Rhodium catalysed diboration of unstrained internal alkenes and a new and general route to zwitterionic [L2Rh(η6-catBcat)] (cat = 1,2-O2C6H4) complexes

Article abstract of DOI:10.1039/a804710c

Reactions of [L₂Rh(acac)] (L = alkene or phosphine) with B₂cat₃ yield the zwitterionic complexes [L₂Rh(η⁶-catBcat)] and [(acac)Bcat] cleanly; [(dppm)Rh(η⁶-catBcat)), the X-ray structure of which is reported, is an excellent catalyst for the diboration of vinylarenes and unstrained internal alkenes cis- and trans-stilbene and trans-β-methylstyrene.

Full text of DOI:10.1039/a804710c

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Rhodium catalysed diboration of unstrained internal alkenes and a new and
6
general route to zwitterionic [L2Rh(h -catBcat)] (cat = 1,2-O2C6H4)
complexes†
Chaoyang Dai,a Edward G. Robins,b Andrew J. Scott,c William Clegg,c Dmitri S. Yufit,b Judith A. K. Howardb
and Todd B. Marder*a,b
a Department of Chemistry, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
b Department of Chemistry, University of Durham, Durham, UK DH1 3LE. E-mail: Todd.Marder@durham.ac.uk
c Department of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne, UK NE1 7RU
Reactions of [L2Rh(acac)] (L = alkene or phosphine) with
6
B2cat3 yield the zwitterionic complexes [L2Rh(h -catBcat)]
O
O
6
O
O
O
O
and [(acac)Bcat] cleanly; [(dppm)Rh(h -catBcat)], the
RhL2
+
B
O
O B
X-ray structure of which is reported, is an excellent catalyst
for the diboration of vinylarenes and unstrained internal
alkenes cis- and trans-stilbene and trans-b-methylstyrene.
[(acac)RhL2]
B2cat3
Catalysed 1,2-diborations of alkynes, 1,4-diboration of
1,3-dienes and a,b-unsaturated ketones, as well as additions of
B–Si and B–Sn bonds to alkynes, a,w-diynes and enynes have
been the subject of a recent review.1 These reactions are
catalysed by platinum or palladium complexes and usually a
single catalytic pathway leads to a single product. In contrast,
the catalysed diboration of alkenes can lead to up to nine
products owing to the competition between B–C reductive
O
O
O
O
–
B
+
B
O
O
O
O
Rh+
(acac)Bcat
L
L
[RhL2(η6-catBcat)]
2
elimination
and
b-hydride
elimination
from
the
1a L2 = (C2H4)2
b L2 = (C8H14 2
c L2 = cod
d L2 = (PPh2Me)2
e L2 = dppm
f L2 = dppe
g L2 = dppp
h L2 = dppb
LnM(Bcat)[CHRCHRA(Bcat)] intermediate formed by alkene
)
insertion into the M–B bond. Initially, we examined2
6
i L2 = dppf
[(dppb)Rh(h -catBcat)], [dppb = Ph2P(CH2)4PPh2] an out-
standing hydroboration catalyst,3 for the addition of B2cat2 to
4-vinylanisole and obtained the desired 1,2-diboration product
4-MeOC6H4CH(Bcat)CH2(Bcat) in 44% yield. The remaining
products included 23% of 4-MeOC6H4CH(Bcat)CH3 and 22%
of the unusual 2,2,2-tris(boronate) ester 4-MeOC6H4CH2C(B-
cat)3, both arising from intermediates generated by the
b-hydride elimination process. A catalyst system composed of
[AuCl(PEt3)] + Cy2P(CH2)2PCy2 gave exclusive formation of
the 1,2-bis(boronate) ester; however, catalyst activity and
stability were lower than desired. Miyaura and coworkers4
reported the addition of B2pin2 (pin = OCMe2CMe2O) to
terminal alkenes and cyclic alkenes having internal ring strain
using a catalytic amount of Pt(dba)2 at 50 °C, but attempts to
diborate internal alkenes such as stilbene were unsuccessful.
Iverson and Smith5 reported similar results using Pt(cod)2 or
Pt(norbornene)3 as catalyst precursors at ambient temperatures.
Clean diboration was observed for norbornene and norborna-
diene, but not for other internal alkenes, apparently as a result of
complications arising from b-hydride elimination. In addition,
neither of the base-free Pt systems is appropriate for modifica-
tion with chiral ligands. We report herein the first catalyst
system capable of diborating internal alkenes including cis- and
trans-stilbene, and trans-b-methylstyrene without significant
by-products.
Scheme 1 Synthesis of zwitterionic rhodium complexes
(C8H14)2 1b, cod 1c, (PPh2Me)2 1d, dppm 1e, dppe 1f, dppp =
Ph2P(CH2)3PPh2 1g, dppb 1h, dppf = 1,1-bis(diphenylphos-
phino)ferrocene 1i] and (acac)Bcat 2 as evidenced by 1H, 11B,
and 31P NMR spectroscopy, full details of which will be
reported elsewhere.
Of particular interest is the fact that
4 mol% of
6
[(dppm)Rh(h -catBcat)] 1e,‡ prepared in situ from
[(dppm)Rh(acac)] and B2cat3 in THF, and whose molecular
structure§ is shown in Fig. 1, catalyses the diboration of
(addition of B2cat2 to) vinylarenes, norbornene and the
6
Several zwitterionic [L2Rh(h -catBcat)] complexes had been
3
prepared previously3 by addition of HBcat to either [L2Rh(h -
2-Me-allyl)] or [L2Rh(acac)] precursors; however, with L =
arylphosphine the reaction had to be carried out under
hydroboration conditions (i.e. in the presence of excess alkene
6
and HBcat) in order to isolate the [(dppb)Rh(h -catBcat)]
cleanly. In addition, this approach is obviously inappropriate for
L2 = (alkene)2 or diene. We have now found that reactions
(Scheme 1) of [L2Rh(acac)] with B2cat3 yield quantitatively the
6
Fig. 1 View of the molecular structure of [(dppm)Rh(h -catBcat)] 1e with
ellipsoids shown at 50% probability and H atoms omitted for clarity.
Selected distances (Å) and angles (°) Rh(1)–P(1) 2.2332(5), Rh(1)–P(2)
2.2134(5); P(1)–Rh(1)–P(2) 72.938(17).
6
zwitterionic complexes [L2Rh(h -catBcat)] [L2 = (C2H4)2 1a,
Chem. Commun., 1998
1983

