Complexes of borane and N-heterocyclic carbenes: A new class of radical hydrogen atom donor

Article abstract of DOI:10.1021/ja804150k

Calculations suggest that complexes of borane with N-heterocyclic carbenes (NHC) have B-H bond dissocation energies more then 20 kcal/mol less than free borane, diborane, borane-THF, and related complexes. Values are in the range of popular radical hydrogen atom donors like tin hydrides (70-80 kcal/mol). The resulting prediction that NHC borane complexes could be used as radical hydrogen atom donors was verified by radical deoxygenations of xanthates by using either AIBN or triethylborane as initiator. Copyright

Full text of DOI:10.1021/ja804150k

Published on Web 07/09/2008
Complexes of Borane and N-Heterocyclic Carbenes: A New Class of Radical
Hydrogen Atom Donor
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†
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Shau-Hua Ueng, Malika Makhlouf Brahmi, Etienne Derat, Louis Fensterbank,*
Emmanuel Lacoˆte,*,† Max Malacria,*,† and Dennis P. Curran*,‡
Institut de chimie mole´culaire (FR 2769), Laboratoire de chimie organique (UMR CNRS 7611),
UPMC UniV Paris 06, C. 229, 4 place Jussieu, 75005 Paris, France, and Department of Chemistry,
UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260
Received June 2, 2008; E-mail: louis.fensterbank@upmc.fr; emmanuel.lacote@upmc.fr; max.malacria@upmc.fr; curran@pitt.edu
Trialkyltin hydrides are often used as radical hydrogen donors,
but the toxicity of such tin reagents detracts from their use in
chemical processes that respect the principles of safe, sustainable
synthesis. Many alternatives have been introduced to break the
“tyranny of tin”,1 yet its reign continues even if with weakened
dominance.2 Borane (BH3) is an attractive source of hydrogen
because of its high hydrogen content, but the B-H bond dissocia-
tion energy (BDE) of borane is much too high (106.6 kcal/mol3a,b
)
Figure 1. Calculated B-H BDEs of selected NHC-BH3 complexes
for radical hydrogen transfer reactions. Lewis bases reduce the B-H
BDE of the resulting complexes,4 and radical reactions of amine
boranes and phosphine boranes have been studied by Roberts and
others.5,6
(UB3LYP/LACVP* level).
Table 1. NHC Borane Reduction of Xanthates Initiated with AIBN
Calculations by Rablen show only a modest reduction in BDE
for borane complexes of amines and phosphines (BDEs, 94-104
kcal/mol).4 These bonds are much stronger than the X-H bonds
of popular radical hydrogen donors like Bu3Sn-H (74 kcal/mol)
and (Me3Si)3Si-H (79 kcal/mol).7 These calculations also suggest
that borane complexes of Lewis bases with the potential for
π-conjugation have considerably lower BDEs because of delocal-
ization of spin density in the resulting ligated boryl radicals
(L-BH2 ·). However, many of the complexes that were calculated
(with HCN or formaldehyde, for example) are transitory in solution.
Further, such ligands are reduced by BH3.
We hypothesized that complexes of N-heterocyclic carbenes and
borane (NHC boranes) would have weakened B-H bonds due to
π-conjugation and perhaps be better hydrogen donors than amine
and phosphine boranes. However, only a few NHC boranes are
known, and their chemistry is little explored.9 Thus, we first tested
the hypothesis with BDE calculations at the UB3LYP/LACVP*
level. For reference, the BDE of BH3 at this level is 111.7 kcal/
mol, and complexation of BH3 with THF lowers the BDE by less
than 2 kcal/mol (109.9 kcal/mol). The structures and BDEs of three
representative NHC boranes are shown in Figure 1. The NHC
ligands dramatically reduce the BDEs, and the calculated values
(74-80 kcal/mol) are squarely within the range of popular tin and
silicon hydrides.7
New NHC borane complex 2 and known complex 38c were easily
assembled from reaction of BH3-THF with the corresponding
carbenes. The compounds were air-stable, white crystalline solids.
The data from an initial series of Barton-McCombie deoxygenations
with 2 and 3 are summarized in Table 1. In a typical experiment,
xanthate 6, NHC borane 2 (2 equiv), and AIBN (0.5 equiv) were
heated in refluxing benzene for 2 h. After standard workup and
flash chromatography, deoxy sugar 7 was isolated in 61% yield
(entry 8). A range of secondary xanthates were deoxygenated in
comparable yields (entries 1-7).
a Run using 2 equiv of NHC borane complex. b Reaction took 10 h.
c Run using 2.5 equiv of NHC/borane complex. d SM recovered, 20%.
e Reaction took 4 h. f E/Z ratio was ∼3:1. g Diastereomeric ratio was
2.6:1.
Reduction of simple xanthates 4a,b with complex 3 and 2 equiv
AIBN gave 5a,b in about 65% yield (entries 1,2). Similar yields
were obtained with complex 2 (entries 3-5) even though less AIBN
was used (20-50%). Little reaction occurred when the AIBN level
was reduced to 10% or below, and no reaction occurred when the
NHC borane was omitted. The cyclopropyl carbinyl radical
precursor 4c provided the open product 5c (entry 6), while the
† UMR CNRS 7611.
‡ University of Pittsburgh.
9
10082 J. AM. CHEM. SOC. 2008, 130, 10082–10083
10.1021/ja804150k CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 2. NHC Borane Reduction of Xanthates unitiated with
Et3B/O2
These early results show that NHC boranes are competent radical
hydrogen atom donors. By changing the NHC ligand and the borane
substituents, there is promise to modulate both the BDE of the B-H
bond (see Figure 1) and the steric environment about the B-H
bond. In the immediate future, an increased understanding of these
preliminary results will expedite development of the class. For
example, what is the rate constant for hydrogen transfer from NHC
boranes? And what is the fate of the NHC boryl radical that is
presumably produced? Open questions aside, the promise is clear.
To fashion a new tin or silicon hydride, especially a chiral one,
can be an arduous process. In contrast, NHCs can be ligated to
boranes to make new complexes in one step. Many achiral and
chiral NHCs are already available, and the class is growing rapidly
thanks to applications in metal and organocatalysis.11
Acknowledgment. We thank UPMC, CNRS, IUF, and ANR
(grant BLAN0309 “Radicaux verts”) for funding. DPC thanks l’e´tat
et la re´gion Ile-de-France for a Chaire Blaise Pascal and NSF for
funding. We thank Drs. Julien Coulomb and Jason Clyburne for
helpful discussions. We thank Dr. Lise-Marie Chamoreau for
solving the X-ray structure.
Supporting Information Available: Experimental procedures and
characterization of all new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
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a Run using 2 equiv of NHC/borane complex. b Recovered SM, 21%
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Finally, the BD3 analogue of 2 was prepared and used to reduce
6 under the conditions of Table 2, entry 10. The isolated yield of
7 was only 27%, but it contained 70% deuterium at the deoxy
1
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result (reduced yield, substantial but incomplete labeling) presages
a substantial isotope effect.
JA804150K
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J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008 10083