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Sigma-Bonded Organochromium Compounds. Hydrogen Transfer Reactions

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Sigma-Bonded Organochromium Compounds. Hydrogen Transfer Reactions
BY H. H. ZEISS AND R. P. A. SNEEDEN [*I
The preparation and properties of two classes of organochromiunr compounds and their
interconversion have been studied actively in recent years. These are the a-bonded organochromium compounds in which alkyl, aryl, or araikyl groups are covalently bonded to
chromium (11 or 111) and the n-arenechromium complexes in which “aromatic” nuclei are
covalently bonded to chromium (0 or I ) . From certain reactions of the a-bonded organochromium compounds, evidence has accumulated which shows that the chromium atom
acts not only as a coordination center, but also as a hub for hydrogen transfer and HID
exchange 11.21. a-Organochromium compounds may act as sources of radicals and carbanions. The present paper deals with the preparation and properties of D-bonded organochromium cumpounds ; particular attention is given to hydrogen transfer reactions and
rearrangements to bis(x-arene)chromium complexes.
I. Preparation
Solvated a-bonded organochromium compounds may
be prepared by the action of certain organometallic
compounds on chromium halides, e.g., unsolvated
CrC13 or CrCI3.3 tetrahydrofuran, provided particular
attention is paid to solvent, temperature, and stoichiometry of the reaction. Phenyl-lithium I31 and phenylsodium [41, for example, react with chromium trichloride according to eq. (a) and (b) to give lithiumand sodium-containing organochromium complexes
of varying composition. In some cases organolithium
reagents give lithiula-free complexes [ 5 ] , e.g. ( I ) .
The most convenient and perhaps cleanest route to
a-bonded organochromium compounds is by the
action of organornagnesium halides on CrC13.3THF
in tetrahydrofuran [2,7-10,13,141 or in diethyl ether
[eq. (c)]. This is a general and most versatile
method for the preparation of such compounds.
3 RMgX+ CrCl3.3THF + R3Cr.xTHFf MgXC1.yTHF
(c)
R = phenyl [7]; a- and @-naphthyl[8]; ethyl [9]; mesityl [10,14b];
ally1 [11,12]; vinyl [13]; benzyl [2]; p-tolyl [14]; p-F-, p-C1-,
p-Br-, p-C6H5-C&
[14a]; m-cl-, m-CH3-C&
[14a]; 2-thiophenyl [14a]; o-tolyl [14b]; methyl 191.
By varying the stoichiometric ratios of the reactants,
sohated organochromium compounds of the type,
RnCrC13-n’xS (n = 1,2 or 3) R = e.g. benzyl[*.15a,
161 E. Kurras, Dissertation, Universitat Jena, 1959.
[7] W. Herwig and H . Zeiss, J. Amer. chem. SOC.79,6561 (1957).
[8] W. Herwig, W. Metlesics, and H. Zeiss, I. Amer. chem. SOC.
81, 6203 (1959).
[9] M . Tsutsui and H . Zeiss, J. Amer. chem. SOC.81,6090 (1959).
However, in the reaction between triphenylaluminum
and chromium(II1) chloride in tetrahydrofuran only
[*] Prof. Dr. H. H. Zeiss and Dr. R. P. A. Sneeden
Monsanto Research S.A.
Binzstr. 39
8045 Zurich (Switzerland)
I11 H . H. Zeiss, Bull. SOC.chim. France 1963, 1500.
[2] F. Glocking, R. P . A . Sneeden, and H. H. Zeiss, J. organomet.
Chemistry 2, 109 (1964).
[3] Fr. Hein and R . M’eiss, Z. anorg. allg. Chem. 295, 145 (1958).
[4] Fr. Hein, B. Heyn, and K. Schmiedeknecht, Mber. dtsch.
Akad. Wiss. Berlin 2, 552 (1960); Fr. Hein and K. Schmiedeknecht, J. organomet. Chemistry 5, 454 (1966); 6, 45 (1966).
(51 Fr. Hein and D.Tifle, Z. anorg. allg. Chem. 329, 72 (1964).
Angew. Chem. internat. Edit.
/ Voi. 6
(1967)
1 No. 5
[lo] W. Metfesics and H. Zeiss, J. Amer. chem. SOC.81, 4117
(1959).
[ll] W. Oberkirch, Dissertation, Technische Hochschule Aachen,
1963.
[12] E. Kurras and P. Klimsch, Mber. dtsch. Akad. Wiss. Berlin
6, 736 (1964).
(131 H.-P. Throndsen, W. Metlesics, and H. Zeiss, J. organomet.
Chemistry 5, 176 (1966).
1141 R . P. A . Sneeden and H . Zeiss, J. organomet. Chemistry 4,
355 (1965).
[14a] G . Stolze and J . Huhfe, J. organomet. Chemistry 5, 545
(1966).
[14b] G. Stoke, J. organornet. Chemistry 6, 383 (1966).
