Reactions at the boron-carbon double bond of methyl(methylidene)boranes

Article abstract of DOI:10.1002/zaac.200300396

The addition of Lewis bases L to the methyl(methylidene)-boranes MeB=CA₂(A = SiMe₃) and MeB=CAA′ (A′ = SiMe₂Cl) gives the adducts MeB(L)=CA₂ (1a, b; L = trimethylpyridine, PMe₃) and MeB(L)=CAA′ (1c, d; L = di- and trimethylpyridine), respectively. Alcohols and amines HX are added to the BC double bond to give boranes MeB(X)-CHA₂ (8a-c; X = OiPr, OtBu, NiPr₂); MeB=CAA′ and HNMe₂ react in the ratio 1:2, yielding MeB(X)-CHA(SiMe₂X) (2d; X = NMe₂). From MeB=CA₂ and BH₃, the five-membered ring [-CA ₂-BH-CA₂-BMe(Hm)₂BMe-] (2e; 2:1) or the six-membered ring [-CA₂-BH(Hμ)₂BMe-CA ₂-BH(Hμ)₂BMe-] (2f; 1:1) are formed, both containing double hydrogen bridges; the product 2f crystallizes in the space group P1. The metallocene trihydrides [Cp₂MH₃] add to the BC double bond under formation of a double hydrogen bridge to give [Me(A ₂CH)B(Hμ)₂MCp₂] (2g, h; M = Nb, Ta). MeB=CA₂ can be chloroborated, -stannated, and -phosphated with E-Cl to yield the boryldisilylmethanes MeB(Cl)-CA₂-E (2i-1; E = EtClB, tBuClB, Me₂ClSn, Cl₂P). The alkyloboration and -alumination with E-R leads to the boryldisilylmethanes MeBR-SiA₂-E (2m-o; E-R = Me₂B-Me, Et₂B-Et, Cl₂Al-Et) and the bromination to MeB(Br)-CA₂Br. (2+2) Cycloadditions are achieved, when MeB=CA₂ is reacted with unsaturated molecules a=b, yielding four-membered rings [-BMe-CA₂-b-a-] [4a-d; a=b = fluorenone, bis(methoxycarbonyl)ethyne (reacting at both of the C=O bonds), phenylisocyanate (reacting at the C=O bond), N-isopropylacetoneimine], or with triple bond systems RC≡Z, yielding four-membered rings [-BMe-CA₂-Z-CR-] (4e-g; RC≡Z = PhC≡CPh, AC≡CCl, tBuC≡P). With a series of six molecules with an element-oxygen double bond, a primary (2+2) cycloaddition is followed by a metathetical splitting of the transient four-membered rings 4h-m. One of the metathesis products is MeB≡O, which is identified as boroxene (MeBO)₃, the other component is an alkene RR′C=CA ₂ [starting from MeB=CA₂ and PhCHO, PhC(Me)O] or an alkene RR′C=CAA′ (starting from MeB=CAA′ and PhCHO, tBuCHO) or the methylidene phosphorane Ph₃P=CA₂ (starting from MeB= CA₂ and Ph₃PO) or the dicarbadicobaltatetrahedrane [(CA)₂{Co(CO)₃}2] {starting from MeB=CA₂ and [Co₂(CO)₈]}. The (2+3) cyclodaddition of MeB=CA ₂ to the azide X₂PN₃ (X = NiPr₂) as 1,3-dipole gives the five-membered ring [=BMe-CA₂-N=N-NX=] (5a) and to RN₃ the rings [=BMe-CA=N-NA-NR=] (5′b, c; R = iBu, A; formed from the cycloadducts 5b, c by migration of A); analogously, [=BMe-CA′=N-NA-NA=] (5′d) is formed from MeB=CAA′ and AN ₃. Finally, the nitrone O-NMe=CHPh and MeB=CA₂ or MeB=CAA′ give the corresponding (2+3) cycloadducts 5e, f, respectively. All of the products were characterized by their ¹H, ¹¹B, and ¹³C NMR spectra.

Full text of DOI:10.1002/zaac.200300396

Reactions at the Boron-Carbon Double Bond of Methyl(methylidene)boranes
Peter Paetzold*, Ulli Englert, Rudolf Finger, Thomas Schmitz, Alexander Tapper, and Ralf Ziembinski
Aachen, Institut für Anorganische Chemie der Technischen Hochschule
Received December 4th, 2003.
Abstract. The addition of Lewis bases L to the methyl(methylidene)-
boranes MeBϭCA2 (A ϭ SiMe3) and MeBϭCAAЈ (AЈ ϭ SiMe2Cl)
gives the adducts MeB(L)ϭCA2 (1a, b; L ϭ trimethylpyridine,
PMe3) and MeB(L)ϭCAAЈ (1c, d; L ϭ di- and trimethylpyridine),
respectively. Alcohols and amines HX are added to the BC double
ate (reacting at the CϭO bond), N-isopropylacetoneimine], or with
triple bond systems RCϵZ, yielding four-membered rings
[ϪBMeϪCA2ϪZϭCRϪ] (4eϪg; RCϵZ ϭ PhCϵCPh, ACϵCCl,
tBuCϵP). With a series of six molecules with an element-oxygen
double bond, a primary (2ϩ2) cycloaddition is followed by a meta-
thetical splitting of the transient four-membered rings 4hϪm. One
of the metathesis products is MeBϵO, which is identified as borox-
ene (MeBO)3, the other component is an alkene RRЈCϭCA2 [start-
ing from MeBϭCA2 and PhCHO, PhC(Me)O] or an alkene
RRЈCϭCAAЈ (starting from MeBϭCAAЈ and PhCHO, tBuCHO)
or the methylidene phosphorane Ph3PϭCA2 (starting from MeBϭ
CA2 and Ph3PO) or the dicarbadicobaltatetrahedrane
[(CA)2{Co(CO)3}2] {starting from MeBϭCA2 and [Co2(CO)8]}.
The (2ϩ3) cyclodaddition of MeBϭCA2 to the azide X2PN3 (X ϭ
bond to give boranes MeB(X)ϪCHA2 (8aϪc;
OtBu, NiPr2); MeBϭCAAЈ and HNMe2 react in the ratio 1:2,
yielding MeB(X)ϪCHA(SiMe2X) (2d; NMe2). From
MeBϭCA2 and BH3, the five-membered ring
X ϭ OiPr,
X
ϭ
[ϪCA2ϪBHϪCA2ϪBMe(Hm)2BMeϪ] (2e; 2:1) or the six-mem-
bered ring [ϪCA2ϪBH(Hµ)2BMeϪCA2ϪBH(Hµ)2BMeϪ] (2f; 1:1)
are formed, both containing double hydrogen bridges; the product
¯
2f crystallizes in the space group P1. The metallocene trihydrides
[Cp2MH3] add to the BC double bond under formation of a double
hydrogen bridge to give [Me(A2CH)B(Hµ)2MCp2] (2g, h; M ϭ Nb,
Ta). MeBϭCA2 can be chloroborated, -stannated, and -phosphated
with EϪCl to yield the boryldisilylmethanes MeB(Cl)ϪCA2ϪE
(2iϪl; E ϭ EtClB, tBuClB, Me2ClSn, Cl2P). The alkyloboration
and -alumination with EϪR leads to the boryldisilylmethanes
NiPr2)
as
1,3-dipole
gives
the
five-membered
ring
[ϭBMeϪCA2ϪNϭNϪNXϭ] (5a) and to RN3 the rings
[ϭBMeϪCAϭNϪNAϪNRϭ] (5Јb, c; R ϭ iBu, A; formed from
the cycloadducts 5b,
c by migration of A); analogously,
[ϭBMeϪCAЈϭNϪNAϪNAϭ] (5Јd) is formed from MeBϭCAAЈ
and AN3. Finally, the nitrone OϪNMeϭCHPh and MeBϭCA2 or
MeBϭCAAЈ give the corresponding (2ϩ3) cycloadducts 5e, f,
respectively. All of the products were characterized by their 1H, 11B,
and 13C NMR spectra.
