Oxovanadium(IV) complex-mediated catalysis in Mukaiyama-type aldol additions

Article abstract of DOI:10.1055/s-1999-2729

Condensation of vanadyl sulfate with 2,2'-biphenol in the presence of N- containing ligands provides oxovanadium(IV) complexes which serve as efficient catalysts in Mukaiyama aldol reactions to afford aldol products in 52-100% yields with diastereoselecti

Full text of DOI:10.1055/s-1999-2729

816
LETTER
Oxovanadium(IV) Complex-mediated Catalysis in Mukaiyama-type Aldol
Additions
Chien-Tien Chen,* Sang-Wen Hon, Shiue-Shien Weng
Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan 117, Republic of China
Fax +11-886-2-29324249; E-mail: chefv043@scc.ntnu.edu.tw
Received 15 February 1999
(such as Et3N, N-methylimidazole (NMI), and 1,2-dime-
Abstract. Condensation of vanadyl sulfate with 2,2’-biphenol in the
thylimidazole (DMI)), Scheme 1.14 A green solution of
presence of N-containing ligands provides oxovanadium(IV) com-
the respective vanadyl complex 1 was resulted in 2 hours
plexes which serve as efficient catalysts in Mukaiyama aldol reac-
and may be used directly after concentration. The alterna-
tive method, which follows the procedure similar to that
of oxotitanium(IV),13c requires direct condensation of va-
nadyl isopropoxide15 and 2,2’-biphenol in refluxing benz-
ene. A light brown salt 2 obtained after solvent removal
was used for the aldol studies. The green color of the va-
nadyl complexes 1 obtained from the first method is char-
acteristic for many penta-coordinated vanadium(IV)
species.16 Presumably, all these N-containing ligands can
satisfy the valence requirement around V(IV). All these
complexes were subjected to catalysis for the Mukaiyama
aldol addition since the varying modes and strengths of
ligand coordination around V(IV) may have a decisive in-
fluence on the reactivity and stereoselectivity in the aldol
process.
tions to afford aldol products in 52–100% yields with
diastereoselectivities of up to 90/10 (anti/syn)
Key Words: Lewis acids, 2,2'-biphenol, N-methylimidazole, dia-
stereoselective, b-hydroxyesters
Mukaiyama-type aldol reactions have constituted one of
the most salient tactics in carbon-carbon bond formation.
The diversity of Lewis acids,1 and the great opportunities
for the judicious combination of either aldol components
have elicited tremendous endeavors toward the stereose-
lective and asymmetric variants of this type of process.2
During the search of new functional metal catalysts,
chemists have recently switched gears toward the first row
transition metals other than d0–Sc(III),3 d0–Ti(IV)4 and
d10–Zn(II),5 such as Fe(II/III)5,6 and Cu(II).7 Among them,
the d9-Cu(II) salts have resurged as extremely mild Lewis
acids in various asymmetric aldol-type additions.8 How-
ever, the complimentary, paramagnetic d1-V(IV) species
has received very little attention.9 Vanadium(IV) com-
plexes are commonly employed as pre-catalysts for sul-
fide and olefin oxidation when combined with O2 or
peroxide co-oxidants,10 although the moderately electro-
philic nature of oxovanadium complexes toward
nitrogen11 and oxygen12 functionalities has been well doc-
umented in the literature. Spurred by the recent findings
on the use of oxotitanium(IV) complexes as Lewis acids
in Michael and Mukaiyama aldol reactions,13 we have ini-
tiated a study toward the catalytic profile of the corre-
sponding vanadium(IV) complexes. Herein, we describe
our preliminary findings as to their uses in the Mukaiyama
aldol reactions.
Scheme 1
Aldol additions between benzaldehyde and silyl ketene
acetal (SKA) derived from g-butyrolactone were first ex-
amined in the presence of 10-20 mol% of the respective
vanadyl catalyst with varying solvent polarity (entry 1–4),
Table 1. The reaction was found to proceed more smooth-
ly with 1a at -20 °C in CH2Cl2 or in toluene/CH2Cl2, lead-
ing to the corresponding b-hydroxylactones-3a in 53%
yield with moderate anti preference (80/20, anti/syn).
When the reaction was conducted at -50 °C, the selectivity
was improved to 88/12, albeit with lower conversion
(40%). It should be pointed out that vanadyl sulfate alone
does not catalyze the reaction due to its poor solubility in
aprotic, organic solvents. The reaction proceeded slug-
gishly in polar coordinating solvents (e.g., CH3CN, diox-
ane, and THF). The aldol adducts were produced in less
than 15% yield with slightly higher diastereocontrol (84/
16, anti/syn).
We have selected 2,2’-biphenol as a model template for
complex formation in view of its structure and reactivity
similarities to the chiral variant, 2,2’-dihydroxy-1,1’-bi-
naphthyl. Although the analogous titanium(IV) complex-
es have been characterized to be monomeric in nature,13c
the aggregation behavior of the vanadyl substrates in so-
lution has never been identified. Vanadyl biphenolate
complexes with varying ligand attributes were synthe-
sized from two different routes. The first one involves
