Synthesis of borophosphonate cage compounds: Influence of substituent and concentration effects on product distribution in condensation reactions of aryl phosphonic acids and boronic acids

Article abstract of DOI:10.1021/om501101q

Aryl borophosphonate cage compounds [ArPO₃BAr′]_n with n = 4 or 6 were synthesized by condensation reactions of ArP(O)(OH)₂ and Ar′B(OH)₂. (3,5-^tBu₂-Ph)P(O)(OH)₂ (1) reacts with a

Full text of DOI:10.1021/om501101q

Article
pubs.acs.org/Organometallics
Synthesis of Borophosphonate Cage Compounds: Influence of
Substituent and Concentration Effects on Product Distribution in
Condensation Reactions of Aryl Phosphonic Acids and Boronic Acids
Qian Liu, Nathan D. Contrella, Alexander S. Filatov, and Richard F. Jordan*
Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637, United States
S
* Supporting Information
ABSTRACT: Aryl borophosphonate cage compounds [Ar-
PO3BAr′]n with n = 4 or 6 were synthesized by condensation
reactions of ArP(O)(OH)2 and Ar′B(OH)2. (3,5-tBu2-Ph)P-
(O)(OH)2 (1) reacts with arylboronic acids that contain
electron-withdrawing substituents to form borophosphonate
tetramers [Ar1PO3BAr2]4 (Ar1 = 3,5-tBu2-Ph; Ar2 = o-Br-Ph, o-
CF3-Ph, p-CF3-Ph, p-CHO-Ph; 3a−d) and hexamers
[Ar1PO3BAr2]6 (Ar2 = p-CF3-Ph, p-CHO-Ph; 4c−d) in 80−
93% NMR yield. For Ar2 = p-CF3-Ph and p-CHO-Ph, both
products were observed, with the tetramer being favored under
dilute reaction conditions and the hexamer favored under
concentrated reaction conditions. Interconversion between
tetramer (3c or 3d) and hexamer (4c or 4d) was observed at room temperature and above in toluene. The phosphine
phosphonic acid (2-PPh2-Ph)P(O)(OH)2 (6) reacts with arylboronic acids that contain electron-withdrawing substituents to
form tetramers [Ar1PO3BAr2]4 (Ar1 = 2-PPh2-Ph; Ar2 = p-CF3-Ph, p-CHO-Ph; 7c−d) in 70−75% NMR yield. The reactions of 1
or 6 with (p-tolyl)B(OH)2 (2f), and the reaction of 6 with (o-CF3-Ph)B(OH)2 (2b), yield only trace amounts of
borophosphonate cage compounds and instead afford the corresponding [ArBO]3 boroxines and condensation products with
unknown structures.
compounds, which are isoelectronic and isostructural with
polyhedral oligomeric silsesquioxanes (RSi)8O12 (B, POSS),2,3
have attracted interest due to their potential applications as
building blocks for more complex structures and materials and
as scaffolds for multicenter catalysts. Several borophosphate
compounds of general formula [EtOPO3BR′]6 (C) have also
been reported and adopt hexameric cage structures that are
similar to the cubanoid structure observed for borophospho-
nates and POSSs.4
INTRODUCTION
■
Borophosphonates of general formula [RPO3BR′]4 adopt
three-dimensional cubic cage structures (Chart 1, A), in
which the edges are formed by B−O−P bonds.1 These
Chart 1
Borophosphonate cages have been synthesized by several
methods (Scheme 1). Roesky and co-workers reported that
alkyl (R = tBu, Me, Et) and phenyl phosphonic acids react with
trialkylboranes (R′ = Et, sBu) by alkane elimination to generate
borophosphonates in 66−95% yield (Scheme 1, route a).1
Roesky, Mason, and their co-workers used this method to
generate analogous structures with other Group 13 metals (Al,
Ga, In).1a,5,6 The reaction of tBuP(O)(OH)2 with GaMe3 under
mild conditions (room temperature in THF) yields the
tetrameric gallophosphonate cage [tBuPO3GaMe]4. In contrast,
t
the reaction of BuP(O)(OH)2 with the bulkier gallium alkyl
GatBu3 yields different condensation products depending on
the reaction conditions and provides insight into the stepwise
nature of the growth process. The single-ring dimer [tBu-
Received: October 31, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om501101q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
tetrameric borophosphonate cage (Scheme 1, route b), though
the yield was only 14%.10,11
Scheme 1
Recently, Severin and co-workers reported that condensation
reactions of tBuP(O)(OH)2 and R′B(OH)2 in refluxing
toluene, using a Dean−Stark trap to remove the H2O
byproduct, generate tetrameric borophosphonate cages in
high yield (Scheme 1, route c).12 A wide range of R′B(OH)2
reactants are converted to cage products, including alkyl (R′ =
nBu, nPr, Me, Cy) and sterically undemanding aryl (R′ = Ph, p-
tol, p-CHO-Ph, 3,5-(CHO)2-Ph) boronic acids. However,
similar reactions of the sterically smaller alkylphosphonic
acids nBuP(O)(OH)2 and nHexP(O)(OH)2 with boronic
acids produced mixtures of condensation products that could
not be separated and were not characterized. Reactions of
PhP(O)(OH)2 with boronic acids gave insoluble products
which were also not characterized. Severin concluded that
phosphonic acids that are both sterically demanding and
solubilizing are required for a controlled condensation reaction.
Scheme 2 illustrates several possible condensation pathways
of phosphonic acids with boronic acids. In addition to
tetrameric cages [RPO3BR′]4, 1:1 borophosphonate products
with different structures and degrees of oligomerizations are
possible. In particular, hexameric cages analogous to C (Chart
1) may be expected when sterically small RP(O)(OH)2 and
R′B(OH)2 reactants are used. Boronic acids are well-known to
reversibly and rapidly self-condense to boroxines (R′BO)3, with
condensation being particularly favored for electron-donating
R′ groups.13 Arylboroxines are destabilized by ortho sub-
stituents, which distort the planar structure14 and reduce the
overlap between the π-orbitals of the aryl ring and the empty
boron p orbital. Finally, phosphonic acids are also known to
self-condense to linear or cyclic phosphonic acid anhydrides
(H[OP(O)R]nOH or [RP(O)O]n).15
(OH)PO2GatBu2]2 is formed at −50 °C in THF, the trimer
(tBuGa)2(tBu2Ga)(O3PtBu)2{O2P(OH)tBu} (Chart 1, D) is
observed after 15 h in refluxing toluene, and the tetrameric cage
[tBuPO3GatBu]4 is ultimately formed after 2 h in refluxing
digyme.6b,7,8 A mixture of tetrameric and hexameric alumi-
nophosphonate cages, [tBuPO3AlMe]4 and [tBuPO3AlMe]6, is
formed when the small trialkylaluminum AlMe3 is used.9 In a
similar system, in addition to tetrameric and hexameric cages,
[MePO3AltBu]4 and [MePO3AltBu]6, an interesting [Me-
PO3AltBu]10 decamer consisting of two tetramers (each with
an open edge) linked through a cyclic dimer was also
This paper describes the synthesis of [ArPO3BAr′]4 and
[ArPO3BAr′]6 borophosphonate cages that contain aryl
substituents at both P and B via Severin-type condensation
reactions. The product distribution is influenced by the steric
observed.5b,c Kuchen reported that BuP(O)(OSiMe3)2 reacts
t
with PhBCl2 by Me3SiCl elimination to form the corresponding
Scheme 2
B
dx.doi.org/10.1021/om501101q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 3
properties and solubility of the phosphonic acid, the steric and
electronic properties of the boronic acid, and the concen-
trations of the reactants.
