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On the Nature of Pyrazolylborane. An Ab InitioIGLONMR Study

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120 1
2 c - Monoclinic P2Jn, a = 9.698(2), b = 23.753(4), c = 9.869(2) A, fl =
118.55", V = 1 9 9 6 . 8 ( 7 ) A 3 , 2 = 4 , e s ~ , s d1=. 0 9 2 g ~ m - ~ , p = O . O 6 r n m - ~ .
Mo,. radiation (graphite monochromator), 3-50" in 2 0 . 3532 unique
intensities, 2664 observed [(Fot 4~s(F)], R=0.059, R, = 0.065, 257 refined parameter, T = 115 K. Further information on the crystal structure
investigation is available on request from the Fachinformationszentrum
Karlsruhe, Gesellschaft fur wissenschaftlich-technische Information mbH,
D-7514 Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository
number CSD-320069, the names of the authors, and the journal citation.
M. Yalpani, R. Koster. R. Boese, unpublished.
Thecalculated B-N bond length in Me,NBMe, is 1.422 A: M J S . Dewar.
M. L. McLee, J. Am. Chem. Soc. 99 (1977) 5231.
For Me,NBMe, the observed B-N bond length is 1.40 and 1.43 A for the
two independent molecules found in the crystal: J. Bullen, N. H. Clark, J
Chem SOC.A 1977, 992.
2d, i.,,
220 ( E = 1.5 x lo3); 2a, %,,
, 240 (3.1 x lo3); 2b = 2c, A,, =
255 nm (2.8 x 10))
Program MOPAC, Version 3.12, QCPE N o 455.
The version of this manuscript submitted on October 10,1989 also included an MNDO calculation for the hypothetical unsubstituted parent compound of 4. The results led to an intermediate (activated complex) structure with C , geometry (interplanar N,B/N,C3 angle 37.1") with bond
lengths and energies of similar magnitudes as are reported by Schleyer et
al. [15] using ab initio methods. Since our results evoked the criticism of
one of the referees, who insisted on a C,, geometry, we omitted their
discussion in the present version of this communication. Finally, since the
IGLO NMR calculations in [15] do not take account of the severe intermolecular interactions present in the real molecules 2a-c, their relevance
to the values actually found is open to question.
The calculated structural data (bond distances and angles) are in good
agreement with the experimental values found by X-ray diffraction.
P. von R. Schleyer, M. Buhl, Anpew. Chem. 102 (1990) 320; Angen. Chem.
Inr. Ed. EngI. 29 (1990) 304.
Fig. 2. Dependence of the differences in the enthalpy of formation AAH:"
- AHfCF(acyclic))on the substituents of the pyrazolyl groups of
boranes 2a-d.
and thermodynamic stability of species 4 relative to the corresponding acyclic structure ( 3 / 3 )upon further increase in
the internal crowding of the molecule. Hitherto, all attempts
to introduce a substituent substantially larger than a methyl
or ethyl group have failed at the P-diketone, starting material
Experimental Procedure
2a: A solution of l a (1.5g, 8.3 mmol) and (9H-9-BBN), (l.Og, 4.15 mmol)
dissolved in 10 mL of toluene was heated to reflux for 18 h, during which time
200 mL of H, gas evolved. The solvent was removed in vacuo and the solid
residue recrystallized from hexane by slow cooling to - 78 "C. Crystalline 2 a :
2.4 g (96%). m.p. 89-90°C. MS: miz 300 (Me,B,, 90%) 285 (43, 271 ( 4 3 ,
257 (6% 243 (100). IlB NMR (64.2 MHz, CDCI,): 6 = 63.1 (h,,, = 350 Hz).
'HNMR(200.1 MHz,CDCI3):6=6.09(s,1H),2.33(br,2H),1.X8(m,12H),
1.26 ( s , IXH). "C NMR (50.3 MHz, CDCI,, T = 311 K): 6 = 162.8 (s, 2C),
On the Nature of Pyrazolylborane.