MeBRϪSiA2ϪE (2mϪo; EϪR
ϭ
Me2BϪMe, Et2BϪEt,
Cl2AlϪEt) and the bromination to MeB(Br)ϪCA2Br. (2ϩ2) Cyclo-
additions are achieved, when MeBϭCA2 is reacted with unsatu-
rated
molecules
aϭb,
yielding
four-membered
rings
[ϪBMeϪCA2ϪbϪaϪ] [4aϪd; aϭb ϭ fluorenone, bis(methoxycar-
bonyl)ethyne (reacting at both of the CϭO bonds), phenylisocyan-
Keywords: Boron carbon double bond; (2ϩ2) Cycloaddition;
Metathesis; (2ϩ3) Cycloaddition
Reaktionen an der Bor-Kohlenstoff-Doppelbindung von Methyl(methyliden)boranen
Inhaltsübersicht. Die Addition von Lewisbasen L an die Methyl-
(methyliden)borane MeBϭCA2 (A ϭ SiMe3) und MeBϭCAAЈ
(AЈ ϭ SiMe2Cl) führt zu den Addukten MeB(L)ϭCA2 (1a, b; L ϭ
Trimethylpyridin, PMe3) bzw. MeB(L)ϭCAAЈ (1c,d; L ϭ Di- und
Trimethylpyridin). Alkohole und Amine HX addieren sich an die
BC-Doppelbindung unter Bildung von MeB(X)ϪCHA2 (2a-c; X ϭ
OiPr, OtBu, NiPr2); MeBϭCAAЈ und HNMe2 reagieren im Ver-
hältnis 1:2 zu MeB(X)ϪCHA(SiMe2X) (2d; X ϭ NMe2). Aus
MeBϭCA2 und BH3 bilden sich der fünfgliedrige Ring
[ϪCA2ϪBHϪCA2ϪBMe(Hµ)2BMeϪ] (2e; 1:2) oder der sechsglie-
drige Ring [ϪCA2ϪBH(Hµ)2BMeϪCA2ϪBH(Hµ)2BMeϪ] (2f;
1:1), die beide Wasserstoff-Doppelbrücken enthalten; der Sechsring
aϭb ϭ Fluorenon, Bis(methoxycarbonyl)ethin (an beide CϭO-
Bindungen wird addiert), Phenylisocyanat (an die CϭO-Bindung
wird addiert), N-Isopropylacetonimin] oder mit Dreifachbindungs-
systemen RCϵZ zu Vierringen [ϪBMeϪCA2ϪZϭCRϪ] reagiert
(4eϪg; RCϵZ ϭ PhCϵCPh, ACϵCCl, tBuCϵP). Mit sechs Ver-
bindungen mit einer Element-Sauerstoff-Doppelbindung folgt auf
eine primäre (2ϩ2)-Cycloaddition eine metathetische Ringspal-
tung. Eines der Metatheseprodukte ist MeBϵO, das als Boroxin
(MeBO)3 nachgewiesen wird, das andere ist ein Alken RRЈCϭCA2
(aus MeBϭCA2 und PhCHO, PhMeCO) oder ein Alken RRЈCϭ
CAAЈ (aus MeBϭCAAЈ und PhCHO, tBuCHO) oder das Methyli-
denphosphoran Ph3PϭCA2 (aus MeBϭCA2 und Ph3PO) oder das
Dicarbadicobaltatetrahedran [(CA)2{Co(CO)3}2] {aus MeBϭCA2
und [Co2(CO)8]}. Die (2ϩ3)-Cycloaddition von MeBϭCA2 an das
Azid X2PN3 (X ϭ NiPr2) als 1,3-Dipol ergibt den Fünfring
[ϭBMeϪCA2ϪNϭNϪNXϭ] (5a) und an das Azid AN3 die Fünf-
ringe [ϭBMeϪCAϭNϪNAϪNRϭ] (5Јb, c; R ϭ iBu, A; hervorge-
gangen aus den Cycloaddukten 5b, c durch Wanderung von A);
entsprechend bildet sich [ϭBMeϪCAЈϭNϪNAϪNAϭ] aus
MeBϭCAAЈ und AN3. Schließlich entstehen aus MeBϭCA2 bzw.
MeBBϭCAAЈ und dem Nitron OϪNMeϭCHPh die (2ϩ3)-Cy-
cloaddukte 5e, f. Alle Produkte wurden mit Hilfe ihrer 1H-, 11B-
und 13C-NMR-Spektren charakterisiert.
¯
2f kristallisiert in der Raumgruppe P1. Die Metallocentrihydride
[Cp2MH3] addieren sich an die BC-Doppelbindung unter Bildung
einer Wasserstoff-Doppelbrücke zu [Me(A2CH)B(Hµ)2MCp2]
(2g, h; M ϭ Hf, Ta). MeBϭCA2 kann mit EϪCl zu Boryldisilylme-
thanen MeB(Cl)ϪCA2ϪE chloroboriert, -stanniert und -phospha-
niert werden (2iϪl; E ϭ EtClB, tBuClB, Me2ClSn, Cl2P). Die Al-
kyloborierung und -aluminierung mit EϪR ergibt die Boryldisilyl-
methane MeB(R)ϪCA2ϪE (2mϪo; EϪR ϭ Me2BϪMe, Et2BϪEt,
Cl2AlϪEt) und die Bromierung MeB(Br)ϪCA2Br (2p). (2ϩ2)-
Cycloadditionen beobachtet man, wenn MeBϭCA2 mit ungesättig-
ten Molekülen aϭb zu Vierringen [ϪBMeϪCA2ϪbϪaϪ] [4aϪd;
508
 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
DOI: 10.1002/zaac.200300396
Z. Anorg. Allg. Chem. 2004, 630, 508Ϫ518

Reactions at the Boron-Carbon Double Bond of Methyl(methylidene)boranes
Introduction
In akylideneboranes XBϭCR2, the two-coordinated boron
atom may obtain an electron octet by forming a coordin-
ative XB-π bond, when a lone pair at the ligand X is avail-
able, e.g. when X represents an amino group [1, 2]. Other-
wise, non-classical structures are formed. Describing bonds
qualitatively by localized orbitals, the term “non-classical”
means, that (3c2e) bonds are required in addition to (2c2e)
bonds in order to supply boron with an electron octet. A
broad variety of corresponding carbon-boron species had
been reviewed by Berndt [3]. The short-lived parent mol-
ecule HBϭCH2 could be characterized in an argon matrix
by infrared spetroscopy [4]. We had reported on a relatively
simple derivative, MeϪBϭCA2 (A ϭ SiMe3) [5], which can
be isolated and characterized before slowly cyclodimerizing
at room temperature. The same is true for the derivatives
MeϪBϭCAAЈ (AЈ ϭ SiMe2Cl) and MeϪBϭCAAЉ (AЉ ϭ
SiMe2Br), which are formed from HalϪBϭCA2 by a rapid
exchange of Hal and Me along the BϪCϪSi skeleton
(Hal ϭ Cl, Br) [6]. The stabilizing effect of the silyl groups
was explaind by the ability of the CSi-(2c2e) bond to be
expanded to a BCSi-(3c2c) bond, thus providing boron with
an electron octet [3].
It has become usual to test the reactivity of multiply
bonded semimetal species by a series of reactions: the
addition of Lewis bases to the unsaturated semimetal; the
addition of unpolar or polar species XЈϪX to the BϭC
bond; the application of the BϭC bond as a dipole in cyclo-
additions with 1,1 unsaturated systems a [(2ϩ1) cycload-
ditions, e.g., when a is a carbene], with 1,2 unsaturated sys-
Addition of Lewis bases
The presumed SiϪC-σ electron donation towards a BSiC-
3c2e bond is weak enough to be easily replaced by the lone
pair of methylpyridines as Lewis bases and even of the less
basic trimethylphosphane [Eqs. (1a), (1b)]. The B atoms in
the pyridine adducts are distinctly more shielded than in
the starting boranes, giving a high-field shift of the NMR
signals from δ ϭ 69.5, 70.8 (MeBϭCA2, MeBϭCAAЈ) to
δ ϭ 41Ϫ42 (Table 1). The rotation around the BC double
1
bond axis is restricted, making two different H NMR sig-
nals for the protons of the non-equivalent SiMe3 groups
observable. Coalescence temperatures of 43 and 52 °C for
the adducts of di- [5] and trimethylpyridine (1a), respec-
tively, to MeBϭCA2 correspond to a rotation barrier of
about 70 kJ/mol, comparable to the barrier around the
CϪN bond in carboxamides. The two different silyl groups
of MeB(L)ϭCAAЈ cause isomers, which are found in the
tems aϭb [(2ϩ2) cycloadditions], or with 1,3 unsaturated
.. ..
systems a
b
c [(2ϩ3) cycloadditions]. A generalized and
partly hypothetical application of these five types of reac-
tions to the methyl(methylidene)borane MeBϭCR2 is
shown in Eqs. (1) to (5) in the following scheme.
1
ratio 58:42 (1c) and 57:43 (1d), according to the H NMR
A few reactions of the boranes MeBϭCA2, MeBϭCAAЈ,
or MeBϭCAAЉ have already been investigated: the stabiliz-
ation by cyclodimerization [a reaction of type (4)] [5, 6]; the
addition of the Lewis base 2,6-dimethypyridine [a reaction
of type (1)] [5], the addition of the silylene SiMe2 [a reaction
of type (3); the primary silaboracyclopropane spon-
taneously adds a second molecule MeBϭCA2 to give the
corresponding 1-sila-3,4-diboracyclopentane] [7]; the ad-
dition of the ketone Ph2CO or the iminoborane
tBuBϵNtBu [reactions of type (4)] [5]. We report here on
the reactions of MeBϭCA2 and MeBϭCAAЈ with Lewis
bases, with molecules XЈϪX, with unsaturated molecules,
containing double and triple bonds, and with 1,3 dipolar
agents, according to Eqs. (1), (2), (4), and (5).
intensities. The adduct 1b, however, gives only one SiMe3
NMR signal. A broadening of this signal below Ϫ40 °C
indicates that the rotation barrier is rather small. Besides
the rotation around the BϭC bond axis, however, a rapid
intermolecular exchange of PMe3 molecules, similar to the
intermolecular exchange of PMe3 found for BMe3(PMe3)
[8], could give rise to the pseudo-equivalency of the SiMe3
groups of 1b.