[l5a] R. P. A . Sneeden and H.-P. Throndsen, Chem. Commun.
1965, 509.
435
15bV16,171[*I, phenylC61, methyl 1161, p-tolyl[16aJ, and
mixed organochromiumcompounds RR'RCr .xS have
become accessible.
Another interesting route to organochromium species
is based on the reaction of chromium(n) salts with
certain organic halides W 1 9 1 (eq. (d)). This reaction
has been used to prepare the compounds (3)-(6/
[2oa-2Ocl as well as alkylchromium compounds [2OdJ.
compounds decompose rapidly in this solvent at room
temperature. Solutions of C & C H ~ C ~ C ~ ~ in
. XtetraS
hydrofuran can be diluted with water and stored at
-20 "C.Alternatively C6HsCH2CrC12.xTHF may be
extracted quantitatively into water from ethereal
suspension. Properties of the aqueous solution [15b717J,
i.e. reducing power towards FeC13, formation of benzaldehyde on oxidation, and the visible spectrum are
the same as those of a complex C ~ C ~ ~ ( C ~ H S C H ~ ) ~ H ~ O
formed directly from benzyl chloride and chromous
salts [181.
The thermal decomposition of the benzyl- and mesitylcompounds as in eq. (e) involves homolysis of the
carbon-chromium bond giving 1,2-diphenylethane
and bimesityl, respectively, and chromium(@chloride.
It has been reported that CH3CrC12.xS can be recrystallized from warm tetrahydrofuran 1161.
[email protected]
5 .H 2 0
A
RCrClrxS + R-R
R = Benzyl, mesityl
More recently this method has been adapted to the
preparation of the crystalline monoorganochromium
compounds RCrCly3pyridine (R = benzyl, o-chlorobenzyl and p-chlorobenzyl)[15bl.
+ CrC1z.xS
(e)
The monoorganochromium dichlorides RCrC12.xS
react with mercuric chloride in tetrahydrofuran [eq.(f)]
to give the corresponding organomercuricchloride and
CrC13.3THF.
RCrClynTHF+ HgClz
--f
RHgCl+ CrCly3THF
(f)
II. Properties
The monoorganochromium dihalides RCrClZxS differ
markedly in thermal stability. Whereas the phenyl
and p-tolyl derivatives may be recrystallized from
tetrahydrofuran (-10 to 20 "C), the benzyl and mesityl
[15b] R. P. A . Sneeden and H.-P. Throndsen, J. organomet.
Chemistry 6, 542 (1966).
[16] E. Kurras, Mber. dtsch. Akad. Wiss. Berlin 5, 378 (1963).
[16a] J. J. Duly, R. P. A . Sneeden, and H. H. Zeiss, J . Amer.
chem. SOC. 88, 4287 (1966).
(171 J. K . Kochi and D. Buchanan, J. Amer. chem. SOC. 87, 853
(1965).
[*I Deuteriotoluenes [a] from the hydrolysis of benzylchromium
compounds RnCrC13-n.xS in D20 at 0 C".
3
2
1
11.2
10.6
11.7
88.7
0.1
86
89.3
0.06
88
88.3
0.00
92
Therefore, cs-bonded organochromium compounds
can be regarded as sources of carbanions or of
radicals :
63
R e . . . CrClyxS
c-f
RCrC12.xS
f-t
Re..
. . CrClzxS
The triorganochromium compounds, R3Cr.xS, undergo both heterolyticand homolytic carbon-chromium
bond cleavage. Thus, triphenylchromiumis hydrolysed
to benzene and reacts with mercuric chloride to give
phenylmercuric chloride [71. Triallylchromium is converted on warming to diallylchromium(n) and presumably 1,5-hexadiene[121. Depending on the nature
of group R and of the coordination, these substances
may also be regarded as sources of carbanions, of
radicals, and even of carbonium ions.
III. Reactions
1. Reactions with Ketones
Organochromiumcompoundsdiffer from their organomagnesium counterparts in reactions with ketones.
The latter may act either as a source of alkyl or aryl
groups in forming tertiary carbinols or as conventional
436
Angew. Chem. internat. Edit. J Vol. 6 (1967)
1 No. 5
organic bases in effecting self-condensation of ketones 1211. Organochromium compounds, on the other
hand, may serve to do both simultaneously.
4) Compounds which are formed when one molecule
of the acetylene component combines with one or two
of the groups bonded to chromium. For example, trimesitylchromium and dimethylacetylene give 2-mesityl-2-butene as well as hexamethylbenzene[IOJ. Bicyclo[2.2.l]hepta-2,5-diene is mono and diphenylated
by triphenylchromium giving exo-5-phenylbicyclo[2.2.l]hept-2-ene and exo,exo-2,5-diphenylbicyclo[2.2.1 Iheptane [231.
