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Cationic Terminal Borylene Complexes Interconversion of Amino and Alkoxy Borylenes by an Unprecedented MeerweinЦPonndorf Hydride Transfer.

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Angewandte
Chemie
Boron Chemistry
DOI: 10.1002/ange.200600714
Cationic Terminal Borylene Complexes:
Interconversion of Amino and Alkoxy Borylenes
by an Unprecedented Meerwein–Ponndorf
Hydride Transfer**
Deborah L. Kays (ne Coombs), Joanna K. Day,
Simon Aldridge,* Ross W. Harrington, and
William Clegg
Dedicated to Professor Tony Downs
on the occasion of his 70th birthday
The fundamental reactivity of transition-metal complexes
containing metal–boron double bonds has recently become an
area of considerable research activity,[1] reflecting both the
development of viable synthetic approaches to such systems
(for example, terminal borylene complexes LnM=BX),[2–4] and
the desire to compare patterns of reactivity with other
multiply bonded second-period ligand systems (for example,
carbenes and alkylidenes).[5] Within this area, diverse chemistries are emerging, ranging from simple nucleophilic sub[*] Dr. D. L. Kays (ne Coombs), J. K. Day, Dr. S. Aldridge
Centre for Fundamental and Applied Main Group Chemistry
School of Chemistry, Cardiff University
Main Building, Park Place, Cardiff, CF10 3AT (UK)
Fax: (+ 44) 2920-874-030
E-mail: AldridgeS@cardiff.ac.uk
Dr. R. W. Harrington, Prof. W. Clegg
School of Natural Sciences (Chemistry)
Bedson Building, University of Newcastle
Newcastle upon Tyne, NE1 7RU (UK)
[**] We thank the EPSRC for funding (including that for the National
Crystallography Service) and for access to the National Mass
Spectrometry Facility, and the CCLRC for the award of synchrotron
beamtime.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. 2006, 118, 3593 –3596
stitution and addition processes dominated by the electrophilic character of the two-coordinate boron center,[4] to
synthetically valuable borylene-transfer chemistry involving
both transition-metal and wholly organic substrates.[2b, g, 6] We
have recently demonstrated the first examples of M=B
metathesis chemistry; reaction of the cationic aminoborylene
complex [CpFe(CO)2=B(NiPr2)]+[BArf4] (1; Cp = C5H5 ;
Arf = 3,5-(CF3)2C6H3) with Ph3E=X (E = P, As; X = S, O)
yields [CpFe(CO)2(EPh3)]+[BArf4] and [XB(NiPr2)]n (X = O,
n = 3; X = S, n = 2).[7]
The widespread application of metathesis processes
involving substrates with multiple bonds to carbon[5b,c] has
led us to explore the reactivity of 1 towards C=E multiple
bonds. Given that the key step in M=B metathesis towards
P=O, P=S, and As=O linkages has been shown to be the
precoordination of the oxygen or sulfur donor center to
boron,[7] it seemed logical to first examine the reactivity of 1
towards polar C=E bonds, such as those present in ketones or
imines. Such an investigation also probes the extent to which
borylene reactivity can be tuned by modification of the
pendant X substituent; the arylborylene complex
[Cp*Fe(CO)2=B(C6H2Me3-2,4,6)]+ (Cp* = C5Me5) has been
shown to react with ketones by substitution of the borylene
ligand at the iron center, yielding for example, [Cp*Fe(CO)2(h1-OCPh2)]+.[4b] The reaction of 1 with benzophenone
proceeds by neither metathesis nor ligand displacement, but
by an entirely unprecedented ligand transformation process:
Meerwein–Ponndorf b-hydride transfer from an isopropyl
substituent of the aminoborylene ligand to the coordinated
ketone generates the first example of an imine-donorstabilized borylene complex, and the first cationic alkoxyborylene system.