All of the solid adducts of methylated pyridines to MeBϭ
CA2 and MeBϭCAAЈ exhibit an orange colour. The elec-
tronic situation around the boron atom is similar to that in
the crystalline red adducts of pyridine to borabenzene,
BC5H5, or 2-boranaphthalene, BC9H7 [9]: The nitrogen ad-
jacent to boron is incorporated into an aromatic and boron
either into an aromatic or into an “olefinic” π-electron
system. The red colour of the boraarene adducts had been
attributed to a charge-transfer from a HOMO, which is lo-
calized at the boraarene π-electron orbitals, to a LUMO,
localized at the pyridine π-orbital system. Accordingly, the
orange colour of the methylpyridine adducts of type 2 is
* Prof. Dr. P. Paetzold
Institut für Anorganische Chemie
Technische Hochschule Aachen
D-52056 Aachen, Germany
E-mail: peter.paetzold@ac.rwth-aachen. de
Z. Anorg. Allg. Chem. 2004, 630, 508Ϫ518
zaac.wiley-vch.de
 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
509

P. Paetzold, U. Englert, R. Finger, T. Schmitz, A. Tapper, R. Ziembinski
The structure of the products was identified by character-
1
1
istic H, 11B, and 13C NMR shifts, the intensity of the H
NMR signals and the expected NMR coupling schemes
(Table 2). One set of signals was observed for the alkoxy-
boranes 2a, b. The aminoborane 2c exhibited H and 13C
1
NMR signals of two different iPr groups at room tempera-
ture in the ratio 1:1, indicating a restriction of the rotation
around the BN double bond. We interpret two 13C NMR
signal sets in the ratio 85:15 at Ϫ30 °C by adopting con-
formers with respect to the central BϪC axis, the rotation
around which is restricted for sterical reason, i.e. an overlap
of the cross-sections of one of the isopropyl and the silyl
groups. The rotation around the BN double bond of 2d is
also restricted, as indicated by two 1H NMR signals for one
of the NMe2 groups. The two diastereotopic silicon-bound
Me groups of SiMe2(NMe2) give different signals in the 13
NMR spectrum.
C
probably due to charge-transfer from the BϭC-π electrons
to the pyridine LUMO.
Addition of Hydridic Agents
The borane MeBϭCA2 reacted with BH3(thf) in a 2:1 ratio,
according to Eq. (2d), when a 1:1 ratio was applied and
the solution quickly warmed to room temperature. Two BH
bonds of one molecule BH3 are added to two molecules
MeBϭCA2, according to the polarity of the correcponding
bonds: hydride adds to boron and the boron of BH3 to
carbon, thus forming a BϪCϪBϪCϪB chain, the boryl
ends of which then bind to one another by two BHB
bridges to give 2e. A comparable product (2eЈ) had been
formed by the addition of BH3(thf) to the diazadiboretane
(RBNtBu)2, according to Eq. (2dЈ) [11]. Typical for the
structure of 2e are two IR bands at ν ϭ 2455 and
1533 cmϪ1, representing vibrations of the terminal and the
bridging hydrogen atoms. Two 11B NMR signals in the ratio
2:1 are found at characteristic values, δ ϭ 27.3 and 83.4
[12].
Addition of Molecules XЈ؊X
Addition of Protic Agents
The addition of the alcohols iPrOH and tBuOH and of the
amine iPr2NH to MeBϭCA2 proceeds in a straight-forward
way, according to eqs. (2a) and (2b), respectively. The ad-
dition of tBuOH is accompanied by the formation of the
diboryl oxide [Me(CHA2)B]2O, formed presumably from
Me(HA2C)B(OH) and unreacted MeBϭCA2, after the
elimination of isobutene from 2b. The aminoborane 2c had
been previously synthesized from the amino(methylene)bor-
ane iPr2NϭBϭCA2 by first adding LiMe to the BC double
bond and subsequent protolysis with HCl [10]. Upon reac-
tion of Me2NH with MeBϭCAAЈ, the amine is added to
the BC double bond, and the SiCl bond simultaneously un-
dergoes aminolysis, giving the aminoborane 2d [Eq.(2c)].
510
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Reactions at the Boron-Carbon Double Bond of Methyl(methylidene)boranes
Applying a 1:2 ratio of MeBϭCA2 and BH3(thf) and
conducting the reaction at Ϫ78° yielded the 1:1 product 2f,
according to Eq. (2e). The structure of 2f follows from the
followed by a second hydroboration step or a head/tail
cyclodimerization, respectively. With niobocenium- or tan-
talocenium trihydride, [Cp2MH3], the orientation of the ad-
dition to the BC double bond of MeBϭCMe2 is reversed:
a proton is added to carbon, and the metal (Nb or Ta)
is added to boron via two MHB (3c2e) bonds [Eq. (2f)].
Metal(V) is formally reduced by HϪ (ǞHϩ) during this
process to give metal(III), thus maintaining the 18e shell at
the metal. The bonding situation is not unusual and is well
known, e.g., in [Cp2NbH2BH2] [16]. The NMR spectra of
the products are in accord with a structure, where the
C2BM coordination sphere around boron defines a mirror
plane, that makes the two silyl groups equivalent, whereas
the two η5-Cp groups are situated one cis and the other
trans to the disilylmethyl group. The BH2M plane is ortho-
gonal to the mirror plane, making the bridging H atoms
equivalent. For details of the NMR spectra see Experimen-
tal Section!
1
NMR spectra. The H NMR intensity ratio of 18:3:2:1 is
in accord with a 1:1 reaction of the starting components.
The 1H NMR signal at δ ϭ 0.58 (BMe; 3J ϭ 3 Hz) is related
to δ ϭ 1.97 (µ-H) by a 2D-1H/1H NMR cross-peak. The
1H NMR signal at δ ϭ 3.91 is assigned to the terminal H
atom, BHt; its coupling pattern depends on temperature as
a consequence of the 11B quadrupole relaxation: a quartet
at 90, a doublet at 25 and a singlet at Ϫ10 °C. The coupling
constant is typical for terminal H atoms (1J ϭ 146 Hz). The
two 11B NMR signals are found at δ ϭ 17.5 and 33 in the
expected range, comparable to those of triethyldiborane,
HEtBH2BEt2 (δ ϭ 17.2, 34.3 [12]). A 2D-11B/1H NMR
1
spectrum relates the 11B signal at δ ϭ 17.2 to the H signal
of Ht and Hµ, and the 11B signal at δ ϭ 33 to that of Hµ
only. An average coupling of J ϭ 92 Hz is found for the 11
NMR doublet at δ ϭ 17.2.
B
The molecular structure of 2f was finally confirmed by
a crystal structure analysis (Fig. 1). The centrosymmetric
molecule is built up by a planar BBCBBC-six-membered
ring with ring angles of 121.21(10) (B1), 111.61(10) (C2)
and 127.18(10)° (B2), giving 720.00° as the sum of the six
ring angles. The ring ligands H1 and C2 are situated in the
ring plane; the boron atoms are planarly coodinated. The
plane through B1, B2Ј, H2 and H3 is almost orthogonal to
the ring plane [89.2(7)°]. The bond lengths and angles are
comparable to those in similar molecules with a central
Chloroboration, -stannation, and -phosphation
BH2B
unit,
e.g.,
in
trans-bis(trisyl)diborane(6)
(A3C)HBH2BH(CA3) [13, 14].
The double bond of MeBϭCA2 can be chloroborated with
RBCl2 [Eq. (2g)], chlorostannated with Me2SnCl2 [Eq.
(2h)], and chlorophosphated with PCl3 [Eq. (2i)] to yield
the diborylmethanes 2i, j, the borylstannylborane 2k, and
the borylphosphanylmethane 2l, respectively. Again, the
NMR spectra allow a unique structural identification of the
products 2iϪl (see Table 3 for details). The colourless phos-
phanylborane 2l is extremely sensitive towards moisture. A
coupling of the SiMe and BMe protons with the phos-
phorus nucleus is observable through four bonds. The chlo-
roboration of iminoboranes, RBϵNRЈ, proceeds in an
analogous manner as that of MeBϭCA2 [17].
Fig. 1 Structure of 2f (PLATON [15], 50 % displacement ellip-
soids); selected distances/pm and angles/°:
B1ϪB2Ј 184.65(19), B1ϪC2 158.68(18), C2ϪB2 159.23(18), B1ϪC1
158.66(19), B2ϪH1Ј 105.4(19), C2ϪSi1 190.52(15), C2ϪSi2 190.3(2),
B1ϪH2 132.9(17), B1ϪH3 130.6(17), B2ϪH2Ј 127.7(17), B2ϪH3Ј 123.1(18);
B2ЈϪB1ϪC2 121.21(10), B1ϪC2ϪB2 111.61(10), C2ϪB2ϪBlЈ 127.18(10),
C1ϪB1ϪB2Ј 113.53(10), C1ϪB1ϪC2 125.24(11), C2ϪB2ϪH1Ј 120.9(10),
B1ϪB2ЈϪH1 111.9(10)
The first step in the reactions (2d) and (2e) is certainly a
hydroboration of the BC double bond of MeBϭCA2,
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P. Paetzold, U. Englert, R. Finger, T. Schmitz, A. Tapper, R. Ziembinski
Alkyloboration and -alumination
We report here on the cyclodimerization of MeBϭCA2
with a ketone [9-fluorenone; Eq. (4a)], a carboxylic ester
[MeOOCϪCϵCϪCOOMe; Eq. (4b)], an isocyanate
[PhNCO; Eq. (4c)], and a ketoneimine [Me2CϭNiPr; Eq.
