Synthesis and solution and solid-state structures of tris(pentafluorophenyl)borane adducts of PhC(O)X (X = H, Me, OEt, NPri2)

Article abstract of DOI:10.1021/om9710327

Reaction of the highly electrophilic borane B(C₆F₅)₃ with the carbonyl Lewis bases benzaldehyde, acetophenone, ethyl benzoate, and N,N-diisopropylbenzamide led to isolation of the crystalline adducts 1-H, 1-Me, 1-OEt, and

Full text of DOI:10.1021/om9710327

Organometallics 1998, 17, 1369-1377
1369
Syn th esis a n d Solu tion a n d Solid -Sta te Str u ctu r es of
Tr is(p en ta flu or op h en yl)bor a n e Ad d u cts of P h C(O)X
(X ) H, Me, OEt, NP r i )
2
Daniel J . Parks,† Warren E. Piers,*,† Masood Parvez,† Reinaldo Atencio,‡ and
Michael J . Zaworotko‡
Departments of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta
T2N 1N4, Canada, and Saint Mary’s University, Halifax, Nova Scotia B3H 3C3, Canada
Received November 24, 1997
Reaction of the highly electrophilic borane B(C6F5)3 with the carbonyl Lewis bases
benzaldehyde, acetophenone, ethyl benzoate, and N,N-diisopropylbenzamide led to isolation
of the crystalline adducts 1-H, 1-Me, 1-OEt, and 1-NP r , respectively, in good to excellent
yields (63-89%). Equilibrium measurements and exchange experiments indicated that the
order of basicity (from highest to lowest) of these bases toward B(C6F5)3 follows the order
N,N-diisopropylbenzamide > benzaldehyde > acetophenone > ethyl benzoate. The solution
and solid-state structures were probed to rationalize these observations. In solution, the
borane coordinates to the carbonyl lone pair syn to H and Me in the aldehyde and ketone
adducts, as indicated by 1H/19F NOE difference experiments. The same coordination
geometry was observed in the solid state upon X-ray diffraction analysis of the two adducts.
The added front strain associated with the ketone adduct (C-O-B )133.6(3)° vs 126.7(5)°
for the benzaldehyde complex) accounts for the observed order of basicity with these two
bases. For ethyl benzoate and N,N-diisopropylbenzamide, the borane coordinates syn to
the phenyl group in both solution and the solid state. In addition to the carbonyl oxygen-
boron interaction, the two complexes engage in a π-stacking interaction between one of the
borane C6F5 rings and the syn phenyl group. In addition to the structural proof of this
interaction in the solid state, variable-temperature 19F NMR experiments suggest it is
important in the solution structures of these adducts as well.
In tr od u ction
both solution and the solid state, is thus essential for
the design of catalysts and the understanding of their
Since the recognition in 1960 that AlCl3 dramatically
accelerates Diels-Alder cycloadditions,1 Lewis acids
(LA’s) have played an important role in the catalysis of
organic transformations involving carbonyl functions.
While the details of this LA effect can be complicated,
it is generally assumed that LA’s serve to activate
carbonyl-containing substrates through coordination to
the carbonyl oxygen atom, further polarizing the CdO
double bond and rendering the function more reactive
than it is in the absence of the LA.
In addition to this electronic effect, the alteration of
the steric environment around the carbonyl function
upon coordination to the LA can influence the trajectory
of incoming reagents as they approach the coordinated
carbonyl compound.2 This allows for the design of
stereo-, regio-, and chemoselective reactions; further-
more, in many instances, the LA need only be present
in catalytic quantities, making practical the use of
structurally complex chiral LA’s for asymmetric trans-
formations.3 An intimate understanding of the carbo-
nyl-LA interaction from a structural point of view, in
mode of action.4
Boranes constitute a particularly important class of
LA’s in organic synthesis, BF3 being the quintessential
example. Although several detailed NMR studies exist
which probe the solution structure of carbonyl-borane
adducts,5 only two structurally characterized examples
(3) For selected recent examples, see: (a) Corey, E. J .; Guzman-
Perez, A.; Loh, T.-P. J . Am. Chem. Soc. 1994, 116, 3611. (b) Corey, E.
J .; Letavic, M. A. J . Am. Chem. Soc. 1995, 117, 9616. (c) Hayashi, Y.;
Rohde, J . J .; Corey, E. J . J . Am. Chem. Soc. 1996, 118, 5502. (d) Evans,
D. A.; Murry, J . A.; von Matt, P.; Norcross, R. D.; Miller, S. J . Angew.
Chem., Int. Ed. Engl. 1995, 34, 798. (e) Ishihara, K.; Gao, Q.;
Yamamoto, H. J . Am. Chem. Soc. 1993, 115, 10412. (f) Ishihara, K.;
Kurihara, H.; Yamamoto, H. J . Am. Chem. Soc. 1996, 118, 3049. (g)
Heller, D. P.; Goldberg, D. R.; Wulff, W. D. J . Am. Chem. Soc. 1997,
119, 10551.
(4) (a) Shambayati, S.; Crowe, W. E.; Schrieber, S. L. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 256. (b) Shambayati, S.; Schrieber, S. L. In
Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon: Oxford, U.K., 1991; Vol 1, pp 283-324.
(5) (a) Hunt, I. R.; Rogers, C.; Woo, S.; Rauk, A.; Keay, B. A. J . Am.
Chem. Soc. 1995, 117, 1049. (b) Frateillo, A.; Onak, T. P.; Schuster, T.
P. J . Am. Chem. Soc. 1968, 90, 1194. (c) Henriksson, U.; Forse´n, S. J .
Chem. Soc. D 1970, 1229. (d) Hartman, J . S.; Stilbs, P.; Forse´n, S.
Tetrahedron Lett. 1975, 3497. (e) Schuster, R. E.; Bennett, R. D. J .
Org. Chem. 1973, 38, 2904. (f) Fratiello, A.; Stover, C. S. J . Org. Chem.
1975, 40, 1244. (g) Stilbs, P.; Forse´n, S. Tetrahedron Lett. 1974, 3185.
(h) Fratiello, A. Kubo, R.; Chow, S. J . Chem. Soc., Perkin Trans. 2 1975,
1205. (i) Torri, J .; Azzaro, M. Bull. Soc. Chim. Fr. 1978, 286. (j) Childs,
R. F.; Mulholland, D. L.; Nixon, A. Can. J . Chem. 1982, 60, 801. (k)
Fratiello, A.; Schuster, R. E. J . Org. Chem. 1972, 37, 2237. (l) Fratiello,
A.; Vidulich, G. A.; Chow, Y. J . Org. Chem. 1973, 38, 2309.
