C O M M U N I C A T I O N S
on larger scales and compared the findings with those obtained at 1 atm
of nitrogen (Table 3). The results suggest that the effects of carrying out
reactions at reduced pressure are not significant. Only the coupling of S5
carried out with 8W over 15 h suggests that reduced pressure may be
required to maintain high %Z for long periods. Some loss of monomer at
∼0.5 mm naturally is observed over extended times (∼14% measured in
the case of 8W after 15 h).
method if OR′′′ is sufficiently large (eq 1); formation of Z-product
in the primary step obviously is required for a Z-selective reaction.
One possible indirect mode of forming E-product is for the Z-product
to be isomerized through reaction with a M ) CHR1 species to afford
a trisubstituted metallacycle that contains two adjacent trans substit-
uents. Such a process is likely to be relatively slow in many
circumstances for steric reasons5 because two R1 groups must be
oriented toward the large OR′′′ in the metallacyclobutane, if that
metallacyclobutane is the only type that forms. A second possible
indirect mode is for the reverse of eq 1 to be fast (ethenolysis8), but
only if the monomer is reformed and recoupled many times in the
presence of ethylene, and if a “mistake” that results in formation of
E-product in any single step (eq 1 and immediately above) is thereby
magnified. On the basis of the results summarized in Table 3, rapid
and repeated ethenolysis is probably not the main pathway giving rise
to E-products with the catalysts and substrates explored here. Therefore,
at this stage we propose that E-product forms primarily through
isomerization of the initial Z-product.
Formation of high %Z homocoupled acyclic products, as described
herein, has, to the best of our knowledge, never been observed. The
experiments detailed here have evolved as a consequence of our discovery
and investigation of MAP catalysts.1-6,8 We expect that the results will
continue to depend sensitively upon a combination of steric factors within
each catalyst and substrate, and upon experimental conditions.
It should be noted that intermediate metallacyclobutanes in ruthenium-
based metathesis catalysts that contain two chlorides9 are proposed to be
14 electron TBP species with the chlorides in axial positions.10 Axial
chlorides would not be able to control the substitution pattern in the
ruthenacyclobutane in the manner that we have achieved with the MAP
complexes described here. No comparable Z-selectivities in homocoupling
reactions have been reported for ruthenium-based catalysts.
Table 3. Effect of Reduced Pressure on Z-Content (1 mol % cat.)a
Substrate
Catalyst
P (mmHg)
t (h)
% Conv
%Z
S5
S5
S5
S5
S5
S5
S5
8W
∼0.5
∼760
∼0.5
∼760
∼0.5
∼760
10
∼760
∼0.5
∼760
∼0.5
0.2 (15)
1.5 (15)
0.6 (16)
0.6 (16)
5 (21)
5 (21)
19
19
2
2
14
25 (>98)
84 (86)
36 (34)
24 (24)
7 (22)
10 (27)
62
42
64
52
70
>98 (>98b)
97 (88)
61 (61)
61 (59)
>98 (>98)
>98 (>98)
88
90
94
96
95
8W (2%)
c
12Mo
12Mo
4W
4W
3Mo
c
S7
3Mo
S10
12Mo(2%)
a Reaction scale ∼200 mg, neat substrate, catalyst added as a solid.
b 86% yield after 15 h. c 12Mo ) Mo(NAr)(Pyr)(CHR2)(Mes2BitetOMe).
The results of reactions performed at 80-120 °C (bath temp), larger
scales, and lower catalyst loadings are shown in Table 4. In several cases,
the remaining monomer was removed in Vacuo and the Z-product yields
were established. A number of reactions proceed with >90% Z-selectivity
and in good yield.
Table 4. Reactions Carried out at Elevated Temperaturesa
Substrate
Catalyst
%
t (h)
°C (Bath)
%Conv.
