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Diastereogenic Addition of Crotylmetal Compounds to Aldehydes.

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Volume 21
. Number 8
August 1982
Pages 555-642
International Edition in English
Diastereogenic Addition of Crotylmetal Compounds to Aldehydes**
By Reinhard W. Hoffmann*
Dedicated to Professor Georg Wittig on the occasion of his 85th birthday
In an ideal synthesis, the formation of the CC bonds as well as the creation of the chiral
centers and of the final functionality would be carried out simultaneously in each step. Accordingly, stereoselective CC bond forming reactions are required. Instead of a pair of diastereomers, a synthesis should yield only one of the two possible diastereomers. Such selectivity is often achieved in the addition of crotylboron, -aluminum, -tin, -titanium, and
-chromium compounds to aldehydes. Depending on whether one starts from E- or 2-crotyl
compounds, one diastereomeric adduct is formed preferentially. In this case the newly
formed CC bond carries a methyl substituent. However, the synthetic goal is often a compound carrying an oxygen function at the site of the new CC bond. Compounds of this type
can be obtained through addition of heteroatom-substituted allylboronic acid esters or
analogous reagents to aldehydes.
1. Introduction
For several decades stereoselective synthesis belonged to
the domain of alicyclic and heterocyclic chemistry, being
concerned in particular with the generation of chiral centers having the desired configuration during the synthesis
of sugars, steroids, or alkaloids. In contrast, stereoselective
synthesis of open-chain systems has been intensely investigated only during the last few years. This surge in interest
was caused by new efforts to synthesize macrolide and polyether antibiotics, as well as some pheromones containing
stereochemically defined p-methyl alcohol or vic-diol
functionalities. Many improvements have been reported
within a short span of time and numerous methods are
now available['].
As an example, consider some syntheses of exo-brevicomin 3, which is a sex-attractant of the bark beetle Dendroctonus brevicomis. Being a bicyclic acetal, brevicomin
can be prepared from the ketodiol 2. Diastereomerically
pure 2'"'' is needed in order to obtain isomerically pure 3.
Following a classical synthetic strategy"' the carbon skeleton would be constructed first. In the case of 1 this would
involve the synthesis of the stereochemically uniform cissubstituted double bond constituting the two adjacent prochiral centers. Further steps-epoxidation with m-chloroperbenzoic acid and opening to the trans-diol 2 -give rise
to the two neighboring chiral centers with the desired relative configuration via "internal asymmetric induction""'.
In such a typical reaction sequence the carbon skeleton
is first formed, followed by creation of uniform prochirality, and finally of the chiral centers themselves. In princi-
I"]Prof. Dr. R. W.Hoffmann
Fachbereich Chemie der Universitat
Hans-Meerwein-Strasse, D-3550 Marburg (Germany)
Based on a plenary lecture given at the GDCh Hauptversamrnlung in
Hamburg on September 19, 1981.
Angew. Chem. Int. Ed. Engl. 21 (1982) 555-566
[***I In
this article only one enantiomer of the diastereomers is shown in
each case.
0 Verlag Chemie GmbH. 6940 Weinheim. 1982
0570-0833/82/0808-0555$ 02.50/0
~ 1 . "were
~ ~ developed so that the aldol addition could be
carried out in such a way that of the two possible diastereomeric adducts 10 and 11, only one-e.g. 10, and not
11 - is formed (Scheme 1).
OH 0
ple, it would be more economical if the carbon atom skeleton and the chiral centers were formed in a single step. For
this purpose, two prochiral C atoms would have to be
linked to each another, e. g. the negatively charged C atom
in the organolithium compound 5 and the C atom of the
carbonyl group in the aldehyde 4. However, this reaction
sequence did not give the desired diastereomer only, but
rather a mixture of the two possible diastereomers in the
ratio I :1.3"'.
Scheme 1. Y=alkyl or a heteroatom-bonded group, M=metal
The addition of allylmetal compounds to aldehydes
(Scheme 2) is structurally and, probably also, mechanistically analogous to the aldol addition (Scheme 1). Accordingly, in the case of the reaction of aldehydes with allylmetal derivatives 12 or 14, a high degree of diastereoselectivity is achieved. Because reactions of this kind are only
marginally covered in reviews of aldol
it is
appropriate at this point to review them in their own
3 + Diastereomer
CC bond forming reactions of much greater diastereoselectivity are, therefore, required. This holds, for example,
for the addition of the allylboronic acid esters 6 to the aldehyde 7, which resulted in a diastereoselectivity of 15 : l
in favor of the required adduct 8. This, in turn, was easily
converted into exo-brevicomin 314].
Scheme 2. Y =alkyl or a heteroatom-bonded group, M=metal.