* To whom correspondence should be addressed. Phone: 403-220-
5746. FAX: 403-289-9488. E-mail: wpiers@chem.ucalgary.ca.
† University of Calgary.
‡ Saint Mary’s University.
(1) Yates, P.; Eaton, P. J . Am. Chem. Soc. 1960, 82, 4436.
(2) Yamamoto, H. Chem. Commun. 1997, 1585.
S0276-7333(97)01032-7 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 03/06/1998

1370 Organometallics, Vol. 17, No. 7, 1998
Parks et al.
Ta ble 1. Com p a r a tive Sp ectr a l Da ta for Ad d u cts 1
a n d th e F r ee Liga n d s
have been reported in the literature, to the best of our
knowledge. These are the BF3 adducts of the aldehydes
benzaldehyde6 and 2-methylacrolein.7 A growing num-
ber of reports have appeared in the literature concerning
the application of perfluorinated arylboranes8 and -borin-
ic acids9 to synthetic organic problems. In particular,
the highly electrophilic tris(pentafluorophenyl)borane,
B(C6F5)3,10 is finding utility as an LA catalyst for various
reactions. This borane offers several chemical advan-
tages over BF3,8f,g albeit at a higher economic cost;
however, it is now commercially available.
During the course of our studies on the use of B(C6F5)3
as a carbonyl hydrosilation catalyst,8b we found that it
forms stable, crystalline adducts with a variety of
carbonyl-containing compounds. In this paper, we
present the detailed solution and solid-state structures
of the adducts formed between B(C6F5)3 and benzalde-
hyde, acetophenone, ethyl benzoate, and N,N-diisopro-
pylbenzamide.
X
dataa
H
CH3
2.3
212.8
197.0
15.8
1603
1686
-83
OEt
19.2
175.3
166.2
9.1
1669
1718
-49
NPri
2
δ(11B)
5.0
-0.05
174.0
170.8
3.2
1570
1628
δ(13CO,1)
δ(13CO,free)
∆
199.4
192.1
7.3
1620
1702
ν
CO(1)
CO(free)
ν
∆
-82
-58
a
δ in ppm; ν in cm-1. ∆ is the difference between δ or ν for
adducts 1 and free carbonyl substrate.
as the LA exacerbates the inherent polarization of the
CdO bond. This shift is strongest for 1-Me and is
dampened somewhat if X is more electron donating as
in 1-NP r . Furthermore, the carbonyl stretching fre-
quency in the IR spectra of adducts 1 are red-shifted in
comparison to the free substrates by 49-83 cm-1, as
would be expected for the lowered bond order of the
CdO bond upon activation by the LA.
Resu lts a n d Discu ssion
Ad d u ct Syn th esis a n d Rela tive Ba sicities. The
adducts of B(C6F5)3 and benzaldehyde (1-H), acetophe-
none (1-Me), ethyl benzoate (1-OEt), and N,N-diisopro-
pylbenzamide (1-NP r ) were prepared by addition of 1
equiv of the carbonyl compound to a toluene solution of
scrupulously dried borane (eq 1).
In solution, the equilibria depicted in eq 1 lie far
toward adducts 1. As the measured equilibrium con-
stants Keq indicate, the order of basicity of these ligands
toward B(C6F5)3, from most to least basic, is N,N-
diisopropylbenzamide > benzaldehyde > acetophenone
> ethyl benzoate. Although we were unable to measure
Keq quantitatively for the amide ligand,12 exchange
experiments indicate that it is the most basic toward
B(C6F5)3 of the four carbonyl compounds. For example,
the benzamide rapidly displaces benzaldehyde to form
1-NP r and free PhC(O)H. This order of basicity is
further supported by observed exchange rates between
free and bound carbonyl bases and in the results of other
competition experiments. Homo exchange rates and the
equilibrium constants for hetero exchange experiments
are given in Table 2. For 1-H, 1-Me, and 1-OEt, homo
exchange is kinetically rapid; the most rapid homo
exchange rate was observed for PhC(O)OEt, the least
basic, most weakly bound substrate. However, when
excess benzamide is added to a solution of 1-NP r , no
exchange between free and bound ligand is observed on
the NMR time scale at room temperature. Hetero
exchange experiments show that ethyl benzoate is
readily displaced by benzaldehyde or acetophenone
when 1-OEt is treated with 1 equiv of either of the more
basic substrates. In the hetero exchange experiments,
equilibrium is reached essentially upon mixing of the
reagents, qualitatively attesting to the lability of these
systems.
Adduct formation is rapid and, upon workup, the
adducts 1 are isolated in 63-89% yield as white crystal-
line solids. NMR and IR spectral data for the products
of eq 1, a selection of which is presented in Table 1, are
consistent with adduct formation. The 11B NMR chemi-
cal shifts are in the region associated with neutral, four-
coordinate boron nuclei.11 Upon coordination, the 13C
resonance for the carbonyl carbon shifts downfield by
3.2-15.8 ppm in comparison to the same shift in the
free ligand. This reflects the deshielding of this carbon
(6) Reetz, M. T.; Hu¨llmann, M.; Massa, W.; Berger, S.; Radmacher,
P.; Heymanns, P. J . Am. Chem. Soc. 1986, 108, 2405.
(7) Corey, E. J .; Loh, T.-P.; Sarshar, S.; Azimioara, M. Tetrahedron
Lett. 1992, 33, 6945.
(8) (a) Parks, D. J .; Spence, R. E. v H.; Piers, W. E. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 809. (b) Parks, D. J .; Piers, W. E. J . Am. Chem.
Soc. 1996, 118, 9440. (c) Ishihara, K.; Hananki, N.; Yamamoto, H.
Synlett 1993, 577. (d) Ishihara, K.; Funahasi, M.; Hanaki, N.; Miyata,
M.; Yamamoto, H. Synlett 1994, 963. (e) Ishihara, K.; Hananki, N.;
Yamamoto, H. Synlett 1995, 721. (f) Ishihara, K.; Hanaki, N.; Funaha-
si, M.; Miyata, M.; Yamamoto, H. Bull. Chem. Soc. J pn. 1995, 68, 1721.
(g) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 345.
(9) Ishihara, K.; Kurihara, H.; Yamamoto, H. J . Org. Chem. 1997,
62, 5664.
The order of basicity observed here does not follow
the order of proton basicities for these compounds13 but
(12) Proton NMR techniques were used to measure all equilibrium
constants and exchange rates; because of the more complex, temper-
ature-dependent 1H NMR spectra of free N,N-diisopropylbenzamide,
Keq for this substrate could not be determined accurately using this
methodology.
(13) March, J . Advanced Organic Chemistry, 3rd ed.; Wiley: New
York, 1985; pp 220-222.
