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Vinylidene complexes in organic synthesis"
B.GITA AND G.SUNUARARMAN**
Depanmenr of Chcmntry. lndian Institute of Technology, Madras 600 036, India.
Received on January 21, 1994
Abstract
Appl~cationof vmylidenc ~omplcxesin urganic syntbcsis 1s discussed. Mechanism of forma0011from q' alkyrle
cornplexea of triu~sitianmetala, preparation from q 2 alkyne, acerylide, carbma, carhyr~zor acyl complexes as well
as rhe structure o f thc vinylidme complexes art. outlined. Reactions of vinylidem cuniplexcs with electraphiles and
nucieophiles are alao detailed. In addilion, catalyt~crcactiuns invulvmg trans~tionnrelai complexes for the
syntheals of a variety of organic compounds and polymcrs from acetylanic compounds which mvuke an in siru
formation ot vmylidene intermediates are d~scussed~n detatl.
Key words: q2-alkyne-vinylidenen rearrangement, vmylidene complexes
1. Introduction
The term vinylidene describes the first member of the homologous series of unsaturated
carbenic species of the generic formula C,Hz. The parent vinylidene, CzH2, has never been
isolated as it undergoes an exlremely rapid 1,2-hydrogen shift to give acetylene (eqn 1).
: C=C<H
H
.
H-C E
(1)
C-H
However, it has been shown that it is possible to statilise vinylidene as ligands in
transition metal complexes. Some chemical transformations which relate vinylidene to other
monohapto ligands are shown in Scheme 1'.
Stcpwise reduction of an ethynyl iron complex to a neopentylidene unit has been reported
and here it was concluded by IH-NMR studies that a vinylidene complex is an intermediate
(Scheme 2)'. A series of cationic vinylidene complexes ha~rebeen isolatcd containing the
'CpFeL?' fragment (Fig. I)'. A few examples of mono. hi, tri and tetra nuclear metal
co~nplexes~-~
are shown in Fig. 2 and review articles are available in literature detailing their
synthesis and structure8 lo.
*
Dedicated to Proi C . N. R. Rao an the occasmn of hls sixtieth binhday.
** Author tor correspondence.
ir
9 GiTA %\o C . SUYDARARAIAN
Sziit\?i 2 Conieiuon or ethynyl iron to a neopenrylidene ~roncomplex viu a vinylidene colnplch
Several reactions of organometallic reagents may also contain intermediacy of vinylidene
complexes. Such reactions have proved to he highly useful in the synthesis of organic
compounds which otherwise require many cumbersome steps, as shall be shown later. In this
paper, the mechanism of vinylidene reamngement, a few preparative methods, thc structure
and several reactions of the vinylidene complexes will be discusqed. For the sake of brevity,
we restrict ourselves to the chemistry of mononuclear M-C vinylidene complexes, as
comprehensive literature is available for heterovinylidenes".
3. Isomerisation of I-alkynes t o vinylidenes o n metal templates
Terminal nlkynes lsomerise to the col~espondingvinylidene via two possible mechanistic
pathways. In4he first case, a stepwise process, I-alkyne undergoes an oxidative addition at
the metal centre (before or after n-coordination) to give a hydrido-alkynyl complex. This can
VINYLIDENE
a: L,L'
b: L,L'
c: L,L'
COMPLEXES
=
=
=
IN ORGANIC SYNTHESIS
dppe; R,RS=CH3,X=S03F
dppe, R=CH3, R'=H; X=PFs
dppe; R,R'=H, X=PF6
Mononuclear
Hornobinuclear
Heterobinuclear
Trinuclear
[Mn] = Mn (CO)z (q5-CsHs)
[Re] = Re (CO)n (~1~-CsHs!s
FIG2 Some erarnples of mono to mulo nuclear complexes w ~ t hvmylidenc ligands
further rearrange to the vinyl~denevia a 1.3 hydrogen shift from the metal to Cp (eqn 2).
A
M
[I
+
R-CEC-H-[M]-
/
1
I
H
I
-[M]-C--C-R-[M]=C=C<R
(2)
358
B GITA
\\D
G. SUNDARARAJAN
Alrematively, a concerted shift of hydrogen atom from C, to Cp as the metal approaches C,
can also be envisaged (eqn 3).