(4d)].
The alkyloboration of MeBϭCA2 with BR3 gives the dibor-
ylmethanes 2m, n [Eq. (2j)]. Whereas the chloroboration is
faster than the alkyloboration upon reacting RBCl2 with
MeBϭCA2, the situation is reversed, when (EtAlCl2)2 is ap-
plied. The structure of the ethyloalumination product 2o
[Eq. (2k)] is concluded from the NMR spectra. The dimeric
structure is in accord with the 27Al NMR signal at δ ϭ 122,
typical for four-coordinate aluminium, and the 11B NMR
signal is found in a range typical for trialkylboranes, not of
alkyldihaloboranes (Table 3). The product 2o is thermolab-
ile.
The characterization of the products 4aϪd by NMR
makes use of routine arguments. Moreover, the chemical
shifts of 4a are very similar to those of the cycloadduct
from MeBϭCA2 and OϭCPh2, which differs from 4a only
by a PhϪPh connection through ortho-C atoms. In the case
of 4b, a 2:1 addition is achieved, even when the components
are applied in a 1:1 ratio. The 13C NMR shift at δ ϭ 92.3
shows that the CC triple bond is still present in 4b. The
polar CϭO and not the unpolar CϵC bond of the starting
component had reacted with the BϭC bond. Whereas single-
bonded molecules EϪX attack isocyanates preferably at the
CN double bond, it is the CO double bond of PhNϭCϭO
that undergoes the cycloaddition (4c). The 11B NMR shift
at δ ϭ 57.3 is found in the expected range. The analogous
oxoniaboratacyclobutene from the addition of MeBϭCA2
and OϭCPh2 gives an 11B NMR shift at δ ϭ 58.7, whereas
the boron nucleus of the azoniaboratacyclobutene 4d with
a nitrogen ring atom resonates at a distinctly higher field-
strength, δ ϭ 45.9. The mass-spectrometric fragmentation
of 4c gives mass peaks for the fragments MϪMeBO (59 %)
and MϪMeBOϪMe (100 %), which clearly show that bo-
ron is bound to oxygen in 4c. Analogously, the mass-spec-
trometric fragmentation of 4d yields the peak of
MϪMeBNPrϪMe (100 %), establishing that the cycload-
dition (4d) and the subsequent fragmentation together are
representing a metathesis reaction.
Addition of Bromine
Bromine is sponaneously added to the BC double bond of
MeBϭCA2 at Ϫ78 °C, according to Eq. (2l). The colourless
solid product 2p can be sublimated. It was characterized by
elemental analysis and its NMR spectra.
We had taken into account a 1,2-dyotropic exchange of
Me and Br along the BϪC bond of 2p to give
Br2B-CMeA2. Reactions of this type can be achieved ther-
mally or also catalyzed by acids, when electronegative
groups are bound to a carbon atom adjacent to boron [18].
Heating 2p in boiling toluene for 18 h, however, did not
give any reaction.
Similar cycloadditions are given by MeBϭCA2 and the
unpolar ethyne PhCϵCPh or the polar ethyne ACϵCCl or
the phosphaethyne tBuCϵP [Eq. (4e)]. Although the for-
mation of 4b had taught us, that the CϭO bond of
MeOOCϪCϵCϪCOOMe is more reactive than the unpo-
lar CϵC bond, diphenylethyne undergoes a quantitative 1:1
cycloaddition with MeBϭCMe2. The product 4e gives the
expected NMR data. The mass-spectrometric fragmen-
tation follows a cycloreversion reverse to the formation (4e),
whereas metathetical fragments could not be detected, in
accord with a still strong double bond in the ring skeleton
of 4e.ϪThe alkyne ACϵCCl adds to MeBϭCA2 in a regio-
[2؉2] Cycloadditions
Amino(methylene)boranes like iPrNϭBϭCA2 had been
subjected to (2ϩ2) cycloadditions with a variety of mol-
ecules with a carbon oxygen double bond like aldehydes,
ketones, and carboxylic acid derivatives [10]. The methyl-
(methylene)boranes MeBϭCA2 and MeBϭCAAЈ had been
subjected to a [2ϩ2] cycloaddition with Ph2CϭO and
MeBϭCA2 also with the iminoborane tBuBϵNtBu, and
their stabilization by cyclodimerization represents also a
[2ϩ2] cycloaddition [5, 6].
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Reactions at the Boron-Carbon Double Bond of Methyl(methylidene)boranes
selective manner, according to the polarities of the BϭC
and CϵC bonds and also according to the sterically prefer-
able way, with the three bulky silyl groups so remote from
each other as possible.ϪThe regioselective addition of
tBuCϵP, documented by only one set of NMR signals, fol-
lows again the polarities of the unsaturated components
(δϩ at boron and phosphorus; δϪ at carbon) and is also in
accord with the more farvourable steric situation. The 13C
NMR signals of the ring and of the BMe carbon atoms are
not observable, even not at Ϫ50 °C, a common situation in
the coordination sphere of 11B nuclei. The three observable
13C NMR signals (SiMe3, tBu) exhibit the expected coup-
lings with the 31P nucleus. The principal structure of the
viscous oily 4g could be confirmed by a crystal structure
analysis, after crystallizing 4g by a miniature zone melting
process with a focussed infrared radiation; we do not pub-
lish the data, however, because some unusually anisotropic
displacement parameters indicate disorder [19].
In analogy to Eqs. (4f), (4g), the oxophosphorane Oϭ
PPh3 splits the BC double bond of MeBϭCA2 by meta-
thesis, according to Eqs. (4h) and (4i). The 11B NMR signal
of δ ϭ 55.2 in the reaction solution indicates the transient
presence of the cycloadduct 4l. A metathetical reaction also
proceeds, when MeBϭCA2 is reacted with [Co2(CO)8]. It is
presumably one of the bridging CO units, to which MeBϭ
CA2 is added to give the intermediate 4m [Eq. (4j)], which
subsequently eliminates MeBO and CO to yield the well-
known dicarbadicobaltatetrahedrane [(CA)2{Co(CO)3}2]
[21].
Aldehydes and ketones like PhCHO, tBuCHO, PhMeCO,
Ph2CO, give [2ϩ2] cycloadducts with iPr2NϭBϭCA2 and
the relatively bulky Ph2CO even with MeBϭCA2 [5, 6]. We
were not able, however, to isolate cyclodadducts from the
methyl(methylene)boranes MeBϭCA2 or MeBϭCAAЈ and
the oxoalkanes PhCHO, tBuCHO, PhMeCO, according to
Eq. (4f), although these cycloadducts have certainly be
formed as the intermediates 4hϪk, detectable in the reac-
tion solution by an 11B NMR signal in the range δ ϭ
57Ϫ60. Instead of 4hϪk, we isolated the corresponding ole-
fines as the products of a metathesis, according to Eq. (4g),
and characterized them as usual by NMR methods. The
oxoborane MeBϵO as the second metathesis product spon-
taneously cyclotrimerizes to methylboroxene, which could be
identified as a volatile material by its characteristic 11B NMR
peak. The possibility of MeBϵO to appear as a short-lived
intermediate is well established in the literature [20].
[2؉3] Cycloadditions
Azides RN3 had been applied as 1,3-dipolar agents in reac-
tions with the BC double bond of the amino(methylene)-
borane iPr2NϭBϭCA2. Instead of a cycloadduct of type 5,
according to Eq. (5a), an azaboriridine (R ϭ Ph, Bzl) or
the corresponding (aminoboryl)diazoalkane (R ϭ A) had
been isolated, both obviously formed by reaction (5b) or
(5c), respectively. A third possibitity would have been the
formation of the amino(imino)borane iPr2NϭBϭNR and
the diazoalkane A2CN2 by cycloreversion of the primary
cycloadduct. A reaction of this type had been observed, at
least as a side-reaction, when an azide had been applied on
the methylenesilane Me2SiϭCA2 [22], but had not taken
place with Pr2NϭBϭCA2 and azides.
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P. Paetzold, U. Englert, R. Finger, T. Schmitz, A. Tapper, R. Ziembinski
reasonable fragmentation of a ring of type 5. A cation of
A2CN2 is not detected in these mass spectra, in contrast to
that of 5a. The same arguments reveal the structure of the
product 5Јd, when AN3 is reacted with Me-BϭCAAЈ. Only
one of the groups A and AЈ migrates from carbon to nitro-
gen, since only one isomer corresponds to the NMR signals.