gradual addition of a solution of 2,2’-biphenol (1 equiv.)
in CH3CN (with or without Et3N) to a solution of vanadyl
sulfate trihydrate in warm MeOH by the action of ligands
Synlett 1999, No. 6, 816–818 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Oxovanadium(IV) Complex-mediated Catalysis in Mukaiyama-type Aldol Additions
817
Table 1. Aldol Reactions between Aldehydes and Enol Silanes Cata-
lyzed by Various Oxovanadium(IV) Complexes
In addition to the parent benzaldehyde, 5 different aromat-
ic aldehydes of varying election demands were also exam-
ined (entry 10-14). The chemical yields of the resultant b-
hydroxylactones were in the range of 86–100% except in
the case of 3c (R = 4-MeOC6H4), whose yield was de-
creased by about 15%. In addition, the diastereocontrols
were all moderate (76/23–90/10, anti/syn). On the other
hand, much poorer conversion yields (20–76%) were ob-
served when the vanadyl catalyst 2 generated from va-
nadyl isopropoxide was employed. The diastereocontrols
of the aldol additions fell within the range of 50/50 to 35/
65, but with slight preference of the opposite syn isomer.
The oxovanadium(IV) complex-mediated catalysis by 1b’
was also amenable to aliphatic aldehydes (e.g., 3-phenyl-
propanal and cyclohexylcarbaldehyde,20 entry 15 and 16).
TMS ketene acetals of all three different substitution pat-
terns (un-, mono-, and di-substituted) as well as silyl enol
ethers are all capable aldol components, as represented in
entry 17-19. A typical reaction procedure is described for
benzaldehyde and SKA derived from g-butyrolactone
with catalyst 1b’: To a solution of vanadyl sulfate trihy-
drate (1 mmol) and NMI (2 mmol) in hot MeOH (5 mL)
was added a solution of 2,2'-biphenol (1 mmol) in warm
CH3CN (2 mL). The reaction mixture which turned dark
green gradually was heated at 50 °C for 2 hours. After
having been cooled to ambient temperature, the resultant
vanadyl complex was concentrated in vacuo to give a
glassy brownish green solid which was used directly with-
out further purification. The vanadyl catalyst (10 mol%)
dissolved in CH2Cl2 (5mL) was mixed with benzaldehyde
(2 mmol) and the mixture was cooled to -20 °C.21 Silyl
ketene acetal (4–6 mmol, 2–3 equiv.) was added and the
reaction mixture was stirred for 6–12 hours. After having
been quenched with saturated, aqueous NaHCO3 (5 mL),
the reaction mixture was separated and the aqueous layer
extracted with CH2Cl2 (3 x 15 mL). The combined organic
extracts were concentrated and hydrolyzed in 1N HCl/
THF solution (5 mL, 0 °C, 2 hours). After usual workup
procedure, the aldol products 3a were isolated by column
chromatography on silica gel (1/4, EtOAc/hexane).
1
b
aAnti/syn diastereomeric ratios determined by H NMR analysis.
Isolated yields. c The values in parenthesis are the resluts from cata-
lyst 2. d Toluene/CH2Cl2 (1/1) was used as solvent. e Two equivalents
of NMI were added. f Two equivalents of DMI were added. g See re-
fercence 18. h Reactions were performed at rt for 48 h.
In conclusion, we have documented the first example of
the Mukaiyama aldol reaction mediated by oxovanadi-
um(IV) biphenolate complexes. The complexes can be
easily generated from relatively cheap, commercial va-
nadyl sulfate. More importantly, ease of incorporation of
chiral auxiliaries and/or ligands22 as well as ease of mod-
ulation of the metal counterions make oxovanadium(IV)
an attractive alternative to other transition metals. Investi-
gations are underway as to the scope of vanadium(IV) in
other modes of catalytic reactions as well as in asymmet-
ric modifications of the aldol and related processes.
Significantly better chemical yields (72% and 66%) and
diastereocontrols (90/10 and 87/13) were observed when
the reactions were carried out with catalyst 1b and 1c (en-
try 5 and 7).17 It should be noted that roughly equal anti-
syn preference was found in this test aldol reaction cata-
lyzed by common Lewis acids (e.g., SbCl5, entry 8) or by
silyl-X (e.g., Me3SiOTf, entry 9).18 The resulting salt
((Et3NH)2SO4) in the catalyst preparation could also inter-
fere with the aldol performance. Best results (93% yield,
90/10) were finally obtained by using catalyst 1b’ where
biphenol was not pre-treated with Et3N (entry 6). Con-
trasting results were observed when the reactions were
catalyzed by 1a with 2 extra equivalents of NMI or DMI
as an additive (ca. 10% yield, entry 3 and 4),19 indicative
of the importance of the ligand attributes on the catalytic
activity of the vanadyl (IV) complexes in solution.10b
Acknowledgement
We are grateful to the National Science Council (NSC 87-2113-M-
003-008) of the Republic of China for a generous support of this re-
search.
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C.-T. Chen et al.
LETTER
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Synlett 1999, No. 6, 816–818 ISSN 0936-5214 © Thieme Stuttgart · New York