RESULTS AND DISCUSSION
■
Reaction of (3,5-tBu2-Ph)P(O)(OH)2 (1) with Arylbor-
onic Acids That Contain Electron-Withdrawing Sub-
stituents. Based on the guidelines outlined by Severin noted
above, (3,5-tBu2-Ph)P(O)(OH)2 (1), which incorporates
t
solubilizing Bu groups and is not sterically hindered at the
phosphorus center, is an attractive candidate for controlled
condensation reactions with boronic acids. Compound 1 reacts
in toluene with (o-Br-Ph)B(OH)2 (2a) and (o-CF3-Ph)B-
(OH)2 (2b), which contain electron-withdrawing substituents
in the ortho position, to generate the tetrameric cages
[(3,5-tBu2-Ph)PO3B(o-Br-Ph)]4 (3a) and [(3,5-tBu2-Ph)PO3B-
(o-CF3-Ph)]4 (3b) in 84% and 93% yield, respectively, as
determined by 31P{1H} NMR. Compounds 3a and 3b were
isolated in 48−50% yield by recrystallization from hexane.
High-resolution mass spectral data, multinuclear NMR data,
and an X-ray crystallographic analysis of 3a (vide infra)
established that 3a and 3b have tetrameric cage structures as
shown in Scheme 3.
The reactions of 1 with (p-CF3-Ph)B(OH)2 (2c) and (p-
CHO-Ph)B(OH)2 (2d), which contain electron-withdrawing
substituents in the para position, are less selective. These
sterically unhindered boronic acids react to form mixtures of
tetrameric and hexameric borophosphonate cages, and
interestingly, the concentration of the reactants strongly
influences the product distribution. The reaction of 1 with 2c
at low initial concentration ([1]0 = [2c]0 = 2.5 mM) generates
[(3,5-tBu2-Ph)PO3B(p-CF3-Ph)]4 (3c) as the major product,
along with a small amount of [(3,5-tBu2-Ph)PO3B(p-CF3-Ph)]6
(4c) as shown by 31P{1H} NMR analysis of the product
mixture (Figure 1a). The yields of 3c and 4c were determined
to be 81% and 7%, respectively, by 1H NMR. 4c is less soluble
than 3c and was removed by precipitation with hexane followed
by filtration. 3c was isolated in analytically pure form by
removing the solvent from the filtrate and washing with hexane.
However, the isolated yield of 3c was low (12%), due in part to
competitive conversion to 4c during workup. In contrast, the
reaction of 1 with 2c at high initial concentration ([1]0 = [2c]0
= 25 mM) produces 4c as the major product (72%, vs 16% 3c)
(Figure 1b). 4c was isolated by recrystallization from toluene in
49% yield.
Figure 1. 31P{1H} NMR spectra of product mixtures from the reaction
of (3,5-tBu2-Ph)P(O)(OH)2 (1) and (p-CF3-Ph)B(OH)2 (2c) in
toluene. (a) Low initial concentration of reactants: ([1]0 = [2c]0 = 2.5
mM); NMR solvent = C6D6. Peak assignments: [(3,5-tBu2-Ph)PO3B-
(p-CF3-Ph)]4 (3c): δ −6; [(3,5-tBu2-Ph)PO3B(p-CF3-Ph)]6 (4c): δ
−1. (b) High initial concentration of reactants: ([1]0 = [2c]0 = 25
mM); NMR solvent = toluene. Peak assignments: 3c: δ −7; 4c: δ −1.
[(3,5-tBu2-Ph)PO3B(p-CHO-Ph)]4 (3d) was the major product
(81%, 1H NMR), and only a small amount (5%) of the
hexamer [(3,5-tBu2-Ph)PO3B(p-CHO-Ph)]6 (4d) was formed.
Compound 3d was isolated by precipitation from the reaction
Similar results were obtained for the reaction of 1 with 2d. At
low initial concentration ([1]0 = [2d]0 = 2.5 mM), the tetramer
C
dx.doi.org/10.1021/om501101q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
mixture in 44% yield. At high initial concentration ([1]0 =
[2d]0 = 25 mM), a mixture of 3d (44%) and 4d (36%) is
formed. The conversion of 3d to 4d is slower than the
conversion of 3c to 4c noted above, but 4d was enriched to
52% when the product mixture from the high concentration
reaction was cooled and left at room temperature for 18 h and
was isolated by recrystallization from toluene in 36% yield.
Similarly, NMR studies show that 1 reacts with PhB(OH)2 (2e)
at low initial concentration ([1]0 = [2e]0 = 2.5 mM) to yield
donating methyl substituent, results in complete consumption
of the starting materials but generates a mixture of products
(Scheme 5). The 31P{1H} NMR spectrum of the reaction
mixture contains a sharp signal at δ −8.0, in the range expected
for [(3,5-tBu2-Ph)PO3B(p-tolyl)]4 (3f), and a broad resonance
at δ 20 to −10 (Figure 2a). The presence of 3f was confirmed
1
[(3,5-tBu2-Ph)PO3BPh]4 (3e) as the major product (73%, H
NMR).
Interconversion of Hexameric and Tetrameric Cages.
To probe the hexamer/tetramer interconversion further, the
1
isomerization of pure 4c was studied by H NMR. Heating a
solution of 4c in toluene-d8 at 105 °C results in slow (3 weeks)
formation of an equilibrium mixture of 3c and 4c (Keq = [3c]3/
[4c]2 = 2.4(1) × 10−3 M, Scheme 4). This equilibrium shifts
Scheme 4
toward 4c at room temperature (Keq = [3c]3/[4c]2 = 1.2(1) ×
10−3 M), which is consistent with the expectation (based on
stoichiometry) that 3c should be entropically favored. The
conversion of pure 4c to the equilibrium 4c/3c mixture is much
slower than the interconversion of 3c and 4c observed in the 1/
2c reaction mixture, suggesting that the latter process is
catalyzed by other species in the mixture (e.g., RP(O)(OH)2,
H2O, other condensation products).