Triphenylchromium reacts with cyclohexanone 1221 to
give 1-phenylcyclohexanol, 2-cyclohexenylcyclohexanone and 2-phenylbicyclohexyl-1',2-diol(7). Similarly,
triethyl- and triphenylchromium with 3-pentanone
give respectively 3-ethylpentan-3-01and 3,5-diethyl-4methylheptane-3,S-diol ((8), R = C2H5), and 3phenylpentan-3-01 and S-ethyl-4-methyl-3-phenylhep- 5) Bis(n-arene)chromium complexes formed from
trimers of types 2) and 3).
tane-3,5-diol ((8), R = C6H5).
The formation of 2-cyclohexenylcyclohexanonefrom
cyclohexanone and triphenylchromium is evidence of
the organochromium compound acting as a coupling
center. Finally, the formation of (8) (R = c6H5)in the
same reaction is evidence that triphenylchromium
acts simultaneously as a coupling center[*] and a
phenylating agent.
The isolation of compounds of type 3) (see Table 1)
provided the first evidence that organochromium
compounds promote hydrogen abstraction. Although
no intermediates have been isolated yet, the first step
may tentatively be represented by the following
equilibrium :
R,Cr.xS s R-H... . . . . RzCrH.x.3
The mono- and dimeric forms of the triphenylchromium complex and the monomeric form of the
trimethylchromium complex [(9b), ( l o b ) , and ( l l b )
2. Reactions with Disubstituted Acetylenes and Olefins
C.sH5
Organochromium compounds act as coordination
centers in their reactions with disubstituted acetylenes,
e.g., dimethyl- and diphenylacetylene[I]. In general,
five main types of products are obtained:
/9b)
1) Yellow, viscous polymers formed by the usual
ionic polymerization of acetylenes.
2) Hexasubstituted benzenes, e.g., hexamethyl- or
hexaphenylbenzene, formed by trimerization of the
acetylene component.
3) Tetrasubstituted cyclic products formed when two
molecules of the acetylene combine with one of the
groups attached to chromium with concomitant
hydrogen abstraction (see Table 1).
#
Products
R3CrxS
R
Type 2)
Phenyl
Phenyl
P-Naphthyl
(CH3)6Ce
(C,5H5)6C,5
(CH3)6C,5
Ethyl
Methyl
Vinyl
Ally1
f~HS)6G
I
Type 3)
(GHs)aC,5
1,2.3,4-tetramethylnaphthaIene
1,2,3,4-tetraphenylnaphthalene
I,2,3,4-tetramethylanthracene
I ,2,3,4-tetramethylphenanthrene
1,2,3.4-tetraphenylbenzene
1,2,3,4-tetraphenylcyclopentadiene
1,2,3,4-tetramethylbenzene
(cH3)6G
pentamethyl benzene
(qH5)6%
___-.
[21] M. Kharasch and 0.Reinmuth: Grignard Reactions of Nonmetallic Substances. 2nd Edit., Prentice-Hall, New York 1954.
I221 R. P. A. Sneeden, T. F. Burger. and H. Zeiss, J. organomet.
Chemistry 4, 397 (1965).
[*] Evidence that the chromium atom itself acts as coupling
center is provided by the facts that (i) the phenyl groups are all
accounted for in the final products and (jj) no toluene can be
detected in the reaction products.
Angew. Chem. internat. Edit. 1 VoI. 6 (1967) 1 No. 5
respectiv&] have been considered as rearrangement
components.
-___
I231 W. Metlesics, P. J. Wheatley, and H. Zeiss, J. Amer. chem.
SOC.84, 2327 (1962).
1241 w.Herwig and H . Zeiss, J. Amer. chem. SOC.80,2913 (1958).
[25] R . P. A. Sneeden and H . Zeiss. unpublished.
437
In effect, the organochromium compound R3Cr.xS
can be regarded as a potential source of an aryne
[(9h), when R is an aryl group] or of a carbene [ ( l l h )
when R is an alkyl group]. In such species, rapid
reaction with “two molecules of acetylene” would
occur to give compounds of type 3).
This concept also implies that a c-bonded organochromium(m) hydride (12) is either in equilibrium
with, or can proceed to, the x-arenechromium(1)
species.
:cc,
(12)
S”
I
rt
H
HSC6
3. Rearrangement
c-Bonded arylchromium compounds undergo an
unusual reaction in rearranging to n-arenechromium
complexes; thus, on treatment with oxygen-free diethyl ether at 20 OC, (C6H&Cr.3THF [261 and
( ~ - C H ~ C ~ H ~ ) ~ C I(141
- . ~are
T Hconverted
F
into black
intermediates which, on hydrolysis, give bis(x-arene)chromium complexes. These complexes may be isolated
as their tetraphenylborate salts, which are stable in air.
