Highly Z-selective metathesis homocoupling of terminal olefins

Article abstract of DOI:10.1021/ja908098t

(Chemical Equation Presented) Mo and W MonoAryloxide-Pyrrolide (MAP) olefin metathesis catalysts can couple terminal olefins to give as high as >98% Z-products in moderate to high yields with as little as 0.2% catalyst. Results are reported for 1-hexene, 1-octene, allylbenzene, allyltrimethylsilane, methyl-10-undecenoate, methyl-9-decenoate, allylB(pinacolate), allylOBenzyl, allylNHTosyl, and allylNHPh. It is proposed that high Z-selectivity is achieved because a large aryloxide only allows metallacyclobutanes to form that contain adjacent cis substituents and because isomerization of Z-product to E-product can be slow in that same steric environment.

Full text of DOI:10.1021/ja908098t

Published on Web 10/30/2009
Highly Z-Selective Metathesis Homocoupling of Terminal Olefins
Annie J. Jiang,† Yu Zhao,† Richard R. Schrock,*,† and Amir H. Hoveyda‡
Department of Chemistry 6-331, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467
Received September 29, 2009; E-mail: rrs@mit.edu
Highly reactive monoaryloxide-pyrrolide (MAP) olefin metathesis
catalysts of Mo1 (1) have been prepared from bispyrrolide precursors2 in
situ and employed for enantioselective olefin metathesis reactions, such
as a total synthesis of quebrachamine (when R ) 2,6-i-Pr2C6H3, R′ )
Me, R′′ ) Br).3 Cross-metatheses with 1 (when R ) 1-adamantyl) were
found to be Z-selective, as well as enantioselective, as a consequence, we
proposed, of restrictions imposed on substituents within the metallacy-
clobutane in the TBP intermediate by a “large” axial aryloxide in
combination with a “small” imido group.4 The above principle guided
the synthesis of related complexes 2 (R′ ) H (2H) or Me (2Me)), which
have been shown to catalyze the Z-selective metathesis of selected internal
Z-olefins and to polymerize substituted norbornadienes to yield >99% cis
and >99% syndiotactic polymers.5 We report here that the principles of
Z-selectivity by MAP catalysts can be extended to homocoupling of
terminal olefins.
is not necessary for highly Z-selective couplings of a terminal olefin and
(ii) W-based complexes deliver higher %Z than Mo complexes. Examples
of higher selectivity furnished by W include 3W versus 3Mo and 5W versus
5
Mo.7 Compound 4W6a is as effective as 3W, and 95% Z-product is obtained
at low conversion. The Z-product isomerizes to E with time and conversion
(e.g., see 5W). The results for the homocoupling of 1-hexene are similar
to those shown in Table 1 in all cases; for example, catalyst 4W gave
95% Z-5-decene at 33% conversion after three days.
Table 1. Homocoupling of 1-Octene (S2)a
Catalyst
time
% Conv
%Z
2Me
3Mo
3W
Mo(NAd)(Me2Pyr)(CHR2)(OHIPT)b
Mo(NAr)(Pyr)(CHR2)(OHIPT)b
W(NAr)(Pyr)(CHR2)(OHIPT)b
W(NAr)(Pyr)(C3H6)(OHIPT)
3 h
43
80
88
33
58
38
72
68
40
88
95
70
93
88
20 m
26 h
3 h
15 m
30 m
2 h
4W
5Mo
5W
Mo(NAr)(Pyr)(CHR2)(Mes2Bitet)*b
W(NAr)(Pyr)(CHR2)(Mes2Bitet)*b
a 4 mol % cat in C6D6 at 22 °C; R2 ) CMe2Ph; Ad ) 1-adamantyl; Ar )
2,6-i-Pr2C6H3; OHIPT is the aryloxide shown in structure 2; Mes2Bitet is the
ligand in structure 1 with R′′ ) mesityl. b Prepared in situ; see Supporting
Information.
The principles of Z-selective homocoupling of R1CHdCH2 by a
syn,rac-MAP complex are shown in eq 1. (Only syn isomers1 have been
observed to date in NMR or X-ray studies.) The terminal olefin enters