This example illustrates the trend in present day stereoselective synthesis to form not only a new CC bond but
also the relative (and possibly also the absolute) configuration of the newly formed chiral centers in a single reaction
step. Thus, by avoiding refunctionalization steps one approaches the "ideal synthesis"[51in which individual segments of the molecular skeleton are linked in such a way
that the final functionality and steric structure are formed
simultaneously. Because of the high selectivity
(> 100 : I)[''. I 'I in the aldol addition of metal enolates 9 to
aldehydes, this diastereogenic CC bond forming
is of particular current
It took a considerable length of time, however, before
the early observations of Dubois et al.''*l and House et
These reactions are not only structurally, but also synthetically equivalent to the aldol addition since the adducts
13 and 15, being homoallylic alcohols, can be transformed
into aldol derivatives. The newly formed alcohol group is
often protected by esterification in these reactions. The
double bond is cleaved by ozonolysis and transformed into
p ",
Na104, RuCls
H :
Scheme 3.
Angew. Chem. Inr. Ed. Engl. 21 (1982) 555-566
an alcohol or an aldehyde by reductive work-up[l5].Oxidative work-up, on the other hand, leads to formation of carboxylic acids[‘61.Typical examples are given by the trans- ’ ~which
led to
formations of a cyclic l a ~ t o n e - o l e f i n [ ’ ~16a
the Prelog-Djerassi lactone 16 (Scheme 3).
These refunctionalizations turned out not to be more
bothersome than the corresponding reactions required
after aldol addition of special enolate reagents to aldehydes (Scheme 4).
Allylmetal compounds can exist as either monohapto(ql-) or trihapto-(q3-)bound forms, each of which should
be configurationally stable. However, E/Z isomerization is
possible for the monohapto-compounds 12 and 14 via a
metallotropic rearrangement to 17. Typical examples of
this are the fluxional dialkyl(ally1)boron ~ o m p o u n d s [ ~ ~ . ~ ~ ~ .
The transition states for these rearrangements should resemble the trihapto-structures 18 and 19 (M = BR2). E/Zisomerizations are also conceivable with the frzhapfo-compounds 18 and 19, if the monohapto-structure 17 is accessible from them. For example, this is the case in allyllithium compound^[^*^^^^ where rapid E/Z isomerization occurs130.311
Scheme 4
At this point a word on the stereo-nomenclature of diastereomeric pairs, such as Wll, is in order. If one attempts to read the original literature on aldol additions
quoted in the review^['.^-^], one is confronted with a BabyIonic confusion. The unambiguous R*,S*-nomenclature is
too cumbersome for easy reading of formulas and for verbal communication. Research groups concerned with the
aldol addition predominantly use the terms threo for 11
and erythro for 10[7.201.
Unfortunately, this designation is
just the opposite of what would result from the usual
Fischer projection. Therefore, when using the terms erythro
and threo it is first necessary to establish to which nomenclature forms the audience is accustomed. Because of this
dilemma, other designations have been suggested. These
are derived either from structures of the products, such as
u/I[”~, syn/anti‘*’’, p / r ~ [ ’ ~or
] , from the structures of the
educts, such as Ik/uIp11, r e r e / r e ~ i [ ’ ~ ’ ~or. ~heterofacial/ho~~,
Although none of these descriptions is ideal,
the syn/anti nomenclature will be used here for the sake of
ease of communication, because in the most frequently encountered simple cases structures can be recognized or assigned at a glance. Thus 10 would be a syn compound, because in the “sawhorse” formalism the groups of interest,
OM and Y , are on the same side of the main backbone (the
newly formed CC bond).
2. Configurational Stability of
Allylmetal Compounds
In the addition reactions in Scheme 2, the basic idea is
the production of a specific relative configuration in the
product by transformation of the prochirality of the allylmetal compound, e. g. E in 12, or Z in 14, in a stereoselective reaction. Therefore, a preparative application is
usually possible only when each E/Z stereoisomer of the
y-substituted allylmetal compound is independently accessible. Furthermore, each of these should be configurationally stable under the reaction conditions and, ideally, also
on storage.
Angew. Chrm. In[. Ed. Engl. 21 (1982) 555-566
In a simplified discussion it is not necessary to consider
the ion-pair character of allylmetal compounds. Rather, it
suffices to state that, in order to be of use for diastereoselective CC bond formation, only allylmetal compounds
possessing an energy barrier > 20 kcal/mol for the mutual
interconversion of the monohapto- and trihapto-forms can
be used. Examples of trihapto-compounds fulfilling this requirement are dicarbonyl(cyclopentadieny1)iron derivat i v e ~ [ ~a ~wide
] , range of nickel
or dicyclopentadienyltitanium compounds’341.Allylpotassium compounds should also be mentioned here129.351.
Among the
monohapto-allylmetal compounds, the silanes should have
the lowest tendency to metallotropic is~merization[~~l.
configurational stability decreases on going from the germanium to the tin compound^[^^,^^^, which often undergo
E/Z isomerization below 100°C or in the presence of
Lewis bases.
Of the allyl compounds of the third group elements,
those of boron are the best known. In such compounds the
tendency to metallotropic rearrangement can be controlled
through proper choice of the other substituents at boron:
dialkyl(a1lyl)boron compounds rearrange rapidly at room
and can be handled without isomerization only below - 78 0C1391.
n-Donor substituents at boron
raise the energy of the vacant boron p orbital to such an
extent that rearrangement to the trihupto-structure is rendered difficult or does not occur. Although a single OH
group at boron is insufficient to suppress the borotropic
shift at -20 0C1401,
an amine substituent renders the compounds stable up to 150 0C[411.