106.5 (d, l C ) , 33.6 (t, 4C), 30.5 (4, 6C). 30.0 (s, 2C). 28.5 (br., d, 2C). 22.8
An Ab Initio/IGLO/NMR Study
(t, 2C). I3C NMR (75.5 MHz, C,D8, T = 173 K): 6 = 165.1 ( s , 1C), 161.9
(s, 1 C ) . 108.0 (d, 1 C), 33.9 (t, 4C), 32.8 (s, 1 C), 32.5 (s, 1 C), 31.4 (q, 3C), 29.9
By Paul von Rague Schleyer* and Michael Biihl
(q. 3C). 28.4 (br.. d, 2C). 23.5 (t. 2C).
2b: (9H-9-BBN), (0.53 g, 2.2 mmol) and 1 b (0.84 g. 4.3 mmol) were allowed to
Dedicated to Professor Dr. Giinther Wilke on the occasion
react as above for 24 h to give 1.3 g (94%) of a colorless solid. Purified by
of his 6Sth birthday
sublimation in vacuum; m p. 70-71 "C. MS: m / z = 314 (Me, B,, 80%), 299
(20). 285 (40). 257 (100). "B NMR (64.2MHz. CDCI,). 6 =64.2
As described in the preceding communication by Yalpani
(hl;z = 500 HZ). ' H N M R (200.1 MHz,CDCI,): d = 1.95 (s, br., 12H); 1.27 (s,
al., diorgano(pyrazoly1)boranes substituted with bulky
3H); 1.39 ( s , 20H). 13CNMR (50.3 MHz, CD,CI,, T = 311 K): 6 = 157.4 ( s ,
2C),115.8(s,1C),33.8(t,4C),32.9(s,2C),30.1(br.,d,2C),29.9(q,6C),22.8 groups undergo degenerate fluxional isomerization in solution.['] Although the available crystal structures show the
(t, 2C), 12.7 (9, 1 C) "C NMR (75.5 MHz, C,D,O, T = 163 K): 6 = 158.5
BR, group to be classically bonded to a single nitrogen (but
(t. 2C), 13.6 (q, 1 C).
with varying degrees of twist), the possibility was considered
2 c : (9-H-9-BBN), (0.84 g, 3 4 mmol) and 1 c (1.36 g, 6.5 mmol) were allowed to
that a symmetrical structure might be present in solution.
react as above for 24 h to give 1.9 g (88%) of a colorless solid. Purified by
This species would involve simultaneous bonding between
sublimation in vacuum; m.p. 82-83°C. MS: m/z = 328 (Me. B,, loo%), 285
(65). 271 (8% 245 (85), 232 (90). "B NMR (64.2 MHz, CDCI,). 6 = 65.3
the boron and both nitrogens. We were intrigued by this
(h,,, = 550 Hz). 'H NMR (200.1 MHz, CDCI,): 6 = 2.67 (q, 2H), 1.95 (br.,
novel system, and also by the possibility of using the com2H). 1.88(m. 12H). 1.36(s. lXH), 1.14(t,3H). "CNMR(50.3MHz,CDC13,
bined a b initio/IGLO/NMR chemical shift methodr2. to
T = 3 1 1 K ) : 6 = 157.9(s,2C),122.3(s,lC),33.8(t,4C),33.2(s,2C),30.8(q,
probe the nature of the highly substituted species in solution.
6'21%30.0 (br., d, 2C), 22.8 (1, 2C), 17.8 (t. lC), 16.5 (q, 1C).
Received: October 2, 1989;
revised: December 14, 1989 [Z 3571 IE]
German version: Angen. Chem. 102 (1990) 318
CAS Registry numbers:
l a , 1132-14-5; 1 b, 18712-47-5; Ic, 125281-21-2;2a, 125303-72-2;2b, 125281-
22-3; 2C, 125281-23-4;(9H-9-BBN),, 21205-91-4.
Handbuch der Anorganuchen Chemie, 22.
[l] K. Beeker in: Gmelin
Erganzungsn?erk. Borverbindungen, Teil 4 , Springer, Berlin 1975, p. 99ff.
[2] A. Meller in: Gmelin Handbook oflnorganic Chemistry, B3rd Suppl. Vol. 3 ,
Boron Compounds. Springer, Berlin 1988, p. 148.
[3] K. Niedenzu in J. F. Liebmann, A. Greenberg (Eds.): Mofecular Structure
and Energetics, Voi. 5 , VCH Veriagsgeselischaft, Weinheim 1988, p. 357.
[4] Monomeric diamino(pyrazoly1)boranes of the type PzB(NR,), are known
[F. Alam. K. Niedenzu. J. Organomer. Chem. 243 (1983) 191: in these,
however, the participation of lone-pair electrons of the three neighboring
nitrogen atoms in x bonding with the boron atom largely eliminates the
Lewis acidity of the latter.
[5] B. Wrackmeyer. R. Koster in: Houben- Weyl-Miiller. Methoden der Organisrhen Chemie, Vol. XII1/3c, 4th ed., Thieme, Stuttgart 1984. p. 493.
[6] M. Yalpani, R. Boese, R. Koster, Chem. Ber. 122 (1989) 19.
@> VCH Vedaxsgesellschaft mbH. 0-6940 Weinheim. 1990
This new combination of methods, which involves comparison of chemical shifts calculated for several structural models with experimental data, is quite successful for the elucidation of structural
We calculated the structural parameters of 1-3, three basic forms of the parent pyrazolylborane, at reasonably sophisticated a b initio levels.[41The geometries were optimized
with the polarized split valence 6-31G* basis set; key
parameters are included in Figure 1. One set of single point
Prof. P. van R. Schleyer, Dipl.Chem. M. Biihl
Institut fur Organische Chemie der Universitat Erlangen-Niirnberg
Henkestrasse 42, D-8520 Erlangen (FRG)
[**I This work was supported by the Deutsche Forschungsgemeinschaft, the
Fonds der Chemischen Industrie, Convex Computer Incorporated and the
Volkswagenstiftung. We thank the Kernforschungsanstalt Jiilich for a
grant of computer time, and are indebted to Prof. W Kutzelnigg and his
group for the development of the IGLO program, M. Schrndler for the
Convex version, and U . Fleischer for discussions. We thank Prof. Yulpani
and his associates for discussions and for additional experimental information.
OS70-0833/90/0303-0304 S 02.SOiO
Angew. Chem. Int. Ed. Engl. 29 (1990) No. 3
1.421 (1.428)
1.361 (1.381)
1 371 (1.386) 0
1.345(1 366)
1.484(1 486) A\NIN
I .342 (1.356)
1.431 (1.421)
1.322(1 352)
Fig. I . The 6-31G* optimized geometries of 1-3 (C, symmetry). The
MP2(FU)/6-31G* parameters are given in parentheses
energies were obtained with a larger basis set, 6-31 +G*,
which adds a set of diffuse s and p functions on the non-hydrogen atoms, and includes electron correlation at the second order Mdler-Plesset (MP2) level using the frozen core
(FC) approximation. Electron correlation favors the bridged
structure (3) preferentially and lowers its relative energy below that of 2 (Table 1). The effect of correlation results from
the more delocalized character of the bonding in 3 with its
three-membered ring. Consequently, 1-3 were reoptimized
at the MP2(FU)/6-31G* correlated level (FU = full). This
changed the geometries only slightly (see Fig. 1, MP2 values
are given in parentheses). Our final relative energies
(Table 1) include the zero point (ZPE) corrections, calculated at 6-31G*. At MP2(FU)/6-31G*//MP2(FU)/6-3lG*
ZPE(6-31G*), 3 lies 12.4 kcalmol-' above 1 and the energy
of 2 is a little higher (18.1 kcalmol-').