Figure 2. 31P{1H} NMR spectra (in C6D6) of product mixtures from
(a) the reaction of (3,5-tBu2-Ph)P(O)(OH)2 (1) and (p-tolyl)B(OH)2
(2f) and (b) the self-condensation of (3,5-tBu2-Ph)P(O)(OH)2 (1). *
= [(3,5-tBu2-Ph)PO3B(p-tolyl)]4 (3f, δ −8).
The product mixture from the dilute 1/2c reaction ([1]0 =
[2c]0 = 2.5 mM) is enriched in 3c relative to the expected
equilibrium concentration ([3c]/[4c]: obs, 17; calcd, 2.6; see
the Supporting Information for details). Conversely, the
product mixture from the concentrated reaction ([1]0 = [2c]0
= 25 mM) is enriched in 4c ([3c]/[4c]: obs, 0.33; calcd, 1.0).
These deviations from the equilibrium ratios suggest that
kinetic factors influence the product distribution. The high
reactant concentrations favor intermolecular condensations
relative to intramolecular reactions and produce larger
(hexameric) aggregates.
by MALDI-MS, but ions derived from other mixed
condensation products were also observed by MALDI-MS
and APCI-MS analysis of the reaction mixture. For comparison,
the self-condensation of 1 under the same reaction conditions
in the absence of boronic acid generates products that exhibit
broad 31P{1H} resonances at δ ca. 15, 9, and 0 (Figure 2b).
These results suggest that the reaction of 1 and 2f generates, in
addition to 3f, other mixed borophosphonate condensation
products as well as condensation products derived from 1. The
1H NMR spectrum of the product mixture contains prominent
resonances for p-tolylboroxine (5f), corresponding to a 20%
yield.
Reaction of 1 with (p-tolyl)B(OH)2. The reaction of 1
with (p-tolyl)B(OH)2 (2f), which contains an electron-
Scheme 5
D
dx.doi.org/10.1021/om501101q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Reaction of (2-PPh2-Ph)P(O)(OH)2 (6) with Arylboronic
Acids. The reactions of (2-PPh2-Ph)P(O)(OH)2 (6)16 with
boronic acids were explored because the corresponding cages
are potentially interesting as ligands. Compound 6 reacts
cleanly with (p-CF3-Ph)B(OH)2 (2c) and (p-CHO-Ph)B-
(OH)2 (2d) to afford the tetrameric products [(2-PPh2-
Ph)PO3B(p-CF3-Ph)]4 (7c) and [(2-PPh2-Ph)PO3B(p-CHO-
Ph)]4 (7d), even at high concentration (Scheme 6). 7c and 7d
Steric Limitations on Cage Formation. The reaction of 6
with (o-CF3-Ph)B(OH)2 (2b) is unselective, generating a
mixture of products. The reactants are fully consumed, but
multiple species are observed by 31P{1H} NMR (multiple
resonances in the range δ 51 to −15). A weak [M + H]+signal
for [(2-PPh2-Ph)PO3B(o-CF3-Ph)]4 (7b) is observed in the
APCI mass spectrum of the reaction mixture, and (o-CF3-Ph)-
boroxine is formed in 40% yield, as determined by 1H NMR. It
is likely that the presence of ortho substituents on both the
phosphonic acid and boronic acid disfavors cage formation in
this case.
Scheme 6
Solid-State Structures. The solid-state structure of 3a
(Figure 3) is similar to those of [tBuPO3BAr′]4 (Ar′ = p-tol, p-
are formed in 75% and 70% NMR yield, respectively, and were
isolated by recrystallization from toluene in 34−40% yield. The
31P{1H} NMR spectra of 7c and 7d each contain phosphonate
resonances in the range δ −9 to −10 and a phosphine
resonance at δ −14. The diphenylphosphino substituent is thus
able to solubilize the reaction intermediates and is sufficiently
sterically demanding to favor the selective formation of
tetramers.
The reaction of 6 with (p-tolyl)B(OH)2 (2f) is similar to that
of 1. The reactants are fully consumed, but only a trace amount
of [(2-PPh2-Ph)PO3B(p-tolyl)]4 (7f) is formed, as determined
by 31P{1H} NMR and confirmed by MALDI-MS. The major
products are p-tolylboroxine (25%) and condensation products
of unknown structures.
Figure 3. Molecular structure of 3a. Hydrogen atoms are omitted.
Four of the Bu groups are disordered over several positions; in these
cases, the positions of the methyl carbons were refined isotropically,
and one position of the methyl carbons is shown as a sphere. Average
bond lengths (Å) and angles (o) (number of data, range): P−O:
1.522(7) (12, 1.511(2)−1.533(2)); B−O: 1.483(7) (12, 1.472(3)−
t
Role of Boroxine Formation. As noted above, 1 and 6
react with PhB(OH)2 and aryl boronic acids that contain
electron-withdrawing substituents (Br, CHO, CF3) to afford
high yields of tetrameric or hexameric cages, but when p-
tolylboronic acid is used, only low yields of cage products are
formed and p-tolylboroxine is the major boron-containing
product. In principle, phosphonic acids can also condense with
boroxines to form borophosphonate cages. Indeed, the reaction
of 1 with o-CF3-phenylboroxine generates tetrameric cage 3b in
high yield (85% by 19F NMR). In contrast, the reaction of 1
with p-tolylboroxine yields cage 3f in only 10% NMR yield,
with 20% of the p-tolylboroxine remaining after the reaction.
These results suggest that the relative stabilities of
borophosphonate cages and boroxines influence the product
distribution. Electron-withdrawing substituents on the boron
aryl group stabilize the “borate-like” four-coordinate boron
centers in the borophosphonate cages and destabilize the three-
coordinate boron centers in the boroxines.13 In contrast,
electron-donating groups have the opposite effect, and so cage
formation is disfavored in these cases. This effect is less
important when alkylphosphonic acids rather than arylphos-
phonic acids are used. Severin reported high yields of cage
1.496(3)); P−Cipso: 1.766(2) (4, 1.766(3)−1.767(2)); B−Cipso
:
1.600(3) (4, 1.595(4)−1.603(3)); P−O−B: 141.2(4.5) (12,
135.7(1)−149.1(2)); O−P−O: 111.39(1.05) (12, 113.39(9)−
119.62(9)); O−B−O: 108.7(0.8) (12, 107.6(2)−110.8(2)).