Allylboron derivatives with two oxygen functions at boron, e . g . , the crotylboronic acid esters 25 and 26, are sufficiently thermally stable that the E- and Z-derivatives are
separately manipulable at room temperat~re[~’-~I.
Consequently, allylboron compounds containing one oxygen and
one nitrogen substituent, or even two nitrogen substituents,
are particularly resistant to borotropic rearrangement[44,451.
On the other hand, with these compounds and the allylboronic acid esters as well, the tendency to isomerize like
other organometallic compounds in the presence of Lewis
a ~ i d becomes
~ ~ ~apparent;
~ - ~ this,
~ ~of course, limits the
choice of conditions for their preparation.
Allylmagnesium, like the organozinc compounds-and
by inference also the organocadmium compounds-undergo rapid metallotropic shifts 12 + 14, even at low temp e r a t u r e ~ [ ~ ~Thus,
. ~ ' ~ . apart from special
are hardly useful as reagents in diastereoselective addifor the allyllitions to aldehyde^^'"^'^. The same
thium corn pound^^^^^^^^, particularly if not only one of the
stereoisomers 12 or 14 is to be used. Consequently, further
discussion will be limited to allylmetal compounds which
are configurationally stable per se at room temperature and
also under the conditions of reactions with the aldehydes.
Monohapto bonded a l l y l c h r ~ m i u m ~allylzirc~nium[~~~,
and allyltitanium c o m p o ~ n d salso
~ ~ belong
~ ~ ~ ~to~ this
3. Stereochemistry of the Addition Reaction
An allylic rearrangement is always observed in reactions
between allylmetal compounds such as 12 or 14 and aldehydes under kinetically controlled conditions (Scheme 2).
Thus, it is reasonable to propose a six-membered cyclic
transition state, especially when the metal can coordinate
to the carbonyl group of the aldehyde as a Lewis acid due
to the presence of a low-lying vacant orbital, i. e. when it is
~ ~order
~ . to delineate the ocable to form a t e - c ~ m p l e x e s [In
currence and the limits of diastereoselectivity one can refer
to the concepts used in the interpretation of the aldol reaction[','l. As in the aldol addition, the transition state comprises a cyclic arrangement of the reacting atoms with conformations similar to that of cyclohexane, i.e. chair or
boat. Thus, diastereoselectivity depends on the relative energies of the diastereomeric transition states and these can
be influenced by conformational factors in the same way
as in cyclohexane. This is illustrated (cf. Is]) for the addiL
Scheme 5. CI, CZ=chair-like, BI, BZ=boat-like transition states
tion of a Z-allylmetal derivative to an aldehyde. Both, the
chair-like transition states CI and C2, as well as boat-like
transition states B1 and B2 are considered in Scheme 5.
In the simplest case the addition is diastereoselective
when one of the transition states is approximately > 2
kcal/mol more stable than the others. For the discussion of
the factors determining the relative energies of the transition states it is necessary to assume that at least two more
ligands, L, are bound to the metal. The steric requirements
of R and L are of decisive importance for the diastereoselectivity, because in the chair-like transition state C2 a 1,3diaxial interaction exists between them. If this interaction
is larger than the gauche-interaction between R and R' in
the transition state C1, then C1 will be preferred. Of the
three conceivable boat-like conformations, the one having
the aldehyde-oxygen and the allyl-CH groups in the bow
and stern positions are preferred. Only these transition
states (B1 and B2) are considered here. Of these, transition
state B2 should be significantly more stable than B1 as a
result of the 1,3-diaxial R/L and R/R' interactions. However, because of the eclipsing groups, boat transition states
are inherently disfavored with respect to the chair transition states. However, the energy difference between boat
and chair transition states should not be very large, since
studies of the Claisen rearrangement, which takes place uia
similar transition states, have shown that the boat-like
transition states are only 2-3 kcal/mol less stable than the
corresponding chair forms[601.Therefore, boat-like transition states may well represent the principal path of the
Claisen rearrangement as a result of steric interactions between substituentsl']. Consequently, boat-like transition
states should also be considered in the additions of allylmetal compounds discussed here.
These conclusions were derived for the reactions of the
2-allylmetal compounds 14. A similar analysis may be
made for the reactions of the corresponding E-compounds
of type 12.
Further points to be noted include: a high degree of
diastereoselectivity may be expected in additions to aldehydes, but not to ketones, because the diastereoselectivity
stems from different steric interactions between the groups
R and the aldehydic H with the metal ligands. Therefore,
the size of L should be significant for the degree of diastereoselectivity. Fortunately, the size of L can be chosen at
will (see e. 9. l6']). The R/L-interaction and, hence the diastereoselectivity are at a maximum when the length of the
oxygen-metal bond in the transition state is at a minimum.