%Z
%Yield
S1
S2
4W
0.4
0.4
0.2
0.2
0.2
0.2
2
48
24
3
80
120
120
120
100
90
100
100
100
100
100
100
90
72
94
>98
63
94
28
>97
97
46
74
46
95
95
86
77
93
88
86
95
87
91
94
>98
91
94
58
78
77
56
65
26
4W
10W
4W
Acknowledgment. We are grateful to the National Science
Foundation (CHE-0554734 to R.R.S.) and to the National Institutes
of Health (Grant GM-59426 to R.R.S. and A.H.H.) for financial
support.
S3
4
10W
24
18
23
16
1
18
24
18
24
b
S4
S5
S6
S7
13W
b
13W
b
13W
1
80
4W
0.2
0.2
4
4
4
Supporting Information Available: Experimental details for the
synthesis of all compounds and metathesis reactions. This material is
10W
5W
S8
S9
42
90
36
4W
10W
50
a Reaction scale ∼0.5 g to ∼4 g; catalyst was dissolved in ∼1 mL of
benzene, and substrate was added in one portion. The mixture was refluxed
under N2. b W(NAr′)(Pyr)(C3H6)(OHIPT).
References
(1) Schrock, R. R. Chem. ReV. 2009, 109, 3211.
(2) (a) Hock, A.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2006, 128,
16373. (b) Singh, R.; Schrock, R. R.; Mu¨ller, P.; Hoveyda, A. H. J. Am. Chem.
Soc. 2007, 129, 12654.
We propose that the efficiency of the mechanism of formation of
Z-product shown in eq 1 depends upon a ligand combination that allows
only a syn,syn-R,R1/ꢀ,R1 metallacyclobutane to form from a syn alkylidene.
Therefore, a large OR′′′ ligand is required to form Z-product with high
selectivity, as we proposed previously. A “small” imido group is not
required, most likely because the steric demands of a syn,syn-R,R1/ꢀ,R1
metallacyclobutane are less severe than the steric demands of an all syn,
trisubstituted metallacycle.5
A critical question relates to the mechanism of formation of
E-product. Three possible direct ways of forming E-product are (i)
approach of monomer to the syn alkylidene to yield a metallacycle
with R1 pointed toward OR′′′; (ii) reaction of monomer with a highly
reactive (unobservable) anti alkylidene (in equilibrium with a syn
alkylidene) to give a trans disubstituted metallacyclobutane inter-
mediate; and (iii) approach of the monomer in a manner different
from that shown in eq 1 to generate a different type of metallacy-
clobutane. On the basis of what we know at present we propose
that a significant amount of E-product is not formed through a direct
(3) (a) Malcolmson, S. J.; Meek, S. J.; Sattely, E. S.; Schrock, R. R.; Hoveyda, A. H.
Nature 2008, 456, 933. (b) Sattely, E. S.; Meek, S. J.; Malcolmson, S. J.; Schrock,
R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 943. (c) Meek, S. J.;
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(4) Ibrahem, I.; Yu, M.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2009,
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(5) Flook, M. M.; Jiang, A. J.; Schrock, R. R.; Mu¨ller, P.; Hoveyda, A. H. J. Am.
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(6) (a) Jiang, A. J.; Simpson, J. H.; Mu¨ller, P.; Schrock, R. R. J. Am. Chem. Soc.
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(7) See Supporting Information for other examples and syntheses of all new
catalysts (*).
(8) Marinescu, S. C.; Schrock, R. R.; Mu¨ller, P.; Hoveyda, A. H. J. Am. Chem.
Soc. 2009, 131, 10840.
(9) Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, 2003;
Vols. 1-3.
(10) (a) Romero, P. E.; Piers, W. E. J. Am. Chem. Soc. 2005, 127, 5032. (b) Romero,
P. E.; Piers, W. E. J. Am. Chem. Soc. 2007, 129, 1698. (c) Webster, C. E.
J. Am. Chem. Soc. 2007, 129, 7490. (d) van der Eide, E. F.; Romero, P. E.;
Piers, W. E. J. Am. Chem. Soc. 2008, 130, 4485. (e) Wenzel, A. G.; Grubbs,
R. H. J. Am. Chem. Soc. 2006, 128, 16048.
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