(10) Massey, A. G.; Park, A. J . J . Organomet. Chem. 1966, 5, 218.
(11) Kidd, R. G. In NMR of Newly Accessible Nuclei; Laszlo, P., Ed.;
Academic Press: New York, 1983; Vol. 2.

Tris(pentafluorophenyl)borane Adducts of PhC(O)X
Organometallics, Vol. 17, No. 7, 1998 1371
Ta ble 2. Exch a n ge Ra tes a n d Equ ilibr iu m
Con sta n ts for Hom o- a n d Heter oexch a n ge
Exp er im en ts
X
Y
T (K)
kex (s-1
)
Keq (×10-2
)
H
H
290
265
203
295
295
295
295
390
119
811
CH3
OEt
OEt
Me
OEt
H
CH3
OEt
Me
H
18(2)
5.4(5)
0.95(7)
<1
H
NPri
2
rather parallels their observed BF3 affinities14 and is
in keeping with the notion that the order of carbonyl
basicity is determined mainly by steric effects for these
neutral borane Lewis acids.15 In the case of the ben-
zamide ligand, however, the superior π-donating abili-
ties of the NR2 group render the CdO group more
electron rich, and despite being the most sterically
demanding ligand, the benzamide is the strongest base
toward B(C6F5)3.
Solu tion a n d Solid -Sta te Str u ctu r es of 1-H a n d
1-Me. Since borane LA’s are not strong π acceptors,
adducts with carbonyl groups tend to assume a bent
geometry, utilizing the carbonyl HOMO, which is es-
sentially an unhybridized oxygen 2p orbital.16 Thus, for
unsymmetrically substituted carbonyl functions such as
those employed here, there are two possible coordination
sites for B(C6F5)3 to occupy, i.e. I or II. For aldehyde
F igu r e 1. 1H/19F NOE difference spectra for 1-Me (1H
observe, 19F irradiate): (A) proton NMR spectrum of 1-Me;
(B) NOE difference spectrum with irradiation of ortho
fluorines (-136.0 ppm); (C) NOE difference spectrum with
irradiation of para fluorines (-157.0 ppm); (D) NOE
difference spectrum with irradiation of meta fluorines
(-164.6 ppm).
and especially ketone bases, the site of coordination
tends to be dictated by the relative steric attributes of
the two carbonyl substituents. Consequently, in borane
adducts formed from aldehydes, the LA coordinates syn
to the aldehyde proton almost exclusively. In the case
of acetophenone, it is predicted a priori that the borane
will coordinate syn to the methyl group on steric
grounds.
Spectroscopic studies on 1-H and 1-Me strongly
suggest that these adducts adopt such structures in
solution. For 1-Me the 13C chemical shift for the methyl
carbon is shifted 3.0 ppm upfield in comparison to that
found for free benzophenone. This upfield shift is
characteristic for the R-carbon of the group syn to the
borane in complexed ketones.5d Perhaps more convinc-
ingly, in 1H/19F NOE experiments in which the reso-
nance for the ortho fluorine atoms was selectively
irradiated, a strong enhancement of the resonances for
the aldehyde and methyl protons was observed in the
1H NMR spectra of 1-H and 1-Me, respectively. Figure
1 shows the spectra for 1-Me. A weaker enhancement
for the ortho protons of the phenyl ring was also
observed in both cases. Although this could be inter-
preted as being indicative of a syn:anti isomerization
process, molecular models of these two adducts suggest
that the ortho protons and ortho fluorines can acheive
nonbonded contacts of between 2 and 3 Å even when
B(C6F5)3 is syn to X, well within the range of distances
at which NOE effects may be expected.17 Furthermore,
no evidence for an isomer with B(C6F5)3 anti to X was
1
found in the low-temperature H or 19F NMR spectra
of either 1-H or 1-Me.
Syn coordination was confirmed via analysis of the
solid-state structures of 1-H and 1-Me. The molecular
structures of these adducts are depicted in Figures 2
and 3, respectively, and selected metrical parameters
(14) Maria, P.-C.; Gal, J .-F. J . Phys. Chem. 1985, 89, 1296.
(15) Rauk, A.; Hunt, I. R.; Keay, B. A. J . Org. Chem. 1994, 59, 6808.
(16) Linear coordination geometries are possible when the LA is a
strong π-acid. See ref 4b and: Sun, Y.; Piers, W. E.; Yap, G. P. A.
Organometallics 1997, 16, 2509.
(17) Derome, A. E. Modern NMR Techniques for Chemistry Research;
Pergamon: Oxford, U.K., 1987.

1372 Organometallics, Vol. 17, No. 7, 1998
Parks et al.
Ta ble 3. Selected Metr ica l P a r a m eter s for
Ad d u cts 1
X
H
CH3
OEt
NPri
2
Bond Distances (Å)
O(1)-B(1)
1.610(8) 1.576(5) 1.594(6) 1.52(1)
1.241(7) 1.242(5) 1.253(5) 1.32(1)
1.427(8) 1.464(6) 1.469(6) 1.49(2)
1.631(9) 1.626(6) 1.642(6) 1.64(2)
1.609(9) 1.634(6) 1.626(7) 1.62(2)
1.634(9) 1.631(7) 1.612(7) 1.65(2)
C(19)-O(1)
C(19)-C(20)
C(1)-B(1)
C(7)-B(1)
C(13)-B(1)
C(19)-C(26)
C(19)-O(2)
C(19)-N(1)
O(2)-C(26)
N(1)-C(26)
N(1)-C(29)
1.478(6)
1.301(5)
1.28(1)
1.469(5)
1.51(1)
1.51(1)
Bond Angles (deg)
F igu r e 2. ORTEP diagram of the adduct 1-H.