This will be the case in endothermic or weakly exothermic
reactions in which the transition state occurs late on the
reaction coordinate[621.However, the ultimate attainable
metal-oxygen bond length is the most decisive factor. According to this criterion, as shown by Evans et aZ.I8], diastereoselectivity should decrease along the series, metal=B (B-Ogl.4 A), Si (Si-Ogl.6 A), Ti ( T i - 0 ~ 1 . 7
A), Sn (Sn-0 22.1 A), Zr (Zr-0 = 2.1 A). The above considerations refer to metals which can coordinate to the aldehydic carbonyl group as Lewis acids. It is questionable
whether or not this is possible for M = R3Be1631.If so, this
reaction would imply a nucleophilic substitution with retention at a saturated first row element (boron).
Angew. Chem. Int. Ed. Engl. 21 (1982) 555-566
The addition of allylmetal compounds to aldehydes can,
according to the nature of the metal, occur even at low
temperatures, or, in the case of trialkyltin derivatives, only
at temperatures above 100 0C[641.Such additions are catalyzed by Lewis acids such as boron trifluoride etherateIb5].
Essentially only the Lewis acid catalyzed addition is
The catalysis may derive
known for allyltrialkylsilanes[661.
from an activation of the carbonyl group through coordination of the Lewis acid with the oxygen atom. In these
cases it is not certain whether the reactions still take place
via cyclic transition states since the aldehyde group is already complexed with the catalyst. In these cases openchain transition states as in SE2'-reactions may well be
more favorable than cyclic ones[671.Support for this interpretation comes from the fact that allylic rearrangement is
no longer the rule in catalyzed additions of crotyltin compounds to quinonesi681,because both SE2' and SE2 reactions are possible for open-chain transition states16*'. Of
such open-chain transition states (see 20 and 22) the most
Pure Z-crotylbis(dimethy1amino)borane 23 has also
been obtained from Z-crotylpotassium; the yields were ca.
The boronic acid amides 23 and 24 could be converted in situ with a 1,2-diol into the reactive but thermally
only slightly stable boronic acid esters 25 and 26f441.
Analogous chiral crotylboronic acid esters (e. g. 67 and 69) are
obtainable using optically active 1,2-diol~[~~].
The dimethyl
ester of 2-crotylboronic acid can be advantageously generated in situ from Z-~rotylpotassiurn[~~1
and allowed to react
directly with aldehydes. E- and 2-Boronic acid esters such
as 27 and 28 carrying two a-alkyl groups, e. g. CH3, can be
obtained in good yields from a-bromoisopropylboronic
acid esters[42,"!
favorable will be those in which the least steric interaction
occurs along the newly formed bond. Stereoconvergence
can occur in such cases, so that predominantly one and
the same diastereomeric product 21 arises from the two
stereoisomeric allylmetal compounds 12 and 14.
4. Diastereogenic CC Bond Formation
In this section we will describe the preparative aspects
of the addition of crotylmetal compounds to aldehydes,
considering the synthesis of the stereochemically pure reagents as well as the diastereoselectivity in their reactions
with aldehydes.
4.1. Boron as Key Atom
4.1.1. Crotylboronic Acid Esters
The crotylboronic acid esters 25 and 26 can be prepared
from the reaction of crotylmagnesium chloride with chlorobis(dimethylamino)borane, which gives a mixture of
isomeric methylallylboronic acid amide~[~'].
with ZnBrz at 120 "C gives a mixture of 23 and 24, which
can be separated by distillation at 100 torr using a spinning
band column.
Angew. Chem. Int. Ed. Engl. 21 (1982) 5.55-566
The crotylboronic acid esters thus obtained undergo virtually quantitative addition to aldehydes below 0 0C[711.
Cleavage of the boric acid esters formed using triethanolamine gives rise to the homoallylic alcohols 29 and 30, the
1,2-diol, and the boratrane 31. In favorable cases the homoallyl alcohol and the diol can be separated by distillation. If this is not possible, the homoallyl alcohol must be
separated by gas or column chromatography.
In the addition of the 2-crotylboronic acid ester 25 to
aldehydes, the diastereoselectivity obtainable was directly
related to the isomeric purity of 25 (see Table l)[44.701.
each case the diastereomer of type 29 predominated; this
is formed via the chair-like transition state C1. This stereoselective CC bond formation has been applied in the
preparation of the pheromone 33 of the drugstore beetle,
whereby the syn-isomer 32 was obtained in 70% enantiomeric excess (e. e.)[*lusing the chirally modified crotylboronic acid ester 69[l6](Scheme 6).
e.e., enantiomeric excess is a measure of the enrichment of one enantiomer relative to a racemic mixture. A 70% e.e. corresponds to a 85%
content of the enantiomer concerned. Here, diastereomeric purity refers
to the amount of the appropriate diastereomer in the isomeric mixture
The addition of the E-crotylboronic acid ester 26 to aldehydes also took place in a diastereoselective manner via
transition state C1 (see Table 1). Thus, the complementary
32, 93 %
D i a s t e r e o m e r i c purity
D i a s t e r e o m e r i c purity
9 5%
Table 1. Diastereoselectivity in the formation of the homoallylic alcohols 29
and 30.