The reaction of 1 with benzophenone (Scheme 1) proceeds by an initial coordination of the ketone at the electrophilic boron center, yielding the thermally fragile intermediate [CpFe(CO)2B(NiPr2)(OCPh2)]+[BArf4] (2). Although 2
is labile even at 30 8C, its multinuclear NMR spectra at
50 8C are consistent with an adduct of the type
[CpFe(CO)2B(NiPr2)(OEPhn)]+[BArf4] .[7] Thus, for example, the detection of inequivalent isopropyl groups by 1H and
13
C NMR spectroscopy (in contrast to the equivalent isopropyl groups in 1), and of a significantly upfield-shifted
11
B NMR resonance (2: dB = 49.0 ppm; 1: dB = 93.5 ppm) are
consistent with the formation of a trigonal-planar boron
center (from the linear two-coordinate boron center of 1) by
coordination of the Ph2CO Lewis base. Very similar spectroscopic changes are observed on formation of the crystallographically characterized boron-bound Ph3PO adduct
[CpFe(CO)2B(NiPr2)(OPPh3)]+[BArf4] (3 a; Scheme 2): a
11
B NMR resonance at dB = 48.9, and two 1H NMR signals
for the methyl substituents of the inequivalent isopropyl
groups at dH = 1.06 and 1.20 ppm.[7]
On warming to room temperature, quantitative conversion of intermediate 2 is detected, and the species formed (4)
displays a more complex pattern of two doublets (both
integrating to three hydrogen atoms), two singlets (also both
integrating to three hydrogen atoms), and a septet (integrating to one hydrogen atom) in the alkyl region of its 1H NMR
spectrum. Such a pattern is consistent with the formation of a
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Zuschriften
Scheme 1. Reaction of 1 with benzophenone. Reaction conditions: a) 1, benzophenone (1 equiv), CH2Cl2,
to room temperature, sonication for 1.5 h, 36 % isolated yield.
78 to
50 8C; b) subsequent warming
contains only peaks due to ions without a coordinated imine
group
(for
example,
[CpFe(CO)2B{OC(H)Ph2}]+
+
([M N(iPr)CMe2] ) at m/z = 371.1), the loss of a weakly
bound ligand under such conditions is amply precedented.
Furthermore, the structure of 4·CH2Cl2 in the solid state has
subsequently been determined, and it confirms the formulation of 4 as [CpFe(CO)2B{OC(H)Ph2}{N(iPr)CMe2}]+[BArf4]
(Figure 1).
Scheme 2. Contrasting predominant resonance forms for 3 a and
imine-stabilized complexes 4 and 5.
coordinated imine (iPrN=CMe2). Compared to the 1H NMR
spectrum of the “free” imine (dH = 1.10 (d, 6 H), 1.85 (s, 3 H),
2.00 (s, 3 H), 3.60 ppm (sept, 1 H)),[8] the chemical shift
differences and the splitting of the isopropyl methyl resonances in the alkyl region of the spectrum of 4 are consistent
with the electronic and steric effects of coordination. Furthermore, the OC(H)Ph2 fragment implied by the hydridetransfer process that gives rise to the coordinated imine
(Scheme 1) is evidenced by signals at dH = 6.24 ppm and dC =
89.3 ppm in the 1H and 13C NMR spectra of 4 (compare to
dH = 5.92 ppm and dC = 74.2 ppm for Ph2C(H)OH). The
11
B NMR chemical shift for the boron center in the cation
of 4 (dB = 64.0 ppm) is also consistent with a base-stabilized
borylene complex (compare to dB = 52.1 ppm for the pyridine-stabilized ethoxyborylene complex OsK [B(NC5H4NL H)(OEt)]Cl(CO)(PPh3)2[3b]). The carbonyl stretching frequencies (ñ(CO) = 2012, 1969 cm 1) in the IR spectrum of 4 are
intermediate between those observed for base-free cationic
iron–carbonyl complexes with two-coordinate aminoborylene
ligands (1: ñ(CO) = 2070, 2028 cm 1;[7] [CpFe(CO)2=B(NCy2)]+[BArf4] (Cy = C6H11): ñ(CO) = 2070, 2021 cm 1 [9])
and those for neutral complexes with three-coordinate
aminoboryl ligands (for example, CpFe(CO)2{B(NiPr2)Cl}:
ñ(CO) = 2001, 1941 cm 1 [7]). Although the mass spectrum of 4
3594
www.angewandte.de
Figure 1. Molecular structure of one of the two crystallographically
independent cations in 4·CH2Cl2. Thermal ellipsoids are set at 50 %
probability. Hydrogen atoms (with the exception of H114) are omitted
for clarity. Selected bond lengths [H] and angles [8]: Fe1 B1 2.000(4),
Fe1 C101 1.752(4), B1 O103 1.343(4), O103 C114 1.454(3), B1 N1
1.570(4), N1 C111 1.281(4), N1 C108 1.494(4); Fe1-B1-N1 122.1(2),
Fe1-B1-O103 131.6(2), N1-B1-O103 106.2(3), N1-C111-C112 124.5(3),
N1-C111-C113 119.9(3), C112-C111-C113 115.6(3), O103-C114-C115
109.5(2), O103-C114-C121 106.7(2), C115-C114-C121 114.7(2).