the coordination sphere trans to the pyrrolide (Pyr)5,6 to generate an
intermediate metallacyclobutane with adjacent R1 substituents pointing
away from the axial OR′′′ group; this scenario holds if OR′′′ is sufficiently
large to prevent formation of any metallacycle in which R1 is oriented
toward OR′′′. Loss of Z-R1CHdCHR1 yields a methylene complex with
an inverted configuration at the metal center (SfR in eq 1). A productive
metathesis reaction between the methylene species and R1CHdCH2 then
yields ethylene and reforms (S)-M(NR)(CHR1)(Pyr)(OR′′′). Inversion of
configuration at the metal center is a consequence of each forward
metathesis step.5,6 Inversion itself should not be an important feature of a
homocoupling reaction, but if OR′′′ is enantiomerically pure, then both
diastereomers must be Z-selective.
a
Table 2. Screening of Substrates S3-S9
Sn
Catalyst
t (h) % Conv %Z
S3 3W
S3 4W
S3 6W
W(NAr)(Pyr)(CHR2)(OHIPT)b
3
14
3
3
1.5
3
3
1.5
3
1.5
3
40
52
62
33
69
33
30
70
33
24
52
91
94
93
98
92
90
94
96
98
98
98
W(NAr)(Pyr)(C3H6)(OHIPT) (2%)
W(NArCl)(Pyr)(CHR2)(Mes2Bitet)*b
S4 7Mo Mo(NAd)(Me2Pyr)(CHR2)(Br2Bitet)*b
S5 8W
S6 9W
S7 3W
W(NAr′)(Pyr)(CHR2)(Mes2BitetOMe)*b
W(NAr′)(Pyr)(CHR2)(OHIPT)*b
W(NAr)(Pyr)(CHR2)(OHIPT)b
S7 10W W(NArCl)(Pyr)(CHR3)(OHIPT)*
S7 7Mo Mo(NAd)(Me2Pyr)(CHR2)(Br2Bitet)b
S8 11W W(NArCl)(Pyr)(CHR3)(Mes2Bitet)*b
S9 11W W(NArCl)(Pyr)(CHR2)(Mes2Bitet)*b
a 4 mol % cat in C6D6 at 22 °C in NMR tube; R2 ) CMe2Ph; ArCl
)
2,6-Cl2C6H3; Ar′ ) 2,6-Me2C6H3; Br2Bitet is the ligand in structure 1 with R′′
) Br; Mes2BitetOMe is the methyl-protected analogue of the ligand in structure
1 with R′′ ) Mesityl; R3 ) t-Bu. b Prepared in situ.
A selection of some of our findings in connection with initial screening
of substrates S3-S9 is shown in Table 2.7 Note that 7Mo and 3W both give
high %Z for S7, although a direct comparison of Mo and W is not possible
since 1-adamantyl imido species of W are not known. It should be noted
also that moderate conversion is satisfactory since the product can be
separated easily from the unreacted substrate, which can be recycled.
To test the degree to which ethylene might be deleterious to Z-
selectivity, we explored reactions involving several of the higher boiling
substrates under a 0.5 or 10 mmHg vacuum with 1 or 2 mol % catalyst
Initially, we concentrated on small-scale experiments involving 1-hex-
ene (S1) or 1-octene (S2; Table 1; eq 2) with 4 mol % catalyst in a closed
system (NMR tube or vial). On the basis of extensive screening (see
Supporting Information) we conclude that (i) a relatively small imido group
† Massachusetts Institute of Technology.
‡ Boston College.
9
16630 J. AM. CHEM. SOC. 2009, 131, 16630–16631
10.1021/ja908098t CCC: $40.75  2009 American Chemical Society

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
available free of charge via the Internet at http://pubs.acs.org.
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).
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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
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trisubstituted metallacycle.5
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approach of monomer to the syn alkylidene to yield a metallacycle
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reactive (unobservable) anti alkylidene (in equilibrium with a syn
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