Full Paper
into an NMR spectrometer preheated (to 40, 50, 60 or 708C) for
10 min to allow thermal equilibration. The NMR tube was removed
from the spectrometer and 1 was added. The tube was shaken to
ensure dissolution, and then returned to the spectrometer (still at
temperature) for 11B{1H} analysis. Analysis by linear least-squares fit-
ting of a standard Eyring plot (see Supporting Information, Fig-
ure S10) gave ln(k/T)=10.9–8.1ꢁ103/T (R2 =0.996) and thus DH° =
its borane adduct are both volatile and, unless the phosphine
is also volatile, are readily separated from the product at the
end of reaction by evaporation. Notably, pyrrolidine is similarly
volatile and inexpensive, but significantly more reactive than
diethylamine (Table 2) and should be considered as an advan-
tageous alternative. In contrast, for the more alkyl-rich phos-
phines (n=0, 1) the decomplexation reaction is much less effi-
cient. For these systems, there is considerable advantage to
use cyclic non-hindered or bridgehead amines, such as DABCO
(7),[12] in an apolar solvent. If thermally labile phosphine P-ster-
eogenicity is not an issue, then the solvent should ideally be
of a suitable boiling point to allow reaction at higher tempera-
ture, for example toluene, xylene or mesitylene. Crucially, to
gain kinetic benefit from the bimolecularity of the process,
both reactants (R3P·BH3 and 7) should be present at as high an
initial concentration as is practical.
16.1 kcalmolÀ1 and DS° =À25.6 calKÀ1 molÀ1
.
Test for cross-over during reaction of triphenylphosphine
borane (1) with quinuclidine (2)
Complex 1 (5.8 mg, 0.021 mmol), amine 2 (3.1 mg, 0.028 mmol),
and [D15]Ph3P (5.8 mg, 0.021 mmol) were added to a vacuum-dried
NMR tube. Dry toluene was then added to give a total volume of
0.7 mL. The NMR tube was placed in a preheated (308C) NMR spec-
trometer for 31P{1H} reaction monitoring. No significant loss of the
absolute intensity of the [D15]Ph3P peak was observed over the
course of 8 h, during which >80% of 1 had been converted to
Ph3P. Simulation of the temporal concentration of 1, using Dyno-
chem 2011 v4, with the following bimolecular equilibrium rate law:
Àd[1]/dt=(k([1][2]À[[Dn]Ph3P][3]K); n=0,15), gave a second-order
We are currently exploring the use of the borane transfer re-
action for the parameterisation of reactant descriptors for
linear free energy relationships, across a series of amines and
phosphines. We will report on this, as well as a predictive
model for P-to-N borane transfer kinetics, in due course.
rate constant k=2.4ꢁ10À3 mÀ1 À1, consistent with that determined
s
in the absence of [D15]Ph3P.
Experimental Section
Acknowledgements
Kinetics of reaction of triphenylphosphine borane (1) with
quinuclidine (2)
University of Edinburgh and the Bristol Chemical Synthesis
DTC (EPSRC grant EP/G036764/1) provided financial support to
N.P.T. We also thank Dr. Oren Sherman (Cambridge, UK), Dr.
David Fox (Warwick, UK), and Prof. Jeremy Harvey (Leuven, Bel-
gium) for valuable discussion and preliminary DFT studies.
The following procedure is typical. Complex
1
(14.0 mg,
0.05 mmol) was added to an oven-dried Schlenk tube under an at-
mosphere of nitrogen. Dry toluene was added. The mixture was
stirred and heated to 308C until complete dissolution was ach-
ieved, to give a 0.02m solution of 1. A known mass of 2 was then
added to the solution, (t=0) and after dissolution (normally occur-
ring within a matter of seconds), a sample was transferred via pip-
ette to an NMR tube. This was placed in a preheated (308C) spec-
trometer for 11B{1H} NMR reaction monitoring. Kinetic data were
obtained with [2]0 in the range 0.01–0.73m; an additional set of re-
Keywords: boranes
·
phosphane ligands
·
phosphine
boranes · protecting groups · reaction mechanisms
[1] a) J. Bayardon, S. Jugꢂ, in Phosphorus(III) Ligands in Homogenous Cataly-
sis: Design and Synthesis (Eds.: P. C. J. Kamer, P. W. N. M. van Leeuwen),
Wiley, New York, 2012, pp. 366–382; b) M. Hurtado, M. Yꢃnez, R. Her-
rero, A. Guerrero, J. Z. Dꢃvalos, J.-L. M. Abboud, B. Khater, J.-C. Guille-
b) D. H. Nguyen, J. Bayardon, C. Salomon-Bertrand, S. Jugꢂ, P. Kalck, J.-C.
have been tested as retinal ganglion cell signal activators. Extracellular
amines are proposed to mediate borane transfer to liberate the free
phosphine in vivo: M. Almasieh, C. J. Lieven, L. A. Levin, A. Di Polo, J.
actions were performed at
a higher initial concentration of
1 ([1]0 =0.04m), using [2]0 =0.1m in toluene at 308C. Full data are
presented in the Supporting Information, Figure S1. In all cases, ki-
netic simulations, performed using Dynochem 2011 v4, using the
following bimolecular equilibrium rate law: Àd[1]/dt=k([1]
[2]À[Ph3P][3]/K) gave excellent correlations with experimental data
with k=2.6ꢁ10À3 mÀ1
[2]). The kinetics of bimolecular equilibrium reaction of a large
excess of Ph3P with 3 (i.e., the reverse reaction) was then em-
ployed to determine K.
s
À1, when K is large; that is, Àd[1]/dtꢂk([1]
Determination of activation parameters
Variable temperature analysis was carried out using the same pro-
cedure as above with [1]0 =0.02m and [2]0 =0.04m in toluene, to
[4] a) T. Imamoto, T. Oshiki, T. Onozawa, T. Kusumoto, K. Sato, J. Am. Chem.
J. P. GenÞt, Tetrahedron Lett. 1990, 31, 6357–6360; f) E. F. Clarke, E.
Soc. 2011, 133, 5740–5743; h) M. Revꢂs, C. Ferrer, T. Leꢅn, S. Doran, P.
obtain the second-order rate constants (k [mÀ1 À1]) for BH3 transfer.
s
Reactions were performed every 108C from 30–708C inclusive.
Analysis by linear least-squares fitting of a standard Eyring plot
(see Supporting Information, Figures S3 and S10) gave ln (k/T)=
15.8–8.3ꢁ103/T (R2 =0.996) and thus DH° =16.6 kcalmolÀ1 and
DS° =À15.8 calKÀ1 molÀ1. Reactions between 1 and triethylamine
were also performed every 108C from 30–708C inclusive. Stock sol-
utions of triethylamine in dry toluene were prepared, and a set
volume transferred to a J. Youngs NMR tube. This was then placed
Chem. Eur. J. 2015, 21, 1 – 7
5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