The solid-state structure of 4·CH2Cl2 not only confirms the
gross structural features implied by the spectroscopic data,
but also suggests that a description of 4 as a base-stabilized
borylene complex (that is, one involving significant contributions from a resonance form incorporating Fe=B and N=C
double bonds) is most appropriate (Scheme 2). As such, 4
represents only the second example of a terminal alkoxyborylene complex,[3b] and the first such cationic species; it also
represents the first example of an imine-donor-stabilized
borylene complex. The bonding in 4 contrasts with that found
in the superficially similar complex [CpFe(CO)2B(NiPr2)(OPPh3)]+[BArf4] (3 a), which has been shown to best be
viewed as a boryl species in which the positive charge is
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 3593 –3596
Angewandte
Chemie
largely localized on the pendant phosphorus center
(Scheme 2).[7] Thus, the two sets of C–N distances for the
iPrN=CMe2 fragment in 4 are consistent with the distinct
C=N double (1.281(4) and 1.283(4) H, for the two distinct
cations in the asymmetric unit) and C N single bonds
(1.494(4) and 1.508(4) H) implied by a coordinated imine
moiety. In addition, the B N distances (1.570(4) and
1.575(5) H) associated with this fragment are significantly
longer than the B N distances typically found for conventional aminoboryl complexes (for example, CpFe(CO)2{B(NiPr2)Cl}: 1.389(5) H[7]), being much more akin to those
found for donor–acceptor adducts of imines with borane
Lewis acids (for example, Ph(H)C=N(CH2Ph)·B(C6F5)3 :
1.627(3) H[10]). The Fe B distances for 4 (2.000(4) and
2.001(4) H) are shorter than those found in either neutral or
cationic boryl systems featuring the B(NiPr2) moiety (for
example,
CpFe(CO)2{B(NiPr2)Cl}:
2.054(4) H;
3 a:
2.057(4) H[7]), but are somewhat longer than those found in
base-free systems featuring a two-coordinate boron center
and a M=B double bond (for example, [CpFe(CO)2=B(NCy2)]+[BArf4] : 1.859(6) H[9]). The smaller extent of bondlength contraction (compared to M B single bonds) on
formation of a M=B double bond in a base-stabilized
borylene complex, however, reflects trends previously
observed for related osmium–borylene systems.[3] Finally,
the metric parameters for the BOC(H)Ph2 unit are indicative
of O B and O C single bonds;[11] the presence of the
hydrogen atom at C114 is reflected in the narrowing of the
O-C-Cipso and Cipso-C-Cipso angles (sum of angles = 330.68
(mean)) compared to those of benzophenone (sum of
angles = 3608).[12]
The contrasting reactivity of 1 towards E=O (E = P, As)
and C=O bonds presumably reflects the differing electronic
structures of the first-formed boron-bound adducts
[CpFe(CO)2B(NiPr2)(OEPhn)]+[BArf4] (Scheme 3). In contrast to the Ph3PO (3 a) and Ph3AsO (3 b) adducts of 1,[7] the
corresponding benzophenone adduct (2) possesses a pendant
atom E (the carbonyl carbon) with significant electrophilic
character. In essence, the cationic borylene complex 1 serves
as a Lewis acid to activate the benzophenone carbonyl group
to nucleophilic attack, in this case by the b hydride of a
proximal isopropyl group from the amino substituent. As
such, this chemistry represents an unprecedented ligand
modification by a Meerwein–Ponndorf reduction.[13] The
degree of electrophilic character at the carbonyl carbon
atom can be shown to be a key factor in facilitating hydride
transfer. Thus, the reaction of 1 with iPrN=CMe2 leads to the
formation of the imine adduct [CpFe(CO)2B(NiPr2){N(iPr)CMe2}]+[BArf4] (5; see Supporting Information for
crystallographic data). In this case, however, no hint of
hydride transfer is observed by 1H NMR spectroscopy, even
at 100 8C, presumably owing to the less electrophilic nature of
the imine substrate compared to benzophenone.