R
Borh these mechanisms may be valid but which one predominates may depend on the
nature of the metal-containing species. Also as a rule of thumb, q2-alkyne complexes of d4
and d' metai centres do not isomerise to the vinylidene analog, while the rearrangement is
highly favoured in d6 complexes, possibly due to the unfavourable 4e-2-centre d-rr intera~tion'~.'~.
Evidence for acetylene-hydride intermediate can be found in the reaction of the rhodium
compiex, l?hCl(~f-HC?Ph)( P R h On treatment with pyridme, the qZ-alkynecomplex is converted
to the hydrid*acetylide, a species that rearranges to the vinylidene on addition of cyclopentadienide
anion. The intermediate Rh(C2Ph) (Py) (PR3)2 can be isolated if the reaction is carried out at
0°C (Scheme 3)14.
SCHEME3. Vinylidene formation I.ra a metal-hydndo-acetyiIde complex
On the other hand, in the acetylene-vinylidene transformation in the diiridium complex,
the first such example involving a dimetallic complex, the sequence of steps envisaged by the
authors could be suggestive of a concerted pathway for the rearrangement (Scheme 4)15.
3. Preparation of vinylidene complexes
Mononuclear vinylidene complexes can be obtained by any one of the four methods, viz., from
li) $-bound terminal alkynes, (ii) carbene or carbyne complexes, (iii) metal-acetyiides, or
(ivl by dehydration of metal-acyl complexes.
3.1. From terminal nlkynes
A 1,2-hydrogen shift on the acetylenes attached to a metal centre results in the formation of
a vinylidene. As the rearrangement is very facile, detection of q 2 alkyne complex is generally
elusive, while in the case of a non-terminal alkyne like Zbutyne, the q2-bound complex has
been isolated (eqn 4)M.
VlNYLlDENE COMPLEXES IN ORGANIC SYNTHESIS
Scirrvr 4 Vmyhdcnc form~rionby a concerted mechanlm m a bismdium complex
As a prelude to polyrncrisation reactions involving vinylidene intcmcdiates, like
rne?athetical polyrnerisations. more than one alkyne can get incorporated into the vinylidene
Ilgand. One such example is given in eqn 5".
Cationic vmylidene complexes are mmunc to ligand exchange reactions; howcver, mixed
ligand vinylidene complexes can be oblained by performing Lhe exchange reactions on the
corresponding acetylides followed by protonation'? Cbiral vinylidene complexes of the
formula, [11~-CsHs]Re (NO) (PPhl) (=C=CRR') +X-where R, R'=H, CtI? CsHs have also been
synthesised. Geometrical isomerism has been observed in these complexes and various
reaction? have also been performed on
Anionic vinylidenc complexes have been observed to be formed by the reaction of
molybdenum and tungsten alkylidene complexes with a basd9.
B. GITA AND G. SUNDARARAJAN
360
3.2. From carbyne or carbene complexes
Vinylidene derivatives can be obtained readily from metal-carbyne complexes possessing
P-hydrogens when treated with strong bases like 11-butyllithium (eqn 6Y9.
Chlorocarbene complexes of Iron(Q porphyrins'" (on reaction with DDT)undergo elimination
of HC1 to give the diarylvinylidene complexes (eqn 7)20.21.
z
.
(Por) Fe [CCICH(C,H,CI),]
(Po,) Fe = C = C (C,H,CI),
(7)
3.3. From metal acecylides
An efficient way to obtain disuhstituted vinylidene complexes is by the addition of
electrophiles to metal acetylides, accessed by reacting acetylides with the metal halide
(Scheme 5)'?.
[RU]+=C= C <
Y
y2
R
[Ru]
SCHEME
5. Vinylidem complexes from metal acctylides.
-C
'x&
3.4. From acyl complexes by dehydration reaction
The acetyl complex of rhenium, Ke(NO)(PPhn) q5-C5Hs (COCHzR) on treatment with triflic
anhyride resulted in the protonation of the acyl group to yield a mixture of stable
VWYLIDENE COMPLEXES IN ORGANIC SYNTHESIS
361
hydroxycarbene and a vinylidene from dehydration of an oxycarbene. Deprotonation of the
vinylidene/hydroxycarbene mixture so obtained by potassium tertiary butoxide base led to a
1:1 acetylide/acyl complex mixture which on further reaction with triflic anhydride resulted
in protonation of the acetylide to give the vinylidene (Scheme 6)'7,'8.