C(19)-O(1)-B(1), θ
O(1)-C(19)-C(20)
C(19)-C(20)-C(25)
C(19)-C(20)-C(21)
O(1)-B(1)-C(1)
O(1)-B(1)-C(7)
O(1)-B(1)-C(13)
C(1)-B(1)-C(7)
C(1)-B(1)-C(13)
C(7)-B(1)-C(13)
O(1)-C(19)-C(26)
O(1)-C(19)-O(2)
O(2)-C(19)-C(20)
O(1)-C(19)-N(1)
N(1)-C(19)-C(20)
∑[C-B(1)-C]
126.7(5) 133.6(3) 138.2(4) 131(1)
123.4(6) 115.8(4) 127.2(4) 121(1)
121.6(6) 119.0(4) 118.8(4) 118(1)
118.3(7) 122.0(4) 120.5(4) 120(1)
103.8(5) 102.3(3) 100.8(3) 104(1)
103.7(5) 105.9(3) 107.2(3) 111(1)
107.9(5) 109.9(3) 107.5(4) 109(1)
113.0(6) 113.3(4) 115.3(4) 103(1)
110.9(6) 107.3(3) 108.3(4) 113(1)
116.3(6) 117.1(4) 116.3(4) 114(1)
122.8(4)
118.5(4)
114.3(4)
118(1)
120(1)
330
340.2
Torsion Angle, φ (deg)
C(20)-C(19)-O(1)-B(1) 4.6(6) 4.2(4)
337.7
339.3
15.6(8)
3(1)
a higher level of front strain in the more sterically
confined ketone complex. The angle φ (approximated
by the dihedral angle X-C(19)-O(1)-B(1)) is about 4°
for each adduct. Although an “in-plane” (φ ) 0°)
geometry might be expected to be energetically the most
favorable, it has been shown by computations19 and in
structural studies involving carbonyl adducts of other
LA’s4b that the potential surface for distortions of LA
binding out of the carbonyl plane is quite shallow for
up to φ ≈ 15°.
F igu r e 3. ORTEP diagram of the adduct 1-Me.
for all of the adducts reported herein are collected for
easy comparison in Table 3; the angles θ and φ are as
defined in III.
The phenyl rings of these two adducts are essentially
in conjugation with the CdO double bond, twisting only
a few degrees out of the plane defined by O(1), C(19),
and C(20). The dihedral angles O(1)-C(19)-C(20)-
C(25) are 6(1) and 5(1)° for 1-H and 1-Me, respectively.
Because the phenyl rings are “in-plane”, a close non-
bonding contact between one of the ortho fluorine atoms
and an ortho phenyl proton occurs, in accord with the
solution 1H/19F NOE experiments. The F(5)-HC(25)
distance in 1-H is only 2.85 Å, while the F(15)-HC(25)
distance in 1-Me is ∼2.45 Å; free rotation about the
B-Cipso carbons would likely shorten these contacts.
It has been postulated that aldehyde protons in
complexes between aldehydes and borane LA’s can
engage in C-H‚‚‚O or C-H‚‚‚F hydrogen bonds (e.g.
IV).20 These interactions may be important in orienting
The structures of 1-H and 1-Me are quite similar, the
slight differences being attributable to the different
steric requirements of the X groups. The B-O lengths
are not significantly different from those found in the
BF3 adducts of benzaldehyde (1.591(6) Å6) and 2-me-
thylacrolein (1.587(8) Å7). The carbon-oxygen bond
lengths are slightly longer than the distances typical of
aldehydes and ketones, ∼1.22 Å.18 The angle θ for 1-H
is about 6° smaller than the corresponding angle in
1-Me, while the boron atom is more severely pyrami-
dalized in the latter (∑[C-B-C] ) 337.7° for 1-Me and
340.2° for 1-H). These observations are consistent with
(19) LePage, T. J .; Wiberg, K. B. J . Am. Chem. Soc. 1988, 110, 6642.
(20) (a) Corey, E. J .; Rohde, J . J .; Fischer, A.; Azimioara, M. D.
Tetrahedron Lett. 1997, 38, 33. An alternative explanation for this
conformational preference involves an anomeric effect of n(O) f σ*-
(B-F) character.20b Computations suggest that both effects may
contribute to the stability of this conformation. (b) Mackey, M. D.;
Goodman, J . M. Chem. Commun. 1997, 2383.
(18) Sutton, L. E., Ed. Tables of Interatomic Distances and Config-
uration in Molecules and Ions; The Chemical Society: London, 1958.

Tris(pentafluorophenyl)borane Adducts of PhC(O)X
Organometallics, Vol. 17, No. 7, 1998 1373
some ambiguity therefore is associated with the proton-
fluorine NOE experiments on 1-OEt and 1-NP r .
In these two complexes, then, the borane is likely
coordinated syn to the (relative to H or Me) bulky phenyl
group in solution. This accounts at least partially for
the lower observed basicity of ethyl benzoate toward
B(C6F5)3 in comparison to benzaldehyde or acetophe-
none. However, since the amide substrate will displace
even benzaldehyde in competition for B(C6F5)3, the steric
properties of the group syn to the borane do not fully
account for the observed basicities.
aldehyde substrates when they are bound to borane
LA’s, particularly chiral borane catalysts.21 The alde-
hyde proton in 1-H appears to engage in a similar, albeit
weaker, C-H‚‚‚F contact with one of the B(C6F5)3 ortho
fluorines. F(10) points up at the aldehyde hydrogen
(F(10)-H(1)-C(19) ) 91.3°) such that the F(10)-H(1)
distance of 2.56 Å22 is within the sum of the van der
Waals radii of H (1.20 Å) and F (1.47 Å)23 but is longer
than the C-H‚‚‚F separation of 2.35 Å found in the BF3
2-methylacrolein complex.7 Given the greater flexibility
associated with B(C6F5)3, the F(10)-H contact is re-
markably close; however, no evidence that this interac-
tion is important in the solution behavior of 1-H was
uncovered.
Solu tion a n d Solid -Sta te Str u ctu r es of 1-OEt
a n d 1-NP r . Syn:anti coordination geometries in borane
adducts of esters and amides are also influenced by
steric effects. Since OR and NR2 are both capable of
π-donation to the electron-deficient carbonyl carbon, the
alkyl groups R are coplanar with the carbonyl function.
In amides this places RZ in steric conflict with any LA
coordinating syn to this substituent (V). For esters, this
The steric requirements of the phenyl group are
attenuated in the adducts 1-OEt and 1-NP r since the
X groups are able to π-donate to the carbonyl carbon,
allowing the phenyl group to rotate out of the plane
defined by Cipso-C-O. This phenomenon is expecially
evident in 1-NP r and aids in rationalizing the com-
pound’s solution behavior and solid-state structure. The
proton NMR spectrum of free N,N-diisopropylbenzamide
is severely broadened due to exchange of the E and Z
isopropyl groups about the C(O)-N bond, which has
partial double-bond character. Upon B(C6F5)3 coordina-
tion, this exchange becomes much slower on the NMR
time scale, and two sharp sets of resonances for the E
and Z isopropyl groups are observed in the 1H NMR
spectrum of 1-NP r . Thus, coordination of the LA
significantly raises the barrier associated with this
exchange by rendering resonance structure IX relatively
more important in 1-NP r vs 1-OEt.