Frequency calculations at 6-31G* indicate both 1 and 3 to
be minima. Transition state 2 is involved in BH, rotation,
while 3 is an intermediate in the interconversion between 1
and its degenerate counterpart. The barrier we calculate for
the parent species (our best value is at least 12.4 kcal/mol) is
in line with the experimental observations on the more
crowded derivatives in solution.[*]The bulky substituents
lower the barrier and, as indicated from the crystal structures, twist the borane moieties from the preferred coplanar
arrangement (1) to intermediate conformations tending towards 2. The geometries shown in 1 and 2 are in reasonable
Table I . Absolute [a] and relative [b] energies for 1-3.
Level of theory
1 (CS.
6-31 +G*l/6-31G*
MP2(FC)/6-31 G*//6-31G*
MP2( FU)/6-31G*//6-31G'
ZPE(6-31G*) [c]
Findl [d]
2 (CS,
3 (CS,
[a] In a.u. [b] In parentheses, kcalmol-I. [c] Zero point energy, kcalmol-',
scaled by 0.89 as recommended in reference [4]. [d] Relative energies in
kcdlmol-' at MP2(FU)/6-31G*+ZPE.
Angew. Chem. h i .
Ed. Engl. 29 (1990) No. 3
agreement with the X-ray data,"] when one takes the expected repulsions due to the substituents into account.
We now address the two principal questions which were
posed by the experimental results, but remained unanswered.
Even though 3 is inherently less stable than 1, could it be
uossible that the bulky substituents result in a preference for
the symmetrical form in solution? If this symmetrical form
be present, what is its electronic structure? This second question can be answered by examining an ab initio wave function, but the calculated geometry (see 3) also i s revealing.
Since the C-C and C-N bond lengths in the pyrazole ring in
3 have about the averaged values of those in 1 and 2, there
is no indication of any significant change in the basic 6 7 ~
aromatic character of the pyrazole ring. The B-N bond
lengths in 3,1.590 8, at 6-31G* (the MP2/6-31G* lengths are
nearly the same), are about 0.2 8, longer than the single B-N
distance in 2, but are ca. 0.1 8, shorter than the dativec5]
N + B bond length, 1.690 A, calculated at the same level for
H3NBH3.[41Hence, the geometry of 3 indicates a classical
three-membered ring arrangement. When the B-N-N angle
is decreased, the nitrogen lone pair electrons in 2 interact
with the vacant p-orbital on boron and a three-membered
ring structure for 3 results.
Natural population analysis[61confirms this picture. As
expected, B-N bond order (0.88) is highest in 1, because of
the double bond character. The partial ionic nature of the
bonding reduces the bond order, as calculated using
Wiberg's definition.[" The B-N bond order of 0.71 in 2 represents the value for a single B-N bond given by this method
of analysis. The B-N bond order of 0.54 in 3 is only slightly
smaller despite the three-membered ring structure and partial dative character. The natural chargec6]on boron is largest in 2 ( + O M ) ; the lower value 10.64 in 1 results from the
7[: back donation from the ring nitrogen to boron. The boron
charge (+0.49) in 3 is lowest of all. Note that the sign is
positive, despite the formal negative charge which one would
write classically for a tetravalent borate ion. This i s general
behavior. When the atoms involved differ in electronegativity (as do B and N in the present case), the formal charges
which satisfy Lewis octet requirements are not consistent
with charges derived from quantum mechanics wave functions.['] (See below for an example.)
The IGLO "B-NMR chemical shifts, calculated for 1-3
using the MP2/6-31G* geometries, showed large variations
with structure. These IGLO calculations employed a double
zeta (DZ) basis set for most of the atoms,[21but a more
sophisticated triple zeta + polarization basis for boron. This
better description of the nucleus in question employing "locally dense basis sets" has been proposed by Chestnut.cg1
While perfect agreement with the experimental values obtained on substituted derivatives cannot be expected, the
general accord is pleasing. Thus, 6 (' B) (vs. BF,.OEt,) for
planar 1 (47.6) and for perpendicular 2 (62.0) may be compared with the values (63.1, 64.2) found experimentally for
twisted structures.