C=0
I
C
(cF3so2 ) 2 0
CH2
I
R
I
K
R
+
GOH
1
y"'
R
H
[Re] = Re (NO) (PPh,) (11-C, H5)
R = H, Me, Ph, 1-ClaH7
SCHEME
6 . Vmylidene complex lrom acyl-metal complex
Similarly, the acylate anion from Mn(C0)s (q5-CjHj,)with MeLi, on reaction with a
proton sponge like, 1,8-bis(dimethy1amino) naphthalene, gives the vinylidene complex as an
intermedi* but it is the binuclear complex that is isolated finally (eqn 8)'?.
R GITA n m G SUNUAKARAJAN
362
The pseudo-tetrahedral complexes of (11'-Cp) (acyl) (CO) (PR?) Fe compkxes also react
rapidly with niflic anhydride to afford a variety of cationic cornplexes viu the intermediacy
of cationic carbene complexes (Fig. 3)".
,R
Fe+=
oc/ :'
C=C
L'
CF,SO,
R
''
R = Me; L = PPh3
R = H; L = PPh3
R = H; L = PMenPh
Fri.. 3. Some a i rhe vinylidene complexes obtained from (qi-Cp) (acyl) (CO) (PRI) Fe complex
4. Structure o f the vinylidene complexes
Free vinylidenes, if they exist, can he either in the singlet or triplet state and may be
represented as follows:
singlet
singlet
tnplet
In the metal complexes, either of the species is bonded to the metal atom via a ligand to
metal 0-donor bond and a metal to ligand n-acceptor bond. Back donation of the electron
density ro the ?(*orbitals of the C-C multiple bond systems can thus take place. The M=C=C
group is almost linear. The angle at C, is between 1.25 and 1.41 %1 which corresponds to a
b n d order between two and three. The M-C bond length is consistent with a bond order of
around t w ~ ~ ~ ' .
The IR spectra of all these complexes invariably show a characteristic y e c between 1620
and 1680 cm-I. The 'H-NMR spectrum does not give any information on the vinylidene as
such. However, I3C-NMR spectral data generally show a highly deshielded carbon at 320380 ppm (i.e., the C,) and Cp between 90 and 140 ppm. Mass spectral data for some of the
complexes are also a~ailable*~.
UV visible spectroscopic studies and electrochemical studies
on such vinylidene complexes have been canied out and are well documentedz9. Structural
parameters of several complexes are available and a typical crystal structure of vinylidene
complex is shown in Fig. J30.
VlNYLlDENE COMPLEXES IN OKGANTC SYNTHESIS
FIG 4 ORTEP diagram of
bonds.
ii
vmylidene cotnplcx showing the linear arrangement of the metal-carhan-carbon
Compounds having vinylidene ligands which have groups like Ph3Ge or P h 6 n instead of
hydrogen atom have been synthesised from the manganese complex, CpMn(CO)(THF).
Various acetylenes like H-CrC-Ph, PhsSi-C-C-Ph, and Ph&-C=C-Ph form the vinylidene
complex readily from the corresponding q2-alkyrre complexes (Scheme 7)5.This Type of
rearrangement is observed in the coordination sphere of rhodium with various trimethylsilyl
acetylenes3'.
R
/
CI-Rh=C=C
I/
C.,
L = PPrj
R = Me, Ph, COOEt, COOSiMe3
SCHEME7. q2-alkyne to vmylldene rearrangrmenr with tnalkylsdyl groups
5. Reactions of vinylidene complexes
It has been shown by theoretical calculations that the clectron deficiency at C, renders it
susceptible to mdeophilic attack and the localization of electron density in the metal-carbon
double bond and on Cp causes the chemical reactivity to be oriented towards ele~trophiles~~.
This prediction is very well borne by experimental results and it may also be conveniently
said that barring addition reactions of vinylidenes with olefins or alkynes, their chemistry is
B. GITA m o G. SUNDARARAJAN
364
dominated by nucleophilic attack on C , carbon, be it inter- or intra-molecular reactions with
the intermediacy of vinylidene complex implied or confirmed.
5.1. Elecrrophilic addition to P-carbon
Vinylidenes react with electrophiles at the B-carbon leading to the formation of thc carbyne
complex. The fomation of the carbyne complex could follow any of the steps outlincd in
Scheme 833.
