Journal of the American Chemical Society
Article
Nonetheless, Ph PhCH CH CO Me can be alternatively
dimethyl-1-butene, even warming at 80 °C. It is interesting to
2
2
2
2
prepared in 1 h by the simple use of hydrated tetraethy-
mention that enones as substrates have been typically
14
4j,21
lammonium hydroxide.
The related acrylonitrile (ACN), with a more electron
hydrophosphanated by palladium complexes as catalyst,
whereas a pincer-cobalt complex has been recently reported as
suitable catalyst for the hydrophosphanation of the underex-
15
withdrawing group (EWG) than methylacrylate (MeAC),
4e
was also hydrophosphanated with PHPh . The catalysis was
plored cyclohexenone.
2
found to be slower than for MeAC, requiring 18 min at rt;
higher catalyst loadings (5% mol cat, entry 4) and around 2 h if
more diluted catalytic solutions were used (1% mol cat, entry
From the catalytic solutions, the functionalized phosphanes
DFT Studies. The extraordinary activity of complex 1 in the
hydrophosphanation of methylacrylate (MeAC, entry 2)
prompted us to analyze the first step of the catalytic cycle by
DFT-methods, i.e., the reaction between 1 and MeAC, whose
5
). These results were quite unexpected, since in a classic
outer-sphere phospha-Michael addition, a more EWG at the α-
position of the olefin is expected to produce an increase in the
reaction rate. Despite this, complex 1 operates more rapidly
and with higher conversions in the hydrophosphanation of
acrylonitrile with PHPh than other catalysts reported in the
2
−
1
literature. As a matter of fact, a TOF of 30 h has been
1
6
reported by Morris with [Ru(Cp*)(PPh )(Ph PCH
2
2
CHPPh )] as catalyst (1% mol, rt), whereas values of 10 and
2
−
1
6b
2
h
were observed by Glueck with [Pt(dppe)(H C
2
1
7
CHCN)] (10% mol cat., 50 °C) and Hey-Hawkins
(
[Mo(CO) (PH Fc)], 10% mol cat., 66 °C), respectively.
5 2
4g
Moreover, Waterman’s zirconium catalyst, the ruthenium
1
8
complex from Rosenberg, and the nickel complex from
1
9
Webster required around 18−24 h with catalyst loadings of
−10%. Furthermore, no byproducts derived from the
insertion of more than one molecule of alkene (telomerization)
5
8d
were observed in our case.
Similar reaction times were observed if an EWG is
incorporated at the β-position of the olefin, as observed for
dimethyl fumarate as a substrate (fum, entries 6 and 7). A
small influence of the cis/trans geometry on the reaction-rates
was observed, since slightly longer reaction times were
observed for dimethylmaleate (the cis-isomer of dimethyl
fumarate, entry 8). Lower catalytic loadings (e.g., 1% mol cat.)
with these substrates were associated with lateral reactions that
destroyed the catalyst. Nonetheless, complex 1 was robust
enough to allow the reutilization of the catalytic solutions (5%
mol cat.). Thus, after completion of the first catalytic run,
Figure 1. DFT computed (B3LYP-D3, 6-311G(d,p)/LanL2TZ(f))
Gibbs energy profile for the reaction of [Rh(Tp)H(PMe )(PPh )]
3
2
−
1
(
1) with methylacrylate. Relative ΔG values are given in kcal mol .
2
98
ipso
Only the C of the diphenylphosphanido ligand is shown for clarity.
Selected bond distances (Å) are the following: P−C2 3.567 (1···
MeAC), 2.387 (TS), 1.949 (A), 1.862 (B-MeAC); H−C1 2.617 (1···
MeAC), 2.601 (TS), 2.283 (A). Color code is the following: Rh
(yellow), O (red), N (blue), C (gray), B (orange), H (black). [Rh] =
dimethyl fumarate and PHPh were added again for five
2
“Rh(Tp)(PMe )”, R = CO Me.
3 2
(
Incorporation of a methyl group at the α-position (entry 9)
decreases the reaction rate considerably, which can be
attributed to both the increase of the steric effects and the
decrease of the positive charge on the α-carbon.
After its formation, the initial adduct 1···MeAC evolves to
the intermediate A through a very accessible transition state TS
−
1
(placed 16.7 kcal mol above 1 + MeAC) featuring an
incipient P−C bond. Intermediate A shows an almost strictly
planar five-membered phosphametallacycle Rh−P−C2−C1−
The reaction is also tolerant to aldehydes, a poorly studied
2
0
substrate in hydrophosphanation.
The corresponding
22
phosphanes were obtained in less than 1 h at rt. The catalysis
with cinnamaldehyde (entry 10) with a more EWG than
crotonaldehyde (entry 11) was found to be faster, suggesting
that electronic effects are more important than steric ones.
Small amounts (less than 1% mol) of the corresponding 1-
hydroxy-1,3-diphosphanes derived from the hydrophosphana-
tion of the CO group were observed. Moreover, the
diphosphane from cinnamaldehyde was almost quantitatively
formed (91.4% yield) in 55 min if the catalysis was carried out
H. In addition, the sum of the three bond angles around C1
and C2 is ∑° = 360.0° and 333.0°, respectively. Therefore, C2
has an almost tetrahedral geometry (with the new P−C bond),
2
whereas C1 retains the sp hybridization. Moreover, the
remaining p orbital of C1 directly points at the hydrido ligand,
an indication of interaction between C1 and the hydride,
clearly observed in the HOMO of the complex A (Figure 2).
This additional Rh−H···C1 interaction should stabilize
intermediate A, providing thus an easy path for the P−C
bond formation step. From A, the hydride transfer from
rhodium to C1 takes place directly. This transfer gives the
with a 1:10:40 ratio (1:aldehyde:PHPh ), while 73.6% of the
2
diphosphane was formed when using crotonaldehyde under
similar conditions even after 18 h of reaction.
bis(phosphane)rhodium(I) intermediate, [Rh(Tp)(PMe )-
3
Less-activated substrates with a ketone group such as
benzylideneacetone (entry 12) and cyclohexenone (entry 13)
required longer reaction times, whereas no reaction was
observed with nonactivated substrates, such as styrene or 2,3-
(PRPh )] (R = CH CH CO Me, B-MeAC), which was
2
2
2
2
−
1
found to be −6.5 kcal mol more stable than 1 + MeAC.
The energy profile for this first step in the case of dimethyl
fumarate (fum) was found to be quite similar, although in this
C
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX