A rare η2-butadienyl complex from an alkyne double insertion with double vinylidene rearrangement

Article abstract of DOI:10.1021/ja0297281

A rare η²-butadienyl Ir(III) complex with a weak Ir...C bond is formed from 1-alkyne double insertion with the independent double alkyne to vinylidene rearrangement. A reaction intermediate is isolated, and labeling and crossover experiments indicate the intramolecularity of both alkyne to vinylidene rearrangements. Copyright

Full text of DOI:10.1021/ja0297281

Published on Web 03/01/2003
A Rare η2-Butadienyl Complex from an Alkyne Double Insertion with Double
Vinylidene Rearrangement
Xingwei Li, Christopher D. Incarvito, and Robert H. Crabtree*
Department of Chemistry, Yale UniVersity, New HaVen, Connecticut 06520-8107
Received December 12, 2002; E-mail: robert.crabtree@yale.edu
Rearrangement of η2-1-alkynes RCtCH to metal-bound vi-
nylidenes, RHCdCdMLn, is well known,1 and the resulting
carbenes can undergo vinyl,2a-c alkyl,2d alkynyl, or aryl insertion,2e-h
a reaction that can lead to useful C-C coupling products. Double
insertions are sometimes only apparent: for example, the proposed
double insertions of CO were subsequently shown to proceed via
another pathway;3 true multiple insertions are known for isonitriles
RNdCdMLn.4 Here we report a double insertion of alkynes with
the double alkyne to vinylidene rearrangement together with
mechanistic studies that indicate each alkyne independently un-
dergoes intramolecular rearrangement.
Starting material 1 was obtained (91%) by refluxing [IrH2-
(acetone)2(PPh3)2]SbF6 and 2 equiv of trans-PhCHdCHC(O)Me
in acetone for 4 h.5 1 then reacts (CH2Cl2, 25 °C) with 2 equiv of
RCtCH (R ) Ph, PhCH2) to give the title products 2 (X ) H)
(Scheme 1). Both crystallographic and spectroscopic data support
the presence of a rare η2-butadienyl coordination in 2. As shown
in the crystal structure of 2 (R ) Ph, X ) H)6a (Figure 1), C-12 is
Figure 1. Crystal structure (ORTEP representation) of the cation part of
2 (R ) Ph, X ) H) with 50% thermal probability ellipsoids.6a Hydrogen
weakly bound2c [Ir‚‚‚C(12) ) 2.509(9) Å] to the metal, but C-11
atoms are omitted for clarity. Selected (bond) distances (Å): Ir(1)-O(1)
) 2.204(3); Ir(1)-C(4) ) 1.991(4); Ir(1)-C(11) ) 3.092(8); Ir(1)-C(12)
[Ir‚‚‚C(11) ) 3.092(8) Å, comparable to Ir‚‚‚C(14) ) 3.110(9) Å]
is outside the range of any coordinative interaction. Consistent with
previous η2-allyl or benzyl complexes,7a,b,8 13C NMR spectra of 2
show large (>30 ppm) chemical shift differences between C-11
and C-12.6b The C-11, C-12 double bond is twisted (77.9°, R )
Ph, X ) H) and is almost deconjugated from the C-13, C-14 double
bond. The Ir‚‚‚C(12) bond, readily replaced by CO, is weak (<10
) 2.509(9); Ir(1)-(C13) ) 1.994(4); Ir(1)-C(14) ) 3.110(9); Ir(1)-P(1)
) 2.3788(12); Ir(1)-P(2) ) 2.3698(12); C(11)-C(12) ) 1.3494(10).
Scheme 1 a
1
kcal/mol7a): the VT H NMR of 2 (R ) PhCH2, X ) H) at -85
°C failed to decoalesce the diastereotopic protons in either CH2
group. Such η2-allyl or benzyl complexes have mostly been reported
for early transition metals,7 and only one is known for a late metal
(Ru).8
a (a) CH2Cl2, 0 °C, 1 h, R ) Ph, X ) H, 88%; R ) Ph, X ) D, 86%;
R ) PhCH2, X ) H, 92%; R ) PhCH2, X ) D, 89%. (b) 1 atm CO, CH2Cl2,
25 °C, 5 min, R ) Ph, X ) H, 100%.
The reaction between 1 and RCtCD (R ) Ph or PhCH2) gives
products exclusively labeled (>97%) at the terminal carbons C-11
and C-14. For 2 (R ) Ph, X ) H), the key assignment of the proton
NMR peaks for H-11, H-12, and H-14 follows from the charac-
teristic 16.2 Hz 3J(H-11, H-12) trans H-CdC-H coupling constant
and the 1.6 Hz 4J(H-12, H-14) coupling constant (acetone-d6).
Additional 3J(H, H) couplings between H-11, H-14 and their
adjacent CH2 groups in 2 (R ) PhCH2, X ) H) confirmed this
assignment, which is also consistent with prior work.7a,b