The same black intermediates can be prepared directly by
the reaction of the arylrnagnesium halide and CrCI3 (molar
ratio 3 :1) in diethyl ether [14,261. With p-tolylmagnesium
bromide the final products are toluene and p,p’-dimethylbiphenyl, indicating that in this reaction, and probably in all
cases involving arylchromium compounds, the “dirneric
products” (both free and in the bis(x-arene)chromium complexes) arise by para-coupling of two aryl species within the
confines of the organochromium compound.
The nature of the black intermediate [26al and the mechanism
by which a-bonded arylchromium(Ii1) compounds rearrange
to bis(x-arene)chromium complexes remain unknown.
Clearly both steric and electronic factors as well as solvent
effects influence the rearrangement. Thus the early observation that solvated (mesityl)&r is stable and does not rearrange to a bis(x-arene) complex was taken as evidence that
the two methyl groups ortho to the C-Cr bond impede the
reaction[26b1. However, it has been shown that solvated
(o-tolyl)3Cr, with only one methyl group orrho to the C-Cr
bond, does rearrange to a bis(x-arene) complex. The electronic influence of substituents is illustrated by the observation that, under analogous conditions, the ortho-, para-, and
meta-alkyl substituted compounds rearrange, whereas the
para- and meta-halogen substituted compounds d o
nOt114a, 14bI.
1261 M. Tsutsui and H . Zeiss, J.Amer. chem. SOC.81,1367 (1959).
[26a] H . Zeiss: Organometallic Chemistry. Amer. chem. SOC.
Monograph No. 147, Reinhold, New York 1960, p. 394.
I26bl M.Tsutsui and H. Zeiss, J. Amer. chem. SOC.82, 6255
(1960).
438
IV. Benzylchromium Compounds
1. Reactions
In pure tetrahydrofuran, the tetrahydrofuran adducts of
(CHzC6H5)3Cr, (C6HsCH2)zCrCI,and C6H~cHzCrC12
are readily accessible (see Section I). In pure diethyl
ether, benzylmagnesium chloride and CrC13.3THF,
or CrCl3, (molar ratio 1:l) react at temperatures
lower than -10°C to give monobenzylchromium dichloride which can be extracted into aqueous solution.
With a reactant ratio of 3 : l and at -78 “C,the initial
product is tribenzylchromium [21. This compound is
unstable in diethyl ether and at ca. 20 “C it rearranges
to a black intermediate which, on hydrolysis, gives
hydrogen, toluene, 1,2-diphenylethane, o-methyldiphenylmethane, and n-(o-methyldipheny1methane)-ntoluenechromium(1).
Experiments have shown that when only 1,Z-diphenylethane is formed (hydrolysis of THF/ether mixtures),
little or no n-complex is formed. On the other hand,
when o-methyldiphenylmethane alone, or together
with 1,2-diphenylethane, is formed (hydrolysis of
ethereal solutions), the bis(x arene)chromium complex is also formed. It appears, therefore, that there
is a direct relationship between the formation of the
bis(x-arene)chromium complex and that of o-methyldiphenylmethane.
Formation of o-methyldiphenylmethane indicates that
in the original tribenzyichromium complex two ,benzyl
groups have undergone ortho-coupling. This is in sharp
contrast to the triaryl compounds, in which two aryl
species undergo para-coupling (cf. Section 111.3).
Furthermore, in the case of tribenzylchromiurn, the
formation of toluene and o-methyldiphenylmethane
involves the acquisition of hydrogen by the benzyl
group originally bonded to chromium. The bis(n-arene)
complex can only arise by the acquisition of two
hydrogen atoms by the intermediate formed in the
coupling of two benzyl species within the tribenzylchromium complex. These hydrogen atoms may
originate from the water used in the hydrolysis step
or by internal transfer - their origin will be discussed
in the following section.
2. Deuterolysis (2731
If the hydrogen is acquired during the simple hydrolysis of organochromium compounds or their rearrangement products, then the products of deuterolysis
should be monodeuteriotoluene and monodeuterio-omethyldiphenylmethane, both free and in the xcomplex. However, the results of the deuterolysis
experiments (Table 2 (271 ; Section VI) indicate that
other reactions take place besides those of simple
deuterolysis. The hydrogen deuteride observed may
arise from the deuterolysis of a chromium hydride
[27] R. P. A . Sneeden, F. Glockling, and H . Zeiss, J. organomet.
Chemistry 6, 149 (1966).
[28] R. P. A . Sneeden and H . Zeiss, unpublished.
Angew. Chem. internat. Edit.
Vol. 6 (1967) J No. 5
Table 2. Composition [a] of toluene and o-methyldiphenylmethane isolated from organic products and from bis (x-arene) complex obtained in
the reaction of benzylmagnesium chloride and CrCIy3THF, ratio 3: I, -78' to 20°C in diethyl ether and subsequent deuterolysis.
Experiment No.
