.2]p e n t a d e c-
a n e (14). A solution of 13 (4.90 g, 21.5 mmol) in CH3CN (150
mL) was stirred under a H2S(g) atmosphere (balloon) for 5
days. After evaporation of the solvent in vacuo pure 14 was
obtained as a light yellow solid (5.54 g, 21.1 mmol, 98%): 1H
NMR δ 2.01-2.13 (m, 8H), 2.65-2.82 (m, 8H), 3.99 (s, 2H)
ppm; 13C NMR δ 27.5, 40.5, 85.4 ppm.
3,7,11-Tr ith ia -14,15-d ia za d isp ir o[5.1.5.2]p en ta d ec-14-
en e (15). Compound 14 (5.54 g, 21.1 mmol) was oxidized with
a mixture of Pb(OAc)4 (13.0 g, 29.3 mmol) and CaCO3, (13.0 g,
0.13 mol) in dry CH2Cl2 (160 mL).4a After work up a brown
solid (5.87 g) was obtained, which was recrystallized from hot
ethyl acetate to yield pure 15 (3.39 g, 13.0 mmol, 62%):. 1H
NMR δ 1.88-1.95 (m, 4H), 2.64-2.93 (m, 12H) ppm; 13C NMR
δ 26.4, 40.3, 110.9 ppm.
The PES and RHF/6-31G* data show that ground-state
electronic interactions between the Lpπ(S)- and/or π-MO’s
of 1-6 occur. For 3, 4 and 6 this leads to splitting of the
π-MO’s 3: ∆Iv,j ∼ 0.7 (estimated from bandwidth) and
∆-ꢀj ) 0.81 eV, 4: ∆Iv,j ∼ 0.6 (estimated from bandwidth)
and ∆-ꢀj ) 0.89 eV and 6: ∆Iv,j ) 0.46 and ∆-ꢀj ) 0.56
eV). Moreover, in case of 2 and 4, which both contain
sulfur atoms at their termini, the Lpπ(S) MO’s are also
significantly split [2: ∆Iv,j ) 0.40 eV and ∆-ꢀj ) 0.41 eV
and 4: ∆Iv,j ∼ 0.3 eV (estimated using the two band
maxima) and ∆-ꢀj ) 0.32 eV]. Hence, the interactions
between Lpπ(S) are efficiently mediated over lengths of
8-12 Å.
RHF/6-31G*/NBO analyses on 1-6 show that TB
interactions occur and that the Hax-C-C-Hax PCMO’s
of the cyclohexyl-type rings play a dominant role. The
RHF/6-31G* results of the different conformers of 1-6 in
combination with the He(I) PES results of conformation-
ally locked anti- and syn-7 show that the conformation
of the hydrocarbon skeleton does not affect the magnitude
of the TB coupling, viz. the Hax-C-C-Hax PCMO’s can
be combined with π-type orbitals within all point groups
4-(4-(Tetr a h yd r o-4H-th iop yr a n -4-ylid en e)-1-h yd r oxy-
cycloh exyl)tetr a h yd r o-4H-th iop yr a n -4-ca r boxylic Acid
(17). Tetrahydro-4H-thiopyran-4-carboxylic acid (16,12 0.26 g,
1.80 mmol) and 4-(tetrahydro-4H-thiopyran-4-ylidene)cyclo-
hexanone4a (0.35 g, 1.80 mmol) were coupled to give the
â-hydroxy acid 17 employing a literature procedure.4a How-
ever, the workup procedure was slightly modified. After the
water layer was acidified to ca. pH 1 with concentrated HCl
(32) The calculated (RHF/6-31G*) Lpπ(S) MO’s of a model system
consisting of two dimethyl sulfide and one ethene molecule in the
geometry of 2 and ghost centers on the missing carbon atoms are nearly
degenerate (HOMO, -9.22 eV and HOMO-1, -9.29 eV).30