Additional information was obtained from the R-tetralone series,
where intermediates could be isolated (Scheme 2). A crystallo-
graphically and spectroscopically (VT NMR) characterized C-H
agostic kinetic product 4,9 initially isolated (91%), is a precursor
to the η2-butadienyl complex 5, the thermodynamic product (89%
from 3).
Scheme 2 a
a (a) 2 equiv of PhCCH, CH2Cl2, 25 °C, 15 min. (b) CH2Cl2, 35 °C, 15
h.
with the vinylidene moiety, one of the strongest π-acceptors,10
receiving minimal back-donation when trans to a high trans-effect
ligand in a cationic metal complex (Scheme 3). The kinetic product
(4) is formed from the migratory insertion of the vinyl group anti
to the â-Ph of the vinylidene ligand, which has a free rotation
around the metal-vinylidene bond,10a but 5 is more thermodynami-
cally stable, having the iridium trans to the terminal Ph- group.
Analogues of 4 have been previously seen as stable products,11 and
rearrangement of 4 to 5 very likely goes through an η2-vinyl or a
zwitterionic carbene intermediate.12
The labeling experiment suggests the following proposed mech-
anism (Scheme 3). This involves an alkyne to vinylidene rear-
rangement followed by a hydride insertion, another alkyne to
vinylidene rearrangement, and a migratory insertion of the vinyl
to the vinylidene. In our system, this migratory insertion is favored
9
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J. AM. CHEM. SOC. 2003, 125, 3698-3699
10.1021/ja0297281 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
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Previously proposed mechanisms for alkyne to vinylidene
rearrangement go by (i) RCtC-H oxidative addition13 followed
by either a 1,3-hydrogen shift13f or a bimolecular proton shift,13b
(ii) an R-hydrogen elimination of the metal-vinyl intermediate
(from CtC insertion into M-H),14 or (iii) a concerted intraligand
1,2-hydrogen shift.15 No definitive experimental evidence for path
(iii) has previously been obtained. Our M-H does not scramble
but appears (>97%) as H-12 in 2 (Scheme 1). This eliminates path
(ii) and makes path (i) unlikely because any oxidative addition
should give either a fluxional seven-coordinate IrV(H)(D) species
or, more likely,13f,16 an IrIII(H-D) complex, both leading to H/D
exchange. We carried out two crossover studies, one with PhCH2Ct
CH/PhCtCD, the other with PhCH2CtCD/PhCtCH. To com-
pensate for their different reactivities, we used a 0.8:1.2 ratio of
PhCH2CtCH(D)/PhCtCD(H). All four possible products were
obtained (1H and 2H NMR), but there is essentially no (<5%) proton
scrambling between RCtCH and R′CtCD in both experiments.
This was concluded by identifying the separately distinguishable
H-11 and H-14 1H NMR signals associated with R or R′ and
measuring the D/H ratio. This result is consistent with path (iii),
where each η2-alkyne independently rearranges to a C-H agostic
η2-alkyne, followed by intraligand 1,2-hydrogen shift.
In summary, a rare η2-butadienyl Ir(III) complex is formed via
a concerted alkyne to vinylidene rearrangement. A reaction
intermediate was isolated, and isotope labeling indicates that the
alkyne to vinylidene rearrangements are concerted intramolecular
processes without crossover.
Acknowledgment. We gratefully acknowledge the U.S. Depart-
ment of Energy and the Johnson Matthey Co. for the financial
support of this work.
Supporting Information Available: Experimental procedure for
1
the synthesis of all of the complexes, H and 13C NMR spectra, and
(16) Ge´rard, H.; Eisenstein, O.; Lee, D.-H.; Chen, J.; Crabtree, R. H. New J.
X-ray crystal structure data for 2 (R ) Ph, X ) H) and 4 (PDF and
CIF). This material is available free of charge via the Internet at http://
pubs.acs.org.
Chem. 2001, 25, 1121.
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