can be avoided if the alkyl group assumes an E geometry
as in VI; however the Z geometry (VII) is preferred,
because the steric interactions between RE and Ph are
in fact more severe.24 The presence of a Z substituent
dictates that the coordination site syn to OR or NR2 is
sterically inaccessible as far as a borane is concerned
and coordination of borane anti to the OR or NR2 group
is preferred strongly over the syn isomer. That this
coordination geometry obtains for the solution struc-
tures of adducts 1-OEt and 1-NP r is supported by
proton-fluorine NOE experiments, which show strong
enhancements in the aryl ortho protons upon irradiation
of the ortho fluorine resonances in the 19F NMR spec-
trum. Enhancements are also observed in the signals
due to the OCH2 and the methyl protons of the Z
N-isopropyl group. As explained above, close contacts
between B(C6F5)3 and the anti group also occur in these
compounds. Inexplicably, similar enhancements are
found when the meta and para fluorines are irradiated;
The greater dominance of resonance of IX allows the
phenyl group in 1-NP r to be more flexible with regard
to rotating out of the plane of the carbonyl function. In
addition to allowing for some relief of steric interactions,
we have evidence that this also permits a stabilizing
interaction between the phenyl group and one of the
perfluorophenyl groups of the coordinated B(C6F5)3. It
has been recently recognized that phenyl-perfluorophe-
nyl stacking interactions are quite favorable and may
in fact be used as a tool in crystal engineering.25 These
interactions occur because phenyl and pentafluorophe-
nyl groups have large molecular quadrupole moments
which are of similar magnitude but are opposite in
sign.26 The complementarity of charge distribution
allows for strong stacking interactions in the solid-state
structure of, for example, C6H6/C6F6.
Variable-temperature 19F NMR experiments on sam-
ples of 1-NP r in CD2Cl2 suggest that intramolecular
C6H5/C6F5 stacking may be important in the solution
structures of adducts where B(C6F5)3 is coordinated syn
to a phenyl group.27 Figure 4 shows a series of partial
(21) (a) Corey, E. J .; Rohde, J . J . Tetrahedron Lett. 1997, 38, 37. (b)
Corey, E. J .; Barnes-Seeman, D.; Lee, T. W. Tetrahedron Lett. 1997,
38, 1699.
(22) (a) This value is uncorrected for shortening effects,22b which in
this instance result only in a correction of 0.002 Å. Such effects are
more important when C-H‚‚‚F is near linearity. (b) Churchill, M. R.
Inorg. Chem. 1973, 12, 1213.
(23) Bondi, A. J . Phys. Chem. 1964, 68, 441.
(25) Coates, G. W.; Dunn, A. R.; Henling, L. M.; Dougherty, D. A.;
Grubbs, R. H. Angew. Chem., Int. Ed. Engl. 1997, 36, 248 and
references therein.
(24) (a) George, W. O.; Houston, T. E.; Harris, W. C. Spectrochim.
Acta, Part A 1974, 30, 1035. (b) Kno¨zinger, E. Ber. Bunsen-Ges. 1974,
78, 1199. (c) Kydd, R. A.; Rauk, A. J . Mol. Struct. 1981, 77, 227.
(26) Williams, J . H. Acc. Chem. Res. 1993, 26, 593.

1374 Organometallics, Vol. 17, No. 7, 1998
Parks et al.
undergo restricted rotation at lower temperatures. The
spectra could also be interpreted simply in terms of
arrested rotation without invoking a π-stacking interac-
tion, although one might expect the rotations of all three
rings to freeze out at similar temperatures. It should
also be noted that similar behavior was observed when
this experiment was carried out in d8-toluene, a medium
in which the π-stacking interaction might be expected
to be more effectively solvated. On the other hand, the
chelating nature of the interaction is entropically favor-
able.
Although these NMR spectra studied do not provide
definitive support for π-stacking in solution, the solid-
state structures of 1-OEt and 1-NP r clearly advocate
this feature. The molecular structures of 1-OEt and
1-NP r are shown in Figures 5 and 6, respectively. In
the case of 1-NP r , a low reflection-to-parameter ratio
was obtained, resulting in relatively high esd’s for the
metrical parameters (Table 3). Nonetheless, the struc-
ture is included here because it clearly demonstrates
the phenyl-pentafluorophenyl π-stacking architectural
motif and the structural features of the adduct which
allow such an interaction to be accommodated. Because
of the high esd’s, however, discussion of specific metrical
details for the structure of 1-NP r will be limited.
As expected on the basis of the solution studies
detailed above, the borane is syn to the phenyl group
in both compounds. In the case of the ester adduct, the
ethyl group is in the Z geometry. In both compounds,
the phenyl group is rotated out of conjugation with the
CdO; the dihedral angles O(1)-C(19)-C(20)-C(21) in
1-OEt and 1-NP r are 32.5(7) and -76(1)°, respectively.
Interestingly, the two structures differ significantly in
the angle φ; for 1-OEt this parameter is 15.6°, while
for 1-NP r the borane is essentially in-plane, similar to
1-H or 1-Me. As mentioned above, carbonyl ligation to
LA’s is quite flexible in this regard. It seems entirely
likely, therefore, that in the case of 1-OEt the borane
leaves the carbonyl plane in order that one of the C6F5
groups may more effectively π-stack with the phenyl
group, which does not abandon completely conjugation
with the carbonyl group.
F igu r e 4. Partial 19F NMR spectra of 1-NP r taken at
various temperatures (-127 to -141 ppm, ortho fluorine
region of the spectrum).
19F NMR spectra taken at various temperatures; for
clarity, only the ortho fluorine region of the spectrum
is shown. At room temperature, all of the ortho fluorine
atoms are equivalent. As the temperature is lowered,
the resonances broaden and reemerge such that all six
ortho fluorines are inequivalent at -80 °C. Careful
examination of the spectra allows the conclusion that
the six ortho fluorines undergo coalescence in a pairwise
fashion and that one of the C6F5 rings has its rotation
arrested at significantly higher temperature than the
other two. Similar behavior is observed in the meta and
para resonances. These observations are consistent
with an exchange process which involves rapid rotation
of each C6F5 ring and the B(C6F5)3 group as a whole at
room temperature. As the temperature drops, one of
the C6F5 rings is “trapped” by the phenyl group, while
the other two are yet free to rotate as in X; these
Figure 7 depicts the C6H5-C6F5 stacking in these
complexes in more detail, giving side and top views for
both compounds. For 1-OEt, the two rings are virtually
eclipsed but not entirely parallel as illustrated by the
steady gradient in distances separating the rings from
C
ipso to Cpara. The angle between the planes defined by
C(7)-C(12) and C(20)-C(25) is 26.59°. Although it is
not immediately apparent from the view in Figure 7A,
the plane of the C6F5 ring is tilted away from the
B-C(7)(ipso) vector by about 10°, such that this ring is
leaning toward the C6H5 ring. The separation between
the two rings is in the same range as that observed in
(27) Although the low-temperature limit was not reached in the case
of 1-OEt, severe spectral broadening was observed under similar
conditions, suggesting that π-stacking is a significant solution struc-
tural motif in this adduct as well.

