Excellent Z-Selective Olefin Formation Using Pentacoordinate Spirophosphoranes and Aldehydes. Wittig Type Reaction via Hexacoordinate Intermediates

Full text of DOI:10.1021/ja964122w

5970
J. Am. Chem. Soc. 1997, 119, 5970-5971
Excellent Z-Selective Olefin Formation Using
Pentacoordinate Spirophosphoranes and Aldehydes.
Wittig Type Reaction via Hexacoordinate
Intermediates
The phosphoranes 4Aa-c were prepared from the P-H
9
phosphorane by alkylation with the corresponding haloacetic
esters in the presence of 1,8-diazabicyclo[5.4.0.]undec-7-ene
5
(
DBU). Compounds 4A can be treated with water and
subjected to chromatography without noticeable amounts of
1
0
decomposition.
Treatment of 4A with base followed by
Satoshi Kojima, Ryukichi Takagi, and Kin-ya Akiba*
benzaldehyde furnished cinnamic esters 6a with high Z-
+
+
+
selectivity along with oxidophosphorane 7A-M (M ) Li ,
Department of Chemistry, Faculty of Science
Hiroshima UniVersity, 1-3-1 Kagamiyama
Higashi-Hiroshima 739, Japan
+
+
+
+
31
9a
Na , K ) (for M ) H : P NMR (CDCl3) δ -14.6), also
a stable compound, even though the reactions were carried out
at 0 °C to room temperature (Scheme 1). Isomeric ring-opened
11
+
+
ReceiVed December 2, 1996
8A-M was not detected at all and most of 7A-M could be
separated by filtration before quenching the reaction with water.
A countercation effect was found to exist at 0 °C in THF as the
ratio (Z-6a:E-6a) was 72:28 for the lithium, 96:4 for the sodium,
and 98:2 for the potassium enolates, respectively. Temperature
dependence on the ratio was subtle in THF for both the
potassium enolate (99:1 at -20 °C; 98:2 at 0 °C, 97:3 at room
temperature) and the lithium enolate (75:25 at -45 °C; 72:28
at 0 °C). The effect of solvent polarity was found to be small
for the potassium enolate at 0 °C (98:2 in THF; 97:3 in Et2O;
Recent interest in hexacoordinate phosphorus species has
risen, since their involvement has been implied in various
reactions dealing not only with pentacoordinate but also with
1
tetracoordinate phosphorus compounds. However, synthetic
application of pentacoordinate phosphorus utilizing the hexa-
coordinate state has been limited. Recently, Evans, Jr., et al.
2
have shown that the 10-P-5 phosphoranes 1 are capable of
3
undergoing the Wittig reaction and have observed hexacoor-
dinate species in the reaction mixture at low temperatures with
9
5:5 in toluene). The steric effect of the alkoxy group was also
4
the olefin forming step being the rate-determining step. During
+
negligible in THF at 0 °C [98:2 for 5Aa-K (R ) Et); 97:3
+
for 5Ac-K (R ) t-Bu)].
The reaction of 4Aa was examined with a variety of aromatic
and aliphatic aldehydes employing t-BuOK as base in THF. The
yields of olefins 6 were high (73-83%), and the Z-selectivities
12
were excellent (Z:E ) 96:4 to 98:2, Table 1). It is remarkable
that excellent Z-selectivity was attained easily at temperatures
(
0 °C to room temperature) much higher than those necessary
for the two Z-selective reagents mentioned above and that even
our successful effort to prepare the first example of a stereo-
chemically rigid enantiomeric pair of pentacoordinate phospho-
ranes (2) bearing asymmetry only upon the phosphorus atom
alkyl aldehydes carrying secondary or tertiary carbons adjacent
to the carbonyl group also showed high selectivity.
A competitive deprotonation reaction with (EtO)2P(O)CH2-
7,8,13
5a,b
by reductive removal of the ester moiety in 3, we had also
observed hexacoordinate phosphates.6 This hinted at the
possibility of effecting the Wittig reaction using 10-P-5 phos-
phoranes and prompted us to examine phosphoranes 4. Thus,
we have found that the reaction with aldehydes actually does
proceed and that the olefins are obtained with excellent
Z-selectivity even at 0 °C. As that for stabilized ylides, the
method for stereoselective Z-olefin synthesis has been limited
CO2Et revealed that 4Aa was at least three pKa units less acidic
14
than the phosphonate. This implies that the reverse aldol
reaction which produces the carbanion and aldehyde is ther-
15
modynamically less favorable and thus must be relatively slow.
A competitive reaction with a mixture of benzaldehyde and
p-tolualdehyde showed benzaldehyde to be clearly more reac-
tive, thus indicating that the reaction F value is positive. Since
the F value of the olefin forming step in the Wittig reaction is
7
only to the use of Still’s reagent or the recently reported
diphenyl phosphonoacetate at low temperatures.8
(9) (a) Granoth, I.; Martin, J. C. J. Am. Chem. Soc. 1979, 101, 4618-
4
622, 4623-4626. (b) Perozzi, E. F.; Michalak, R. S.; Figuly, G. D.;
(1) For leading references on hexacoordinate phosphorus, see: (a)
Stevenson, W. H., III; Dess, D. B.; Ross, M. R.; Martin, J. C. J. Org. Chem.
