Alkyne-to-vinylidene transformation on trans-(Cl)Rh(phosphine)2: Acceleration by a heterocyclic ligand and absence of bimolecular mechanism

Article abstract of DOI:10.1021/ja058736p

Alkyne-to-vinylidene transformation on square-planar trans-(Cl)Rh(phosphine)₂ has been proposed to proceed by a mechanism with a key step being bimolecular transfer of hydridic H to an alkynyl carbon. Labeling studies reported here are inconsistent with this pathway. In addition, an imidazolyl substituent accelerates the transformation. Copyright

Full text of DOI:10.1021/ja058736p

Published on Web 02/10/2006
Alkyne-to-Vinylidene Transformation on trans-(Cl)Rh(phosphine)2:
Acceleration by a Heterocyclic Ligand and Absence of Bimolecular
Mechanism
Douglas B. Grotjahn,* Xi Zeng, and Andrew L. Cooksy
Department of Chemistry and Biochemistry, 5500 Campanile DriVe, San Diego State UniVersity,
San Diego, California 92182-1030
Received December 23, 2005; E-mail: grotjahn@chemistry.sdsu.edu
Scheme 1. Proposed Alkyne-to-Vinylidene Mechanisms
One striking advantage of metal-catalyzed processes is the
profound changes in the polarity of substrates when they are
coordinated to the metal. A key step in several catalytic reactions
which fits this class is the transformation of a terminal alkyne (A,
Scheme 1) to a vinylidene ligand (C or D).1 The polarity of alkyne
C1 and C2 is reversed such that C1 of the vinylidene ligand may
be attacked by relatively weak oxygen nucleophiles in an anti-
Markovnikov fashion.1b,c,e,f,2 A number of groups have studied
alkyne-to-vinylidene transformation on [CpRu(phosphine)2]+ frag-
ments,3 a reaction which we are studying in the context of catalytic
hydration of alkynes.2e,g
Here we report that alkyne-to-vinylidene transformation on
significantly different Rh(I) fragments (1 to 2, Scheme 2) is
accelerated when R1 ) Ph is replaced by an imidazol-2-yl group.
Previous beautiful work by Werner and co-workers4a showed that,
in the nonheterocyclic case (1a, R1 ) i-Pr), alkyne-to-vinylidene
transformation appeared to proceed in a two-step process, by way
of a hydrido(alkynyl) isomer (Scheme 1, path a).4 For the second
step of path a, debate has centered on the molecularity of the 1,3-H
shift:1g in general, for group 9 fragments, a unimolecular mechanism
has been put forth,4c but in the case of trans-(Cl)M[phosphine]2
(M ) Rh, Ir), both a major computational study4d and some
circumstantial evidence4a suggest the unusual bimolecular conver-
sion shown at the bottom of Scheme 1. However, here we detail
crossover experiments which are inconsistent with a bimolecular
mechanism, regardless of whether a pendant heterocyclic base is
present or not.
Scheme 2. Ligands and Complexes Used in This Work
Werner’s group readily isolated alkyne complexes of type 1a in
high yield and observed isomerization to vinylidene complexes 2a
within hours at 30-50 °C and also noted that the process “...la¨sst
sich durch Zugabe von Pyridin beschleunigen” (“...may be acceler-
ated by addition of pyridine”).4a Our first attempt to make 1-hexyne
complex 1b-HC2Bu using heterocyclic phosphine 3b led to a
surprising result: mixing 1-hexyne, [(µ-Cl)Rh(cyclooctene)2]2, and
3b in molar ratio 1:0.5:2 led to the immediate and clean formation
of vinylidene complex 2b-CCHBu.5 In fact, the reaction is
essentially a titration, forming a beautiful deep permanganate-purple
solution within seconds.
The obvious acceleration in formation of 2b from 3b relative to
reported reactions using the spectator ligand 3a prompted the more
thorough study described here. For more accurate steric and
electronic comparisons, 3b and 3c were used and cyclooctene-free
materials were made to avoid any complications from the alkene
ligand. Thus, mixing [(µ-Cl)Rh(cyclooctene)2]2 and 3c in a molar
ratio of 0.5:2 led to the formation of a purple solid in 95% yield,
made by Werner et al.6a and formulated as dimer 4 (Scheme 2) in
analogy to the purple complex from 3a characterized by X-ray
diffraction.6b In contrast, when 3b was used, orange-yellow crystals
of complex 5 were obtained in 95% yield. Formulation as the
unsymmetrical monomer 5 with its cis-arrangement of phosphines
2
was concluded on the basis of two 31P NMR doublets, JPP ) 42
Hz.
Addition of 1-hexyne to either 4 or 5 in CD2Cl2 solution at -20
°C led to an equilibrium mixture containing the starting complex,
free alkyne, and mostly the symmetrical alkyne π-complexes 1c-
HC2Bu or 1b-HC2Bu, respectively, in which the alkyne complexes
1
exhibited 31P NMR doublets and diagnostic H NMR resonances
for the terminal alkyne proton.5 With the same total concentrations
of Rh and alkyne, equilibrium lay further to the side of alkyne
complex in the nonheterocyclic case. Observation of these solutions
at -20 °C in the NMR probe showed that the consumption of