The reactivity of aminoborylene complex 1 towards
benzophenone contrasts markedly with that of the related
cationic arylborylene complex [Cp*Fe(CO)2B(C6H2Me32,4,6)]+[BArf4] towards ketones, which has been shown to
proceed by attack at the transition-metal center and displacement of the borylene ligand to give [Cp*Fe(CO)2(h1-OCR2)]+
(R = Ph, Me).[4b] The differing reactivities of 1 and [Cp*Fe(CO)2B(C6H2Me3-2,4,6)]+[BArf4]
towards C=O double
bonds provide convincing evidence of the crucial role
played by the electronic properties of the borylene ligand in
determining the chemistry of the metal complex. Further
comparative studies involving substrates with C=S, C=N, C=P,
and CP bonds will be reported in due course.
Experimental Section
Reaction of 1 with benzophenone (intermediate 2): The reaction of 1
(0.071 g, 0.062 mmol) with benzophenone (1 equiv) in CD2Cl2 (2 cm3)
in a J. Young NMR tube on warming from 78 8C to room
temperature was monitored by multinuclear (1H, 11B, 13C, and 19F)
NMR spectroscopy. On warming to 50 8C, the borylene (dB =
93.5 ppm) and isopropyl methyl (dH = 1.39 ppm) signals assigned to
1 were quantitatively replaced by a broad resonance at dB = 49.0 ppm
and by two doublets at dH = 1.15 and 1.18 ppm. Further warming to
room temperature led to a second set of changes in the 1H and
11
B NMR spectra: the resonance at dB = 49.0 ppm converted into a
signal at dB = 64.0 ppm, and the two methyl signals of the isopropyl
group were replaced by a more complex pattern of two doublets (dH =
1.28 and 1.44 ppm) and two singlets (dH = 2.05 and 2.37 ppm). The
thermally labile first-formed intermediate was identified as 2, on the
basis of multinuclear NMR spectroscopy at 50 8C, whereas the final
product 4 can be prepared in yields of ca. 36 %. 2: 1H NMR
(300 MHz, CD2Cl2, 50 8C): d = 1.15 (d, J = 6.5 Hz, 6 H, CH3 of iPr),
1.18 (d, J = 6.5 Hz, 6 H, CH3 of iPr), 3.44 (sept, J = 6.5 Hz, 1 H, CH of
iPr), 3.96 (sept, J = 6.5 Hz, 1 H, CH of iPr), 4.26 (s, 5 H, Cp), 7.36–8.11
(br m, 10 H, Ph2CO), 7.48 (s, 4 H, p-CH of [BArf4] ), 7.69 ppm (s, 8 H,
o-CH of [BArf4] ); 13C NMR (76 MHz, CD2Cl2, 50 8C): d = 21.6
(CH3 of iPr), 24.6 (CH3 of iPr), 47.9 (CH of iPr), 53.7 (CH of iPr), 85.1
(Cp), 118.4 (p-CH of [BArf4] ), 125.3 (q, 1JCF = 272 Hz, CF3 of
[BArf4] ), 129.6 (q, 2JCF = 31 Hz, m-C of [BArf4] ), 131.0 (m-CH of
Ph2CO), 132.7 (p-CH of Ph2CO), 135.5 (o-CH of [BArf4] ), 136.8 (oCH of Ph2CO), 141.3 (ipso-C of Ph2CO), 162.6 (q, 1JCB = 50 Hz, ipsoC of [BArf4] ), 215.1 ppm (CO), carbonyl carbon of Ph2CO not
observed; 11B NMR (96 MHz, CD2Cl2, 50 8C): d = 7.8 ([BArf4] ),
Scheme 3. Contrasting metathesis and b-hydride-transfer reactions for adducts of the type [CpFe(CO)2B(NiPr2)(OEPhn)]+[BArf4] .