CHO-Ph, and 3,5-(CHO)2-Ph).12 The P2B2O4 rings that
comprise the faces of the cubic core adopt a flattened chair−
chair conformation,17 and the B−O and P−O distances and the
P−O−B, P−O−P, and B−O−B angles are similar to those
observed in previous studies.12
3d also adopts a cubic cage structure. In this case, two types
of cages with the same atom connectivity but different spatial
orientation of the oxygen bridges are cocrystallized, one of
which is shown in Figure 4. These conformational isomers
result from the flexibility of the oxygen bridges. While the cages
are relatively rigid and the boron and phosphorus atoms occupy
essentially the same positions in the two conformers, the
oxygen atoms may lie in or out of the plane passing through the
B−P−B atoms (Figure 5, see the Supporting Information for
further details).
The solid-state structure of 4d is shown in Figure 6. The
hexameric cage features two 12-membered rings and six 8-
membered rings. The 12-membered P3B3O6 rings adopt a
[6′6′] conformation,18 in which four O atoms pucker into the
cage (e.g., O6), while two pucker out (e.g., O7). The inward
pucker generates close contact between the B-aryl and P-aryl
groups that flank the inward-puckered oxygen, forcing these
t
when BuP(O)(OH)2 was reacted with (p-CHO-Ph)B(OH)2
t
(70%) or (p-tolyl)B(OH)2 (67%). The Bu group is more
electron-donating than aryl groups, so it better-stabilizes the
“phosphonium-like” four-coordinate RPO3 centers in the cage
structure.
E
dx.doi.org/10.1021/om501101q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
groups that flank the outward-puckered oxygens. Such steric
interactions would be particularly severe if either the B-aryl or
P-aryl groups contain ortho substituents, which provides an
explanation for the observation that only tetrameric structures
are formed in these cases.
CONCLUSIONS
■
Tetrameric [ArPO3BAr′]4 and hexameric [ArPO3BAr′]6
borophosphonate cages that contain aryl groups at B and P
were synthesized by the condensation reactions of aryl
phosphonic acids and aryl boronic acids using Severin’s
method. Aryl boronic acids that contain electron-withdrawing
substituents afford cages with the highest yields, while aryl
boronic acids with electron-donating groups form mixtures of
condensation products and boroxines. Steric effects strongly
influence the product distribution and yield. When both
starting acids are ortho-substituted, the reaction produces only
trace amounts of tetrameric cages. When either the aryl
phosphonic acid or the aryl boronic acid has an ortho
substituent, the reaction selectively produces the tetrameric
cage. When neither reactant is ortho-substituted, the reaction
produces a mixture of tetramer and hexamer. In this case, the
tetramer is favored at low reactant concentrations, and the
hexamer is favored at high reactant concentrations, reflecting
the relative rates of intramolecular and intermolecular
condensation reactions as the cages are formed. The sensitivity
of hexamer formation to the steric profile of the B and P aryl
groups is attributed to the inward puckering of some of the
oxygen atoms in the molecular structure, which places these
aryl groups in close proximity. The hexameric and tetrameric
cages can interconvert in solution.
Figure 4. Molecular structure of 3d. Hydrogen atoms are omitted. The
oxygen atoms are disordered over two positions, only one of which is
t
shown. One Bu group is disordered over two positions, only one of
which is shown. Average bond lengths (Å) and angles (o) (number of
data, range): P−O: 1.51(5) (3, 1.48(1)−1.57(9)); B−O: 1.47(8) (3,
1.46(1)−1.482(9)); P−Cipso: 1.767(3); B−Cipso: 1.602(4); P−O−B:
142.8(6.9) (3, 135.9(6)−149.7(7)); O−P−O: 110.9(3.1) (3,
107.4(5)−112.8(5)); O−B−O: 108.3(19.9) (3, 87.4(5)−127.1(6)).
Figure 5. Core structures of the two cocrystallized conformational
isomers of 3d. Red = O, orange = P, pink = B.
EXPERIMENTAL SECTION
■
General Procedures. All experiments were performed under a
nitrogen atmosphere using drybox or Schlenk techniques. Nitrogen
was purified by passage through Q-5 oxygen scavenger and activated
molecular sieves. Toluene and hexanes were purified by passage
through BASF R3-11 oxygen scavenger and activated alumina.
Compound 6 was prepared by the literature route.16 Arylboroxines
were prepared by azeotropic distillation of the corresponding
arylboronic acids with toluene for 6 h.19 The following materials
were obtained from commercial sources and used without further
n
purification: 1-bromo-3,5-di-tert-butylbenzene (Aldrich, 97%), BuLi
solution (Aldrich, 2.5 M in hexanes), diethyl chlorophosphate
(Aldrich, 97%), and bromotrimethylsilane (Aldrich, 97%). Phenyl-
boronic acid (Aldrich, 95%), 4-tolylboronic acid (Matrix, 97%), 2-
bromophenylboronic acid (Matrix, 98%), 2-(trifluoromethyl)-
phenylboronic acid (Matrix, 98%), 4-(trifluoromethyl)phenylboronic
acid (Matrix, 97%), and 4-formylphenylboronic acid (Acros, 97%)
were recrystallized from H2O prior to use. Elemental analyses were
performed by Robertson Microlit Laboratories. The solvent content in
1
elemental analysis samples was quantified by H NMR. NMR spectra
were acquired on Bruker DRX-500 or Bruker DRX-400 spectrometers
at ambient temperature unless otherwise indicated. 1H and 13C
chemical shifts are reported relative to SiMe4 and are internally
Figure 6. Molecular structure of 4d·C6H6·2C7H8. Hydrogen and
solvent atoms are omitted. The formyl group is disordered over two
positions, only one of which is shown. Average bond lengths (Å) and
angles (o) (number of data, range): P−O: 1.524(7) (9, 1.514(3)−
1.536(3)); B−O: 1.489(6) (9, 1.479(5)−1.495(6)); P−Cipso: 1.769(5)
(3, 1.767(5)−1.771(4)); B−Cipso: 1.591(7) (3, 1.588(7)−1.593(6));
P−O−B: 137.5(5.1) (9, 127.8(3)−144.2(3)); O−P−O: 110.6(2.8) (9,
104.9(2)−114.2(2)); O−B−O: 107.7(2.4) (9, 102.5(3)−110.1(3)).
