Embedding an allylmetal dimer in a chiral cavity: The unprecedented stereoselectivity of a twofold Wittig [1,2]-rearrangement

Article abstract of DOI:10.1021/ja066462f

After α,α'-dimetalation, both 2,2′-diallyloxy-1,1′-binaphthyl and 2,2′-di-2-methylallyloxy-1,1′-binaphthyl undergo the Wittig rearrangement with perfect diastereoselectivity. When racemic 1,1′-binaphthyl-2,2′-diol ("BINOL") is used as the starting material, it gives rise to a 1:1 mixture of antipodal stereoisomers, whereas enantiomerically pure (M)-2,2′-diallyloxy-1,1′-binaphthyl affords (M)-(S,S)-1,1-(1,1′-binaphthyl-2,2′-diyl)bis(2-propen-1-ol) as the sole product. The (M)-(S,S)/(P)-(R,R) mixture resulting from the rearrangement of racemic 2,2′-diallyloxy-1,1′-binaphthyl can be effectively subjected to a kinetic racemate resolution by applying the Sharpless-Katsuki asymmetric epoxidation. The single-sided Wittig rearrangement of 2-allyloxy-2′-propyloxy-1,1′-binaphthyl proceeds without any diastereoselectivity as this substrate can only be monometalated and hence is incapable of intramolecular aggregate formation which is instrumental for the observed stereoselectivity. Copyright

