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Concerning reversal of diastereoselectivity in the BF3 promoted addition of crotyl—organometallic compounds to aldehydes
C46
Journal of Organometallic Chemistry, 284 (1985) C45-C48 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary communication
CONCERNING REVERSAL OF DIASTEREOSELECTIVITY PROMOTED ADDITION OF CROTYL-ORGANOMETALLIC TO ALDEHYDES
YOSHINORI YAMAMOTO* Department
of Chemistry,
IN THE BF3 COMPOUNDS
and KAZUHIRO MARUYAMA
Faculty
of Science, Kyoto
University,
Kyoto
606 (Japan)
(Received December 7th, 1984)
Summary
The addition of a mixture of benzaldehyde and BFB*OEtz to crotyl-organometallic reagents C,H,MLn (1) (M = Cu, Cd, Hg, Tl, Ti, Zr and V) produces predominantly the erythro homoallyl alcohol as well as the a-adduct, while without BF3 .OEtz the threo isomer is formed preferentially. A recent communication by Reetz and Sauerwald [l] on the reversed diastereoselectivity in BFB-mediated reactions of crotyltitanium reagents with aldehydes has prompted us to report our own findings [ 21 in this area. When we found that the BF3 -mediated reaction of crotyltrialkyltins with aldehydes proceeds in a stereoconvergent manner to produce cry thro homoallyl alcohols predominantly and proposed an antiperiplanar transition state [ 3,4] , we anticipated that a similar reaction of other crotyl-organometallic compounds may exhibit selective inversion from threo to erythro by BF3 coordination to aldehydes (eq. 1). We therefore examined the reaction of benzaldehyde with several crotylmetals in the presence and in the absence of BFJ - OEtz . The results are summarized in Table 1. Ph+
FIJI
+
PhCHO
1 =
'
s
OH
BF?-
Ph$./
threo major
+
1 erythro major 0022-328X/851$03.30
o 1986 Elsevier Sequoia S.A.
(1)
Phb;"‘ =3
C46 TABLE
1
SELECTIVE Entry
INVERSION
1 (MLn)
FROM
with BF 3
without
BF,
‘y-adduct (2)
ratio
cu-adduct (3)
erythro/threo
7la
tMns/cls
erythro/threo
60140 ea/sfi
48162 66144
1 2
MgCl ZnBr
48152 66144
3
B3
30170
cu 6 Cd1 c IIg1 c TlI c TiCl,Cl ZrcP,Cl VCPICl
96/Z 60140 IO/30 9812 66/46 9614 70130
4 6 6 7 6 9 10
three TO erythro =
IO\80 81119 loo/-
30/10
36166 51143 66/36 60160 80/20 34166 66136
67143 66/36 76126 saps 60140 60160 IO/30
46152 50160 34166 6Of60 2Of80 19181 d 43157
a A mixture of PhCHO (1 mmol) and BF, ‘OEt, (2 mmol) in dry ether (1 ml) was added at -78OC to the crotyhnetal reagent (1.1 mmol) in ether (or in etherfTHF). except where otherwise indicated. and the reaction was cmenched at O’C. The reagent was prepared in situ by addition of crotylmagnesium chloride (1.1 mmol) to MLn (1.1 mmol) (ZnBr, in THF. CuI in ether, CdI, in ether, I-MI, in ether, TlI, in ether, TiCp2C1, in ether, ZrCp,Cl, in THF and VCpICl, ln ether). The product ratio and yield were determined by GLC and ‘II NMR spectroscopy. The structure determination was made by comparison with authentic samples [a]. Total yields are almost quantitative except for entries 6.7 and 10. where also small amounts of PbC(O)CH(CH,)CIi=CH, were formed presumably due to the MeerwelnPondorf oxidation. 6 The reagent was made at -3O’C. and PhCHOsBF, was added at this temperature. c The reagent was made at 0°C and PhCHO was added at 0°C. d Data from Y. Yamamoto and K. Meruyama. Tetrahedron Lett., 22 (1981) 2896.