Tris(pentafluorophenyl)borane Adducts of PhC(O)X
Organometallics, Vol. 17, No. 7, 1998 1375
F igu r e 5. ORTEP diagram of the adduct 1-OEt.
F igu r e 7. Chem 3D diagrams depicting the details of the
metrical parameters associated with C6F5/C6H5 stacking
in the adducts 1-OEt and 1-NP r : (A) side view of the
stacking interaction in 1-OEt; (B) top view of the stacking
interaction in 1-OEt (C) side view of the stacking in 1-NP r ;
(D) top view of the stacking in 1-NP r .
plexes. We chose these aromatic ligands to impart
crystallinity upon the adducts and allow for solid-state
structure determination as well as solution studies. We
find that the solution structures of these adducts, on
the basis of extensive NMR investigations, are es-
sentially the same as those found in the solid state. The
most interesting aspect of this study is the finding that
the carbonyl ligation to B(C6F5)3 is augmented by two
types of nonbonded interactions. In the aldehyde ad-
duct 1-H, a weak C-H‚‚‚F hydrogen bond is postulated
on the basis of the solid-state structural analysis. A
more substantial intramolecular interaction was found
in the 1-OEt and 1-NP r adducts, namely a phenyl-
perfluorophenyl π-stacking interaction which arises
because the borane coordinates the carbonyl syn to the
phenyl group. This interaction is strong enough in the
1-NP r complex to observe in solution at low tempera-
ture, implying that such interactions are important not
only in the solid state but under chemically relevant
reaction conditions as well. The role of aromatic
π-stacking in determining the tertiary structure of
complex biological molecules28 and in crystal engineer-
ing29 is only beginning to be appreciated. Our results
show that these are potentially exploitable tools for use
in the design of LA catalysts as well.
F igu r e 6. ORTEP diagram of the adduct 1-NP r .
the diyne structures reported by Coates et al., where
interstack distances of about 3.7 Å were observed.25 In
1-NP r , the rings are closer to being parallel (angle
between planes defined by C(13)-C(18) and C(20)-
C(25) is 12.31°; Figure 7C). Because the NR2 group is
a more effective π-donor than OEt, the phenyl ring in
this adduct more readily twists out of conjugation with
the CdO bond and allows for a stronger π-stacking
interaction. The rings have slipped somewhat from the
more eclisped situation observed in the ester adduct
(Figure 7D), but this may actually enhance the attrac-
tive interaction by allowing for better HOMO-LUMO
overlap in the stacked rings.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. Unless otherwise noted, all
manipulations were carried out under argon using an Innova-
tive Technology System One drybox and/or standard Schlenk
Con clu sion s
Organic carbonyl functions form strong adducts with
the highly electrophilic borane B(C6F5)3, and we have
fully characterized its benzaldehyde, acetophenone,
ethyl benzoate, and N,N-diisopropylbenzamide com-
(28) Hunter, C. A. Chem. Soc. Rev. 1994, 23, 101.
(29) Desiraju, G. R. Crystal Engineering: The Design of Organic
Solids; Elsevier: Amsterdam, 1989; pp 1-25.

1376 Organometallics, Vol. 17, No. 7, 1998
Parks et al.
techniques on double-manifold vacuum lines.30 Toluene, hex-
anes, and THF were dried and deoxygenated using the Grubbs
solvent purification system31 and were stored in evacuated
glass vessels over titanocene32 or sodium benzophenone. Deu-
terated NMR solvents d6-benzene (C6D6) and d8-toluene (C6D5-
CD3) were dried and distilled from sodium/benzophenone ketyl,
and d2-dichloromethane (CD2Cl2) was dried and distilled from
calcium hydride (CaH2). NMR spectra were recorded on
Bruker AC 200, AM 400, and AMX2 300 MHz spectrometers
at room temperature in C6D6 unless otherwise specified.
Proton and carbon spectra were referenced to solvent signals,
boron spectra to external BF3‚Et2O at 0.0 ppm and fluorine
spectra to CFCl3 at 0.0 ppm. NMR data are given in ppm;
13C resonances for the C6F5 groups were not obtained. 1H/19F
variable-temperature NOE experiments were performed on the
AMX2 300 MHz spectrometer in either C6D5CD3 or CD2Cl2
solution. IR spectra were run on a Matteson Instruments 4030
Galaxy Series FT-IR instrument. Elemental analyses were
performed in the microanalytical laboratory of the Department
of Chemistry at the University of Calgary.
Ta ble 4. Da ta Used To Obta in Equ ilibr iu m
Con sta n ts for Eq 1a
X
H
CH3
OEt
δfb
9.658
8.789
8.807
0.0935
0.0211
206
2.079
1.815
1.840
0.0943
0.0959
11.1
1.003
0.745
0.798
0.0933
0.206
1.96
δb
δobs
[PhC(O)X]0
c
Nf
d
Keq
a
b
[B(C6F5)3] ) 0.0938 M, room temperature. In ppm. c Mole
d
fraction of free carbonyl compound. ×10-2
.
Syn th esis of 1-OEt. This compound was prepared using
the same procedure as above for 1-Me with dry ethyl benzoate
(112 µL, 0.781 mmol) and B(C6F5)3 (400 mg, 0.781 mmol),
giving 460 mg of the adduct (89% yield). 1H NMR: 7.59 (m,
3
2H, Hortho); 6.75 (m, 3H, Hpara,meta); 4.06 (q, J H ) 7.1 Hz, 2H,
OCH2); 0.82 (t, 3H, OCH2CH3). 13C NMR: 175.3 (CdO); 135.3,
130.5, 128.8, 127.7 (C6H5); 67.6 (OCH2); 13.9 (OCH2CH3). 19F
NMR: -132.8 (br s, 2F, ortho); -151.7 (br s, 1F, para); -162.4
Acetophenone, benzaldehyde, and ethyl benzoate were
purchased and used after distillation from CaH2. N,N-Diiso-
propylbenzamide was prepared from benzoyl chloride and
diisopropylamine by standard methods. Tris(pentafluorophe-
nyl)borane (B(C6F5)3) was purchased from Boulder Scientific,
dried over trimethylchlorosilane, and sublimed under high
vacuum.
(m, 2F, meta). 11B NMR: 19.2. Anal. Calcd for C27H10
-
BO2F15: C, 48.98; H, 1.52. Found: C, 48.35; H, 1.24. IR (KBr
pellet, cm-1): 1669 (m), 1649 (m), 1519 (s), 1468 (vs), 1297
(m), 1106 (m), 970 (s), 719 (m).