Full text of DOI:10.1021/ja066462f

Published on Web 11/22/2006
Embedding an Allylmetal Dimer in a Chiral Cavity: The Unprecedented
Stereoselectivity of a Twofold Wittig [1,2]-Rearrangement
Manfred Schlosser* and Fre´de´ric Bailly
Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fe´de´rale, BCh,
CH-1015 Lausanne, Switzerland
Received September 7, 2006; E-mail: manfred.schlosser@epfl.ch
Scheme 1. Witting Rearrangement of
1,1′-Binaphthyl-2,2′-bis(diphenylphosphine) (“BINAP”) is argu-
ably the most popular ligand for the catalysis of asymmetric
hydrogenation and isomerization reactions. The bisphosphine was
originally prepared starting from 1,1′-binaphthalene-2,2′-diol
(“BINOL”), a cornerstone of atropisomeric chemistry, through 2,2′-
dibromonaphthalene1 until an improved procedure based on the
palladium-mediated condensation of the BINOL-derived 1,1′-
binaphth-2,2′-diyl bis(trifluoromethane sulfonate) with diphen-
ylphosphine was disclosed.2 This triflate was also found to be
amenable to a palladium-catalyzed methoxycarbonylation, providing
dimethyl 1,1′-binaphthyl-2,2′-dicarboxylate in high yield.3 This
switch from oxygen to carbon functionality intrigued us. We
wondered whether it could not be brought about in a different
manner featuring a Wittig rearrangement as the key step. In fact,
when a solution of 2,2′-di(2-propenoxy)-1,1′-binaphthyl in tetrahy-
drofuran was treated at -75 °C with the LIC-KOR mixture4
(butyllithium + potassium tert-butoxide) before being neutralized
at +25 °C, 1,1-(1,1′-binaphth-2,2′-diyl)bis(2-propen-1-ol) (1a; see
Scheme 1) was isolated indeed in high yield (72% after recrystal-
lization). The mother liquors contained not even trace amounts of
a 3-(1,1′-binaphthyl)-substituted propanal. This would have resulted
from a [1,4]- rather than [1,2]-migration, the former process being
the predominant mode when alkyl allyl ethers, as opposed to aryl
allyl ethers, are subjected to the rearrangement.5
2,2′-Di(2-propenoxy)-1,1′-binaphthyl To Afford
1,1-(1,1′-Binaphth-2,2′-diyl)bis(2-propen-1-ol) (1; 72%)a
a Conditions: (a) LiC4H9 + KOC(CH3)3 in tetrahydrofuran at -75 °C;
(b) temperature increased to +25 °C; (c) H2O.
Scheme 2. Transformation of the Diol 1 to the Bis(allyl chloride) 2
(90%) and Oxidation of the Latter to the Dialdehyde 3 (82%) and
the Dicarboxylic acid 4 (91%)a
a Conditions: (a) Ethanolic hydrogen chloride; (b) OsO4 (1%) + excess
NaIO4 in aqueous THF at 25 °C; (c) NaClO2 + H2O2 in NaH2PO4-buffered
aqueous acetonitrile at +40 °C.
The diol 1a was converted into 1,1-(1,1′-binaphthyl-2,2′-bis(3-
chloro-1-propene) (2). Consecutive Vic-dihydroxylation of the
olefinic double bonds using sodium periodate and osmium tetroxide-
promoted glycol cleavage gave 1,1-dinaphthyl-2,2′-dicarbaldehyde
(3, 63%; Scheme 2).6 The dialdehyde 3 could be reduced with
sodium borohydride to 1,1′-dinaphthyl-2,2′-bismethanol7 and was
oxidized with sodium chlorite to give 1,1′-dinaphthyl-2,2′-dicar-
boxylic acid8 (4, 91%; Scheme 2). Repetition of the entire reaction
sequence employing optically active BINOL9 produced of course
the enantiomerically pure diacid 4.10
Scheme 3. Kinetic Resolution of the Diol Stereoisomers
(M)-(S,S)-1a and (P)-(R,R)-1a by the Sharpless-Katsuki Methoda
Due to its two exocyclic stereocenters, the diol 1 exhibits both
axial and tetrahedral chirality. High crystallinity and a narrow
melting range suggested immediately that a single diastereoisomer
was formed in the course of the Wittig rearrangement. This was
confirmed by X-ray crystallography of the 4-bromobenzoate 1b
(see Scheme 1). The axial chirality (M)11 was found to be associated
with the (S) configuration of the oxygen-bearing side chain and
the (P)11 helicity with the (R) configuration (see Scheme 3).
In the allyl alcohol family, diastereoselectivity can be readily
traded in for enantioselectivity by applying the Sharpless-Katsuki
epoxidation protocol12 for kinetic racemate resolution. In fact, when
the racemic mixture of (M)-(S,S)-1a and (P)-(R,R)-1a was exposed
at -25 °C for 20 h to tert-butyl hydroperoxide, titanium tetraiso-
propoxide, and (+)-diisopropyl (R,R)-tartrate (DIPT) in dichlo-
romethane and in the presence of molecular sieves, the (P)-(R,R)-1
component was completely consumed, with the supposed (R,R)-
a Conditions: (a) (H3C)3COOH + Ti(OiC3H7)4 + (+)-DIPT in CH2Cl2
at 25 °C.
bisepoxide 5 (27%) being isolated as the main product (Scheme
4). Enantiomerically pure (M)-(S,S)-1a was recovered almost
quantitatively (Scheme 3).
Irrespective of its practical potential, we need to rationalize the
origin of this unprecedented diastereoselectivity. The phenomenon
must have to do with intramolecular organometallic aggregation
as the monoallyl ether 6 underwent stereochemically random
rearrangement giving rise to a 1:1 mixture of (R) and (S) alcohols
7 (see Scheme 4). In contrast, the Wittig rearrangement of doubly
metalated 2,2′-di(2-methyl-2-propenoxy)-1,1′-binaphthyl 8 proved
again perfectly diastereoselective, affording the presumed
(M)-(S,S)-9 and (P)-(R,R)-9 structures (Scheme 4).
9
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J. AM. CHEM. SOC. 2006, 128, 16042-16043
10.1021/ja066462f CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 4. Diastereochemically Unselective and Selective
Rearrangement of Metalated
2-(Propenyloxy)-2′-propyloxy-1,1′-binaphthyl and Dimetalated
2,2′-Bis(2-methyl-2-propenyloxy)-1,1′-binaphthyl To Afford
Products 7 (70%) and 9 (82%), Respectivelya
the first new stereocenter should have the (S) configuration when
accompanied by (M) helicity and the (R) configuration when
associated with a (P) axis. The second rearrangement must follow
instantaneously, before any structural reorganization can take place.
The conversion of the dimetalated species 10 into the mono-
rearranged species 12 (see Scheme 5) should weaken the intra-
aggregate stabilization and hence provide an extra driving force
for further transformation. Moreover, metal-π interactions18 may
well be operative in the new intermediate 12 (as shown in Scheme
5) and help to prevent a premature structural collapse of this mixed
aggregate. Consequently, also the second rearrangement step will
proceed under (M) f (S) and (P) f (R) stereocontrol.
Acknowledgment. This work was supported by the Swiss
National Science Foundation, Bern (Grant 20-100’336-02)
and the Bundesamt fu¨r Bildung und Wissenschaft, Bern (Grant
C02.0060).
Supporting Information Available: Working procedures and
complete characterization data for compounds 1a, 1b, and 2-9 and
full crystallographic details of compound 1b (including the associated
CIF tables) are provided. This material is available free of charge via
the Internet at http://pubs.acs.org.
a Conditions: (a) LiC4H9 + KOC(CH3)3 in tetrahydrofuran at -75 °C;
(b) temperature increased to +25 °C; (c) H2O.
Scheme 5. Metamorphosis of the Internal Bisallylmetal Aggregate
10 through the Meisenheimer Complex 11 to the Mixed Aggregate
12, The Intermediate Formed in the First Rearrangement Step
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