The eSythro/threo ratio in the reaction of Mg, Zn, and B is not influenced by the presence of BF, (entries l-3). Reversal of the diastereoselectivity is observed in the reactions of Cu, Cd, Hg, Tl, Ti [5], Zr, and V (entries 4-10). Especially, the use of BFB in the reactions of Cu, Tl, and Zr causes very predominant formation of the erythro isomer. Furthermore, the cu-isomer3 is formed as a by-product, and sometimes as the major product, if BF8 is used. Without BF3 the r-isomer is obtained in an essentially quantitative yield [ 61. The inversed regioselectivity may be explained by tranametallation via an & 2’ process (path b) followed by a rapid reaction with benzaldehyde (path c)
171 (es. 3). (3)
*
PhCHO.BF3
2 (erythro major)
(2)
(c)
l------3 = BF3
p
(3)
Y3 BLn
4
Jr -BLn
-
(d) PhCHO
=5
'
(4)
c47
If such a transmetallation process is involved, the y/a! isomer ratio should depend on the addition sequence of BF3. The addition of 1 equiv. of BF3 to crotyhnagnesium chloride at -78°C followed by the addition of benzaldehyde produced 30% of the y-adduct (erythro/threo = 75/25) and 70% of from 4 the cy-adduct (tram/& = 64/36) (cf. entry 1). Allylic rearrangement to 5 followed by the reaction with benzaldehyde (path d) again produces 2 (eq. 4). In fact, when the mixture of 1 equiv. of BFB - OEtz and crotylmagnesium chloride, prepared at -78”C, was allowed to warm up to room temperature and then cooled to -78”C, a similar reaction with benzaldehyde gave exclusively the r-adduct (erythro/threo = 39/61). We also examined the reaction of isobutyraldehyde (6) to investigate whether in the BFB mediated reaction of an aliphatic aldehyde besides the Tadduct some ar-adduct is also formed. The reaction of crotyl-ZrCp,Cl with 6-BF3 produced 98% of the y-adduct (erythro/threo = 60/40) [8] along with 1% of the aadduct (tram/h = l/l). A similar reaction of crotyl-SnBu, , our original reaction in this series, gave 99% of the y-adduct (erythro/threo = 96/4) along with 0.2% of the a-isomer. Consequently, the r/a ratio is highly dependent upon the structure of the aldehyde, the metal (M), and the reaction conditions, owing to the complex competitive processes (eqs. 2-4). Until now it is not possible to clarify whether 2 is produced via eq. 2 or eq. 4. However, when the formation of the a-isomer can be neglected, the reversed diastereoselectivity must occur via an antiperiplanar transition state (path a, eq. 2). In conclusion, reversal of the diastereoselectivity from threo to erythro is observed for a wide range of crotyhnetals with relatively low Lewis acidity. Furthermore, the use of BFB often induces reversal of the regioselectivity, which is ascribed to the fact that BFB can interact with both nucleophilic sites (Scheme 1). The E-N*BFB interaction produces predominantly the pwythro isomer, and the Nu*BF3 interaction preferentially the a-isomer. A similar double function of Lewis acids has been observed recently with several reagents and reactions, e.g. RCu* BFB [9], R,CuLi* BFB [9], RLi* BF, [lo], allylsilane* TiCL [ 1 l] , and silyl enol ether* Tick, [ 121. E-N
substrate
(X=0)
\
BF
3 reagent
Nu
/
Scheme
1
(R-M) SCHEME
1
References 1 2 3 4
M.T. Reetz and M. Sauerwald, J. Org. Chem.. 49 (1964) 2292. Presented orally at the post ICOS-IV Kyoto symposium. August ‘i982. Y. Yamamoto, I-I. Yata6ai. Y. Naruta and K. Maruyame. J. Am. Chem. Sot.. 102 (1980) 7107: Y. Yemamoto. H. Yata6ai. Y. Ishihara. N. Maeda and K. Maruyama. Tetrahedron, 40 (1984) 2239. The s~nclind transItion state geometry ir proposed for the intramolecular reaction; S.E. Denmark and E.J. Weber, Helv. Chim. Acta. 66 (1983) 1656. For the reaction of crotyIsUane*BF, , see T. Hayashi. H. Ito and M. Kumada. Tetrahedron L&t.. 23 (1962) 4606.
