DOI: 10.1002/chem.201202918
Metal Ions Do Not Play a Direct Role in the Formation of Carbon–Carbon
Triple Bonds during Reduction of Trihaloalkyls by CrII or VII
Ophir Levy and Avi Bino*[a]
Chromium(II) ions are known to reduce alkyl halides,
should be kinetically stable. Furthermore, CrIII complexes
with chromium–carbon double or triple bonds are unprece-
dented and the proposed “(m3-arylidyne)trichromium com-
plex” (Figure 1b) is unrealistic. A tetrahedral geometry
C
RX, and form free R radicals (R) in aqueous solutions
[1]
G
R
(H2O)5]2+
ACHTUNGTERNNUNG .
ACHTUNGTRENNUNG
ꢀ
learn from the products that this reaction proceeds via an
inner-sphere mechanism. When excess Cr2+ is present, a rad-
about the central carbon atom and a Cr C bond length of
about 2 ꢀ imply a Cr···Cr length of 3.26 ꢀ. Such a short dis-
tance does not leave enough space for three full coordina-
ical R reacts with another Cr2+ ion forming a chromium-
[1g]
(III)–alkyl complex: [CrII
E
E
(H2O)5]2+
.
tion spheres of chromiumACTHNGURTENNU(G III) atoms. 2) ChromiumACHTUNGTRENNUNG(III)
Once again, the kinetic stability of chromic ion complexes
allows us to study the bonding and chemistry of these inter-
esting species by isolating and, in some cases, even crystalliz-
ing and structurally characterizing them.[2] Most of the iso-
lated CrIII–alkyl complexes slowly decompose in solution at
room temperature forming Cr2+ and free R radicals that in
chemistry is dominated by octahedral geometry and high ki-
netic stability of its complexes, thus ruling out the possibility
that metal centers are involved in the making of the C–C
triple bond within the metal coordination sphere. 3) The
scheme is also short of explaining the most important and
crucial step in which the carbon–carbon triple bond is ac-
tually formed. In order to couple and form an alkyne, two
alkylidyne ligands (RC3ꢀ) have to be oxidized first to two
carbyne radicals, RC. In the case of the postulated com-
turn produce the corresponding alkane RH and the coupling
[1a,c–e,h]
ꢀ
product R R.
In 2007 Falck, Mioskowski et al. reported that the reac-
tion between a chromous salt and 1,1,1-trichloromethyl aryls
in THF leads to the formation of the corresponding alkynes
ꢀ
plexes (Figure 1), all carbon chromium bonds should be
cleaved homolitically to form kinetically labile CrII species
and RC radicals prior to the coupling process.
as in: ArCCl3 +CrCl2ACHTUGNTRENNNUG
(6 equiv)!ArCꢁCAr (81%).[3] They
offered a scheme in which the carbon–carbon triple bond is
formed by “coupling of two arylidyne moieties, either a (h1-
arylidyne)chromium or a (m3-arylidyne)trichromium com-
plex” (Figure 1). They proposed that these “moieties” are
the result of consecutive a-eliminations of Cl atoms from
[RCl2CCrIII] complexes.[3] This proposed mechanism, howev-
er, suffers several major drawbacks. 1) The existence of the
postulated CrIII–arylidynes in these experiments has never
been substantiated despite the fact that if they were to exist,
In a recent report, we have shown that free carbyne radi-
cals, RC, can be prepared by degradation of metal–alkyli-
dyne complexes in aqueous solution under mild conditions.
These extremely reactive species react with each other and
with solvent molecules and form alkynes and many other or-
ganic molecules.[4] The reaction with water is orders of mag-
nitude slower than the coupling reaction, allowing the car-
byne radicals to travel long distances in solution and meet
with other radicals even at minute concentrations.
such chromium
A
Here we report the results of a study of reactions between
Cr2+ ions and two 1,1,1-trihaloalkyls, namely, bromoform
(tribromomethane, CHBr3) and 1,1,1-trichloroethane
(CH3CCl3) in aqueous solution. Both reactions yield alkynes,
namely, acetylene and 2-butyne, respectively. We show evi-
dence that the alkynes are formed by coupling of free car-
byne radicals in solution and without the assistance of metal
ions, such as CrIII, as previously suggested by Falck, Mios-
kowski et al.[3] The carbyne radicals are formed by reduction
of the trihalide by three Cr2+ ions according to: 3[Cr-
Figure 1. Postulated chromiumACHTNUGTRENNUNG
dyne)-chromium
G
ACHTUNGTRENNUNG
plex.[3]
A
(H2O)5]2+ +RC. Two RC radicals
ACHTUNGTRENNUNG
then couple and form an alkyne: 2RC!RCꢁCR.
[a] O. Levy, Prof. Dr. A. Bino
Institute of Chemistry, The Hebrew University of Jerusalem
Edmond J. Safra Campus, Givat Ram, 91904 Jerusalem (Israel)
[CrACHTNUTGRNEUNG
(H2O)6]2+ in 0.3m HClO4 (for preparation techniques,
see the Supporting Information) or by dissolving CrCl2 in
water. Due to the relatively low solubility of the organic re-
actants in water, large excess of Cr2+ persisted throughout
Supporting information for this article is available on the WWW
15944
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15944 – 15947