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Table 1 [(acac)Rh(dppm)]/B2cat3-catalyzed diboration of alkenesa
containing catalyst system for alkene diboration. Further work
will examine the mechanism of the reaction, the diboration of
other unsaturated substrates and the use of chiral bis(phos-
phine)-containing zwitterionic Rh complexes for asymmetric
diboration reactions.
T. B. M. acknowledges support from NSERC of Canada, the
University of Durham for Special Equipment and Special
Projects Grants, the University of Newcastle upon Tyne through
a Senior Visiting Research Fellowship, and the NSERC/Royal
Society (London) Bilateral Exchange Program, W. C. thanks
EPSRC for an equipment grant, A. J. S. thanks EPSRC for a
studentship, and J. A. K. H. thanks the University of Durham for
a Sir Derman Christopherson Fellowship.
Alkene
Product
Entry
Time/h
Yield(%)
catB
Bcat
Ph
Bcat
a
b
2
87
Ph
catB
2.5
77
C6H4OMe-p
C6H4OMe-p
catB
Bcat
1
6
79
c
C6H4Cl-p
C6H4Cl-p
Notes and References
† Dedicated to Professor Warren Roper on the occasion of his 60th birthday.
Preliminary results were presented at the Fifth Chemical Congress of North
America, Cancun, Mexico, November 1997, Abstract No. 1493.
Bcat
Bcat
d
>99
1
‡ NMR spectroscopic data for 1e in C6D6: 31P{1H}, d 222.97 (d, JRhP
184.6 Hz); 11B{1H}, d 15.76; 1H, d 3.62 (td, 3JRhH 2.0, 2JPH 10.8 Hz, 2 H,
catB
Bcat
Ph
Me
6
6
CH2), 4.74 (m, 2 H, h -C6H4O2), 6.28 (m, 2 H, h -C6H4O2), 6.60 (m, 1 H,
C6H4O2), 6.76 (overlapping m, 2 H, C6H4O2), 6.87–7.13 (overlapping m, 13
H, C6H4O2 and C6H5), 7.65 (m, 8 H, C6H5).
30
>99
92
e
f
Ph
Ph
Me
§ Crystal data: for 1e from C6D6: C37H30BO4P2Rh·C6D6, M = 798.42,
orthorhombic, space group P212121, a = 13.2932(7), b = 15.2327(8), c =
17.8046(10)Å, U = 3605.3(3)Å3, Z = 4, Dc = 1.471 g cm23, m(Mo-Ka)
= 0.606 mm21, T = 160 K. Full-matrix least-squares refinement on F2
(G. M. Sheldrick, SHELXTL manual, Bruker AXS Inc., Madison, WI,
USA, 1994, version 5) anisotropic for all non-H atoms and isotropic for H
(461 parameters) using 8415 unique data (including 3634 Friedel pairs;
26 554 total collected; Rint = 0.0251) from a Bruker AXS SMART CCD
diffractometer (q < 28.46°) gave R1 [I > 2s(I)] = 0.0212, wR2 (all data) =
Ph
catB
Ph
Bcat
Ph
<72
catB
Ph
Bcat
Ph
<72
>99
g
Ph
Ph
0.0473. Residual electron density within ± 0.28 e Å23
For rac-PhCH(Bcat)CH(Ph)(Bcat) from [2H8]THF: C26H20B2O4·C4D8O,
498.16, monoclinic, space group P21/m, a 6.1548(5), b =
.
a All reactions were carried out in THF or [2H8]THF at room temp. in the
presence of 4 mol% catalyst [(acac)Rh(dppm)]/B2cat3, and alkene:B2cat2
= 1:1; product yields determined by 1H and 13C NMR spectroscopy.
M
=
=
19.853(2), c = 10.4004(9)Å, b = 95.933(3)°, U = 1264.1(2) Å3, Z = 2, Dc
= 1.309 g cm23, m(Mo-Ka) = 0.085 mm21, T = 100 K. Full-matrix least-
squares refinement on F2 as above, anisotropic for all non-disordered non-H
atoms, isotropic for H and disordered atoms with disordered H atoms not
included in the refinement (198 parameters) using 2970 unique data (14 317
total collected; Rint = 0.060) (q < 27.50°) gave R1 [I > 2s(I)] = 0.0827,
unstrained internal alkenes cis and trans-stilbene and trans-