C48 6
6 1
8 9
10 11 12
There is lack of agreement with Reetz or Sato in the Ti case (entry 8); F. Sate. K. Ikeda. S. Uima. Ii. Moriya and M. Sato. J. Chem. Soc. Chem. Commun.. (1981) 1140. The difference may be due to the solvent; ether in our ceee and THF in their cases. This means that 1. prepared in situ from crotylmagnesium chloride and MLn. does not take an a-metallyl geometry but possesses a crotyl structure. Y. Yamamoto. N. Maeda and K. Maruyama. J. Chem. Sot. Chem. Commun.. (1983) 142; Y. Yamamoto and K. Meruyama. J. Org. Chem.. 48 (1983) 1664. Another possibility for the inverted repioselectivity is that enhanced electrophilicity at the carbonyl center, caused by coordination of BF, , may force 1 to react at the ar-position. Without BF,, erythro/threo = 88112 (Table 1, footnoted). Y. Yemamoto. S. Yanmnoto. I% Yatagai, Y. Ishihara and K. Maruyama. J. Org. Chem.. 47 (1982) 119. M. Yamaguchi and I. Hirao. Tetrahedron Lett., 24 (1983) 391: M.J. Eis. J.E. Wrobel and B. Ganem. J. Am. Chem. Sot.. 106 (1984) 8693. A. Hosomi and H. Sakurai, Tetrahedron Lett.. 17 (1976) 1296. T. Mukaiyama. K. Banno and T. Narasaka. J. Am. Chem. Sot.. 96 (1974) 7503.
Journal of Organometallic Chemistry, 284 (1985) C45-C48 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary communication
CONCERNING REVERSAL OF DIASTEREOSELECTIVITY PROMOTED ADDITION OF CROTYL-ORGANOMETALLIC TO ALDEHYDES
YOSHINORI YAMAMOTO* Department
of Chemistry,
IN THE BF3 COMPOUNDS
and KAZUHIRO MARUYAMA
Faculty
of Science, Kyoto
University,
Kyoto
606 (Japan)
(Received December 7th, 1984)
Summary
The addition of a mixture of benzaldehyde and BFB*OEtz to crotyl-organometallic reagents C,H,MLn (1) (M = Cu, Cd, Hg, Tl, Ti, Zr and V) produces predominantly the erythro homoallyl alcohol as well as the a-adduct, while without BF3 .OEtz the threo isomer is formed preferentially. A recent communication by Reetz and Sauerwald [l] on the reversed diastereoselectivity in BFB-mediated reactions of crotyltitanium reagents with aldehydes has prompted us to report our own findings [ 21 in this area. When we found that the BF3 -mediated reaction of crotyltrialkyltins with aldehydes proceeds in a stereoconvergent manner to produce cry thro homoallyl alcohols predominantly and proposed an antiperiplanar transition state [ 3,4] , we anticipated that a similar reaction of other crotyl-organometallic compounds may exhibit selective inversion from threo to erythro by BF3 coordination to aldehydes (eq. 1). We therefore examined the reaction of benzaldehyde with several crotylmetals in the presence and in the absence of BFJ - OEtz . The results are summarized in Table 1. Ph+
FIJI
+
PhCHO
1 =
'
s
OH
BF?-
Ph$./
threo major
+
1 erythro major 0022-328X/851$03.30
o 1986 Elsevier Sequoia S.A.