1981, 46, 1049-1053.
Cherkasoz, R. A.; Ploezhaeva. Ups. Khim. 1987, 56, 287-321. (b) Burgada,
R.; Setton, R. In The Chemistry of Organophosphorus Compounds; Hartley,
F. R., Ed.; Wiley-Interscience: Chichester, 1994. (c) Holmes, R. R. Chem.
ReV. 1996, 96, 927-950. (d) Wong, C. Y.; Kennepohl, D. K.; Cavell, R.
G. Chem. ReV. 1996, 96, 1917-1951.
(10) Selected data of phosphoranes (elemental analyses of all compounds
3
1
are within 0.4% of calculated values). 4Aa: mp 91-92 °C; P NMR
31
(CDCl3) -27.0. 4Ab: mp 113-114 °C; P NMR (CDCl3) -27.3. 4Ac:
31
mp 133-135 °C; P NMR (CDCl3) -26.3.
(
2) Perkins, C. W.; Martin, J. C.; Arduengo, A. J.; Lau, W.; Alegria, A.;
Kochi, J. K. J. Am. Chem. Soc. 1980, 102, 7753-7759.
3) For leading references, see: (a) Wadsworth, W. S., Jr. Org. React.
977, 25, 73-253. (b) Cadogan, J. I. G. Organophosphorus Reagents in
(11) Typical reaction conditions are given for the reaction of PhCHO.
To a THF solution (5 mL) of phosphorane 4Aa (505 mg, 0.839 mmol) was
added a THF solution (3 mL) of t-BuOK (90 mg, 0.80 mmol) at 0 °C under
an inert atmosphere. After 30 min of stirring, PhCHO (80 mg, 0.75 mmol)
in THF (3 mL) was added to the yellowish solution. After 3 h of stirring,
the reaction mixture was filtered through Celite to remove most of the
(
1
Organic Synthesis; Academic Press: New York, 1979. (c) Maryanoff, B.
E.; Reitz, A. B. Chem. ReV. 1989, 89, 863-927. (d) Kelly, S. E. In
ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon
Press: Oxford, 1991; Vol. 1, pp 730-817. (e) Johnson, A. W. Ylides and
Imines of Phosphorus; Wiley-Interscience: New York, 1993. (f) Vedejs,
E.; Peterson, M. J. Top. Stereochem. 1994, 21, 1-157. (g) Clayden, J.;
Warren, S. Angew. Chem., Int. Ed. Engl. 1996, 35, 241-270.
+
precipitate (7-K ), and then the reaction was quenched with water.
Extraction with Et2O followed by usual workup and chromatographic
treatment (benzene-SiO2) gave ethyl cinnamate (6, 108 mg) in 82% yield
1
(Z:E ) 98:2 determined by 400 MHz H NMR).
(12) All olefins from alkyl aldehydes, PhCHO, and p-NO2C6H4CHO
exhibited ca. 11-12% NOE signal enhancement between the two cis-vinylic
protons.
(
4) Bojin, M. L.; Barkallah, S.; Evans, S. A., Jr. J. Am. Chem. Soc. 1996,
18, 1549-1550.
5) (a) Kojima, S.; Kajiyama, K.; Akiba, K.-y. Bull. Chem. Soc. Jpn.
1
(
(13) Thompson, S. K.; Heathcock, C. H. J. Org. Chem. 1990, 55, 3386-
3388.
1
995, 68, 1785-1797. (b) Kojima, S.; Kajiyama, K.; Akiba, K.-y.
Tetrahedron Lett. 1994, 35, 7037-7040. (c) Kojima, S.; Nakamoto, M.;
(14) Treatment of a 1:1 mixture of 4Aa and ethyl phosphonate with 0.9
equiv (to one reagent) of t-BuOK lead to deprotonation of only the latter at
the detection limit of 1% (-log{[deprotonated 4Aa][(EtO)2POCH2CO2-
Kajiyama, K.; Akiba, K.-y. Tetrahedron Lett. 1995, 36, 2261-2264.
(
6) Deprotonation of R-unsubstituted â-(hydroxyethyl)spirophosphoranes
-
bearing Martin ligands has been found to quantitatively give hexacoordinate
Et]}/{[4Aa][(EtO)2POCH CO2Et]} ) ca. 3).
phosphates. (a) Kojima, S.; Akiba, K.-y. Tetrahedron Lett. 1997, 38, 547-
50. (b) Kawashima, T.; Watanabe, K.; Okazaki, R. Tetrahedron Lett. 1997,
8, 551-554.
(
(
(15) Vedejs, E.; Fleck, T. J. J. Am. Chem. Soc. 1989, 111, 5861-5871.