1
referenced to residual H and 13C solvent resonances. 31P chemical
shifts are reported relative to externally referenced 85% H3PO4. 19F
spectra were referenced to external BF3·Et2O, and 19F chemical shifts
are reported relative to CFCl3. 11B chemical shifts are reported relative
to externally referenced BF3·Et2O. Coupling constants are reported in
Hz. NMR signals were assigned based on COSY, HMQC, and
1H{31P} experiments as well as trends in chemical shifts and coupling
constants derived from these experiments. C6D6, THF-d8, and toluene-
d8 were dried over Na/benzophenone. CD2Cl2 was dried over P2O5.
Mass spectrometry was performed on an Agilent 6224 TOF-MS
flanking groups to adopt nearly mutually parallel conforma-
tions, as illustrated in Figure 7. This arrangement, in turn,
generates close steric interactions between the B-aryl and P-aryl
F
dx.doi.org/10.1021/om501101q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 7. Alternate view of 4d that illustrates key steric interactions. (a) Side view of 4d in which hydrogen atoms and the aryl groups at the back are
omitted. O6 is inward-puckered and O1, O5, O7, and O8 are outward-puckered. (b) Space-filling view in the same perspective as in (a), in which the
t
aryl-hydrogen atoms are shown but the formyl and Bu substituents are omitted.
instrument (high resolution) or an Agilent 6130 LCMS (low
resolution).
−9.6. 1H NMR (C6D6): δ 8.31 (dd, 3JPH = 17, 4JHH = 2; 2H, H2), 7.96
(dd, 3JHH = 8, 4JHH = 2, 1H, H8), 7.64 (d, 4JHH = 1, 1H, H4), 7.53 (dd,
3JHH = 8, 4JHH = 0.5; 1H, H11), 6.96 (td, 3JHH = 7, 4JHH = 0.5; 1H, H9),
(3,5-tBu2-Ph)P(O)(OH)2 (1). A Schlenk flask was charged with 1-
bromo-3,5-di-tert-butylbenzene (5.384 g, 20.00 mmol) and Et2O (120
3
4
6.81 (td, JHH = 8, JHH = 2; 1H, H10), 1.16 (s, 18H, H6). 13C{1H}
n
NMR (C6D6): δ 151.4 (d, 3JPC = 18, C3), 143.0 (broad, C7), 134.6 (s,
mL) and cooled to 0 °C. BuLi solution (2.5 M, 8.0 mL, 20 mmol)
4
C
B−Ar), 133.6 (s, CB−Ar), 129.5 (s, CB−Ar), 127.3 (d, JPC = 4, C4),
was added via syringe over 5 to 10 min. The mixture was stirred at 0
°C for 30 min and then warmed to room temperature for another 1 h.
The clear colorless solution was cooled to −78 °C, and a solution of
diethyl chlorophosphate (2.9 mL, 20 mmol) in Et2O (30 mL) was
added. The mixture was stirred at room temperature for 18 h to yield a
cloudy colorless solution. The reaction mixture was transferred to a
separatory funnel that contained H2O (50 mL). The aqueous layer was
extracted with Et2O (3 × 50 mL). The combined organic fractions
were washed with brine (20 mL) and dried over MgSO4, and the
volatiles were removed under vacuum to yield a colorless oil. The
crude product was purified by silica gel chromatography, using a 2:1
hexanes:ethyl acetate mixture as the eluent. The product was isolated
as a clear oil and directly dissolved in CH2Cl2 (150 mL).
Bromotrimethylsilane (8.5 mL, 64 mmol) was added at room
temperature, and the reaction mixture was stirred at room temperature
for 2 days. The volatiles were removed under vacuum, and the crude
product was dissolved in MeOH (100 mL) and stirred for 2 h. The
MeOH was removed under vacuum to yield a viscous oil. Hexane was
added to the oil to afford 1 as a white solid. 1 was dried in vacuum
oven (3.721 g, 69%). 31P{1H} NMR (CD2Cl2): δ 24.2. 1H NMR
127.1 (d, JPC = 13, C2), 126.4 (s, CB−Ar), 126.1 (d, JPC = 236, C1),
35.2 (d, 4JPC = 2, C5), 31.3 (s, C6), the CB−Ar resonance is obscured by
the solvent peak (δ 127.8−128.9). 11B{1H} NMR (toluene-d8): δ 3.7.
HRMS (m/z): calcd for [C80H100B4P4O12Br4 + H]+, 1741.3308; found,
1741.3348. EA: calcd for C80H100B4P4O12Br4, %: C, 55.21; H, 5.79.
Found: C, 55.09; H, 5.79.
2
1
[(3,5-tBu2-Ph)PO3B(o-CF3-Ph)]4 (3b). 3b was synthesized analo-
gously to 3a from 1 (811 mg, 3.00 mmol) and 2-(trifluoromethyl)-
phenylboronic acid (569 mg, 2.99 mmol) in toluene (120 mL). The
volatiles were removed under vacuum to afford a white solid. This
material was recrystallized from hot hexane to afford 3b as a white
crystalline solid, which was dried under vacuum (640 mg, 50%).
31P{1H} NMR (C6D6): δ −9.5. 1H NMR (C6D6): δ 8.23 (d, 3JHH = 7,
1H, H8), 8.04 (dd, 3JPH = 12, 4JHH = 2; 2H, H2), 7.65 (d, 4JHH = 1, 1H,
3
(CD2Cl2): δ 11.44 (s, 2H, OH), 7.74 (d, JPH = 15, 2H, H2), 7.68 (s,
1H, H4), 1.34 (s, 18H, H6). 13C{1H} NMR (CD2Cl2): δ 151.5 (d, 3JPC
= 15, C3), 127.7 (d, 4JPC = 2, C4), 126.9 (d, 1JPC = 192, C1), 125.4 (d,
2JPC = 11, C2), 35.3 (s, C5), 31.4 (s, C6). HRMS (m/z): calcd for
[C14H23PO3 + H]+, 271.1463; found, 271.1456.
3
3
H4), 7.62 (d, JHH = 8, 1H, H11), 7.11 (t, JHH = 8, 1H, H9), 7.00 (t,
3JHH = 8, 1H, H10), 1.09 (s, 18H, H6). 13C{1H} NMR (C6D6): δ 151.3
(d, 3JPC = 17, C3), 141.5 (broad, C7), 134.6 (s, C8), 133.3 (q, 2JFC = 31,
C12), 130.9 (s, C9), 127.3 (d, JPC = 4, C4), 126.6 (d, JPC = 13, C2),
4
2
126.4 (q, JFC = 6, C11), 126.1 (q, JFC = 273, C13), 125.8 (d, JPC
=
3
1
1
236, C1), 35.0 (d, JPC = 2, C5), 31.1 (s, C6), the C10 resonance is
obscured by the solvent peak (δ 127.8−128.9). 19F{1H} NMR (C6D6):
δ −56.9. 11B{1H} NMR (toluene-d8): δ 3.6. HRMS (m/z): calcd for
[C84H100B4P4O12F12 + H]+, 1697.6424; found: 1697.6484. EA: calcd
for C84H100B4P4O12F12·0.2 THF, %: C, 59.46; H, 5.94. Found: C,
59.24; H, 6.03.