(1)
Phb;"‘ =3
C46 TABLE
1
SELECTIVE Entry
INVERSION
1 (MLn)
FROM
with BF 3
without
BF,
‘y-adduct (2)
ratio
cu-adduct (3)
erythro/threo
7la
tMns/cls
erythro/threo
60140 ea/sfi
48162 66144
1 2
MgCl ZnBr
48152 66144
3
B3
30170
cu 6 Cd1 c IIg1 c TlI c TiCl,Cl ZrcP,Cl VCPICl
96/Z 60140 IO/30 9812 66/46 9614 70130
4 6 6 7 6 9 10
three TO erythro =
IO\80 81119 loo/-
30/10
36166 51143 66/36 60160 80/20 34166 66136
67143 66/36 76126 saps 60140 60160 IO/30
46152 50160 34166 6Of60 2Of80 19181 d 43157
a A mixture of PhCHO (1 mmol) and BF, ‘OEt, (2 mmol) in dry ether (1 ml) was added at -78OC to the crotyhnetal reagent (1.1 mmol) in ether (or in etherfTHF). except where otherwise indicated. and the reaction was cmenched at O’C. The reagent was prepared in situ by addition of crotylmagnesium chloride (1.1 mmol) to MLn (1.1 mmol) (ZnBr, in THF. CuI in ether, CdI, in ether, I-MI, in ether, TlI, in ether, TiCp2C1, in ether, ZrCp,Cl, in THF and VCpICl, ln ether). The product ratio and yield were determined by GLC and ‘II NMR spectroscopy. The structure determination was made by comparison with authentic samples [a]. Total yields are almost quantitative except for entries 6.7 and 10. where also small amounts of PbC(O)CH(CH,)CIi=CH, were formed presumably due to the MeerwelnPondorf oxidation. 6 The reagent was made at -3O’C. and PhCHOsBF, was added at this temperature. c The reagent was made at 0°C and PhCHO was added at 0°C. d Data from Y. Yamamoto and K. Meruyama. Tetrahedron Lett., 22 (1981) 2896.
The eSythro/threo ratio in the reaction of Mg, Zn, and B is not influenced by the presence of BF, (entries l-3). Reversal of the diastereoselectivity is observed in the reactions of Cu, Cd, Hg, Tl, Ti [5], Zr, and V (entries 4-10). Especially, the use of BFB in the reactions of Cu, Tl, and Zr causes very predominant formation of the erythro isomer. Furthermore, the cu-isomer3 is formed as a by-product, and sometimes as the major product, if BF8 is used. Without BF3 the r-isomer is obtained in an essentially quantitative yield [ 61. The inversed regioselectivity may be explained by tranametallation via an & 2’ process (path b) followed by a rapid reaction with benzaldehyde (path c)
171 (es. 3). (3)
*
PhCHO.BF3
2 (erythro major)
(2)
(c)
l------3 = BF3
p
(3)
Y3 BLn
4
Jr -BLn
-
(d) PhCHO
=5
'
(4)
c47
If such a transmetallation process is involved, the y/a! isomer ratio should depend on the addition sequence of BF3. The addition of 1 equiv. of BF3 to crotyhnagnesium chloride at -78°C followed by the addition of benzaldehyde produced 30% of the y-adduct (erythro/threo = 75/25) and 70% of from 4 the cy-adduct (tram/& = 64/36) (cf. entry 1). Allylic rearrangement to 5 followed by the reaction with benzaldehyde (path d) again produces 2 (eq. 4). In fact, when the mixture of 1 equiv. of BFB - OEtz and crotylmagnesium chloride, prepared at -78”C, was allowed to warm up to room temperature and then cooled to -78”C, a similar reaction with benzaldehyde gave exclusively the r-adduct (erythro/threo = 39/61). We also examined the reaction of isobutyraldehyde (6) to investigate whether in the BFB mediated reaction of an aliphatic aldehyde besides the Tadduct some ar-adduct is also formed. The