Presumably, it is the trimethylsilyl group A that migrates,
because the 13C NMR signal of the ring atom at δ ϭ 128.3
is significantly different from those of 5Јb and 5Јc, men-
tioned above; this C atom should be sensitive to the nature
of the directly bound ligand.
We also reacted N-methylbenzaldimine-N-oxide as a 1,3-
dipolar agent with MeBϭCA2 and MeBϭCAAЈ and
characterized the products 5e, f [Eq. (5e)] by their NMR
spectra. The two silyl groups A are not equivalent in 5e.
None of these reactions was observed, when we applied
azides on MeBϭCA2 or MeBϭCAAЈ. With the azidophos-
phane (iPr2N)2PN3, the primary cycloadduct 5a could be
isolated as a thermally unstable solid. The NMR spectra
support the postulated structure of 5a: Two equivalent silyl
groups are present; the 11B NMR peak (δ ϭ 49.7) is found
in a range typical for dialkyl(amino)boranes (δ ϭ 48Ϫ52
[12]); the 13C NMR signal of the ring-C atom (δ ϭ 45.8)
corresponds to a saturated atom. Moreover, the mass spec-
trum gives the ions of A2CN2 and MeBϵN(PX2) (X ϭ
NiPr2) as fragments. We could not identify the decompo-
sition products, that are formed, when 5a is standing for a
week at room temperature.
1
Diastereomers of 5f are identified in the H NMR spetrum
in the ratio 83:17. The 13C nuclei are not sensitive enough
to make differences between the diasteromers observable,
but they are sensitive enough to make the non-equivalence
of the two methyl groups of AЈ observable.
Experimental
NMR spectra: Bruker WP80/SY (lH; 31P), Bruker WH 270 (13C),
Jeol JNM-PS-100 (11B); standards: SiMe4 (intern; 1H, 13C),
BF3(OEt2) (extern; 11B), H3PO4 (extern; 31P); in CDCl3.
Synthesis of MeBϭCA2 and MeBϭCAAЈ: These methyleneboranes
were synthesized by thermolysis of MeB(OMe)ϪCA3 and
ClB(OMe)ϪCA3, respectively, as described in the literature [5, 6].
We had originally obtained a 4:1 mixture of MeBϭCA2 and the
trisilylmethane HCA3 [5]. By starting from perfectly pure
MeB(OMe)ϪCA3 and thoroughly avoiding any moisture during
the thermolysis, we now obtained pure MeBϭCA2 without any by-
product, as judged from the NMR spectra. The amounts of the
two methyleneboranes, which we applied in the following syntheses,
were concluded from the amounts of thermolyzed starting materi-
als, assuming a 100 % thermolysis yield. Condensed at the walls of
a cooled glass tube after passing the thermolysis tube, the methyl-
eneborane dropped into pentane, placed at the bottom of the tube,
on warming the tube from Ϫ196 to Ϫ78 °C.
The situation is different, when the azides iBuN3 or AN3
are reacted with MeBϭCA2. The isolated products, 5Јb and
5Јc, respectively, are obviously formed from the transient
cycloadducts 5b and 5c by the migration of one of the silyl
groups from carbon to nitrogen [Eq. (5d)]. The five-mem-
bered ring is transformed thereby into an aromatic 6π-elec-
tron system, giving a characteristic change of the 11B NMR
signal to higher field [compare δ ϭ 49.7 (5a) to δ ϭ 31.9
(5Јb), 36.3 (5Јc)] and of the 13C NMR ring atom signal to
lower field [compare δ ϭ 45.8 (5a) to δ ϭ 134.4 (5Јb), 133.0
(5Јc)], in accord with the generally known trend. Two and
three non-equivalent silyl groups are identified by the NMR
data of 5Јb and 5Јc, respectively. Moreover, the mass spectra
of 5Јb and 5Јc show an intensive peak for a fragment, which
is formed by the elimination of NCSiMe3; this is not a
Synthesis of the adducts 1aϪd: The base (2 mmol) was added to
the corresponding methyleneborane (2 mmol) in pentane (5 ml) at
Ϫ78 °C. The colourless adduct with PMe3 precipitates immediately;
the adducts with the methylpyridine bases were stirred in pentane
for several hours and then isolated by removing the solvent in va-
cuo. All of the four adducts crystallized from pentane at Ϫ80 °C.
The adduct 1b was extremely sensitive towards moisture. The or-
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Reactions at the Boron-Carbon Double Bond of Methyl(methylidene)boranes
Table 1 1H, 11B and 13C NMR chemical shifts of the adducts of
MeBϭCA2 (1a, b) and MeBϭCAAЈ (1c, d) with the bases 2,4,6-
trimethylpyridine (1a, d), PMe3 (1b), and 2,6-dimethylpyridine
(1c)a)
mixture of two products is obtained from the alcoholysis with
tBuOH, the product 2b (characterized by NMR data, Table 2) and
the diboryloxide [Me(HA2C)B]2O, obviously formed from 2b by
the well-known elimination of isobutene and subsequent addition
of the intermediate hydroxyborane to unreacted MeBϭCA2.
1a
1b
1cd)
1dd)
{Bis(trimethylsilyl)methyl](diisopropylamino)methylborane (2c):
Excess diisopropylamine (20 mmol) was added to MeBϭCA2
(2 mmol) in pentane (5 ml) at Ϫ78 °C. After 1 h stirring, the solvent
was evaporated in vacuo. The colourless product crystallized from
hexane at Ϫ80 °C (81 %; m.p. 15 °C) and was characterized by
elemental analysis and by the NMR data (Table 2).
1H
SiMe3
SiMe2
BMe
L
Ϫ0.02/0.67
/
0.70
1.57
2.43
6.11
0.45
/
0.36/Ϫ0.28
Ϫ0.01/0.67
0.49/0.56
2.41/2.37
6.48
0.28/Ϫ0.07
Ϫ0.32/0.59
0.47/0.53
1.85/1.85
2.40/2.38
6.43
0.67
0.49b)
6.98
(Dimethylamino){[dimethyl(dimethylamino)silyl](trimethylsilyl)-
methyl}methylborane (2d): The borane MeBϭCAAЈ (8 mmol) in
pentane (5 ml) was aminolyzed with dimethylamine (34 mmol) at
Ϫ78 °C. Stirring at room temperature (6 h), filtration and distil-
lation (42 °C/0.7 Pa) gave pure 2d (52 %), characterized by elemen-
tal analysis (C, H, N) and NMR spectra (Table 2).
11B
13C
40.8
53.4
41.9
40.9
SiMe3
SiMe2
BMe
Si2C
L
4.4/6.1
/
9.0
70.0
20.7
22.0
124.8
151.2
153.1
6.1
/
9.9
34.8
11.3c)
5.4/3.8
7.3/9.4
8.9
69.8
5.5/4.0
7.5/9.5
9.2/9.5
70.2
20.7/20.7
22.0/21.8
125.2/125.4
152.5/152.7
152.6/152.7
22.0
124.5/124.4
139.9/139.9
153.5/153.4
Reaction of MeBϭCA2 with borane: An equimolar amount
(5 mmol) of the borane BH3 in thf (5 ml) was added to MeBϭCA2
in pentane (10 ml) at Ϫ78 °C, the solution was brought to room
temperature and stirred (1 h). In spite of the equimolar amount of
the starting components, the colourless 2:1 product 2e crystallized
from diethylether at Ϫ80 °C (89 %, with respect to MeBϭCA2;
m.p. 88 °C), characterized by elemental analysis (C, H) and NMR
a) In[D8]toluene; 1H NMR of 1a in C6D6; 1H, 11B, 13C NMR of 1b and 13C
NMR of 1a, c, d, at Ϫ30 °C. b) 2J ϭ 9.5 Hz; 31P NMR: δ ϭ 32.6. c) 1J ϭ
27 Hz. d) E/Z or Z/E isomers in the ratio 58/42 (1c) and 57/43 (1d).
1
spectral analysis in [D8]toluene. Ϫ H NMR: δ ϭ 0.18, 0.57 (2 s;
6:1) (signals of BH and BH2B not detected). Ϫ 11B NMR: δ ϭ
27.3, 38.4 (2:1). Ϫ 13C NMR (Ϫ30 °C): δ ϭ 1.1, 4.0 (2 q), 38.0 (s).