4
[(3,5-tBu2-Ph)PO3B(o-Br-Ph)]4 (3a). A Schlenk flask equipped with a
Dean−Stark trap was charged with 1 (324 mg, 1.20 mmol), 2-
bromophenylboronic acid (241 mg, 1.20 mmol), and toluene (48 mL).
The mixture was refluxed for 3 h. The volatiles were removed under
vacuum to afford a white solid. This material was recrystallized from
hot hexane to afford 3a as a white crystalline solid, which was dried
under vacuum (250 mg, 48%). X-ray quality crystals were grown by
slow evaporation of a hexane solution of 3a. 31P{1H} NMR (C6D6): δ
[(3,5-tBu2-Ph)PO3B(p-CF3-Ph)]4 (3c). 3c was synthesized analo-
gously to 3a from 1 (81 mg, 0.30 mmol) and 4-(trifluoromethyl)-
phenylboronic acid (57 mg, 0.30 mmol) in toluene (120 mL). The
volatiles were removed with a rotovap to afford a yellow solid. The
crude product was redissolved in toluene (1 mL), hexane (10 mL) was
added, and a white precipitate formed. The mixture was filtered. The
G
dx.doi.org/10.1021/om501101q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(450 mg, 3.00 mmol), and toluene (120 mL). The reaction mixture
was cooled and left at room temperature for 18 h. The volatiles were
removed under vacuum to afford a white solid. This material was
redissolved in toluene (25 mL), hexane (20 mL) was added, and a
white precipitate formed. The precipitate was recrystallized from
toluene to afford 4d as white powder, which was dried under vacuum
(410 mg, 36%). X-ray quality crystals were grown by slow evaporation
of a toluene solution of 4d at −40 °C. 31P{1H} NMR (THF-d8): δ
−1.6. 1H NMR (THF-d8): δ 9.72 (s, 1H, H11), 7.51 (d, 3JHH = 8, 2H,
H9), 7.44−7.41 (m, 3H, H2 and H4), 7.35 (d, 3JHH = 8, 2H, H8), 1.14
(s, 18H, H6). 13C{1H} NMR (THF-d8): δ 191.7 (s, C11), 151.5 (d, 3JPC
= 17, C3), 149.8 (broad, C7), 136.5 (s, C8), 132.1 (s, C10), 128.7 (s,
filtrate was taken to dryness under vacuum to afford 3c as a yellow
solid. The solid was washed with hexane and dried under vacuum (15
1
mg, 12%). 31P{1H} NMR (THF-d8): δ −6.9. H NMR (THF-d8): δ
7.79 (d, 3JHH = 8, 2H, H8), 7.75 (s, 1H, H4), 7.72 (dd, 3JPH = 16, 4JHH
= 2; 2H, H2), 7.52 (d, 3JHH = 8, 2H, H9), 1.22 (s, 18H, H6). 13C{1H}
3
NMR (THF-d8): δ 152.5 (d, JPC = 17, C3), 132.0 (s, C8), 130.2 (q,
2JFC = 32, C10), 128.8 (d, 4JPC = 4, C4), 125.9 (d, 2JPC = 13, C2), 125.8
(d, 1JPC = 234, C1), 125.5 (q, 1JFC = 271, C11), 124.7 (q, 3JFC = 4, C9),
35.6 (d, 4JPC = 2, C5), 31.2 (s, C6); the C7 resonance was not observed.
19F{1H} NMR (THF-d8): δ −62.9. 11B{1H} NMR (toluene-d8): δ 3.4.
HRMS (m/z): calcd for [C84H100B4P4O12F12 + H]+, 1697.6424; found,
1697.6522.
C9), 128.0 (d, 4JPC4 = 3, C4), 125.9 (d, 1JPC = 214, C1), 125.6 (d, 2JPC
=
12, C2), 35.2 (d, JPC = 2, C5), 31.2 (s, C6). 11B{1H} NMR (toluene-
d8): δ 4.1. HRMS (m/z): calcd for [C126H156B6P6O24 + H]+,
2306.0049; found, 2306.0107.
[(3,5-tBu2-Ph)PO3B(p-CF3-Ph)]6 (4c). 4c was synthesized analo-
gously to 3a from 1 (81 mg, 0.30 mmol) and 4-(trifluoromethyl)-
phenylboronic acid (57 mg, 0.30 mmol) in toluene (12 mL). The
volatiles were removed under vacuum to afford a yellow solid. This
material was recrystallized from hot toluene to afford 4c as a white
powder, which was dried under vacuum (63 mg, 49%). 31P{1H} NMR
[(2-PPh2-Ph)PO3B(p-CF3-Ph)]4 (7c). 7c was synthesized analogously
to 3a from 6 (103 mg, 0.301 mmol) and 4-(trifluoromethyl)-
phenylboronic acid (57 mg, 0.30 mmol) in toluene (12 mL). The
volatiles were removed under vacuum to afford a yellow solid. This
material was recrystallized by diffusion of hexane into a toluene
solution of 7c to afford 7c as a white powder, which was collected by
filtration and dried under vacuum (60 mg, 40%). 31P{1H} NMR
1
3
(THF-d8): δ 0.3. H NMR (THF-d8): δ 7.48 (d, JHH = 8, 2H, H8),
7.45 (s, 1H, H4), 7.42 (dd, 3JPH = 16, 4JHH = 2; 2H, H2), 7.12 (d, 3JHH
= 8, 2H, H9), 1.15 (s, 18H, H6). 13C{1H} NMR (THF-d8): δ 151.5 (d,
3JPC = 16, C3), 132.0 (s, C8), 129.4 (q, 2JFC = 32, C10), 128.0 (d, 4JPC
=
1
2
3, C4), 125.7 (d, JPC = 214, C1), 125.6 (d, JPC = 12, C2), 125.2 (d,
1JFC = 271, C11), 124.3 (d, 3JFC = 4, C9), 35.2 (d, 4JPC = 1, C5), 31.1 (s,
C6); the C7 resonance was not observed. 19F{1H} NMR (THF-d8): δ
−62.8. 11B{1H} NMR (toluene-d8): δ 3.7. HRMS (m/z): calcd for
[C126H150B6P6O18F18 + H]+, 2545.9597; found, 2545.9663.