I
1 [dl
2
3 [gl
4 [hl
5
6 [il a) Filtrate
b) Residue
7 UI
From the bis(x-arene) complex
1
4
6b
7
I
Toluene
Ibl
Do
Dt
D2
D3
50.6
49.5
42.2
62.7
58.9
24.7
23.9
59.4
58
64.5
65.0
44.6
39.1
71.8
41.6
97.4
26
15.9
13.0
45.2
49.2
25.5
28.2
2.2
8.7
10.2
10.8
4.6
5.7
0.8
14.0
0.4
7.5
8.8
10.6
5.2
5.6
1.9
15.0
[fl
17.2
17.0
22.3
98.3
65.3
60.3
57.5
1.35
8.5
10.0
9.6
0.2
9.0
12.2
9.8
I
1
,
[fl
If1
[fl
11.2
74 [el
17 [el
37
44.3
76.2
38.0
97.7
44.6
22.6
36.2
1.7
If1
[f 1
8.1
7.0
3.2
6.2
10.4
21
12.9
28.0
97.5
so
57
50.1
52.2
1.9
D Z
D3
5 [el
3 [el
5
5.3
0.7
9.9
0.6
4
3.6
0.5
10.1
I5
23.3
12.0
0.5
6
10.4
6.7
0. I
I
I
o-Meth yldiphenylmethane
- Dq
1 [el
3
1.8
-
4.9
rr-Complex
Lbl
2.8
2.9
2.4
1.86
0.23
5.7
4.8
1
2.8
1.o
[a] Expressed in relative percentages and corrected for natural isotopic abundance; [bl Quantity expressed inmmoles/lOOrnmolesHSC&HzMgCI;
[c] Representing total of 1,Zdiphenylethane and o-methyldiphenylmethane formed; [dl In this reaction it was shown that Dz (81 %) H D
(18.2 %) and Hz (0.2 %) were formed at the deuterolysis step; [el 1.2-Diphenylethane content included; [fl Not measured; [gl Reaction carried
out in the presence of dihydroanthracene; [h]p-xylene added before deuterolysis: recovered p-xylene was 100 % CsHlo; [i] In these the
crude reaction mixture was filtered, the filtrate and residue being deuterolysed separately; [j] Hydrolysis carried out with water in deuterium
gas atmosphere.
intermediate. The undeuterated toluene and o-methyldiphenylmethane can arise only by a process of
internal hydrogen transfer.
The presence of two or three deuterium atoms in the
toluene and o-methyldiphenylmethane, both free and
in the bis(x-arene) complex, indicate an H/D exchange
reaction.
Experiments in the presence of anthracene and dihydroanthracene (see Table 2 and [271), established
that at no stage during the reaction were benzyl
radicals released into solution. Therefore, the processes of hydrogen abstraction and hydrogen transfer
all occur within the confines of an organochromium
complex. In order to establish that this applies also to
the H/D exchange reaction, it was first necessary to
establish that the latter is promoted neither by the
presence of the bis(x-arene)chromium(O) or-(I) complex, nor by colloidal chromium.
Thus it was found that when tribenzylchromium
rearranges in diethyl ether solution in the presence of
g-xylene and the reaction mixture is deuterolysed, the
p-xylene recovered is deuterium-free (cf. Section VI).
Furthermore, when a mixture of undeuterated x-(0methyldipheny1methane)-x-toluenechromium(0) and
-(I) deuteroxide in benzene/D20 is allowed to interact
with D 2 0 / D 2 , the toluene in the recovered bis(xarene)chromium contains only traces of deuteriotoluenes (cf. Section VI).
toluene and solvated dibenzylchromium(I1). The cisform of the latter can undergo ortho-coupling to
(0-phenylmethylbenzy1)chromium hydride (16).
2. Oxidative coupling of two benzyl groups bonded to
chromium gives undeuterated 1,2-diphenylethane and
benzylchromium(1) (17).
(C6HsCH2)3Cr.xS +
C6H5CH2CH2C6H5 C~HSCH~CX'(I).XS (9)
+
Neither of the reactions 1) or 2) leads directly to the
bisfx-arene) complex, but both effectively reduce the amount
of tribenzylchromium available for the ultimate formation
of bis(x-arene) complex.
3. Two of the benzyl groups in tribenzylchromium
(18) are suitably juxtaposed, for ortho-coupling to the
radical (19) (see Scheme 1).This concept is supported
by the recent observation that the pyridine molecules
which are coordinated to chromium in CrC13 * 3 pyridine
are attacked by phenyl radicals, generated in solution,
giving a mixture of 2-phenylpyridine (45 %), 3-phenylpyridine (19 %) and 4-phenylpyridine (36 %) 1291. Alternatively, ortho-coupling by an ionic mechanism gives
the carbonium ion (20). In both cases a 1,3-hydrogen
shift produces undeuterated o-methyldiphenylmethane
whereas hydrogen transfer to chromium leads to
(0-phenylmethylbenzyl) benzylchromium hydride (21).