reaction of crotyl-ZrCp,Cl with 6-BF3 produced 98% of the y-adduct (erythro/threo = 60/40) [8] along with 1% of the aadduct (tram/h = l/l). A similar reaction of crotyl-SnBu, , our original reaction in this series, gave 99% of the y-adduct (erythro/threo = 96/4) along with 0.2% of the a-isomer. Consequently, the r/a ratio is highly dependent upon the structure of the aldehyde, the metal (M), and the reaction conditions, owing to the complex competitive processes (eqs. 2-4). Until now it is not possible to clarify whether 2 is produced via eq. 2 or eq. 4. However, when the formation of the a-isomer can be neglected, the reversed diastereoselectivity must occur via an antiperiplanar transition state (path a, eq. 2). In conclusion, reversal of the diastereoselectivity from threo to erythro is observed for a wide range of crotyhnetals with relatively low Lewis acidity. Furthermore, the use of BFB often induces reversal of the regioselectivity, which is ascribed to the fact that BFB can interact with both nucleophilic sites (Scheme 1). The E-N*BFB interaction produces predominantly the pwythro isomer, and the Nu*BF3 interaction preferentially the a-isomer. A similar double function of Lewis acids has been observed recently with several reagents and reactions, e.g. RCu* BFB [9], R,CuLi* BFB [9], RLi* BF, [lo], allylsilane* TiCL [ 1 l] , and silyl enol ether* Tick, [ 121. E-N
substrate
(X=0)
\
BF
3 reagent
Nu
/
Scheme
1
(R-M) SCHEME
1
References 1 2 3 4
M.T. Reetz and M. Sauerwald, J. Org. Chem.. 49 (1964) 2292. Presented orally at the post ICOS-IV Kyoto symposium. August ‘i982. Y. Yamamoto, I-I. Yata6ai. Y. Naruta and K. Maruyame. J. Am. Chem. Sot.. 102 (1980) 7107: Y. Yemamoto. H. Yata6ai. Y. Ishihara. N. Maeda and K. Maruyama. Tetrahedron, 40 (1984) 2239. The s~nclind transItion state geometry ir proposed for the intramolecular reaction; S.E. Denmark and E.J. Weber, Helv. Chim. Acta. 66 (1983) 1656. For the reaction of crotyIsUane*BF, , see T. Hayashi. H. Ito and M. Kumada. Tetrahedron L&t.. 23 (1962) 4606.
C48 6
6 1
8 9
10 11 12
There is lack of agreement with Reetz or Sato in the Ti case (entry 8); F. Sate. K. Ikeda. S. Uima. Ii. Moriya and M. Sato. J. Chem. Soc. Chem. Commun.. (1981) 1140. The difference may be due to the solvent; ether in our ceee and THF in their cases. This means that 1. prepared in situ from crotylmagnesium chloride and MLn. does not take an a-metallyl geometry but possesses a crotyl structure. Y. Yamamoto. N. Maeda and K. Maruyama. J. Chem. Sot. Chem. Commun.. (1983) 142; Y. Yamamoto and K. Meruyama. J. Org. Chem.. 48 (1983) 1664. Another possibility for the inverted repioselectivity is that enhanced electrophilicity at the carbonyl center, caused by coordination of BF, , may force 1 to react at the ar-position. Without BF,, erythro/threo = 88112 (Table 1, footnoted). Y. Yemamoto. S. Yanmnoto. I% Yatagai, Y. Ishihara and K. Maruyama. J. Org. Chem.. 47 (1982) 119. M. Yamaguchi and I. Hirao. Tetrahedron Lett., 24 (1983) 391: M.J. Eis. J.E. Wrobel and B. Ganem. J. Am. Chem. Sot.. 106 (1984) 8693. A. Hosomi and H. Sakurai, Tetrahedron Lett.. 17 (1976) 1296. T. Mukaiyama. K. Banno and T. Narasaka. J. Am. Chem. Sot.. 96 (1974) 7503.