Boronic Acid Ester
Bzl = Benzyl
29 : 30
: 3
: 4
: 1 5
: <5
: <5
: 8
: 11
Scheme 6.
4.1.2. Substituted Boronic Acid Esters
Diastereoselective CC bond formation also occurs using
y-alkoxy-substituted allylboronic acid esters: the Z-allylboronic acid esters 6, prepared according to Scheme 7,
add to aldehydes at room temperature (cf. Table 1)14,72,751.
CH30CH20 H
: 11
: 20
: 12
: 20
: 32
98 :
95 :
: <2
diastereoselectivity afforded the homoallyl alcohols with
anti-configuration. This CC bond formation step was used
in a synthesis of 6-multistriatin 34, whereby 52% e.e.
(= 76% of the corresponding enantiomer) was achieved using the chirally modified crotylboronic acid ester 67"''
(Scheme 6). Furthermore, the a,a-disubstituted boronic
acid ester 28 also added to benzaldehyde with 97%diastereoseIectivityIa'.
6a, R = CH,
3 7%
6 ~ R, = CH,OC(CH,)2
6b, R = CH3OCH2
Scheme 7. Tetramethylethylenediamineor potassium fert-butoxide is used as
a base.
An application of this reaction has already been given in
Section 1 (the synthesis of em-brevicomin 3). However,
the reaction between 6a and isobutyraldehyde took place
with somewhat lower diastereoselectivity than in the reactions with benzaldehyde, acetaldehyde, and propionaldehyde. This decrease in diastereoselectivity is even more
pronounced in the addition of the allylboronic acid esters
6b and 6c possessing larger y-substituents, particularly in
the reaction with sterically hindered aldehydes. These limiting factors can be understood in terms of transition states
C1 and C2: sterically demanding groups R and R' destabilize C1 in particular, and to a lesser extent C2. In the latter case, the two groups are farther removed from each
other. As a consequence, a significant portion of the reaction will take place via transition state C2 when steric hindrance is present. The results do not preclude the possibility that the boat-like transition state B2 can also be involved in the reaction.
Angew. Chem. I n f . Ed. Engl. 21 (1982) 555-566
In the addition of boronic acid esters of E-configuration
such as 36, the same steric factors which decrease the diastereoselectivity with sterically demanding aldehydes, such
as in reactions of 6, should now increase the diastereoselectivity. Scheme 8 shows the synthesis of 36.
dues. This was also the maximum diastereoselectivity obser~ed[~~.'~~.
Scheme 8
E-Methoxyallylpotassium generated at - 120 "C can be
trapped in situ giving 35. Under these conditions, isomerization to the 2-conformer is largely suppressed: the product 36 was formed in 60% total yield and with 90% E-configuration. Since 36 reacts more rapidly with aldehydes
than the 2-isomer 6a, crude 36 was mixed with 0.9 equivalents of the aldehyde, whereby virtually only 36 reacted.
The data in Table 1 demonstrate the high diastereoselectivity obtainable in these reactions, particularly in the addition to isobutyraldehyde.
y-Alkylthioallylboronic acid esters 37 and 38 can be
prepared analogously and also add with high diastereoselectivity to
Using a mixture of 38 and its Zisomer, the more reactive E-isomer 38 reacted selectively
with a slight deficiency of the aldehydes[731.The silyl-substituted boronic acid ester 39 also adds diastereoselectively to aldehydes[741.
1% = CcH5
8 3 : 17
K = CH,
8 5 : 15
R = (CH,),CH
8270 68 : 3 2
The higher degree of isomeric purity in educt 42 (and its
stannyl analogues) led to very high diastereoselectivities in
the addition to aldehydes; however, due to side reactions
the chemical yields were only 50-70%. On the other hand,
42 (pyridinium instead of butyl) gave very high chemical
yields and diastereomeric purities of between 85 and
4.2. Crotylaluminum Compounds
Aluminate complexes can be obtained by analogy to 41.
However, the reaction of these complexes with aldehydes
did not give rise to a particularly high degree of diastereo~electivity'~'.~~~.
This is probably due to the fact that the
double bond in crotyl- and heterosubstituted allylaluminates[791does not possess a uniform configuration; however, diastereoselective addition to aldehydes has been
achieved with some heterosubstituted allylaluminates, e. g.
43 i 14.801.
With respect to reactions of other crotylboron compounds, the reaction between 40 and aldehydes is worthy
of mention since a borotropic isomerization can be
avoided when the reaction is carried out at - 100 0C'391.
Diastereomeric purity
Diastereomeric purity
As expected, dialkylcrotylboranes which are not configurationally stable at room temperature react with aldehydes in a nonstereospecific
d A ?