Ϫ IR (in KBr): 2455, 1533 cmϪ1 [ν(BH), ν(BH2B)]. Ϫ When a large
excess of BH3(thf) (12 mmol) was added to MeBϭCA2 (6 mmol)
in pentane (5 ml) at Ϫ78 °C and the solution was stirred at Ϫ78 °C
(6 h), the removal of the solvents in vacuo and the extraction of
the residue with diethylether at Ϫ78 °C gave a solid, from which
the colourless 1:1 product 2f was obtained by crystallization from
toluene at 6 °C (97 %; m.p. 123-124 °C), characterized by elemental
analysis, NMR spectra in [D8]toluene (Varian Unity 500), and by
Table 2 1H, 11B and 13C NMR chemical shifts of the adducts of
HX to MeBϭCA2 (2a-c: X ϭ OiPr, OtBu, NiPr2) and to MeBϭ
CAAЈ [giving MeϪ(Me2N)BϪCHA(SiMe2Nme2), 2d]a)
2a
2b
2cd)
2d
1H
SiMe
BMe
Si2CH
X
0.02
0.03
0.03
0.39
Ϫ0.02, 008g)
0.27
0.36
0.50
b)
b)
0.31
0.45
1.18c)
4.33c)
1.35
1.11e)
1.19f)
3.50f)
3.88e)
2.65h)
2.70i)
2.75i)
1
3
crystal structure analysis. Ϫ H NMR: δ ϭ 0.16 (s), 0.58 (t, J ϭ
1
3 Hz), 1.97 (br), 3.91 (d, J ϭ 146 Hz), in the ratio 18:3:2:1. Ϫ 11B
NMR: δ ϭ 17.5 (d), 33.1 (br) (1:1). Ϫ 13C NMR: δ ϭ 2.3, 6.8, 71.7.
11B
13C
52.2
52.0
45.5
44.8
Crystal structure of 2f: Data were collected on an ENRAF-Nonius
CAD4 diffractometer, using MoKα radiation (λ ϭ 154.184 pm,
j)
SiMe
BMe
Si2CH
X
1.7
5.1
20.6
24.5
67.3
1.8
9.7
21.9
30.7
74.1
2.3/0.8
8.7/8.7
16.7/12.2
21.6/21.6
24.6/25.7
44.6/44.0
48.8/49.6
5.8
¯
graphite monochromator) at 253 K. Space group: P1 (Nr. 2); a ϭ
16.2
38.0
39.2i)
39.3i)
812.5(5), b ϭ 1185.5(3), c ϭ 760.2(2) pm; α ϭ 102.03(5), β ϭ
108.72(5), γ ϭ 98.94(4)°; V ϭ 0.6585(6) nm3; Z ϭ 1; calculated
density: 0.999 g cmϪ3
; µ ϭ
crystal size: 0.85·0.45·0.3 mm3;
2.25 cmϪ1; range of 2Θ: 6Ϫ60°. The structure was solved by direct
methods and refined on reflection intensities [23]. Measured reflec-
tions: 4722; independent data: 3820; observed independent reflec-
tions [I > 2σ(I)]: 3433; refined parameters: 206; R ϭ 0.0396; wR2 ϭ
a) NMR of 2d in C6D6; 13C NMR of 2a؊c at Ϫ30 °C. b) Signal not detected.
c) 3J ϭ 6,1 Hz, d) Signals of two axial-symmetric isomers with respect to rota-
tion around the BϪC axis, resolved in the 13C NMR spectrum in the ratio
85:15. e) 3J ϭ 6.7 Hz. f) 3J ϭ 6.1 Hz. g) In the ratio 3:2. h) SiNMe2 i) Me I
and Me II of BNMe2. j) Three SiMe signals: δ ϭ 0.0 (SiMe3), 1.2, 1.8, (Me I
and Me II of SiMe2Cl).
0.1242; GOF
ϭ 1.075; maximal residue electron density:
265 e nmϪ3. The crystallographic data have been deposited at the
Cambridge Crystallographic Data Centre as supplementary publi-
cation No. CCDC 222554.Copies of the data can be obtained free
of charge on application to CCDC, 12 Union Road, Cambridge
CB2 1 EZ, UK (e-mail: deposit@ccdc.com.ac.uk).
ange-coloured crystals of 1a,c,d gave the expected results on el-
emental analysis. The characteristic NMR data are summarized in
Table 1.
Niobocene- and tantalocene-di-µ-hydrido[bis(trimethylsilyl)methyl]-
methylborane (2g,h): Cp2NbH3 or Cp2TaH3 (3 mmol) in toluene
(5 ml) were added to a solution of MeBϭCA2 (5 mmol) in toluene
(15 ml) at Ϫ78 °C. The solution turned green. After stirring at
room temperature (15 min), volatile materials were removed in va-
cuo. Dark-green crystals were obtained from pentane at Ϫ80 °C
(68 or 84 %, respectively), which were extremely sensitive towards
(Alkoxy)[bis(trimethylsilyl)methyl]methylboranes (2a,b): A slight
excess of the carefully dried alcohols iPrOH or tBuOH, respec-
tively, was given to MeBϭCA2 (5 mmol) in pentane (5 ml) at
Ϫ78 °C. After bringing the solution to room temperature, the prod-
uct 2a was obtained by distillation (23 °C/0.7 Pa; 48 %) and charac-
terized by elemental analysis and NMR spectra (Table 2). A 4:1
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moisture.ϪNMR spectra (2g/2h) in [D8]toluene. Ϫ H NMR: δ ϭ
[Bromo(methyl)boryl]bis(trimethylsilyl)bromomethane (2p): An
equimolar amount (6 mmol) of bromine was dropped into a solu-
tion of MeBϭCA2 in pentane (5 ml) at Ϫ78 °C. The brown colour
immediately disappeared. The colourless product was obtained by
sublimation at 40 °C/1.3 Pa (48 %; m.p. 68 °C) and characterized
by elemental analysis (C, H) and NMR (Table 3).
0.06/0.04, 0. 18/Ϫ0.01, 0.39/0.36, Ϫ14.5/Ϫ14.7, 4.80/4.75, 4.83/4.77
(6 s in the ratio 18:3:1:2:5:5). Ϫ 11B NMR: δ ϭ 53.9/58.1. Ϫ 13C
NMR: δ ϭ 2.7/2.6 (SiMe), 15.1/17.2 (BMe), 24.9/25.7 (Si2CH),
92.9/88.7 (Cp).
Diboryldisilylmethanes (2i,j,m,n): The borane MeBϭCA2 (4 mmol)
was dissolved in pentane (5 ml). A slight excess of the boranes
BMe3, BEt3, EtBCl2 or tBuBCl2, respectively, was added at Ϫ78 °C
(BEt3 at room temperature) through a dropping funnel or (in the
case of BMe3) by condensation. After stirring at room temperature
(1 h; in the case of BEt3: 3 d), volatiles were removed in vacuo. The
products were obtained by sublimation at 40/80/60/80 °C/0.7 Pa in
yields of 64/85/88/78 % as colourless solids [m.p. 212 (2i), 173 (2m),
or 164 °C (2n); 2j is a waxy solid]. The products 2m,n were charac-
terized by elemental analysis (C, H) and all of the four products by
their NMR spectra (Table 3).
Boracyclobutenes (4aϪg): The borane MeBϭCA2 (5 mmol) was
dissolved in pentane (5 ml) or Ϫ in the case of 4a Ϫ in diethylether
(5 ml). The unsaturated components (5 mmol) were added at
Ϫ78 °C, either dissolved in diethylether (10 ml; in the case of
9-fluorenone to give 4a) or in pentane (10 ml; in the case of di-
phenylethyne to give 4e) or without solvent [in the case of bis-
(methoxycarbonyl)ethyne, phenylisocyanate, 2-(isopropylimino)-
propane, chloro(trimethylsilyl)ethyne, tert-butylphosphaethyne to
give 4bϪd,f,g, respectively]. The mixture was stirred at Ϫ78 °C (1 h)
in the case of 4b or at room temperature (30 min) in all of the other
cases. The colour changed in the case of 4a (orange-red Ǟ yellow),
4b (red-brown Ǟ colourless), and 4d (orange-red Ǟ colourless).
Volatile materials were removed in vacuo; in the case of 4g, the
volume of the reaction solution was reduced in vacuo by 50 %. The
products 4a,e crystallized from pentane at Ϫ80 °C; 4e was again
crystallized from diethylether (80 and 86 %, m.p. 38 and 45 °C,
respectively). The product 4b was twice crystallized from diethyl-
ether (54 %; m.p. 89 °C). The products 4d,f,g were transported into
a cooled trap at 55 °C/0.7 Pa, 20 °C/0.1 Pa, 30 °C/1.3 Pa, respec-
tively, and obtained as viscous oils (87, 70, 32 %, respectively). The
product 4c could neither be crystallized nor transported in vacuo
without decomposition; the crude oily material was pure, however.
All of the products were characterized by their NMR data (Table 4)
and elemental analysis.
[Chloro(dimethyl)stannyl][chloro(methyl)boryl]bis(trimethylsilyl)-
methane (2k): Dichlorodimethylstannane (3.2 mmol) in diethylether
(5 ml) was added to MeBϭCA2 (5.9 mmol) in pentane (5 ml) at
Ϫ78 °C. After 1 h stirring, volatiles were removed in vacuo. The
residue was dissolved in pentane (1 ml). Excess Me2SnCl2 crys-
tallized from the solution at Ϫ80 °C and was removed by filtration.
The product was obtained by condensation at 90 °C/0.7 Pa as a
colourless solid (52 %) and was characterized by elemental analysis
(C, H) and NMR (Table 3).