The IGLO "B-NMR chemical shift calculated for 3,
6 = 19.3, effectively rules out symmetrically bridged structures as candidates for the species present in solution. None
of the experimentally observed "B-NMR chemical shifts"]
approach this IGLO value. However, there is good agreement between the IGLO result for 3 and that for numerous
other tetracoordinated boron analogues which involve the
simultaneous attachment of two nitrogen substituents to boron.[''] A simple experimental example, [(CH,),B(NH,),]Br
with 6("B) = 13.9,['0*'11 is representative. The IGLO
G("B)valueis 11.5for [(CH,),B(NH,),]@ using the6-31G*optimized geometry. The computed B-N length is 1.657 8,
Verlagsgesellschafi' mbH. 0-6940 Weinhelm, 1990
0570-0X33/90/0303-030SS 02.SOj0
and the Wiberg bond order is 0.67. The natural charges are
+OX7 on B and - 1.OO on N (vide supra, note signs).
We conclude that the diorgano(pyrazoly1)boranes have
similar geometries in solution and in the solid phase. Although symmetrical structures like 3 are involved in the degenerate rearrangement, they are not present in significant
concentration in solution. Both the electronic structure and
the "B-NMR chemical shift of 3 are normal.['21That is, the
pyrazole ring in the hypothetical 3 retains its 6~ aromatic
character and the B-N bonding is intermediate between single B-N and dative N + B arrangements.
Received: December 20, 1989 [Z 3695 IE]
German version: Angew. Chem. 102 (1990) 320.
CAS Registry number:
pyrazolylborane, 62427-83-2.
[l] M. Yalpani, R. Koster, R. Boese, W. A. Brett, Angew. Chem. 102 (1990)
318; Angew. Chem. Int. Ed. Engl. 29 (1990) 302, the authors did extensive
MNDO calculations; M. Yalpani, private communication.
121 W Kutzelnigg, Isr. J Chem. 19 (1980) 193. M. Schindler, W. Kutzelnigg, L
Chem. Phys. 76 (1982) 1919. Review: W. Kutzelnigg, U. Fleischer, M.
Schindler, N M R Basic Princ. Prog., in press
[3] See, e.g. applications to carbocations: a) M. Schindler, J Am. Chrm. Soc.
109 (1987) 1020; b) M. Bremer, P. von R. Schleyer. K. Schotz, M. Kausch,
M. Schindler, Angew. Chem. 99 (1987) 795; Angew. Chem. h t . Ed. Engl. 26
(1987) 761; c) P. von R. Schleyer, K. E. Laidig, K. B. Wiberg, M. Saunders, M. Schindler, J Am. Chem. Soc. If0 (1988) 300, d) M. Saunders,
K. E. Laidig, K. B. Wiberg, P. von R. Schleyer, ibid.110 (1988) 7652; e) P.
von R. Schleyer, J. W. de M. Carneiro, W. Koch, K. Raghavachari, ibid. 111
(1989) 5475; f) M. Bremer, P. von R. Schleyer, ibid. I 1 1 (1989) 1147; g) M.
Bremer, K Schotz, P. von R. Schleyer, U. Fleischer, M. Schindler, W
Kutzelnigg, P. Pulay, Angew. Chem. I01 (1989) 1063; Angew. Chem. Int.
Ed. Engl. 2X (1989) 1042; h) P. von R. Schleyer, W. Koch, B. Liu, U.
Fleischer, J Chem. Soc. Chem. Commun. f989,1089, Application to boron
NMR chemical shifts: P. von R. Schleyer, M Buhl, U. Fleischer. W. Koch,
Inorg. Chem. 29 (1990) 153.
[4] The Gaussian series of programs with standard procedures and basis sets
were employed. See W. J. Hebre, A. Radom, P. von R. Schleyer, and J. A.
Pople: A h Inilio Molecular Orbital Theory, Wiley-Interscience, New York
[S] A. Haaland, Angew. Chem. 101 (1989) 1017; Angew. Chem. Int. Ed. Engl.