An interpretation of these results must begin with a
consideration of the behavior of tribenzylchromium.
Replacement of the tetrahydrofuran in the initial
tribenzylchromium complex by diethyl ether induces
homolysis of the benzyl-chromium bond. The benzyl
radical remains within the confines of the organochromium compound and reacts either with the
solvate or with a o-bonded benzyl group. This leads
to the following considerations:
Scheme 1 requires that some of the undeuterated
toluene and o-methyldiphenylmethane be present
in the reaction mixture before deuterolysis. This was
confirmed by filtering the crude reaction mixture
and deuterolysing separately the filtrate and residue.
The toluene and o-methyldiphenylmethane found
in the filtrate contain considerably more of the undeuterated species (see Table 2, reaction 6) than do
those found in the residue.
The organic products, isolated after deuterolysis,
always contain more toluene than the combined total
of o-methyldiphenylrnethane and 1,2-diphenylethane
1. Hydrogen abstraction from a molecule of solvate
(diethyl ether or tetrahydrofuran) gives undeuterated
1291 R . J . Gritter and A . W. Godfrey, J. Amer. chem. Soc. 86,
4124 (1964).
Angew. Chem. internat. Edit.
1 Vol. 6 (1967) / No. 5
439
,,,’
H5C6CH2
H,C6 - CH,
attack on
attack of
+
c
solvent
(C6H5CHz)zCr
c, r\>
H5C6CH2
I
I
H5 6
‘
+
ortho-coupling
~
H5C6CHZCHZC6H5
CH,
+
H5C6CHzCr
HZ
117)
/ \
(H5C6cH2-C&CH2) CrH,
(IS)
ortho- coupling
ortho-coupling
(16)
/
(19)
\
HSC,CHz
internal H transfer
e
C
i
CH3&
J
I
H
~
H,
e
C
H
3
,CH2C6H4-CHZC6H5-o
-s
Cr
Cr
II
CH,C,H,-CH,C,H~-
o
f 231
Scheme 1
(cf. Table 2). The relative isotopic compositions of the
toluene and o-methyldiphenylmethane are similar ;
this implies that they are formed from similar but not
necessarily the sameprecursors.Previousexperimentson
the deuterolysis of benzylchromium(IiI)compounds[29al
exclude organochromium(m) compounds as possible
precursors. Although no experimental evidence is
available, reasoning from recent theory concerning
H/D exchange reactions [30,311 suggests that either
an organochromium(1)compound [e.g.,C6H&HZCr-xS
(17)] or an organochromium(n) hydride [e.g.,
O - C ~ H ~ C H ~ C H(16)]
~C~
is H
likely.
ses to the “tautomeric” [321 arene forms (22) and (23)
(Scheme 1). Thus the hydride (21) reacts with DzO to
The relative deuterium contents of the toluene and
o-methyldiphenylmethaneisolated on pyrolysis of the
bis(x-arene) complex are very similar, indicating that
the precursor to the x-complex in which the H/D
exchange occurs, must contain both future arene parts.
Such an intermediate is the organochromium(1n)hydride (21), which is in equilibrium with or progres[29a] See footnote [*I p. 436.
1301 E. Crawford and C . Kernball, Trans. Faraday SOC. 58, 2452
(1962).
[31] J. L. Garnett and W. A . Soiiieh, Nature (London) 201, 902
(1964).
440
Scheme 2
[32] J . Chatt and J . M . Dayidson, J. chem. SOC. (London) 1965,
843, were the first to use the term “tautomerism“ for this type
of system.
Angew. Chem. internat. Edit. 1 VoI. 6 (1967) J No. 5
give the corresponding deuteride [Scheme 2, (24) and
2, ( 2 5 ~ .
A series of H / D exchanges in a o-bonded organochromium deuteride and a series of tautomeric changes
account for the formation of the bis(x-arene) complexes.
V. Structure
Though it had been assumed that the organic groups
are directly bonded to the chromium atom OIJ the
basis of indirect evidence (e.g., deuterolysis, reaction
with HgC12, etc.) there was n o direct evidence for this
assumption. This has now been supplied by single
crystal X-ray structure determinations of Li3Cr(CH3)6.
3 dioxane and ( P - C H ~ C ~ H ~ ) C ~ C ~ ~ . ~ T H F .
The data for the first compound1331 have not been
refined but they show that all six methyl groups are
octahedrally disposed around the chromium atom
(C-Cr bond distance w 2.15 A). The data for
( ~ - C H ~ C ~ H ~ ) C T C ~ ~summarized
. ~ T H F , in Fig. 1
show: i) the p-tolyl group is bonded to the chromium
atom through one carbon atom (C-Cr bond distance
2.104 A & 0.010 A); ii) the length of the chromium
oxygen bond trans to the o-bonded p-tolyl group is
significantly greater than the other two (2.214 f
0.007 A as against 2.045 f 0.008 A). This trans effect
could account for the selective loss of and for the
replacement of one tetrahydrofuran molecule from
(C6H5)3Cre3THF1341 (see Section IV. 2). Reference
whereas the p-tolyl compound is rapidly hydrolysed,
does not reduce FeCI3, and absorbs weakly in the
visible region [16al.