Syn th esis of 1-NP r . This compound was prepared using
the same procedure as above for 1-Me with dry N,N-diisopro-
pylbenzamide (40 mg, 0.195 mmol) and B(C6F5)3 (100 mg, 0.195
mmol), giving 107 mg of the adduct (76% yield). 1H NMR: 6.71
(m, 3H, Hortho,para); 6.58 (m, 2H, Hmeta); 3.10 (m, 2H, NCH); 1.27
(d, 3J H ) 6.9 Hz, 6H, NCHCH3), 0.29 (d, 1J H ) 6.7 Hz, 6H,
NCHCH3). 13C NMR: 174.0 (CdO); 131.2, 131.0, 129.2, 125.6
(C6H5); 54.8, 50.2 (NCH); 20.0, 19.7 (NCHCH3). 19F NMR:
-132.7 (br s, 2F, ortho); -157.6 (m, 1F, para); -164.6 (br s,
2F, meta). 19F NMR (-80 °C, CD2Cl2): -130.5, -132.0 (2F),
-132.9, -134.2, -139.3 (ortho); -158.2, -158.9, -159.5 (para);
-164.7 (2F), -165.1, -165.5 (2F), -166.0 (meta). 11B NMR:
-0.05. Anal. Calcd for C31H19BNOF15: C, 51.91; H, 2.67; N,
1.95. Found: C, 51.55; H, 2.24; N, 1.94. IR (KBr pellet, cm-1):
1650 (m), 1570 (vs), 1519 (vs), 1469 (vs), 1366 (s), 1286 (m),
1096 (vs), 1015 (vs), 794 (vs), 776 (m).
Syn th esis of 1-Me. B(C6F5)3 (195 mg, 0.381 mmol) was
added to a dry 25 mL round-bottomed flask and attached to a
swivel-frit assembly. The frit was evacuated, and toluene (10
mL) was condensed into the flask using a dry ice/acetone bath.
The frit assembly was backflushed with argon; then the
solution was warmed to room temperature. Dry acetophenone
(44 µL, 0.381 mmol) was added to the stirred solution via
syringe under an argon purge. The solution was stirred at
room temperature for 5 min; then the solvent was removed
under reduced pressure, leaving a viscous oil. Hexanes (10
mL) was condensed into the flask at -78 °C, which was then
warmed to room temperature. The flask was sonicated for 20
min, during which time the oil turned into a white precipitate
which was isolated by filtration and washed twice with
hexanes. The solvent was removed under reduced pressure,
and the white precipitate was isolated (210 mg, 87% yield).
1H NMR: 7.52 (m, 2H, Hortho); 6.95 (m, 1H, Hpara); 6.73 (m,
2H, Hmeta); 1.82 (s, 3H, CH3). 13C NMR: 212.8 (CdO); 138.6,
133.8, 131.3, 129.5, (C6H5); 23.5 (CH3). 19F NMR: -136.0 (d,
J ) 20.2 Hz, 2F, ortho); -157.0 (t, J ) 21.1 Hz, 1F, para);
-164.6 (m, 2F, meta). 11B NMR: 2.3. Anal. Calcd for
Mea su r em en t of Keq for Ad d u ct F or m a tion . Equilib-
rium constants for adduct formation as shown in eq 1 were
determined by 1H NMR methods. In 1:1 mixtures of PhC(O)X
and B(C6F5)3, the position of the proton resonance of X may
be used to calculate the mole fraction of free carbonyl sub-
strate, Nf, by using the expression:
C
26H8BOF15: C, 49.40; H, 1.28. Found: C, 49.40; H, 1.00. IR
Nf ) (δobs - δb)/(δf - δb)
(KBr pellet, cm-1): 1647 (s), 1603 (m), 1594 (s), 1564 (s), 1473
(vs), 1369 (s), 1325 (s), 1647 (s), 1287 (s), 1103 (vs), 980 (vs),
768 (vs).
where δobs is the observed chemical shift of X in the sample,
δb is the chemical shift of X in the adduct, and δf is the
chemical shift of X in the absence of B(C6F5)3.33 Once Nf is
known, Keq may be readily calculated.
Syn th esis of 1-H. This compound was prepared using the
same procedure as above for 1-Me with dry benzaldehyde (23.4
µL, 0.23 mmol) and B(C6F5)3 (118 mg, 0.23 mmol), giving 89
mg of the adduct (63% yield). 1H NMR: 8.81 (s, 1H, CHdO);
7.30 (m, 2H, Hortho); 6.88 (m, 1H, Hpara); 6.61 (m, 2H, Hmeta).
13C NMR: 199.4 (CdO); 141.5, 135.1, 130.1, 128.3 (C6H5). 19F
NMR: -133.9 (d, J ) 21.2 Hz, 2F, ortho); -154.3 (t, J ) 20.5
Hz, 1F, para); -162.5 (m, 2F, meta). 11B NMR: 5.0. Anal.
Calcd for C25H6BOF15: C, 48.57; H, 0.98. Found: C, 50.52;
H, 1.04. These data are an average of four analyses; complete
removal of the toluene solvate was apparently not possible.
IR (KBr pellet, cm-1): 1620 (s), 1597 (s), 1575 (s), 1519 (s),
1461 (vs), 1105 (s), 970 (s).
The values for δf were obtained from samples of pure
carbonyl compound of about 0.094 M concentration, while δb
was obtained by adding 10 equiv of B(C6F5)3 to these samples
and measuring the chemical shift of X. No change in this value
was observed upon further addition of B(C6F5)3. Samples for
use in obtaining δobs were prepared from a stock solution of
B(C6F5)3 in C6D6 prepared by dissolving B(C6F5)3 (96 mg, 0.188
mmol) in 2.0 mL of dry C6D6 in a volumetric flask ([B(C6F5)3]
) 0.0938 M). A 0.6 mL aliquot of this solution was placed in
a sealable NMR tube, and the carbonyl compound (0.0563
mmol, [carbonyl] ≈ 0.094 M) was added via syringe. The tube
was flame-sealed; then the 1H NMR spectrum was recorded.
Data obtained for X ) H, CH3, OEt are given in Table 4, along
with calculated values of Nf and Keq.
(30) Burger, B. J .; Bercaw, J . E. Experimental Organometallic
Chemistry; Wayda, A. L., Darensbourg, M. Y., Eds.; ACS Symposium
Series 357; American Chemical Society: Washington, DC, 1987.
(31) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.
(32) Marvich, R. H.; Brintzinger, H. H. J . Am. Chem. Soc. 1971,
93, 2046.