(16) The reaction of ca. 1:1 mixture of benzaldehyde (σp ) 0) and
p-tolualdehyde (σp ) -0.17) with 1 equiv (to one aldehyde) of phosphorane
resulted in a 25:75 mixture of aldehydes and a 78:22 mixture of ethyl
cinnamate and ethyl p-methylcinnamate. This gives an approximation of
ca. 5 times higher reactivity for benzaldehyde.
5
3
7) Still, W. C; Gennari, C. Tetrahedron Lett. 1983, 24, 4405-4408.
8) Ando, K. Tetrahedron Lett. 1995, 36, 4105-4108.
S0002-7863(96)04122-4 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 25, 1997 5971
Scheme 1
Scheme 2
Table 1. Reactions of 4Aa with Aldehydes at 0 °C Using t-BuOK
a
as Base
entry
aldehyde
CHO
4
p-MeOC H CHO
yield (%)
Z:Eb
Z:Ec
1
2
3
4
5
C
6
H
5
82
76
83
77
73
79
75
98:2
97:3
96:4
2:98(60)
2:>98(27)
13:87(40)
28:72(42)
6
p-O
o-O
PhCH
2
NC
NC
6
H
H
4
CHO
CHO
CHO
2
6
4
98:2
2
CH
2
>98:2
>98:2
>98:2
d
6
PhMeCHCHO
t-BuCHO
d,e
7
a
b
Reaction time was set at 3 h and not optimized. Ratios were
1
c
determined by H NMR (400 MHz). Selectivity of the corresponding
1
0-Sb-5 reagent using LDA as base with the total yield in parenthesis.
The reaction mixture was allowed to warm to room temperature and
d
was stirred overnight. e 4Ab was used instead of 4Aa.
known to be negative,17 it follows that the rate-determining step
is the nucleophilic addition of the enolate to the aldehyde. This
is consistent with our unsuccessful attempts to observe inter-
mediates in the reaction by NMR or by low-temperature
quenching experiments. These results are reasonable since such
intermediates cannot be observed for stabilized ylides other than
aldehydes (Table 1). It was necessary to use 2 equiv of lithium
diisopropylamide (LDA) as base to maximize the yields and
the yields of 6 were lower (27-60%) than those for 4A.
Aliphatic aldehydes did not give the expected olefins even when
1
equiv of LDA was used. Surprisingly, the stereoselectivity
was reversed and E-olefins were obtained predominantly (Z:E
28:72 to <2:>98). The higher E ratio with aromatic
18
for specially designed model compounds.
)
Thus, the mechanism of this reaction can be summarized as
follows (Scheme 2). The steric hindrance effected by the gem-
bis(trifluoromethyl) moiety results in the almost exclusive kinetic
formation of anti-9A as opposed to syn-9A. This is also
consistent with the fact that 5A does not react with ketones.
This process is followed by ring closure to form a 12-P-6
hexacoordinate species in which the bonds are expected to be
weakened by attaining higher coordination. The ring opening
of the bidentate used by Evans allows the stabilization of both
the enolate and the adduct leading to facile equilibration,
whereas the Martin ligand is conformationally fixed by the aryl
group and the gem-dimethyl effect thereby lowering possibilities
of retro aldol reaction to occur. Hence, by employing the
present phosphorane 4A, Z-olefins bearing an electron-
withdrawing group are furnished almost exclusively even at 0
aldehydes carrying electron-donating groups reflects the slower
rate in formation of the corresponding electron-rich olefins
compared with that of electron-deficient olefins from the
hexacoordinate intermediates. This implies the possibility for
19
reverse aldol reaction to occur for the adducts of the stiborane.
Further investigation on the mechanism of this reaction and
examination of the scope of 10-P-5 phosphoranes as Wittig
reagents are in process.
Acknowledgment. The authors are grateful to Central Glass Co.
Ltd. for a generous gift of hexafluorocumyl alcohol. Partial support
of this work through Grant-in-Aid for Scientific Research (nos.
0
6740488 and 07740501) provided by the Ministry of Education,
Science, Sports, and Culture, Japan, is heartily acknowledged.
Supporting Information Available: Experimental procedures and
spectroscopic data for phosphoranes 4Aa-c and reaction products (4
pages). See any current masthead page for ordering and Internet access
instructions.
°
C to room temperature.
In order to show the generality of the role of hexacoordinate
intermediates, we prepared the corresponding antimony com-
pounds 4B and examined the Wittig reaction with aromatic
JA964122W
(
17) Kawashima, T.; Kato, K.; Okazaki, R. J. Am. Chem. Soc. 1992,
14, 4008-4010.
18) Kawashima, T.; Kato, K.; Okazaki, R. Angew. Chem., Int. Ed. Engl.
993, 32, 869-870.
(19) The reason why 2 equiv of LDA is optimal for antimony compounds
4B is not clear. Lower amounts of base lead to lower E-selectivity and
thus higher Z-selectivity, although the differences were small (i.e., Z:E )
12:88 (1 equiv of LDA) and 6:94 (2 equiv of LDA)).
1
(
1