- 78OC
The situation is different in the reaction of the trialkylcrotylborate complex 41. Due to the method of preparation, this complex probably contains ca. 80% E-crotyl resiAngew. Chem. Inr. Ed. Engl. 21 (1982) 555-566
+ R
4.1.3. Crotylborate Complexes
R = C6H5
R = CH,
R = (CH,),CH
R = (CH,),C
90% 9 4 : 6
90% 97 : 3
83 : 17
91 : 9
X = CN(iPr),
Crotylaluminum compounds with uniform double bond
configuration, e. g . 44i8'1,can be prepared at low temperatures from stereochemically pure crotylalkali metal compounds and react with aldehydes in situ in a highly stereoselective manner. In the spectacular synthesis of monensinc8'],use was made of this procedure to form one of the
CC bonds.
Heterosubstituted allylaluminates and allylaluminum
compounds have been examined also : differing regioselectivities were observed in the addition reactions of 43['01
and 45i821with aldehydes. This is possibly due to a metallotropic equilibrium in which the aluminate exists in the
extended form, because of coulombic repulsion, and the
aluminum compound 45 is present in an intramolecularly
complexed form.
The stereoconvergence in the BF3-catalyzed reaction was
surprising: both 46 and the corresponding E-isomer 49
added with predominant formation of one and the same
diastereomeric p r o d u ~ t [ ' ~ .The
~ ~ 1preparative
advantage of
stereoconvergence is obvious: instead of using configurationally pure starting materials, 46 or 49, the more readily
available mixture of isomers may be used. The stereoconvergence is not due to an initial rearrangement of 49 to
46 ; rather, open-chain transition states have been considered as possible causes.
4.3. Crotylsilicon Compounds
2-crotyl(trimethy1)silane is obtainable from butadienefs3]
and E- and Z-crotyl(tripheny1)silanefrom the isomeric crotyl chlorides, respectivelyi381(for further syntheses of trialkyl(crotyl)silanes, see i84.851). Trialkyl(allyl)silanes of this
type give good yields in the addition reaction with aldehydes only when Lewis acid catalysis (TiCI,) is usedf661.
The fluoride ion catalyzed addition also takes place in low
yieldsie6'. Little information has been given about diastereoselectivity in these casesi6'].
R = CH,
R = CzH5,
R = (CH3)zCH
9070 4 : 9 6
82% 9 : 9 1
8770 9 : 9 1
9070 9 : 91
The stereoconvergent reaction of 46+49 with 50 was
applied to a stereoselective synthesis of the Prelog-Djerassi
lactone 16"''. However, not all crotyltin compounds react
diastereoselectively with aldehydes; thus, 51 and 52 gave
mixtures of diastereomeric
4.4. Crotyltin Compounds
Stereochemically pure crotyltin compounds can be prepared from the corresponding crotyl chlorides by reaction
with trialkyl-[871
or triaryltin lithium compounds[381.For example, 46 adds diastereoselectively to benzaldehyde at
200 oC[881.
Chloral reacts even at 20 0C[85,881.
" _
4.5. Crotylmagnesium and
Crotylcadmium Compounds
> 99% Dlasterwmenc purity
BF, OEtl
- 78°C
Use of the more reactive trihalotin corn pound^^^^^^^^ allows a decrease in the reaction temperature. Diastereoselectivity is preserved in some cased9']. In other cases, the
dibutyl(croty1)tin chloride adds to aldehydes in such a
manner that the E/Z-ratio of the crotyl residues is no
longer reflected in the syn/anti product ratio[921.
The addition of trialkyl(croty1)tin compounds to aldehydes can also be facilitated using Lewis acid catalysts: 46
adds diastereospecifically to benzaldehyde in presence of
BF3.OEtIs5]. High diastereoselectivity was also reported
for the Lewis acid catalyzed addition of other y-substituted
allyltin compounds such as 47 and 48 to aldehydes'931.
The addition of crotyl-Grignard compounds to aldehydes gave rise to low diastereoselectivity and are thus of
little preparative interestf5'.51.941. Nontheless, some work
with heterosubstituted allylcadmium corn pound^^^^^^^^ indicates the possibility of diastereoselective CC bond formation.
- a Rq
= CsHs
= CH,
= CzH5
= (CH,),CH
: 62
: 50
: 50
: 43
4.6. Crotyllithium Compounds
No diastereoselectivity worth mentioning has been
achieved in the addition of crotyllithium to aldehydes[521.
Angew. Chem. Inr. Ed. Engl. 21 (1982) 555-566
Most likely, this is due to the fact that the lithium (like
magnesium) compounds exist as equilibrating E/Z-isomers. A high degree of diastereoselectivity has occasionally been observed in the addition of y-substituted allyllithium compounds to aldehydes[9s3961;
this probably arises
because the E/Z-equilibrium lies far to the side of one of
the geometric isomers.
selectivity was achieved in the addition of both 56 and the
corresponding iodo but not the chloro complex[581.
4.7. Crotylchromium Compounds
The addition of allyl bromides to aldehydes in the presence of CrCl, has been known for some time. When E-crotyl bromide is used, only diastereomer 54 is produced[ss1.
CrCh_ 1
Very good yields were also reported for the addition of
the triamino(croty1)- or trialkoxy(croty1)titanium compounds 57-60[6'.971.The degree of diastereoselectivity appears to depend on both the spatial demands and the electronegativity of the residues at titanium.
