Borylene metathesis through [2+2] cycloaddition

Article abstract of DOI:10.1002/anie.200702982

(Chemical Equation Presented) From Mn=B to Mn=C: The first example of a concerted borylene metathesis through a clean cycloaddition-cycloreversion sequence is established, emphasizing the decisive influence of boron-bound substituents, and alkyl groups in particular, on the reactivity of borylene complexes. Cyclic reaction intermediates were characterized by X-ray diffraction analysis.

Full text of DOI:10.1002/anie.200702982

Angewandte
Chemie
DOI: 10.1002/anie.200702982
Borylene Complexes
Borylene Metathesis through [2+2] Cycloaddition**
Holger Braunschweig,* Michael Burzler, Krzysztof Radacki, and Fabian Seeler
A variety of synthetic approaches to neutral terminal
=
borylene complexes [LxM BR] (R = N(SiMe3),Si(SiMe 3))
has been developed over the past years on the basis of salt
elimination[1] or transmetalation reactions.[2] Recently,we
also demonstrated that homo-[3] and heterodinuclear[4]
bridged borylene species are susceptible to cleavage reac-
tions,thus yielding in the former case the first terminal alkyl
5
=
borylene species [(h -C5H5)(OC)2Mn BtBu] (1). In addition,
Aldridge et al. have established a versatile route to cationic
goes cycloreversion with clean formation of the metathesis
products.
5
F
= =
iron species such as [(h -C5H5)(OC)2Fe B NR2][BAr ] (2a:
4
R = iPr; 2b: R = Cy; ArF = 3,5-(CF3)2C6H3,Cy = cyclohex-
yl),[5] which was subsequently extended to a corresponding
platinum borylene complex.[6] In contrast to the wide
spectrum of syntheses now available,reactivity patterns,
particularly for neutral complexes,are still restricted to
borylene transfer reactions[7] and the addition of metal
bases.[8] Given the fundamental importance of metal com-
Treatment of [(h -C5H5)(OC)2Mn BtBu] (1) with equi-
molar amounts of benzophenone or DCC immediately
furnished the cyclic products [(h5-C5H5)(CO)2Mn{B(tBu)OC-
(Ph2)}] (3) and [(h5-C5H5)(CO)2Mn{B(tBu)N(Cy)C(NCy)}]
(4),which were isolated in about 70% yield as bright yellow
(3) or colorless (4) crystals upon cooling of the reaction
mixture to À358C [Eq. (1)]. The spectroscopic data are in
agreement with the formulation of 3 and 4 as the products of a
[2+2] cycloaddition. In particular,the resonances in the
11B NMR spectra at d = 72 (3) and 62 ppm (4) are shifted
5
=
plexes [LxM ERn] in metathesis reactions[9] on the one hand,
=
and the presence of a polar metal–borylene linkage with
considerable multiple-bond character in species of the type
[10]
=
[LxM BR] on the other,
the latter species should be
particularly well-suited precursors for borylene metathesis
chemistry. Aldridge et al. have already reported on the
reactions of 2a and 2b with polar substrates such as EE’Phn
(E = O,E ’ = C, n = 2; E = S,E ’ = P, n = 3) and dicyclohex-
ylcarbodiimide (DCC),[11,12] which,however,do not proceed
by concerted [2+2] cycloaddition but in a stepwise fashion
with initial attack of the Lewis basic oxygen and nitrogen
centers,respectively,at the cationic boron center,thus
5
F
4
=
yielding adducts [(h -C5H5)(OC)2Fe B(NR2)(L)][BAr ]
(L = OCPh2,DCC).
[11]
Moreover,substrates such as benzophenone
and
DCC[12] lead to subsequent rearrangement of the initially
formed adducts,while only in the case of SPPh were the
3
products of formal metathesis,[( h5-C5H5)(OC)2Fe(PPh3)]-
[BArF ] and [(R2N)BS]2,observed. [5a]
4
Herein,we report the first example of a concerted
borylene metathesis reaction that proceeds via a structurally
characterized cyclic intermediate that subsequently under-
significantly upfield from that of 1 (d = 144 ppm),thus
À
À
indicating the formation of B O and B N bonds,respectively.
The constitution of both products was confirmed by X-ray
diffraction studies on suitable single crystals.[13] The com-
¯
pounds crystallize in the space groups Pna21 (3) and P1 (4);
[*] Prof. Dr. H. Braunschweig,M. Burzler,Dr. K. Radacki,Dr. F. Seeler
Institut für Anorganische Chemie
both display a Mn-B-X-C (X = O,N) four-membered ring.
The two planar rings (sum of the internal angles 3:359.88; 4:
359.858) are characterized by acute C3-Mn1-B1 angles (3:
59.70(6)8; 4: 61.75(16)8),while the other three angles adopt
Julius-Maximilians-Universität Würzburg
Am Hubland,97074 Würzburg (Germany)
Fax: (+49)931-888-4623
E-mail: h.braunschweig@mail.uni-wuerzburg.de
À
values close to 1008 (Figure 1). The Mn1 B1 distances of
[**] This work was supported by the Deutsche Forschungsgemeinschaft.
We are grateful to BASF AG for a donation of chemicals. Financial
support from the Fonds der Chemischen Industrie is gratefully
acknowledged.
2.1274(16) (3) and 2.185(5) Š (4) are much longer than that in
1 (1.809(9) Š) and resemble those of Mn boryl complexes.[14]
These separations thus not only account for the increased
coordination number of the boron centers but also indicate a
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
À
À
significant loss of Mn B multiple-bond character. The Mn1
Angew. Chem. Int. Ed. 2007, 46, 8071 –8073
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8071

