Helical Triskelion Monophosphites as Ligands
A R T I C L E S
Scheme 1. Two Conformeric Forms of Phosphites P(OR)3
example, Bolm and Sharpless8 have reported the synthesis of a
helical C3-symmetric phospha[2.2.2]cyclophane, but rapid in-
terconversion of the P and M enantiomeric forms due to the
fluxional behavior prohibits their use in asymmetric transition-
metal catalysis. This relates to the early work of Mislow9 on
propeller-like compounds such as bulky triarylmethanes, a
research area which has been extended by other groups.10 If
fluxional compounds of this kind contain additional configu-
rationally stable elements of central, axial, or planar chirality,
any dynamic process that regards the sense of the helicity results
in rapid diastereoisomerization. A number of chiral and possibly
helical C3-symmetric ligands harboring additional central or axial
chirality have been described,7,11 although in many cases the
issue of diastereoisomerism was not addressed. Such ligands
can be mono-, bi-, or tridentate. Some researchers have used
the term triskelion to denote ligands, metal complexes, or
receptors that possess threefold rotational symmetry; they may
be chiral or achiral.12 Double-helical D3-symmetric triskelion
metal complexes also deserve mention.13 In a different approach
to asymmetric catalysis, chiral Lewis acids with helical character
have been described,14 as in the case of a 1,1′-binaphthyl-2,2′-
diol (BINOL)-derived boron compound.14a
Here we focus on a special class of triskelion compounds,
namely, configurationally stable helical C3-symmetric mono-
dentate phosphites P(OR)3, as chiral ligands in transition-metal
catalysis. Phosphites derived from centrally chiral alcohols ROH
have been prepared and used as ligands previously,1,15 but the
possibility of diastereomeric forms due to potential helicity was
not considered, probably because the researchers did not view
the compounds as being so bulky that helical conformers might
be locked in.16 Before our approach to this problem is presented,
it is important to point out that such compounds may exist in
two basically different conformeric forms, which we denote as
syn or anti according to Scheme 1. In the syn conformer, the
first C atoms (CR) in the R groups of P(OR)3 are on the same
side of the P atom as the phosphorus lone pair, while in the
anti conformer, they reside on the opposite side.17 Of course,
within each series, as for example in the case of the syn
compounds, a subset of further conformers is possible, depend-
ing upon the nature of the R groups.
(7) For reviews of chiral C3-symmetric ligands, some of which are
triskelion, see: (a) Moberg, C. Angew. Chem. 1998, 110, 260-281;
Moberg, C. Angew. Chem., Int. Ed. 1998, 37, 248-268. (b) Gibson,
S. E.; Castaldi, M. P. Chem. Commun. 2006, 3045–3062. (c) Gade,
L. H.; Bellemin-Laponnaz, S. Chem.sEur. J. 2008, 14, 4142–4152.
(d) Moberg, C. Angew. Chem. 2006, 118, 4838-4840; Moberg, C.
Angew. Chem., Int. Ed. 2006, 45, 4721-4723. (e) Zhou, J.; Tang, Y.
Chem. Soc. ReV. 2005, 34, 664–676.
(8) Bolm, C.; Sharpless, K. B. Tetrahedron Lett. 1988, 29, 5101–5104.
(9) Mislow, K. Acc. Chem. Res. 1976, 9, 26–33.
In the absence of chirality in the R groups, these monophos-
phites can become chiral if they adopt a helical (twist) form,
giving rise to P or M helicity, as illustrated in Scheme 2.
However, unless such propeller-type compounds are locked in
the P or M form in some way that makes enantiomerization no
longer possible, they cannot be expected to be of any use as
ligands in asymmetric transition-metal catalysis.
(10) (a) Iwamura, H.; Mislow, K. Acc. Chem. Res. 1988, 21, 175–182. (b)
Rappoport, Z.; Biali, S. E. Acc. Chem. Res. 1997, 30, 307–314. (c)
Sedo´, J.; Ventosa, N.; Molins, M. A.; Pons, M.; Rovira, C.; Veciana,
J. J. Org. Chem. 2001, 66, 1579–1589. (d) Wolf, C. Dynamic
Sterochemistry of Chiral Compounds: Principles and Applications;
Royal Society of Chemistry: Cambridge, U.K., 2008.
(11) For examples of C3-symmetric ligands, see: (a) Burk, M. J.; Harlow,
R. L. Angew. Chem. 1990, 102, 1511-1513; Burk, M. J.; Harlow,
R. L. Angew. Chem., Int. Ed. Engl. 1990, 29, 1462-1464. (b)
Wesemann, J.; Jones, P. G.; Schomburg, D.; Heuer, L.; Schmutzler,
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Wadepohl, H.; Gade, L. H. Chem.sEur. J. 2007, 13, 5994–8008. (b)
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(14) For example, see: (a) Kaufmann, D.; Boese, R. Angew. Chem. 1990,
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(16) One of the simplest organic species characterized by helicity is the
helical conformer of n-butane,2b but this is obviously a fluxional
system.
´
2007, 48, 5665–5668. (o) Pinte´r, A.; Haberhauer, G.; Hyla-Kryspin,
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(17) We define the CR atom of the R moiety as the direction of R. It should
be noted that in the case of bulky R groups, the rest of the group can
be positioned either on the same side of the P atom as the lone pair
or on the opposite side; this can lead to confusion. Moreover,
conformers other than the all-syn or all-anti ones are in principle
possible, although we have no direct evidence of their existence in
the present systems.
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