Bis(trimethylsilyl)[chloro(methyl)boryl](dichlorophosphanyl)me-
thane (2l): A slight excess of PCl3 was added to MeBϭCA2
(4 mmol) in pentane (5 ml) at Ϫ78 °C. After stirring at room tem-
perature (20 min) and removal of volatiles in vacuo, the colourless
product crystallized from toluene at Ϫ80 °C (77 %; m.p. 70 °C) and
was characterized by elemental analysis (C, H) and NMR (Tabe 3).
Reaction of MeBϭCA2 with benzaldehyde: An equimolar mixture
(5 mmol) of MeBϭCA2 and PhCHO in pentane (5 ml) was stirred
at room temperature (3 d). An 11B NMR signal at δ ϭ 58.2 indi-
cated the presence of the [2ϩ2] cycloadduct 4h. On removing the
solvent in vacuo, trimethylboroxene, (MeBO)3, was detected among
the volatiles by its NMR signals, indicating a metathesis reaction.
In order to complete the reaction, pentane was replaced by hexane
and the solution refluxed (4 d). A distillation gave the well-known
phenyl-2,2-bis(trimethylsilyl)ethane [24] at 67 °C/1 Pa (38 %).
1,2-Dichloro-µ-dichloro-1,2-bis{[ethyl(methyl)boryl]bis(tri-
methylsilyl)methyl}dialane (2o): Dichloro(ethyl)alane (10 mmol) in
hexane (10 ml) was added to MeBϭCA2 (10 mmol) in pentane
(5 ml) at Ϫ78 °C. After 1 h stirring and removal of volatile com-
pounds in vacuo, 2o was obtained by condensation at 60 °C/0.7 Pa
and purified by crystallization from pentane at Ϫ80 °C (42 %). The
colourless product decomposed slowly, even at Ϫ30 °C, but was
pure, according to the NMR spectrum, measured immediately after
crystallization (Table 3).
1
Ϫ H NMR: δ ϭ 0.04, 0.20 (2 s, 1:1, 18 H), 7.19Ϫ7.28 (5 H), 7.78
(s, 1 H). Ϫ 13C NMR: δ ϭ 0.5, 2.0 (2 q), 146.5 (s), 155.0 (d); ad-
ditional: 3 d, 1 s (Ph).
Table 3 1H, 11B, and 13C NMR chemical shifts of the boryldisilylmethanes MeB(X)ϪC(SiM3)2ϪY (2i؊p)
2i
Cl
BEtCl
2j
Cl
BtBuCl
2k
Cl
SnMe2Cl
2l
Cl
PCl2
2m
Me
BMe2
2n
Et
BEt2
2og)
Et
AlCl2
2p
Br
Br
X
Y
1H
SiMe
BMe
X,Y
0.25
1.07
0.87Ϫ1.38
0.31
1.16
1.04
0.30
0.41e)
1.25f)
/
0.11
0.79
0.79
0.10
0.80
1.00Ϫ1.11
0.29
0.29
1.31
/
1.07
0.99
0.83c)
1.05, 1.34h)
11B
73.1
72.3
71.4
70.6
82.9
85.7
82.4
73.5
13Ca)
SiMe
BMe
Si2C
X,Y
2.6
3.8
3.7
3.6
18.2
3.1
19.2
50.3
19.2
/
3.4
16.0
4.5
1.2
23.0
49.2
/
19.5
50.9
9.9b)
26.0
20.0
58.5
28.8b)
30.1
18.1
40.2
6.6d)
/
16.7
33.3
9.8b)
26.7i)
50.4
/
/
9.7, 9.8b)
23.6, 27.4
/
a) At Ϫ50 °C (except 2p). b) Quartet. c) 2J ϭ 56 Hz (117Sn), 59 Hz (119Sn). d) 1J ϭ 205 Hz. e) 4J ϭ 1.7 Hz. f) 4J ϭ 4.1 Hz. g) Varian Unity 500; in [D8]toluene.
h) t and q; 3J ϭ 7 Hz. i) 27Al NMR: δ ϭ 122.
516
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Reactions at the Boron-Carbon Double Bond of Methyl(methylidene)boranes
Table 4 1H, 11B, 13C NMR chemical shifts of the (2ϩ2) cyclo-
adducts 4a؊g
toluene (10 ml) at Ϫ196 °C. Warming to room temperature gave a
black solution. After 1 h stirring, volatile materials, including the
excess of the starting borane, were removed in vacuo. The dicarba-
dicobaltatetrahedrane, [(CA)2{Co(CO)3}2], a known substance
4a
4b
4c
4d 4e
4fg)
4gg)
1
[21], was twice crystallized from pentane at Ϫ80 °C (81 %). Ϫ H
1H
SiMe 0.03
BMe 0.84
0.09 0.18
0.61 1.06
0.10 0.07
0.14 0.04
0.93 0.53
NMR: δ ϭ 0.32. Ϫ 13C NMR: δ ϭ 1.0 (q), 92.9 (s), 201.1 (s).
0.38 1.02
d)
Etc.
7.11Ϫ7.66 3.70 7.12Ϫ7.50
7.01Ϫ7.46 0.14 1.25j)
3-[Bis(diisopropylamino)phosphanyl]-4-methyl-5,5-bis(trimethylsilyl)-
1,2-diaza-3-azonia-4-borata-l,3-cyclopentadiene (5a): Azidobis-
(diisopropylamino)phosphane [26] (8 mmol) in pentane (5 ml) was
added to MeBϭCA2 (6 mmol) in pentane (5 ml) at Ϫ78 °C. After
stirring the mixture at Ϫ78 °C (30 min), all volatile substances were
removed at Ϫ30 °C/0.7 Pa. Colourless solid product precipitated
from diethylether at Ϫ80 °C and was characterized by elemental
analysis and NMR. Ϫ 1H NMR: δ ϭ Ϫ0.02 (18 H), 0.67 (3 H),
0.97 (d, 12 H), 1.18 (d, 12 H), 3.68 (sept, 4 H). Ϫ 11B NMR: δ ϭ
49.7. Ϫ 13C NMR (Ϫ50 °C): δ ϭ 0.0 (q, SiMe), Ϫ2.4 (q, BMe),
45.8 (s, Si2C), 23.3, 47.1 (q, d, iPr). Ϫ On standing at room tem-
perature, the thermolabile product decomposed slowly. An 11B
11B
58.6 53.8 57.3
45.9 54.2
69.0 41.0
0.18h) 1.7k)
13Ca) SiMe 3.8
BMe 4.6
3.2
5.1
39.8 44.2
153.2 176.0
54.7
0.0
1.8
4.4 1.4
1.7 4.2
33.1 39.9
67.1 158.8
l)
8.65
52.8
71.7
l)
Si2C 42.2
Si2CC 96.4
m)
b)
c)
e)
Etc.
155.2
0.24h) 31.6n)
62.5i) 39.9o)
f)
92.3
a) 4a, d, e at Ϫ30, 4c at Ϫ50 °C. b) 119.8, 125.6, 126,5, 129.2 (4d), 139.7,
147.2 (2s). c) 120.1, 124.5, 129.0 (3d), 137.2 (s). d) 1.36, 1.14, 3.09 (s, d, sept,
in the ratio 6:6:1; 3J ϭ 6,6 Hz.) e) 25.3, 28.2, 42.4 (q, q, d). f) 126.0, 126.8,
128.0, 129.9, 131.5 (6 d), 135.2, 135.4 (2 s). g) Varian Unity 500. h) Arbitrary
assignment of the two SiMe3 signals. i) Observable at Ϫ30 °C (BC). j) 4J ϭ
9.92 Hz. k) 3J ϭ 3.57 Hz. l) Signals not resolved. m) 31P NMR: δ ϭ 301.93.
n) 3J ϭ 205 Hz (tBu). o) 2J ϭ 33.90 Hz (tBu).
1
NMR signal at δ ϭ 33.7 grew, together with a 1:1 set of H NMR
signals in the range typical for SiMe3. An IR band at ν ϭ
2047 cmϪ1 grew simultaneously, indicating a diazo group. In order
to obtain a complete decomposition with a well-defined product,
the mixture was heated in boiling toluene (5 d), yielding an orange,
viscous material, from which no pure product, however, could be
isolated.
Reaction of MeBϭCA2 with acetophenone: Proceeding analogously
as just described makes an 11B NMR signal detectable at δ ϭ 57.5.
The product PhMeCϭCA2 was distilled at 83 °C/1 Pa (55 %).
Ϫ
1H NMR: δ ϭ Ϫ0.23, 0.29 (2 s, 1:1, 18 H), 2.25 (s, 3 H),
3-Isobutyl-4-methyl-2,5-bis(trimethylsilyl)-1,2-aza-3-azonia-4-borata-
3,5-cyclopentadiene (5Јb): Isobutylazide (7 mmol) was added to
MeBϭCA2 (5 mmol) in pentane (5 ml) at Ϫ78 °C. After stirring at
Ϫ30 °C (8 h), an l1B NMR signal at δ ϭ 45.5 was detected in the
reaction mixture, attributed to the primary cycloadduct 5b. After
removal of the volatiles, the product 5Јb was obtained on distil-
lation at 70 °C/0.7 Pa (74 %) as a yellow liquid, characterized by
7.08Ϫ7.32 (5 H). Ϫ 13C NMR: δ ϭ 2.4, 2.7, 30.1 (3 q), 149.3, 164.8
(2 s); additional: 3 d, 1 s (Ph).