Communications
Experimental Section
All manipulations were conducted under an argon atmosphere either
employing standard Schlenk techniques or in a glovebox.
3: Benzophenone (0.018 g,0.082 mmol) was added to a solution
of 1 (0.020 g,0.082 mmol) in toluene (2 mL). The color of the solution
immediately changed from pale to bright yellow. Storage at À358C
afforded yellow crystals (0.025 g,66% yield).
1H NMR (500 MHz,CDCl , À308C): d = 7.9–6.8 (m,10H,Ph),
3
4.29 (s,5H,C 5H5),1.34 ppm (s,9H,
tBu); 13C NMR (126 MHz,
CDCl3, À308C): d = 231.2,225.4 (s,CO),162.0 (Mn-C) 151.1,149.7 (s,
ipso-C),130.0,128.4,127.6,127.1,126.5,125.7,124.8,124.0,122.9,
120.6 (s,Ph),87.5 (s,C 5H5),28.5 ppm (s, tBu); 11B NMR (160 MHz,
CDCl3, À308C): d = 72 ppm (s,broad); IR (benzene): n˜ = 1988,
1906 cmÀ1; elemental analysis calcd (%) for C24H24BMnO3: C 67.63,
H 5.67; found: C 67.12,H 5.95.
Figure 1. Molecular Structure of 3 (left) and 4 (right). Thermal
ellipsoids are set at the 50% probability level; H atoms have been
omitted for clarity. Selected bond lengths [Š] and angles [8] for 3: Mn1-
B1 2.1274(16),B1-O3 1.3478(19),O–C3 1.4655(15),Mn1–C3
2.1791(13); B1-Mn1-C3 59.70(6),O3-C3-Mn1 97.22(8),B1-O3-C3
99.20(10),O3-B1-Mn1 103.67(10); selected bond lengths [Š] and
angles [8] for 4: Mn1-B1 2.185(5),Mn1-C3 2.056(4); C3-Mn1-B1
61.75(16),N1-C3-Mn1 101.2(2),B1-N1-C3 101.2(3),N1-B1-Mn1
95.7(3).
4: Dicyclohexylcarbodiimide (0.034 g,0.164 mmol) was added to
a solution of 1 (0.040 g,0.164 mmol) in pentane (3 mL). Storage at
À358C afforded colorless crystals (0.053 g,72%).
1H NMR (500 MHz,C 6C6,25 8C): d = 4.29 (s,5H,C 5H5),3.48 (m,
1H,Cy),3.20 (m,1H,Cy),1.26 (s,9H,
tBu),2.10–1.05 ppm (m,20H,
Cy); 13C NMR (126 MHz,C 6C6,25 8C): d = 231.1,224.3 (s,CO),151.1
(s,Mn-C),85.8 (s,C 5H5),63.3,63.0 (s,CyN),35.7,35.2,32.8,32.3 (s,
Cy),30.0 (s, tBu),29.6,27.6,27.2,26.6,25.8,25.5 ppm (s,Cy);
11B NMR (160 MHz,C 6C6,25 8C): d = 62 ppm (broad); IR (benzene):
À
C3 distance in 3 (2.1791(13) Š) is in agreement with the Mn
C bond length in [(OC)5Mn(h1-C13H9)] (2.2472(15) Š),[15]
while the decreased coordination number and the sp2 hybrid-
ization of the corresponding carbon atom in 4 impose a
À1
n˜ = 1970,1890 cm
;
elemental analysis calcd (%) for
C24H36BMnN2O2: C 64.01,H 8.06,N 6.22; found: C 63.58,H 7.92,
N 6.44.
Metathesis: Complex 3 (0.015 g,0.035 mmol) was dissolved in
C6D6 (0.4 mL). Within 15 min,the bright yellow solution darkened.
After 6 h the reaction was complete,and NMR spectroscopy revealed
the exclusive formation of 5 (1H NMR: d = 4.37 ppm (s,5H,C 5H5);
À
shorter Mn1 C3 separation of only 2.056(4) Š.
Complex 3 turned out to be extremely sensitive towards
air and moisture but could be stored in the solid state under
argon at À358C for several weeks without decomposition. In
solution at ambient temperature,however,the complex
13
1
=
C NMR: d = 353 ppm (s,Mn C)) and 6 ( H NMR: d = 1.07 ppm (s,
27H, tBu); 11B NMR: d = 33 ppm).
readily undergoes a cycloreversion within 6 h with formation
5
Received: July 4,2007
Published online: September 5,2007
=
of the manganese carbene complex [(h -C5H5)(OC)2Mn
CPh2] (5) and tri-tert-butylboroxine (6),thus completing the
overall metathesis reaction [Eq. (2)].
Keywords: boron · borylene complexes · manganese ·
metathesis
.
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