Angew. Chem. 2006, 118, 3593 –3596
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3595
Zuschriften
49.0 ppm (br, fwhm 1700 Hz, borylene); 19F NMR (283 MHz, CD2Cl2,
50 8C): d = 61.7 ppm (CF3).
4: 1 (0.285 g, 0.248 mmol) and benzophenone (1 equiv) were
dissolved in CH2Cl2 at 78 8C, and the stirred reaction mixture was
warmed to room temperature over 2 h. Sonication for a further 1.5 h
drove the reaction to completion (as judged by 11B NMR spectroscopy). Filtration and concentration of the reaction mixture, followed
by layering with hexanes and storage at 30 8C led to the formation of
4 as colorless crystals suitable for X-ray diffraction. Isolated yield:
0.120 g, 36 %. 1H NMR (400 MHz, CD2Cl2): d = 1.28 (d, J = 6.9 Hz,
3 H, CH3 of iPr), 1.44 (d, J = 6.9 Hz, 3 H, CH3 of iPr), 2.05 (s, 3 H, CH3
of N=CMe2), 2.37 (s, 3 H, CH3 of N=CMe2), 4.07 (sept, J = 6.9 Hz, 1 H,
CH of iPr), 4.81 (s, 5 H, Cp), 6.24 (s, 1 H, CH of Ph2CHO), 7.19–7.32
(m, 10 H, Ph of Ph2CO), 7.47 (s, 4 H, p-CH of [BArf4] ), 7.64 ppm (s,
8 H, o-CH of [BArf4] ); 13C NMR (76 MHz, CD2Cl2): d = 21.0, 21.1
(CH3 of iPr), 23.8, 27.3 (CH3 of N=CMe2), 55.1 (CH of iPr), 85.0 (Cp),
89.3 (CH of Ph2CHO), 117.5 (p-CH of [BArf4] ), 124.7 (q, 1JCF =
272 Hz, CF3 of [BArf4] ), 125.8, 125.9 (p-CH of Ph2CHO), 128.9 (q,
2
JCF = 31 Hz, m-C of [BArf4] ), 128.8, 129.0 (m-CH of Ph2CHO),
129.2, 129.4 (o-CH of Ph2CHO), 134.8 (o-CH of [BArf4] ), 140.2, 140.4
(ipso-C of Ph2CHO), 161.8 (q, 1JCB = 50 Hz, ipso-C of [BArf4] ), 182.1
(C of N=CMe2), 212.7, 213.8 ppm (CO); 11B NMR (96 MHz, CD2Cl2):
d = 7.6 ([BArf4] ), 64.0 ppm (br, fwhm ca. 450 Hz, borylene);
19
F NMR (283 MHz, CD2Cl2): d = 62.7 ppm (CF3); IR (CD2Cl2):
ñ(CO) = 2012, 1969 cm 1; MS (CI): m/z (%): 371.1 (8)
[M N(iPr)CMe2]+. A reproducible elemental analysis for 4 could
not be obtained, possibly owing to the presence of CH2Cl2 solvent in
the crystal lattice. Crystallographic data for 4·CH2Cl2 :
C59H43B2Cl2F24FeNO3, Mr = 1418.3, triclinic, P1̄, a = 12.7448(9), b =
19.0873(14), c = 25.3695(19) H, a = 86.511(2), b = 85.645(2), g =
85.320(2)8, V = 6123.9(8) H3, Z = 4, 1calcd = 1.538 Mg m 3, T =
120(2) K, synchrotron radiation, l = 0.6712 H, 50 972 reflections
collected, which were used in all calculations, R1 = 0.0623, wR2 =
0.1607 (F 2 > 2s(F 2)), R1 = 0.0838, wR2 = 0.1797 (all data), max./min.