It follows therefore that there are certain special
features associated with the benzyl-group when bonded
to metal centers. In the case of chromium it has
already been suggested 121 that the benzyl group, in
analogy to the ally1 group, can act as a bidentate
ligand (cf. (26)). If such a situation pertains, the
reactivity of the ortho position (and of the ortho
hydrogen) will be considerably enhanced. This is
reflected in the recent observations [351 that the reactivity of the ortho carbon atom in the benzyl Grignard
compound is enhanced. Furthermore, in the
case of molybdenum, two benzylmolybdenum
complexes have recently been prepared [361, one
(C~H~CH~)MO(CO)~(C
has
~ H ~been
)
formulated
as a pure o-bonded compound, the other
( C ~ H ~ C H ~ ) M O ( C O ) ~ ( C Sformed
H S ) , by loss of one
carbon monoxide ligand, has been formulated on the
basis of N M R evidence as the -ic-benzylcompound (27).
VI. Experimental
For general reaction conditions, product isolation techniques, and preliminary results of deuterolysis see earlier publications [2,271.
a) Reaction of Benzylmagnesium chloride with
CrCI3.3THF (ratio 3: I ) and Deuterdysis of the
Complex Formed
has been made earlier to the striking differences in
the reactions of the benzyl- and p-tolyl-chromium
systems. In the case of the triorgano-chromium
compounds, coupling of one of the benzyl groups
occurs with the ortho position of another, whereas
in the p-tolyl system one of the p-tolyl groups
couples with another to give exclusively the para
substituted product (cf. Section IV.1). In the case of
the monoorganochromium compounds, the benzyl
compound is stable towards water ( < 0 "), reduces
FeC13, and absorbs strongly in the visible region 115bl,
I331 J. Krouse, Proceedings 9th internat. Conference on Coordination Chemistry, St. Moritz, Sept. 1966, p. 168. Helvetica
Chimica Acta, Basle.
[34] J . Hahle and G. Stolze, Z . Naturforsch. 196, 1081 (1964).
Angew. Chem. internat. Edit.
1 Yol. 6 (1967) No. 5
Ethereal benzylmagnesium chloride (300 ml, 189 mmoles)
was added, over 15 min, to a briskly stirred suspension of
CrCIy3THF (23.6 g, 63 mmoles) in diethyl ether (150 ml)
at -78 "C. After 30 min at this temperature, the dark reaction
mixture was allowed to warm to 20 OC (Gilman Color Test
No. 1, negative). After 3 h at 2OoC, pure, oxygen-free p xylene (13.4 g, 126 mmoles) was added; the reaction mixture
was stirred briskly and DzO added. The final deuterolysis
mixture was filtered through a sinter disc, under nitrogen
pressure, the residue being washed with ether, water, and
hot acetone until the washings were colorless. The combined
etheral layers were dried with Na2S04 and shown by gas
chromatography to contain: tetrahydrofuran (150 mmoles),
toluene (119 mmoles), p-xylene (118 mmoles), o-methyldiphenylmethane and 1,2-diphenylmethane (total 15.4 mmoles,
corrected for 1,Zdiphenylethane in the benzyl-Grignard
solution). The more volatile components were separated by
fractional distillation. The pure compounds, isolated by
preparative gas chromatography, were shown by massspectroscopy [37J to have the compositions: tetrahydrofuran,
CdHsO, 100 %;p-xylene, CsHlo, 100 %; toluene (see Table 2,
[35] R . A . Benkeser and T. E. Johnston, J. Amer. chem. SOC.88,
2220 (1966).
1361 R . B. King and A. Forzogliu, J. Arner. chem. SOC.88, 709
(1966).
[37] We wish to thank Dr. W. E. Koerner and his associates of
the Monsanto Company, Research Center, St. Louis, Mo., USA
for determining the mass spectra.
441
reaction 4). The residue from the above distillation was
further separated by column chromatography into 1,2diphenylethane [m.p. and mixed m.p. 52-53 'C; S(TMS) =
2.87 (CHz)] and o-methyldiphenylmethane, [8(TMS) = 3.91
(CH2) and 2.18 (CH3)I. On many columns the gaschromatographic retention times of these two compounds
are the same. NMR spectroscopy is thus the only reliable
method of analysis. The isotopic composition of these
compounds is given in Table 2, reaction 4. The aqueous
layers were combined with the concentrated acetone extracts, filtered, and treated with NaB(C6H5)4 (5.2 g, in H20
100 ml). The crude x-(0-methyldipheny1methane)-x-toluenechromium(1) tetraphenylborate (2.26 g , 3.52 mmoles) was
pyrolysed to give benzene and biphenyl (from the eB(CgH&
anion), toluene, and o-methyldiphenylmethane. There was
no trace of p-xylene or 1,2-diphenylethane. The isotopic
composition of the toluene and o-methyldiphenylmethane
[6 (TMS) = 3.91 (CH2) and 2.18 (CH3)], are given in Table 2,
reaction 4.