1
3
(THF-d8): δ −9.9, −15.1. H NMR (THF-d8): δ 7.81 (dd, JPH = 16,
3JHH = 8; 1H, H2), 7.53 (dt, 3JHH = 8, 4JPH = 2; 1H, H4), 7.41 (t, 3JPH
=
3JHH = 7, 1H, H5), 7.40−7.32 (m, 3H; H3 and H12), 7.12 (t, 3JHH = 7,
2H, H10), 7.06 (d, 3JHH = 8, 2H, H13), 6.92 (t, 3JHH = 8, 4H, H9), 6.77
(t, 3JPH = 3JHH =7, 4H, H8). 13C{1H} NMR (THF-d8): δ 147.7 (broad,
C11), 143.3 (m, C6), 138.6 (d, 2JPC = 19, C5), 138.0 (d, 3JPC = 14, C3),
133.9 (m, JPC is obscured due to overlap, C2, C4, C8), 133.6 (dd, 1JPC
=
2
236, JPC = 36; C1), 132.3 (s, C12), 129.8 (d, 1JPC = 17, C7), 129.1 (q,
2JFC = 31, C14), 128.7 (m, JPC is obscured due to overlap, C9, C10),
1
3
125.7 (q, JFC = 272, C15), 124.3 (q, JFC = 4, C13). 19F{1H} NMR
(THF-d8): δ −62.5. 11B{1H} NMR (toluene-d8): δ 3.4. HRMS (m/z):
calcd for [C100H72B4P8O12F12 + H]+, 1985.3184; found, 1985.3161.
[(3,5-tBu2-Ph)PO3B(p-CHO-Ph)]4 (3d). 3d was synthesized analo-
gously to 3a from 1 (75 mg, 0.28 mmol) and 4-formylphenylboronic
acid (41 mg, 0.27 mmol) in toluene (110 mL). The volatiles were
removed under vacuum to afford a white solid. This material was
redissolved in toluene (2 mL), hexane (10 mL) was added, and a white
precipitate formed. The solid 3d was collected by filtration, washed
with hexane, and dried under vacuum (46 mg, 44%). X-ray quality
crystals were grown by layering hexane onto a toluene solution of 3d
1
at −40 °C. 31P{1H} NMR (THF-d8): δ −6.9. H NMR (THF-d8): δ
9.92 (s, 1H, H11), 7.82 (d, JHH = 8, 2H, H9), 7.77−7.74 (m, 5H, H2,
3
H4, H8), 1.23 (s, 18H, H6). 13C{1H} NMR (THF-d8): δ 192.0 (s, C11),
[(2-PPh2-Ph)PO3B(p-CHO-Ph)]4 (7d). 7d was synthesized analo-
gously to 3a from 6 (103 mg, 0.301 mmol) and 4-formylphenylbor-
onic acid (45 mg, 0.30 mmol) in toluene (12 mL). The volatiles were
removed under vacuum to afford a white solid. This material was
recrystallized from hot toluene to afford 7d as a white powder, which
was dried under vacuum (46 mg, 34%). 31P{1H} NMR (THF-d8): δ
3
152.5 (d, JPC = 18, C3), 150.9 (broad, C7), 137.3 (s, C8), 132.0 (s,
4
1
C10), 129.1 (s, C9), 128.8 (d, JPC = 4, C4), 125.9 (d, JPC = 234, C1),
125.9 (d, JPC = 13, C2), 35.6 (d, JPC = 2, C5), 31.2 (s, C6). 11B{1H}
2
4
NMR (toluene-d8): δ 3.5. HRMS (m/z): calcd for [C84H104B4P4O16
+
H]+, 1537.6725; found, 1537.6785. ESI-MS (1:1 CH2Cl2:acetonitrile,
LiCl, positive ion scan): 1543.9 ([M + Li]+ = 1543.6).
−9.7, −14.9. 1H NMR (THF-d8): δ 9.85 (s, 1H, H15), 7.80 (dd, 3JPH
=
=
[(3,5-tBu2-Ph)PO3B(p-CHO-Ph)]6 (4d). 4d was synthesized analo-
gously to 3a from 1 (811 mg, 3.00 mmol), 4-formylphenylboronic acid
17, 3JHH = 8; 2H, H2), 7.51 (t, 3JHH = 8, 1H, H4), 7.42 (t, 3JPH = 3JHH
3
6, 1H, H5), 7.37 (d, JHH = 8, 2H, H13), 7.34−7.30 (m, 3H, H3 and
H
dx.doi.org/10.1021/om501101q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3
3
H12), 7.09 (t, JHH = 7, 2H, H10), 6.88 (t, JHH = 8, 4H, H9), 6.78 (t,
REFERENCES
■
3JPH = JHH = 6, 4H, H8). 13C{1H} NMR (THF-d8): δ 192.3 (s, C15),
3
(1) (a) Walawalkar, M. G.; Murugavel, R.; Roesky, H. W.; Schmidt,
H.-G. Inorg. Chem. 1997, 36, 4202. (b) Walawalkar, M. G.; Murugavel,
R.; Roesky, H. W.; Schmidt, H.-G. Organometallics 1997, 16, 516.
(2) Cordes, D. B.; Lickiss, P. D.; Rataboul, F. Chem. Rev. 2010, 110,
2081.
(3) (a) Kuehnle, A. Chem. Abstr. 2006, 144, 276020. (b) Guan, C.;
Yang, H.; Li, W.; Zhou, D.; Xu, J.; Chen, Z.-R. J. Appl. Polym. Sci. 2014,
131, 40847. (c) Kajetanowicz, A.; Czaban, J.; Krishnan, G. R.;
143.4 (m, C6), 138.6 (d, 2JPC = 19, C5), 138.2 (d, 3JPC = 15, C3), 136.6
(s, C12), 134.1 (dd, JPC = 231, JPC = 50; C1), 133.9 (m, JPC is
1
2
obscured due to overlap, C2, C4, C8), 132.5 (s, C14), 129.8 (d, JPC
=
1
17, C7), 128.7 (m, JPC is obscured due to overlap, C9, C10), 128.7 (s,
C13); the C11 resonance was not observed. 11B{1H} NMR (toluene-
d8): δ 2.5. HRMS (m/z): calcd for [C100H76B4P8O16 + H]+,
1825.3485; found, 1825.3476. ESI-MS (1:1 CH2Cl2:acetonitrile,
LiCl, positive ion scan): 1831.5 ([M + Li]+ = 1831.3).
́
́
Malinska, M.; Wozniak, K.; Siddique, H.; Peeva, L. G.; Livingston, A.