H
[Re] = C = c < ~ ,
-
H
BH+
-
H-[Re] = C = C<ph
A
B
*
[Re]
B
C--C
BH+
I
-
H-[Re]
C -C
SCHEME8. Reaction of vinylidene complex with electrophiles: possible modcs of formation of the metal carbyne
complex
5.2. Nucleophilic addition to the a-carbon
One of the most common reactions of the vinylidene is its ability to convert readily to the
metalla-carbene complex upon treatment with alcohols (eqn 9)34.
R'OH
+
[MI = C = C H R
*
[MI = C
<
OR'
CH2 R
(9)
In some ca$es, a cyclic carbene complex was isolated when the substrates like H e C
(CH>),OH react with metal halide complexes. This could occur by a rapid intermolecular
addition of the alcohol function to a short-lived vinylidcne complex (cqn
(See later pan
for more examples with implied vinylidene complex formation.)
VINYLIDENE COMPLEXES IN ORGANIC SYNTHESIS
365
Another interesting reaction of the vinylidene complexes is the formation of a Claisen
system from R'R2C=C=Fe(CO)(PR3) (qS-CsHs)+and the ally1 alcohol, R3(OH)CHCH=CH(Rd)
to give the indicated product (eqn l l ) ? This concept was utilised more effectively in an
Ru-mediated coupling of acetylenes and ally1 alcohol and will be dealt with shortly.
(11)
[Fe] = Fe (CO) (PR'3) (q-CsHs)
p-lactam synthesis has been carried out exploiting the formation of the vinylidene
intermediate from [NMe4][CrC(O)Me(COs)] and the end product was isolated in
87-100% yield (eqn 12)37.
TsCl
Me
(CO),Cr
=6
:
H
M
e
(CO),Cr
=C =
CH2
ArCH = NMe
OTs
6. Reactions thought to proceed via vinylidenes
With the existence of vinyUdene complexes established and their modes of generation
characterized, postulating them as intermediates in reactions involving both a transition metal
complex and a terminal alkyne have become acceptable. Thus the following catalytic
reactions all have in common the suggestion of a vinylidene intermediate, although their
occurrence in the catalytic cycle is only a matter of inference.
B. GITA AND
G. SUNDARARAJAS
6.1. Vinylcar'bamares
Vinylcarbarnates can be synthesised from carbarnates and terminal alkynes and the coupling
reaction is catalysed by RuCli(PR4 (q6-CsMea) (eqn 1313'.
[Ru] = RuCl (PMe+ (q-Cdi6)
6.2.
a
.P
and!or P , y Unsaturated ketones
A general synthetic procedure for obtaining unsaturated ketones was reported recently by
Trost where a ruthenium catalyst med~atescoupling of terminal alkynes and allyl alcohols in
good yields (eqn 14)39.
A variety of acetylenic compounds have been coupled with a series of allyl alcohols and
a sample list is given in Table I. The catalytic cycle proposed by the authors involves the
formation of a vinylidene intermediate arising out of the interaction betueen the alkyne and
the metal catalyst as the key step. The vinylidene, presumably then reacts with the alcohol
to give a 'Chisent-type intermediate (vide eqn 11) which by reductive elimination gives the
product ketone and the metal catalyst, thus regenerating the catalytic cycle (Scheme 9) *,41.
Conventional synthesis of dihydrofurans (cyclic en01 ethers ) normally call for a multi-step
procedure4245.Singly decarbonylated (by me thy la mine-N-oxide) Mo(CO)6 induces cyclization
of 1-alkyn-4-01s to 2,3-diiydrofurans in a single step process with en route formation of a
vinylidene intermediate. The carbonyls of chromium and tungsten, however, give rise to the
corresponding metallacasbenes (eqn 15)46.
VINYLlDENE COMPLEXES IN ORGANIC SYNTHESIS
367
The authors suggest that the cyclo-isomerisation proceeds the following way. An q2alkyne complex formed initially isomerises to the vinylidene which cyclises intra-molecularly
to give an ql-bound cyclic en01 ether. This complex can yield the dihydrofuran by
protonation or can isomerise to the corresponding oxycarbene. In the case of chromium and
tungsten, according to the authors, the activation banier is high compared to molybdenum for
protonation4' and so the formation of dihydrofuran is difficult (vide Scheme 10).