Reaction of MeBϭCAAЈ with benzaldehyde: An equimolar mixture
(16 mmol) of MeBϭCAAЈ and PhCHO was stirred (2 h) in hexane
(15 ml). The product, PhCHϭCAAЈ was obtained by distillation
at 61 °C/0.7 Pa (37 %). Ϫ 1H NMR (two isomers in the ratio 68:32):
δ ϭ 0.28/0.03 (s, 9H), 0.26/0.62 (s, 6 H), 7.21Ϫ7.36 (5 H), 7.86/7.98
(s, 1 H). Ϫ 13C NMR: δ ϭ 0.5/1.8 (q), 5.4/3.8 (q), 144.3 (s), 152.2
(d); additional: 3 d, 1 s (Ph).
1
elemental analysis (C, H, N) and NMR. Ϫ H NMR: δ ϭ 0.25 (s,
9 H), 0.51 (s, 12 H; SiMe, BMe), 0.87 (d, 6 H), 1.97 (mc, 1 H), 3.72
(d, 2 H). Ϫ 11B NMR: δ ϭ 31.9. Ϫ 13C NMR (Ϫ50 °C): δ ϭ 1.6,
2.8 (2 q, SiMe), Ϫ4.2 (q, BMe), 134.4 (s, BCSi), 20.7, 30.7, 55.4 (q,
d, t; iBu).
Reaction of MeBϭCAAЈ with dimethylpropanal: An analogous pro-
cedure gave the olefine tBuCHϭCAAЈ at 30 °C/0.7 Pa (32 %).
Ϫ 1H NMR (two isomers in the ratio 60:40): δ ϭ 0.16/0.29 (s, 9 H),
0.64/0.50 (s, 6 H), 1.12/1.12 (s, 9 H), 6.82/6.98 (s, 1 H). Ϫ 13C
NMR: δ ϭ 1.3/3.6 (q), 7.5/4.4 (q), 30.5 (q), 36.8 (s), 133.4/133.9
(s), 171.6/170.9 (d).
4-Methyl-2,3,5-tris(trimethylsilyl)-1,2-diaza-3-azonia-4-borata-3,5-
cyclopentadiene (5Јc): An equimolar mixture (8 mmol) of MeBϭ
CA2 and azidotrimethylsilane was prepared at Ϫ78 °C and brought
to room temperature. A sublimation at 45Ϫ55 °C/0.7 Pa gave the
product (16 %; m.p. 55Ϫ58 °C), characterized by elemental analysis
1
(C, H, N) and NMR. Ϫ H NMR: δ ϭ 0.27, 0.40, 0.54 (3 s, 1:1:1,
Reaction of MeBϭCA2 with triphenylphophine oxide: A solution of
Ph3PO (4 mmol) in toluene (10 ml) was added to MeBϭCA2
(4 mmol) in toluene (5 ml) at Ϫ78 °C. After stirring at room tem-
perature (30 min), an 11B NMR signal at δ ϭ 55.2 was observed,
attributed to the formation of 4l, besides the signal of (MeBO)3 at
δ ϭ 31.9. After further stirring (18 h), volatile materials were re-
moved in vacuo, pentane was added to the residue, and still unre-
acted Ph3PO was removed by filtration. After the removal of pen-
tane, the known phosphorane Ph3PϭCA2 [25] crystallized from
CHCl3 at Ϫ40 °C. Since traces of the cycloadduct 41 were still pre-
sent, a sublimation (105 °C/0.7 Pa) was performed, that gave the
pure phosphorane (30 %). Ϫ 1H NMR: δ ϭ Ϫ0.21 (s, 18 H),
7.24Ϫ7.84 (5 H). Ϫ 13C NMR: δ ϭ 5.2 (q), 127.8 (d/d), 134.0
27 H), 0.64 (s, 3 H). Ϫ 11B NMR: δ ϭ 36.3. Ϫ 13C NMR (Ϫ30 °C):
δ ϭ 0.3, 1.3, 2.6 (3 q, SiMe3), 3.4 (q, BMe), 133.0 (s, BCSi).
4-Methyl-5-(chlorodimethylsilyl)-2,3-bis(trimethylsilyl)-1,2-diaza-
3-azonia-4-borata-3,5-cyclopentadiene (5Јd): Starting from MeBϭ
CAAЈ and AN3 and proceeding as described for 5Јc, the product
was obtained by sublimation at 50Ϫ60 °C/0.7 Pa (36 %; m.p.
58Ϫ60 °C). Ϫ 1H NMR: δ ϭ 0.41, 0.59, 0.65 (3 s, 1:1:1, 27 H); the
signal for BMe is hidden under the signal of SiMe2Cl (3ϩ6 H),
presumably at δ ϭ 0.65. Ϫ 11B NMR: δ ϭ 36.8. Ϫ 13C NMR:
δ ϭ 0.2, 1.3 (2 q, SiMe3), 5.6 (q, SiMe2Cl), Ϫ3.8 (q, BMe), 128.3
(s, BCSi).
1
(d/d), 130.6 (d), 134.7 (d, J ϭ 79.8 Hz).
2,5-Dimethyl-3-phenyl-4,4-bis(trimethylsilyl)-1-oxonia-2-aza-5-borata-
5-cyclopentene (5e): N-Methylbenzaldimine-N-oxide, PhCHϭ
NMeϪO, (4.5 mmol) in toluene (5 ml) was added to MeBϭCA2
(5 mmol) in pentane (5 ml) at Ϫ78 °C. After stirring the mixture at
Reaction of MeBϭCA2 with octacarbonyldicobalt: The borane
MeBϭCA2 (6 mmol) was condensed to [Co2(CO)8] (3 mmol) in
Z. Anorg. Allg. Chem. 2004, 630, 508Ϫ518
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 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
517

P. Paetzold, U. Englert, R. Finger, T. Schmitz, A. Tapper, R. Ziembinski
room temperature (2 h), volatiles were removed in vacuo. The prod-
uct crystallized from pentane at Ϫ80 °C (92 %; m.p. 42 °C) and was
characterized by elemental analysis (C, H, N) and NMR. Ϫ 1H
NMR: δ ϭ Ϫ0.10, 0.27 (2 s, 1:1, 18 H), 0.52 (s, 3 H), 2.73 (s, 3 H),
4.27 (s, 1 H), 7.24Ϫ7.59 (5 H). Ϫ 11B NMR: δ ϭ 51.9. Ϫ 13C NMR:
δ ϭ 1.8, 2.4, 46.2 (3 q), 38.6 (s), 78.7 (d); additional: 3 d, 1 s (Ph).
[9] R. Boese, N. Finke, J. Henkelmann, G. Maier, P. Paetzold, H.
P. Reisenauer, G. Schmid, Chem. Ber. 1985, 118, 1644.
[10] A. Tapper, T. Schmitz, P. Paetzold, Chem. Ber. 1989, 122, 595.
[11] S. Küpper, U. Englert, P. Paetzold, Heteroatom Chem. 1990,
1, 479.
[12] H. Nöth, B. Wrackmeyer in NMR Basic Principles and Pro-
gress (P. Diehl et al., Eds.), Vol. 14, Springer, Berlin 1978.
[13] S. S. Al Juaid, C. Eaborn, P. B. Hitchcock, K. K. Kundu, M.
4-(Chlorodimethylsilyl)-2,5-dimethyl-3-phenyl-4-(trimethylsilyl)-1-
E. Molla, J. D. Smith, J. Organomet. Chem. 1990, 385, 13.
[14] P. Paetzold, L. Geret, R. Boese, J. Organomet. Chem. 1990,
385, 1.
[15] A. L. Spek, PLATON 94, University of Utrecht, The Nether-
lands, 1994.
oxonia-2-aza-5-borata-5-cyclopentene (5f):
A procedure as de-
1
scribed for 5e gave the product 5f (58 %; m.p. 82 °C). Ϫ H NMR
(diasteromers in the ratio 83:17): δ ϭ 0.07/0,57 (SiMe3), 0.30/0.55
(SiMe2), 0.66/0.59 (BMe), 2.73/2.71 (NMe), 4.58/4.27 (PhCH),
7.23Ϫ7.62 (Ph). Ϫ 11B NMR: δ ϭ 51.9. Ϫ 13C NMR: δ ϭ 2.0
(SiMe3), 5.8 (Me I of SiMe2), 6.0 (Me II of SiMe2), 45.7 (NMe),
78.7 (PhCH); additional: 3 d, 1 s (Ph); the signals of BMe and
Si2CB were not detected.
[16] C. R. Lucas, M. L. H. Green, J. Chem. Soc., Chem. Commun.
1972, 1005.
[17] P. Paetzo1d, Adv. Inorg. Chem. 1987, 31, 123.
[18] P. Paetzold, H. Grundke, Synthesis 1973, 635.
[19] We are greatly indebted to R. Boese (Universität-Gesamthoch-
schule Essen, Germany) for the collection and interpretation
of the crystal diffraction data.
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 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
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