G.; Grela, K. ChemSusChem 2013, 6, 182. (d) Leng, Y.; Liu, J.; Zhang,
C.; Jiang, P. Catal. Sci Technol. 2014, 4, 997. (e) Pescarmona, P. P.;
Masters, A. F.; van der Waal, J. C.; Maschmeyer, T. J. Mol. Catal. A:
Chem. 2004, 220, 37. (f) Abbenhuis, H. C. L.; van Herwijnen, H. W.
G.; van Santen, R. A. Chem. Commun. 1996, 1941.
(4) Tonnemann, J.; Scopelliti, R.; Severin, K. Eur. J. Inorg. Chem.
̈
2013, 5071.
(5) (a) Mason, M. R.; Matthews, R. M.; Mashuta, M. S.; Richardson,
J. F. Inorg. Chem. 1996, 35, 5756. (b) Mason, M. R. J. Cluster Sci. 1998,
9, 1. (c) Mason, M. R.; Perkins, A. M.; Ponomarova, V. V.; Vij, A.
Organometallics 2001, 20, 4833.
(6) (a) Mason, M. R.; Mashuta, M. S.; Richardson, J. F. Angew.
Chem., Int. Ed. 1997, 36, 239. (b) Mason, M. R.; Perkins, A. M.;
Matthews, R. M.; Fisher, J. D.; Mashuta, M. S.; Vij, A. Inorg. Chem.
1998, 37, 3734.
(7) Keys, A.; Bott, S.; Barron, A. R. Chem. Commun. 1996, 2339.
(8) Landry, C. C.; Cleaver, W. M.; Guzei, I. A.; Rheingold, A. L.
Organometallics 1998, 17, 5209.
(9) Yang, Y.; Walawalkar, M. G.; Pinkas, J.; Roesky, H. W.; Schmidt,
H.-G. Angew. Chem., Int. Ed. 1998, 37, 96.
(10) (a) Diemert, K.; Englert, U.; Kuchen, W.; Sandt, F. Angew.
Chem., Int. Ed. 1997, 36, 241. (b) Diemert, K.; Kuchen, W.; Mootz, D.;
Poll, W.; Sandt, F. Phosphorus Sulfur Silicon Relat. Elem. 1996, 111,
100.
Interconversion of 4c and 3c. Two J-Young NMR tubes were
charged with 4c (2.7 mg and 3.3 mg, respectively) and toluene-d8 (0.5
mL) and sealed under nitrogen in a glovebox. The tubes were brought
out of the glovebox and heated in a 105 °C oil bath. NMR spectra
were obtained periodically at room temperature. Equilibrium was
reached after 3 weeks. The equilibrium constant Keq = [3c]3/[4c]2 =
2.4(1) × 10−3 M. The tubes were cooled to room temperature and
NMR spectra were obtained periodically. Equilibrium was reached
after 4 weeks. The equilibrium constant Keq = [3c]3/[4c]2 = 1.2(1) ×
10−3 M.
X-ray Data Collection and Structure Refinement. The
diffraction data for 3a, 3d, and 4d were measured at 100 K on a
Bruker D8 VENTURE with PHOTON 100 CMOS detector system
equipped with a Mo-target X-ray tube (λ = 0.71073 Å). Data reduction
and integration were performed with the Bruker APEX2 software
package. Data were corrected for absorption effects using the empirical
methods as implemented in SADABS. The structures were solved and
refined by full-matrix least-squares procedures using the Bruker
SHELXTL software package. Crystallographic data and details of the
data collection and structure refinement are provided in the
Supporting Information.
(11) Similar reactions of dialkylphosphonates with cyclohexyldi-
chloroborane gave mixtures of oligomeric condensation products:
Mortier, J.; Gridnev, I. D.; Guen
́
ot, P. Organometallics 2000, 19, 4266.
(12) Tonnemann, J.; Scopelliti, R.; Zhurov, K. O.; Menin, L.;
̈
Dehnen, S.; Severin, K. Chem.Eur. J. 2012, 18, 9939.
(13) Tokunaga, Y.; Ueno, H.; Shimomura, Y.; Seo, T. Heterocycles
2002, 57, 787.
(14) Brock, C. P.; Minton, R. P.; Niedenzu, K. Acta Crystallogr., Sect.
C 1987, 43, 1775.
(15) (a) Moedritzer, K. J. Am. Chem. Soc. 1961, 83, 4381. (b) Ohms,
G.; Großmann, G.; Schwab, B.; Schiefer, H. Phosphorus Sulfur Silicon
Relat. Elem. 1992, 68, 77. (c) Fuchs, S.; Schmidbauer, H. Z.
Naturforsch., B: Chem. Sci. 1995, 50(b), 855. (d) Diemert, K.;
Kuchen, W.; Poll, W.; Sandt, F. Eur. J. Inorg. Chem. 1998, 361.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystal structure report for compounds 3a, 3d, and 4d and
combined cif file; Procedures for borophosphonate synthesis
and self-condensation of 1; Interconversion of 4c and 3c;
11B{1H} NMR background subtraction; NMR spectra of
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
(e) Kaltbeitzel, A.; Schauff, S.; Steininger, H.; Bingol, B.; Brunklaus,
̈
G.; Meyer, W. H.; Spiess, H. W. Solid State Ionics 2007, 178, 469.
(f) Kudzin, Z. H.; Depczynski, R.; Kudzin, M. H.; Luczak, J.;
Drabowicz, J. Amino Acids 2007, 33, 663.
(16) Reisinger, C. M.; Nowack, R. J.; Volkmer, D.; Rieger, B. Dalton
Trans. 2007, 272.
(17) Anet, F. L. Dynamics of eight-membered rings in the
cyclooctane class. In Dynamic Chemistry; Springer: Berlin, Heidelberg,
1974; Vol. 45, pp 169.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rfjordan@uchicago.edu.
(18) (a) Dale, J. Acta Chem. Scand. 1973, 27, 1115. (b) Goto, H.
̅
Tetrahedron 1992, 48, 7131.
Notes
(19) (a) Chen, F.-X.; Kina, A.; Hayashi, T. Org. Lett. 2005, 8, 341.
(b) Liao, Y.-X.; Hu, Q.-S. J. Org. Chem. 2011, 76, 7602.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Antoni Jurkiewicz for assistance with NMR
spectroscopy, Jin Qin for assistance with mass spectrometry,
and Ian Steele for assistance with X-ray crystallography. This
work was supported by the National Science Foundation
(CHE-0911180 and CHE-1048528).
I
dx.doi.org/10.1021/om501101q | Organometallics XXXX, XXX, XXX−XXX