residual electron density 0.91/ 1.03 e H 3. CCDC-296849 (4·CH2Cl2)
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
5: The imine iPrN=CMe2 (0.039 mL, 1 equiv) was added to a
solution of 1 (0.338 g, 0.294 mmol) in CH2Cl2 at 78 8C, and the
reaction mixture was warmed to room temperature over 20 min. At
this point, the reaction was judged to be complete by 11B NMR
spectroscopy. Filtration of the reaction mixture, followed by layering
with hexanes and storage at 30 8C led to the formation of 5 as pale
yellow crystals suitable for X-ray diffraction. Isolated yield: 0.110 g,
30 %. 1H NMR (400 MHz, CD2Cl2): d = 1.26 (d, J = 7.0 Hz, 3 H, CH3
of iPr), 1.31 (d, J = 7.1 Hz, 3 H, CH3 of iPr), 1.35 (d, J = 7.2 Hz, 3 H,
CH3 of iPr), 1.38 (d, J = 7.2 Hz, 3 H, CH3 of iPr), 1.56 (d, J = 6.7 Hz,
3 H, CH3 of iPr), 1.58 (d, J = 6.6 Hz, 3 H, CH3 of iPr), 2.23 (s, 3 H, CH3
of N=CMe2), 2.47 (s, 3 H, CH3 of N=CMe2), 3.66 (sept, J = 7.0 Hz, 1 H,
CH of iPr), 4.00 (sept, J = 6.9 Hz, 1 H, CH of iPr), 4.52 (sept, J =
7.1 Hz, 1 H, CH of iPr), 4.91 (s, 5 H, Cp), 7.51 (s, 4 H, p-CH of
[BArf4] ), 7.66 ppm (s, 8 H, o-CH of [BArf4] ); 13C NMR (76 MHz,
CD2Cl2): d = 21.3, 21.5, 24.4, 24.7, 25.1, 25.8 (CH3 of iPr), 24.3, 28.5
(CH3 of N=CMe2), 51.6, 55.1, 55.4 (CH of iPr), 85.0 (Cp), 117.6 (p-CH
of [BArf4] ), 124.7 (q, 1JCF = 272 Hz, CF3 of [BArf4] ), 128.9 (q, 2JCF =
31 Hz, m-C of [BArf4] ), 134.9 (o-CH of [BArf4] ), 161.8 (q, 1JCB =
50 Hz, ipso-C of [BArf4] ), 183.8 (C of N=CMe2), 222.4 ppm (CO);
11
B NMR (96 MHz, CD2Cl2): d = 7.6 ([BArf4] ), 53.7 ppm (borylene); 19F NMR (283 MHz, CD2Cl2): d = 62.7 ppm (CF3); IR
(CD2Cl2): ñ(CO) = 2007, 1951 cm 1; elemental analysis (%) calcd: C
48.99, H 3.55, N 2.24; found: C 48.51, H 3.39, N 2.17. Crystallographic
data for 5 included in Supporting Information.
.
Keywords: boron · borylene ligands · hydrogen transfer · imines ·
iron
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Received: February 23, 2006
Published online: April 24, 2006
3596
www.angewandte.de
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 3593 –3596
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borylenes, meerweinцponndorf, interconversion, transfer, amin, terminal, unprecedented, hydride, complexes, alkoxy, cationic, borylene
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