Importance of open structure of nonmetal based catalyst in hydrogen bond promoted methanolysis of activated amide: Structure dynamics between monomer and dimer enabling recombinant covalent, dative, and hydrogen bonds

Article abstract of DOI:10.1021/ja9029494

(Figure Presented) We disclosed structural dynamics hidden behind a series of aminoorganoboron (AOB) compounds that involved a recombinant of covalent, dative, and hydrogen bonds. A combination process occurred via reorganizing elements and bonds between

Full text of DOI:10.1021/ja9029494

Published on Web 06/04/2009
Importance of Open Structure of Nonmetal Based Catalyst in Hydrogen Bond
Promoted Methanolysis of Activated Amide: Structure Dynamics between
Monomer and Dimer Enabling Recombinant Covalent, Dative, and Hydrogen
Bonds
Shunsuke Oishi,† Junichi Yoshimoto,† and Susumu Saito*,†,‡
Graduate School of Science, Nagoya UniVersity, Chikusa, Nagoya 464-8602, Japan, and Institute for AdVanced
Research, Nagoya UniVersity, Chikusa, Nagoya 464-8601, Japan
Received April 16, 2009; E-mail: saito.susumu@f.mbox.nagoya-u.ac.jp
The importance of an opened structure such as an open dimer
or monomer in organometallic reactions is now gaining recognition
among chemists.1 Formation of open dimers from the more stable
closed dimers (Figure 1, left) or releasing the monomers from those
species is believed to be a key step to push forward chemical entities
into a favorable transition state. However, the significance of such
an open structure in metal-free catalysis, namely in hydrogen bond
promoted catalysis, remains totally obscured.2 We report here that
the open structure is important for catalysis involving subtle
interplay of multiple hydrogen bonds. Open structures were ela-
borated by controlling novel structural dynamics of aminoorga-
noboron (AOB) compounds3 undergoing the monomer-recombinant
dimer equilibrium (Figure 1, right).
both embedded into the structure of a single molecule open-1a,
being isolated as a monomer.
In contrast, if we followed the experimental procedure5 upon
reaction of 2b with B(OMe)3 (Scheme 1), closed-1b was obtained
in ∼20% yield representing an entirely different structure, disclosed
by the XRD analysis.4 The 12-membered heterocycle comprises
multiple hydrogen bonds, in which two N-B--N-H residues make
linkages with two HCl molecules, forming a dimer, closed-1b. The
dimer adopts two boron-centered spiro structures, interacting in a
manner to make a mirror image. The heterochiral R and S interaction
has been commonly observed in chiral organometallic compounds.6
Scheme 1
Figure 1. Open and closed structures of organometallic and non-metal
reagents.
The AOB alkoxide open-1a was readily prepared by dilithiation
of 2a (2 equiv) with n-BuLi (4 equiv) in Et2O at -44 °C for 1 h,
followed by exposure to B(OMe)3 (1 equiv). The resulting mixture
was stirred at 25 °C for 12 h, during which time the mixture became
a white suspension. Making a clear solution by adding excess
CH2Cl2, followed by treatment with hexane, immediately produced
a white precipitate in ∼40% yield (Scheme 1), which was filtrated
and recrystallized from CHCl3/MeOH cosolvents. The obtained
single crystal was subjected to XRD analysis, showing a novel
structure of open-1a,4 in which the multiple elements were arranged
linearlywithinthestructure(openedarray)delineatedasO(δ-)-B(δ+)--
N(δ-)-H(δ+)--N(δ-)-H(δ+) (-: covalent bond; --: noncovalent
bond, in a canonical structure). The alternating array of two different
bonds, namely covalent and noncovalent bonds, was another
characteristic feature. This structure recalls, for example, the open
dimer of lithium diisopropylamide (LDA) that should assume a
transition structure during deprotonation of the R-proton of carbonyl
compounds, in which the interacting elements make a similar
alignment of different bonds alternating as (R-C)(δ-)-H(δ+)--
N(δ-)-Li(δ+)--N(δ-)-Li(δ+).1c Of note is that alternations of
acid(δ+)/base(δ-) elements and covalent/noncovalent bonds were
What we envisaged from these two different structures, open-
and closed-1a (or 1b), was that these two would be interchangeable
by altering acidic or basic environments in their solution state
(Scheme 1). Indeed, by adding NaOMe (1 equiv) to a CH3OH
solution of closed-1a, the structure immediately returned to open-
1a accommodating >99% conversion of closed-1a. In contrast, when
open-1a was treated with 1 equiv of HCl in CD3Cl at 25 °C,