M IB r
Surprisingly enough, this reaction is also stereoconvergentts4].It is not known whether this stereoconvergence is
due to an equilibration of the intermediate crotylchromium compound 53 to the E-isomer. The diastereoselectivity attainable depends both on the solvent (tetrahydrofuran is best) and on the aldehydic residue R[s41.
57, X = NMe,
58, X = NEt,
59, X = O i P r
60, X = OC6H5
The amino compound 58 exists as the pure E-compound[971.In all likelihood, this is also true for the alkoxycompound 60 which gave selectivities of > 90% in several
reactions with a-branched aliphatic aldehydes, favoring
formation of the anti-adducts16'].
The results shown in Scheme 9 indicate that also in the
case of heterosubstituted allyl derivatives, the titanium
compounds are the superior reagents.
4.8. Monohapto-CrotylzirconiumCompounds
50 : 5ot8*]
A fair degree of diastereoselectivity is shown in the addition of the crotylzirconium compound 55 to aldehydesr561.
This may be due to the fact that the reagent, because of the
method of preparation, is an E/Z-mixture.
91 : 9@*1
(i Pr ),
X = CN(iPr),
It = CH,
R = (CH,)2CH
R = CH,O&(CH,),
85% 73 : 27
8870 86 : 14
90% 88 : 12
94 : 6
C p = Cyclopentadienyl
4.9. Monohapto-Crotyltitanium Compounds
The titanium compound 56, which corresponds to 55, is
expected to have the E-configuration. Excellent diastereoAngew. Chem. Inr. Ed. Engt. 21 (1982) 555-566
Scheme 9.
4.10. T~ihapto-CrotyltitaniumCompounds
Although stereoisomeric trihapto-crotylmetal derivatives
are known[32,991,
only titanium compounds 61 with E-configuration have been reacted with aldehydes to date[341.
The additions took place with good diastereoselectivity
and yields.
> 95% D i a s t e r e o m e r i c
5. Cram/anri-Cram Selectivity
Et2O BF,
Up until now, only those diastereogenic reactions have
been discussed in which a prochiral allyl or crotylmetal
compound was added to the prochiral carbonyl group of
an aldehyde. The relative configuration of the two newly
formed chiral centers was of particular interest. If the aldehyde is not achiral then the upper and lower faces of this
group will no longer be an enantiotopic, but rather diastereotopic. Different diastereoisomeric adducts will be
formed in CC bond formations taking place on the re- and
si-faces. An example is the addition to an aldehyde containing a chiral center, such as an a-methyl group, leading
to a pair of isomers such as 62/63 (Scheme 10) with three
chiral centers. The isomers now differ as regards the configuration of the newly formed chiral center (with the hydroxy group) relative to the chiral center carried over from
the aldehyde.
Approach f r o m
below “ r e - face”
reaction partners, namely the aldehyde. A higher degree of
selectivity is attainable when the two faces of the crotylmetal compounds are made diastereotopic as well. This is
possible e. g. through a chiral modification of the reagent
26 to give 67.
“ anti
Scheme 10. The configuration of the second newly formed chiral center is
not considered.
The chiral center in the a-position of the aldehyde
causes a 1,2-asymmetric induction. As a result, the two
diastereomers are formed in different yields. Of course,
reactions leading in high selectivity to only one diastereomer are desirable. The extent of 1,2-asymmetric induction in the addition to a-chiral carbonyl compounds has
been examined, in particular, by Cram el al.[lool.According
to Cram’s rules the isomer 62 should be formed preferentially. Compounds 62 and 63 are, therefore, also designated as Cram and anti-Cram products, respectively.
The Cramlanti-Cram selectivity problem is of course
relevant to the addition of crotyl- and allylmetal compounds to a-chiral aldehydes. Examples of surprisingly
high diastereoselectivity are found in the syntheses of rifamycin by Kishi et al.‘891
and the Prelog-Djerassi lactone 16
by Maruyama et uI.~”].
In the last example, the preferred attack at the re-side of
the aldehyde 50 forming 64 is probably due to complexation (cf. 65). Unfortunately, the Cram/anti-Cram selectivities are often only in the range 2-5 :1 in the addition of
crotylmetal compound^^^^-^^^ 69, ”*
and substituted allylmetal compounds~’0’.’021
to a-chiral aldehydes. The reason
is that the asymmetric induction is due to only one of the
Approach f r o m
above ‘ ‘ $ 1 - face”
>94% D i a s t e r e o m e r i c purity
76 : 24
92 : 8
A proper combination of the chiralities of aldehyde (+)66 and reagent (-)-67 results in a mutual reinforcement
of the individual stereodifferentiations; this “double
stereodifferentiation”[61has proved of value in the aldol
a d d i t i ~ n ~ ~ and
~ ~ .thus
’ ~ ~be] used
to improve the
selectivity to 92 :8 in favor of the diastereomer 68[69J.For
comparison, the addition of rac-67 to (+)-66 gives a selectivity of 76 :24, because this reaction is only subject to single stereodifferentiation from the aldehyde. Surprisingly,
the anti-Cram isomer 70 predominated in the addition of
the Z-crotylboronic acid ester rac-69 to the aldehyde (+)66. In this case, (+)-69 has the “correct” chirality to increase the selectivity in favor of 70 through double stereodifferentiation.