(33) Drago, R. S. Physcial Methods for Chemists, 2nd ed.; Saun-
ders: New York, 1992; p 257.

Tris(pentafluorophenyl)borane Adducts of PhC(O)X
Organometallics, Vol. 17, No. 7, 1998 1377
Ta ble 5. Su m m a r y of Da ta Collection a n d Str u ctu r e Refin em en t Deta ils for 1-H, 1Me, 1-OEt, 1-NP r
1-H
1-Me
1-OEt
1-NP r
formula
fw
cryst syst
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
space group
Z
C
25H6BOF15‚0.5C7H8
C26H8BOF15
632.13
monoclinic
12.3720(18)
10.7021(12)
18.351(3)
C27H10BO2F15
662.16
monoclinic
36.836(15)
7.686(3)
C31H19BONF15
717.28
orthorhombic
18.111(2)
19.247(4)
17.479(2)
664.18
triclinic
12.380(3)
12.476(2)
9.538(2)
100.25(1)
96.89(2)
111.34(1)
1322.5(5)
P1h
18.867(5)
96.40(3)
108.68(3)
2414.7(6)
P21/n
4
5060(3)
C2/c
8
6092(1)
Pbca
8
2
F(000)
658
1.668
0.173
0.042
0.042
2.10
1249.08
1.739
0.18
0.053
0.045
4.32
2624
1.738
0.183
0.045
0.044
2.52
2880
1.564
0.157
0.045
0.038
1.63
d
calc, Mg m-3
µ, mm-1
R
Rw
GOF
Mea su r em en t of Keq for th e Heter oexch a n ge Rea c-
tion s. 1-Me/Ben za ld eh yd e. In a dry NMR tube was added
1-Me (21 mg, 0.0332 mmol) in a measured volume of C6D6.
Benzaldehyde (3.4 µL, 0.0332 mmol) was added to the sample
via syringe. The 1H NMR of the sample was obtained, and
the chemical shifts of the methyl protons of acetophenone (δobs
2.0019 ppm) and the aldehyde proton of benzaldehyde (δobs
9.0414 ppm) were recorded. These values were compared to
the known chemical shifts for the free and fully bound carbonyl
compounds (see Table 4) and the equilibrium constant calcu-
lated.
1-OEt/Acetop h en on e. An identical procedure using 1-OEt
(27 mg, 0.0408 mmol) and acetophenone (4.8 µL, 0.0408 mmol)
yielded chemical shifts for the methyl protons of acetophenone
(δobs 1.8589 ppm) and the methyl protons of ethyl benzoate
(δobs 0.9728 ppm). These values were compared to the known
chemical shifts for the free and fully bound carbonyl com-
pounds and used to calculate Keq.
1-OEt/Ben za ld eh yd e. An identical procedure using 1-H
(23 mg, 0.0370 mmol) and ethyl benzoate (5.3 µL, 0.0370 mmol)
yielded chemical shifts for the aldehyde proton (δobs 8.8971
ppm) and the methyl protons of ethyl benzoate (δobs 1.0000
ppm). These values were compared to the known chemical
shifts for the free and fully bound carbonyl compounds and
used to calculate Keq.
X-r a y Str u ctu r a l Deter m in a tion s. Summaries of data
collection and structure refinement details for each adduct are
given in Table 5.
collected during the course of data collection. Neutral atom
scattering factors were taken from ref 34. Values of R and
Rw are given by R ) (Fo - Fc)/∑Fo and Rw ) [∑(w(Fo - Fc))2/
∑(wFo2)]1/2. All crystallographic calculations were conducted
with the PC version of the NRCVAX program package35 locally
implemented on an IBM-compatible 80486 computer.
1-H, 1-OEt, a n d 1-NP r . Suitable crystals were placed in
glass capillaries, sealed and mounted onto a Rigaku AFC6S
diffractometer. Measurements were made using graphite-
monochromated Mo KR radiation (λ ) 0.710 69 Å) at 23, -103,
and -73 °C for 1-H, 1-OEt, and 1-NP r , respectively. The
structures were solved by direct methods and refined by full-
matrix least-squares calculations. The non-hydrogen atoms
were refined anisotropically; hydrogen atoms were included
at geometrically idealized positions with C-H ) 0.95 Å and
were not refined. The final difference maps were essentially
featureless. All calculations were performed using the TEX-
SAN36 crystallographic software package of Molecular Struc-
ture Corp.
Ack n ow led gm en t. Funding for this work from the
Natural Sciences and Engineering Research Council of
Canada came in the form of a Research Grant to W.E.P.
and a Postgraduate Scholarship to D.J .P. W.E.P. also
thanks the Alfred P. Sloan Foundation for a Research
Fellowship (1996-1998). We also thank Dr. Nils Met-
zler, who performed initial 1H/19F NOE experiments on
1-Me, Professor Arvi Rauk for valuable discussions, and
Dr. Sean Maddaford for providing a sample of N,N-
diisopropylbenzamide for initial experiments.
1-Me. Single crystals suitable for X-ray crystallography
were mounted in thin-walled glass capillaries and optically
centered in the X-ray beam of an Enraf-Nonius CAD-4 dif-
fractometer using graphite-monochromated Mo KR radiation
(λ ) 0.709 30 Å). Unit cell dimensions were determined via
least-squares refinement of the setting angles of 24 high-angle
reflections, and intensity data were collected using the ω-2θ
scan mode in the range 38.6-48.2°. Data were corrected for
Lorentz, polarization and absorption effects but not for extinc-
tion. All structures were solved using direct methods. Aryl
Su p p or tin g In for m a tion Ava ila ble: Listings of posi-
tional and thermal parameters, bond distances and angles,
torsion angles, and nonbonded contacts for 1-H, 2-Me, 1-OEt,
and 1-NP r (52 pages). Ordering information is given on any
current masthead page.
OM9710327
hydrogen atoms were placed in calculated positions (DC-H
)
1.00 Å). Methyl hydrogen atoms were located via inspection
of difference Fourier maps and fixed, temperature factors being
(34) International Tables for X-ray Crystallography; Kynoch Press:
Birmingham, U.K., 1974; Vol. IV.
(35) Gabe, E. J .; Le Page, Y.; Charland, J .-P.; Lee, F. L.; White, P.
S. J . Appl. Crystallogr. 1989, 22, 384.
(36) Crystal Structure Analysis Package, Molecular Structure Corp.,
The Woodlands, TX, 1985, 1992.
based upon the carbon atom to which they are bonded.
A
weighting scheme based upon counting statistics was used
2
with the weight modifier k in kFo being determined via
evaluation of variation in the standard reflections that were