structural change completed within a few seconds, forming the
4
1
structure closed-1a, ascertained by H and 13C NMR analysis.
Reversible change was controllable by merely supplying either acid
or base! Analogously, addition of KOH to a H2O solution of closed-
1b afforded open-1b as the sole product. Going back and forth
among these structural changes, covalent, dative, and hydrogen
bonds were cleaved and/or reorganized.7
Further experiments demonstrated that either an open or a closed
structure was formed preferentially when a subtle modification of
the basicity of phenoxides (thus pKa of phenol) was adjusted upon
reaction with closed-1b (Scheme 2). For example, predominant
formation of open-4a was detected by 1H NMR analysis using Na-
3a (closed/open ) 4:96; [4a] ) ca. 40 mM), whereas closed-4g8
was formed preferentially with Na-3g (closed/open ) ca. 60:40).
More precise switching of phenoxides through Na-3b to Na-3f,
which were derived from the parent phenols of an inherent pKa in
† Graduate School of Science.
‡ Institute for Advanced Research.
9
8748 J. AM. CHEM. SOC. 2009, 131, 8748–8749
10.1021/ja9029494 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
H2O ranging from 7.15 to 10.21,9 affected the dynamic behavior
yielding a mixture of two structures open- and closed-4 in a different
ratio.4 A Hammett plot of the logarithm of equilibrium constant
Keq ([open-4]/[closed-4]) vs the σp- parameters for the substituents
of phenols was approximately linear and yielded a negative F-value
of -1.12 (Figure 2a). It is obvious that the ratio of the dynamic
structures is proportional to the pKa of phenols. The more basic
the phenoxide, the more stable the open structure. We next
attempted the methanolysis of Evans’s N-acyl oxazolidinone 5a10
as a model reaction to see any differences in catalytic activity among
a series of AOB phenoxides 4a-4g (Scheme 3). Plotting the
logarithm of the relative apparent rate ([6a(%)]/[6a(%) with 4b]
calculated from isolated yields of 6a after reacting 5a for 1 h at 25
°C in MeOH; [4a] ) ca. 10 mM) vs log Keq conformed again to a
linear free-energy relationship, suggesting that open-4 should be
more responsible for a higher reaction rate (Figure 2b).
of the alternative amide bond of 5b giving 6c was reasonably
prevented. This could be ascribed to the almost neutral pH conditions,
and/or to some molecular recognition events, in which a specific
functional group such as a ꢀ-dicarbonyl unit was favorably discrimi-
nated by very active species. In fact, more neutral catalyst open-1a
gave a selectivity superior to NaOMe (5 mol %, pH ) 9-10 in
MeOH), which facilitated further fragmentation.
Scheme 4
In summary, we disclosed structural dynamics hidden behind a
series of AOB compounds that involved a recombinant of covalent,
dative, and hydrogen bonds. A combination process occurred via
reorganizing elements and bonds between the two major structures
open- and closed-1 (or 4), which were chemically switchable
through precise adjustment of either acidic or basic conditions. The
structural dynamics favoring an open structure seem to be more
important for catalysis, as represented by the methanolysis of
activated amides. Within the open structures, a linear alignment of
alternating covalent and noncovalent bonds should be well orga-
nized in such a way that relatively acidic (δ+) and basic (δ-)
elements can cooperate effectively in a transition state.
Scheme 2
Scheme 3
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research on Priority Areas “Advanced Molecular
Transformations of Carbon Resource” from the Ministry of Educa-
tion, Culture, Sports, Science and Technology, Japan, as well as
by Asahi Glass Foundation. S.S. also greatly appreciates Prof. R.
Noyori (Nagoya Univ. and RIKEN) for his valuable suggestions.
Methanolysis of aldol product 5b10 proceeded as well in the presence
of 5 mol % of open-1a at -25 °C (pH ) 7-8 in MeOH; [cat] ) 25
mM) to give methyl ester 6b in 90% yield without racemization
(Scheme 4).4 The cyclic structure and the chirality of oxazolidinone
6a remained intact,4 which was recovered in 91% yield. The cleavage
Supporting Information Available: Full experimental details and
1H, 13C NMR spectra of all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
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Figure 2. Linear free-energy relationships: (a) log Keq vs σp-; (b)
log(relative apparent rate) vs log Keq.
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