b) Benzylmagnesium chloride and CrCl3.3THF (molar
ratio 3: I ) , Filtration and Deuterolysis
Ethereal benzylmagnesium chloride and CrC1373THF, (molar
ratio 3:1), were allowed to interact under the conditions
described above. The reaction mixture was filtered though a
sinter disc under nitrogen pressure. The residue was suspended in pure, dry, oxygen-free diethyl ether and the
resulting suspension and the filtrate were deuterolysed. The
products were isolated and estimated as described in the
previous experiment. The distribution of toluene, 1,2-diphenylethane, and o-methyldiphenylmethane, and the bis(n-arene) complex between the filtrate and the residue is
given in Table 2. The isotopic composition of these products
is also given in Table 2, reaction 6.
C)
(p-Tolyl) Cr(ur) Cly3THF
Equimolar ratios of p-tolylmagnesium chloride and
CrC13.3THF were allowed to react in tetrahydrofuran at
-50 "C. On warming to 20 OC (3 h) the pink crystalline solid
gradually dissolved, to give finally a clear green solution.
This was cooled to -20 OC (24 h), and the yellow-green solid
which had formed was removed by filtration under argon.
Crystallization of this substance from tetrahydrofuran was
carried out in a Schlenk tube, in vacuum, in the temperature
range -30" to 20'C. The results of elemental analysis,
molecular-weight determination (found 425.2, based on
measured density d y = 1.315; calc., 430.4[401), and the paramagnetic nature of the compound (3.87 B.M. [3*J)show it to
be (ptolyl)Cr(111)C1,.3THF. Optical data: Amax(THF) 445
nm; emax137.
d) Attempted HID Exchange in Bis(arene)chromium
Complexes
Crude x-(0-methyldipheny1methane)-n-toluenechromium(1)
hydroxide ( 5 mmoles) (from the benzyl Grignard compound
and CrC13-3THF, ratio 3 :1) was reduced in benzene (as the
chromium(0) complex) with alkaline sodium dithionite [391.
The aqueous layer was replaced by D20 (50 ml) and dry
oxygen was bubbled through the system. The color passed
from the benzene into the D20 layer, indicating the formation of the chromium(1) deuteroxide. Metallic sodium was
added cautiously until the color was evenly distributed
between the benzene and D20. The apparatus was flushed
with Dz, sealed, and left for 24 h at 20 "C. Dry oxygen was
again bubbled through the system and the bis(n-arene) complex recovered quantitatively from the D20 layer as its
tetraphenylborate. The toluene, obtained by the pyrolysis
of the crude salt, was isolated by preparative gas-chromatography and had the composition[37]: C7H8, 97.2; C7H7D,
2.6; C ~ H ~ D0.2
Z ,%.
Received: October 27th, L965; revised: February 13tb. 1967 [A 573 IE]
German version: Angew. Chem. 77,401 (1965)
[38] Kindly determined by Drs. G. and S. Olivd of MRSA,
Zurich, the value is for a tetrahydrofuran solution, and is based
on a molecular weight of 430.4.
[39] W. Herwig and H. Zeiss, Liebigs Ann. Chem. 606,209 (1957).
[40] Kindly determined by Dr. J . J. DaIy of MRSA Zurich, by
an X-ray evaluation of the cell dimensions.
By D . Seebach [*I
Dedicated to my tutor, Professor Rudolf Criegee,
on his 65th birthday
Little is known about metalated orthothioformates "1. By
adding equimolar amounts of butyl-lithium [21 to the orthothioformates ( l a ) to ( I f ) in tetrahydrofuran at -78 "C c3.41,
we obtained solutions of the lithium compounds (2a) to (2f).
HydIolysis of the metalated compounds (2) with D20
The comaffords the deuterated orthoesters (la') to (If').
pounds (2) are also formed conveniently from orthothiocarbonates (3) and butyl-lithium. (Previously, a carbonsulfur bond cleavage of this kind had only been observed with
sp-hybridized carbon 151.) Thus, the orthothioformate ( I d )
(b.p. = 74 OC/O.35 mm,
= 1.6180) is isolated in 70 % yield
on cleavage of (3d) (m.p. = 32.3-32.7 "C [61) and subsequent
hydrolysis. We found the following order of decreasing
(2d)
(2f) > (2.)
thermal stability 11 71 : (2a) > (2.)
(cf. Table 1).
>
442
-
Angew. Chem. internat. Edit.
Vol. 6 (1967)
/ No. 5
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