b-methylstyrene at room temp. (Table 1).¶ Syn-addition of the
B2 unit to the alkene was evident in the NMR spectra of the
norbornene diboration product (entry d). A crystal structure§ of
the trans-stilbene diboration product (entry f) is also consistent
with syn-addition. The disorder observed in the crystal structure
results from the apparent superposition in space of the two
enantiomers of the racemic compound; attempts to solve the
structure based on the meso-model gave an unreasonable central
C–C bond length. Likewise, in CD2Cl2, the signal for the unique
benzylic C–H proton at d 3.71 is distinct from that for the cis-
stilbene diboration product (entry g) which occurs at d 3.78 the
latter thus being assigned to the meso compound. Diboration of
trans-b-methylstyrene (entry e) proceeds in > 99% yield,¶
generating two adjacent and distinct chiral carbon centres.
Significantly reduced hapticity of the p-coordinated cate-
cholate must be required in order to generate vacant sites for
alkene and B–B activation. Although reaction times were found
to be somewhat longer than in THF, the diborations can also be
carried out in less polar C6D6 suggesting that complete
dissociation into L2Rh+ and [Bcat2]2 is unlikely. The success of
the dppm based catalyst system compared with the dppb system
indicates that the relative rates of B–C reductive elimination vs.
b-hydride elimination are a sensitive function of the bite angle
of the chelating phosphine ligand.
wR2 (all data) = 0.1931. Residual electron density within ± 0.572 e Å23
.
CCDC 182/949.
¶ A representative procedure for the diboration of trans-b-methylstyrene: in
a N2-filled glove-box, [(acac)Rh(dppm)] (0.010 mmol) and B2cat3 (0.010
mmol) were charged into a 20 ml vial and dissolved in THF (0.5 ml). The
solution was stirred rapidly for ca. 5 min and then a solution of trans-
b-methylstyrene (0.250 mmol) in THF (0.5 ml) was added. Finally, B2cat2
(0.250 mmol) was added portionwise and the resulting reaction mixture
allowed to stir rapidly at room temperature. Aliquots (1 ml) were removed
regularly to monitor the disappearance of alkene via GC–MS. Crude
product was isolated by reduction of the THF volume by ca. 50% followed
by addition of n-hexane (2–3 ml). Spectroscopic data for PhCH(Bcat)CH-
1
(Me)(Bcat) in C6D6: H NMR, d 1.12 (d, 3 H, J 7.5 Hz), 2.39 (dq, 1 H, J
11.4, 7.5 Hz), 3.05 (d, 1 H, J 11.4 Hz), 6.69 (m, 4 H), 6.88 (m, 4 H), 7.02
(m, 1 H), 7.14 (m, 2 H), 7.29 (m, 2 H). 11B{1H} NMR, d 35.6 (br s, 2B).
HRMS. Calc. for C21H18B2O4: m/z 356.1391. Found m/z 356.1391.
1 T. B. Marder and N. C. Norman, Top. Catal., 1998, 5, 63.
2 R. T. Baker, P. Nguyen, T. B. Marder and S. A. Westcott, Angew. Chem.,
Int. Ed. Engl., 1995, 34, 1336; Angew. Chem., 1995, 107, 1451.
3 S. A. Westcott, H. P. Blom, T. B. Marder and R. T. Baker, J. Am. Chem.
Soc., 1992, 114, 8863.
4 T. Ishiyama, M. Yamamoto and N. Miyaura, Chem. Commun., 1997,
689.
5 C. N. Iverson and M. R. Smith III, Organometallics, 1997, 16, 2757.
This is the first report of the catalysed diboration of
unstrained internal alkenes and of an efficient phosphine-
Received in Cambridge, UK, 22nd June 1998; 8/04710C
1984
Chem. Commun., 1998