28 (1989) 992.
[6] Review: A. E. Reed, L. A. Curtis, F. Weinhold. Chem. Rev. 88 (1988) 899.
[7] K. Wiberg, Tetrahedron 24 (1968) 1083.
[S] A. Greenberg, R. Winkler, B. L. Smith, J. Liebman, J. Chem. E ~ K C59
(1982) 367.
191 D. B. Chestnut, K. D. Moore, J Compur. Chem. 10 (1989) 648.
[lo] H. Noth, B. Wrackmayer: Nuclear Magnetic Resonance Spectroscopy of
Boron Compounds, Springer, Berlin 1978.
[ l l ] K. Maier, Disserfation, Universitat Marburg 1971
[12] We also examined two planar symmetrially-bridged C, structures like 3,
with perpendicular and with coplanar BH, groups. The latter was
85 kcalmol-' higher in energy than 1 at 6-31G+//6-31G* and possessed
three imaginary frequencies. The perpendicular form was a transition state
for the degenerate interconversion of 3, and was 2.4 kcalmol higher in
energy than the latter at our final level.
For example, the rates of the acid-catalyzed cyclization of
nerol['] and the acid-catalyzed dehydration of 1,I-diphenyl1-0ctadecanol ['I are strongly affected by surface pressure
changes, since these reactions require the involvement of
remote functionalities, i.e. the remote double bond in nerol
and the P-hydrogens in the 1,I -diphenyl-I -octadecanol, for
reaction. High surface pressures disfavor this participation
and the rate is correspondingly lowered. By contrast, reactions taking place at a single center, e.g. the chromic acid
oxidation of 1-phenyl-l-hexadecano1" and the base-catalyzed hydrolysis of an octadecyl
are not significantly
affected by surface pressure changes. We report here the
effect of surface pressure on the mechanism of the acid-catalyzed hydrolysis of I-phenylhexadecyl ethanoate 1 to 1phenyl-I -hexadecanol2 in monolayers at an air/sulfuric acid
\'C L
P h
\'C &Ph
-I-- a
The investigation of reactions in monolayers requires that
the substrate forms films stable within the time scale of the
experiment. The monolayer film of I-phenylhexadecyl ethanoate amply satisfies this requirement. Compressed films
( R > 15.0 mN m- ') over water produce negligible reduction
in n (< 5 %) over a 30 min period. The surface pressure-area
(R-A)isotherm (Fig. 1) is characteristic of amphipatic systems; the limiting area (obtained by extrapolating the R-A
isotherm to R = 0) of 0.85 nm2 is larger than for simple alkyl
chains such as octadecanoic acid, where the limiting area is
0.21 nm2,[51because of the phenyl group inhibiting further
compression. 1,l -Diphenyl-1-octadecanol has a limiting
area of 0.72 nm2.[23Interestingly, the surface pressure-area
isotherm in Figure 1 exhibits a just discernible plateau region
Switching Reaction Mechanism
by Monolayer Compression: An Ester Hydrolysis
By Jamil Ahmad* and Kenneth Brian Astin'
Monolayers provide a novel opportunity to examine the
reactivity of molecules constrained to a plane, where the
geometrical requirements of transition states may be inferred
from the reactivity of films at varying surface pressures (or
areas per molecule). The method is suitable for the investigation of neighboring group and proximity effects." - 31
Prof. Dr. J. Ahmad, Prof. Dr. K. B. Astin,
Department of Chemistry, University of Bahrain
P.O. Box 32038, Isa Town (Bahrain)
VCH Yerlagsgesellschafi mhH, 0-6940 Weinherm,1990
A [nm2 molecule']+
Fig. 1. K-Aisotherm of 1-phenylhexadecylacetate at 25 "C over water, using a
Wilhelmy balance (initial film area 300 cm2,final area 70 cm2,compression rate
24 cmz min- ').
0570-0X33/90/0303-03063 02 SO10
Angew. Chem Inl. Ed. Engl. 29 (1990) No 3
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