70 : 70
5 : 95
Similarly, the reaction of 50 with (+)-69 occurs with a
diastereoselectivity of 3 8 1 : < 19 thanks to the double
Angew. Chem. Int. Ed. Engl. 21 (1982) 555-566
The Prelog-Djerassi lactone 16
was easily obtained from the adduct, although in this case
the stereoselectivity was not as high as in the addition of
chiral enolates1'0.221
to 50.
6. Conclusion
(101 S. Masamune, M. Hirama, S. Mori, S. A. Ali, D. S. Garvey, J . Am.
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(111 D. A. Evans, J. Bartroli, Tetrahedron Lett. 23 (1982) 807.
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Chem. SOC.95 (1973) 3310.
(141 Cf. also Y. Yamamoto, K. Maruyama, Heterocycles 18 (1982) 357.
[IS] R. W. Hoffmann, W. Helbig, Chem. Ber. 114 (1981) 2802.
(161 R. W. Hoffmann, W. Ladner, K. Steinbach, W. Massa, R. Schmidt, G .
Snatzke, Chem. Ber. 114 (1981) 2786.
1171 K. Maruyama, Y. Ishihara, Y. Yamamoto, Tetrahedron Lett. 22 (1981)
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(191 R. W. Hoffmann, W. Ladner, unpublished results.
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When an anti-adduct such as 13 is desired, of the many
possibilities for diastereoselective addition of crotyl residues to aldehydes, the reaction of the rnonohapto-chromium 53 or -titanium compounds 56-60 should be the
method of choice. In order to obtain the other diastereomer 15, the use of the crotyltin compound 46 or the Zcrotylboronic acid ester 25 is the most advantageous. If alkyl groups other than methyl are desired as appendages at
the new CC bond, the requisite tributylstannyl or the
chromium(r1) derivatives are probably most readily prepared. The diastereoselective positioning of heteroatoms,
e.g. oxygen, sulfur, or silicon at the branching point, is
particularly well achieved using allylboronic acid esters.
It is clear, therefore, that crotylmetal compounds such as
c 1.
12 or 14 are equal or even superior to the enolate reagents
[29] M. Schlosser, M. Stahle, J. Organornet. Chem. 220 (1981) 277.
9 (Scheme 1) with respect to both structural variety and
(301 M. Schlosser, J. Hartmann, J. Am. Chem. SOC.98 (1976) 4674.
diastereoselectivity in addition reactions to aldehydes
(311 T. B. Thompson, W. T.Ford, J. Am. Chem. SOC.101 (1979) 5459 and literature cited therein.
(Scheme 2). Another advantage is the high chemoselectiv(321 A. Cutler, D. Ehntholt, W. P. Giering, P. Lennon, S. Raghu, A. Rosan,
ity of the allylboronic acid esters and the allylchromium
M. Rosenblum, J. Tancrede, D. Wells, J . Am. Chem. SOC.98 (1976)
both of which react selectively at the alde3495 and literature cited therein.
(331 L. S. Hegedus, J. Organomef.Chem. Libr. I (1976) 329: P. Heimbach,
hyde function of a ketoaldehyde (cf. 743). The chemoseP. W. Jolly, G. Wilke, Adv. Organomet. Chem. 8 (1970) 29.
lectivity of allylsilicon[66~,allyltin[651,and trihapto-crotylti(341 F. Sato, S. Iijima, M. Sato, Tetrahedron Leu. 22 (1981) 243; cf. H.
Lehmkuhl, S. Fustero, Liebigs Ann. Chem. 1980, 1371.
tani~m[~'lreagents is less pronounced, since these react
(351 M. StBhle, J. Hartmann, M. Schlosser, Helu. Chim.Acfa 6 0 (1977) 1730
with aldehydes and ketones under comparable conditions.
and literature cited therein.
Allylboronic acid esters are also the reagents of choice for
(361 J. Slutsky, H. Kwart, J. Am. Chem. SOC.95 (1973) 8678; T.H. Chan, 1.
asymmetric induction through chiral rn~dification~'~.'~.~~~. Fleming, Synfhesis 1979, 761.
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However, these have not yet reached the standard set by
Chem. SOC.97 (1975) 843.
[38] E. Matarasso-Tchiroukhine, P. Cadiot, J. Organomet. Chem. 121 (1976)
the enolate reagents using chirally modified enolbori-
The work described here originatingfrom our own group is
due, in particular, to the initiative and persistence of Dr. H.J . Z e g , W. Ladner, and Dr. B. Kernper to whom I express
my thanks. Thanks are also due to the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for
continued support.
Received: December 28, 1981 [A 420 IE]
German version: Angew. Chem. 94 (1982) 569
Translated by Dr. Edeline Wentrup-Byrne, Marburg
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