6.4. Polymerisation of alkynes and cyclic olefins
It was demonstrated by Katz not long ago that metal carbenes catalyse polymerisation of
alkynes and strained cyclic ole fin^“^. AS vinylidene complexes are also structurally, in part,
metallacarbenes, they should be able to initiate such polymerisation reactions. It should also
be noted that stable vinylidene complexes may not be good catalysts. For instance, the
cationic complex containing the phenyl vinylidene ligand, viz., {trans-[FeCI(C2HPh)(depe)z]
PF6, depe =1,2-bis (diethyl phosphino)ethane) is very stable and shows no tendency to react
with alcohols to form alkoxy c a ~ b e n e s ~Thus,
~ . vinylidene complexes of metal carbonyls
containing no other stabilising ligand could very well be good catalysts for polymerisation of
alkynes and cyclic olefms. This was demonstrated by Geoffroy who showed that presence of
preformed metal carbene is unnecessary for initiating such reactionsSoand can be accomplished
by photolysis of metal carbonyls in the presence of a terminal alkyne, the key step suggested
Table I
Some of the acetylenes and ally1 alcoholr coupled by Ru complex (vide Scheme 9) to yield unsaturated
ketones ltaken from ref. 391
AceNIenr
Alcohoi
Pmducr
Time
Yield
herein being the rearrangement of q2-alkyne to vinylidene preceded by photochemically
aided loss of carbon monoxide ligand. In the presence of excess alkyne and extended
irradiation addition of alkyne molecule to the vinylidene and propagation of polymerisation
VINYLIDENE COMPLEXES IN ORGANlC SYNTHESIS
10. Proposed mechanism for the cydo-isomensation of 1-dkyn-4-01s to 2,3-dihydrofurans by Mo (CO)S
SCHEME
(NNle?).
reaction becomes straightforward (see Scheme 11). It is of pedantic interest to note here that
acetylene requires irradiation only for initiation of polymerisation while phenylacetylene
when used as the monomer calls for continuous irradiation through the c o m e of the reaction.
Ph
W (CO),
P h - C I C - H
~--------+
hu, -CO
.
c o w ,
Ph
(~OI~w=c-c<~
-0-+
H
Ph
Ph-CEC-H
Ph
b
(co),w=c=c<
h", -GO
Ph
1
----
--4
(CO)
w-c;
;
Cl-.
-
H
SCHEME
11. Mechmisrn of polymerisation of rcrminal alkynes by W(C0)6 under photolytic condnions
B. GITA
370
AND
G . SUNDARARAJAN
The need for a vinylidene intermediate for initiating metathetical polymerisation is
supported by the observation that internal alkynes and strained cyclic olefins which could be
polymerised readily by, say, metallacarbenes, do not react under these conditions to give
polymers. However, addition of a small amount of an initiator like phenylacetylene triggers
the ring-opening-polymerisation reaction of norbornene, possibly via the initial generation of
the vinylidene (Scheme 12)5'.
w (CO),
P h - C E G H
(CO)5W = C =
hexane, h i
SCHEUE
12. Polynarbomene from W(C0)s hv system with phenylacetylene as initiator.
7. Conclusion
From a pristine organometallic reation q2-alkyne to vinylidene rearrangement has grown into
a synthetically useful tool. Many reactions need to be unfolded that utilise such vinylidene
species because of their simplicity in generation; one needs just a transition metal complex
with a vacant coordination site and a terminal alkyne, and its high utility, as a variety of
elecbophiles and nucleophiles can add on to the vinylidene as have been shown earlier. Since
many such reactions involving vinylidenes are catalytic in nature and as the products obtained
are novel and difficult to obtain otherwise, the importance of this rearrangement has been
recognised in the recent literaturei2. It is only hoped that many such useful reactions can be
uncovered, like, for instance, application of c h i d vinylidenes for synthesis of c h i d
compounds.
Acknowledgement
GB thanks the Department of Science and Technology for a research fellowship.
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In this context, it is apt to recall a rather provocative
comment made by Prof. Dieter Seebach in this article:
'The discovery of truly new reactions is likely to be
limited to the maLn of transition-metal organic chemisuy,
which will almost certainly provide us with additional
'maacle reagents' & the y e m to come'.