alkyne π-complex occurred 9.6 times more rapidly in the imidazole
case, but the reaction order could not be determined.5 Hydrido-
(alkynyl) complexes or other intermediates were not detected in
these reactions.
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10.1021/ja058736p CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. Results of Crossover Experiments Forming 2a
by the imidazol-2-yl phosphine studied (b vs c or a). Of greater
significance for vinylidene chemistry, in general, double crossover
experiments clearly are inconsistent with significant involvement
of a bimolecular mechanism for fragments derived from the three
ligands studied, including P(i-Pr)3. Future reports will attempt to
clarify the mechanism of alkyne-to-vinylidene transformations in
these and related systems, using a combination of experimental and
theoretical approaches.
Acknowledgment. The NSF is thanked for generous support
of this and related work, and Dr. Ashok Krishnaswami of JEOL
USA for the D NMR results.
Supporting Information Available: Details of compound prepara-
tion and characterization. This material is available free of charge via
the Internet at http://pubs.acs.org.
References
a Upper bounds given are estimated detection limits.
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The acceleration does not appear to be a matter of an elec-
tronic effect of the heterocycle, which was probed using alkyne
π-complexes 1c- or 1b-H13C13CH, formed from H13C13CH and
either 4 or 5 in CD2Cl2 at -20 °C. Analysis of the ten-line alkynyl
proton AA′XX′ pattern revealed that 2JCC for the coordinated alkyne
was identical in each case (117.0 Hz), as were the other C-H and
H-H couplings within experimental uncertainty. The strong back-
bonding of the metal to the alkyne π-system is obvious by noting
the medium-dependent value of 2JCC ) 166-170 Hz for acetylene
itself.7 Moreover, when the solutions of acetylene complex were
allowed to warm to room temperature, 2b- and 2c-13C13CH2 with
2JC1-C2 ) 58.0-58.7 Hz were seen, again consistent with the same
degree of back-bonding of metal to vinylidene.8
In an effort to establish whether the isomerization of alkyne to
vinylidene in this system is bimolecular or unimolecular, a series
of double crossover experiments were performed (Scheme 3). We
note that a double crossover experiment was attempted on hydrido-
(alkynyl)iridium species of type B with the P(i-Pr)3 ligand, but rapid
H-D exchange between the hydridic starting materials prevented
a conclusion regarding molecularity.4a In our work, adding an
equimolar mixture of terminal alkynes DCC(CH2)3CH3 and
HCCCHMe2 (0.6 equiv of each relative to the amount of Rh) to
either 4 or 5 at -20 °C produced mixtures of alkyne π-complexes
1, which evolved to vinylidene complexes (2) as the mixtures were
warmed. Significantly, with an estimated detection limit of 5-10%,
no crossoVer was detected, regardless of ligand type, inconsistent
with significant bimolecular 1,3-H shift.
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(5) Complete details of experiments and compound characterization are in
Supporting Information.
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2
(8) For examples of changes JC1-C2 as a function of ligands, see: Moigno,
D.; Callejas-Gaspar, B.; Gil-Rubio, J.; Werner, H.; Kiefer, W. J.
Organomet. Chem. 2002, 661, 181-190.
However, because the propensity for bimolecular reaction could
be a function of hindrance on the alkyne or the phosphine and
because the previous studies4a,d focused on acetylene itself and the
P(i-Pr)3 ligand, we allowed a 1:1 mixture of 1a-HCCH and 1a-
D13C13CD to convert to vinylidene complexes 2 at 25-30 °C in
C6D6. Remarkably, formation of 2a-CCH2 and 2a-13C13CD2 is
consistent with an absence of crossover even in this system.9
Notably, our estimated limit of detection of D in 2a-CCH2 is 1%.
In summary, our preliminary results show an intriguing accelera-
tion of alkyne transformation on these square-planar Rh(I) centers
(9) The absence of D in 2a-CCH2 formed from 1a in the presence of the
D13C13CD isotopomer is the most compelling evidence against crossover.
See Supporting Information for full details. Rearrangement of pure 1a-
D13C13CD in silylated and dried NMR tubes gave 2a-13C13CD2 along
with 10-20% of the 13C13CHD isotopomer, which complicated the analysis
somewhat. In these experiments, the source of the H could not be
determined, but it would not appear to be extraneous water because the
amount of H incorporation was unaffected by pretreating the NMR tube
with D2O before drying, or by the presence or absence of 1a-HCCH.
Moreover, the vinylidene protons of 2-CCH2 did not exchange with D2O
at room temperature in C6D6.
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