Stereoselective synthesis of trisubstituted alkenes is a long-standing challenge in organic chemistry, due to the small energy differences between E and Z isomers of trisubstituted alkenes (compared ...with 1,2-disubstituted alkenes). Transition metal-catalyzed isomerization of 1,1-disubstituted alkenes can serve as an alternative approach to trisubstituted alkenes, but it remains underdeveloped owing to issues relating to reaction efficiency and stereoselectivity. Here we show that a novel cobalt catalyst can overcome these challenges to provide an efficient and stereoselective access to a broad range of trisubstituted alkenes. This protocol is compatible with both mono- and dienes and exhibits a good functional group tolerance and scalability. Moreover, it has proven to be a useful tool to construct organic luminophores and a deuterated trisubstituted alkene. A preliminary study of the mechanism suggests that a cobalt-hydride pathway is involved in the reaction. The high stereoselectivity of the reaction is attributed to both a π–π stacking effect and the steric hindrance between substrate and catalyst.
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The intramolecular “inverse” frustrated Lewis pairs (FLPs) of general formula 1‐BR2‐2‐(Me2N)2C=N‐C6H4 (3–6) BR2=BMes2 (3), BC12H8, (4), BBN (5), BBNO (6) were synthesized and structurally ...characterized by multinuclear NMR spectroscopy and X‐ray analysis. These novel types of pre‐organized FLPs, featuring strongly basic guanidino units rigidly linked to weakly Lewis acidic boryl moieties via an ortho‐phenylene linker, are capable of activating H−H, C−H, N−H, O−H, Si−H, B−H and C=O bonds. 4 and 5 deprotonated terminal alkynes and acetylene to form the zwitterionic borates 1‐(RC≡C‐BR2)‐2‐(Me2N)2C=NH‐C6H4 (R=Ph, H) and reacted with ammonia, BnNH2 and pyrrolidine, to generate the FLP adducts 1‐(R2HN→BR2)‐2‐(Me2N)2C=NH‐C6H4, where the N‐H functionality is activated by intramolecular H‐bond interactions. In addition, 5 was found to rapidly add across the double bond of H2CO, PhCHO and PhNCO to form cyclic zwitterionic guanidinium borates in excellent yields. Likewise, 5 is capable of cleaving H2, HBPin and PhSiH3 to form various amino boranes. Collectively, the results demonstrate that these new types of intramolecular FLPs featuring weakly Lewis acidic boryl and strongly basic guanidino moieties are as potent as conventional intramolecular FLPs with strongly Lewis acidic units in activating small molecules.
Novel intramolecular “inverse” frustrated Lewis pairs have been prepared and their ability to activate small molecules investigated.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The selective formation of antimony-carbon bonds via organic superbase catalysis under metal- and salt-free conditions is reported. This novel approach utilizes electron-deficient stibine, Sb(C6F5)3, ...to give upon base-catalyzed reactions with weakly acidic aromatic and heteroaromatic hydrocarbons access to a range of new aromatic and heteroaromatic stibines, respectively, with loss of C6HF5. Also, the significantly less electron-deficient stibines, Ph2SbC6F5 and PhSb(C6F5)2 smoothly underwent base-catalyzed exchange reactions with a range of terminal alkynes to generate the stibines of formulae PhSb(C≡CPh)2, and Ph2SbC≡CR R = C6H5, C6H4-NO2, COOEt, CH2Cl, CH2NEt2, CH2OSiMe3, Sb(C6H5)2, respectively. These formal substitution reactions proceed with high selectivity as only the C6F5 groups serve as a leaving group to be liberated as C6HF5 upon formal proton transfer from the alkyne. Kinetic studies of the base-catalyzed reaction of Ph2SbC6F5 with phenyl acetylene to form Ph2SbC≡CPh and C6HF5 suggested the empirical rate law to exhibit a first-order dependence with respect to the base catalyst, alkyne and stibine. DFT calculations support a pathway proceeding via a concerted σ-bond metathesis transition state, where the base catalyst activates the Sb-C6F5 bond sequence through secondary bond interactions.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Combinations of Ni(acac)2 with phosphine ligands were found to catalyze the regioselective hydroboration of N-heteroarenes with pinacolborane, affording N-borylated 1,4-reduction products. ...Preliminary mechanistic studies have focused on the isolation and study of potential intermediates in the catalytic cycle.
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C U soon: Clusters containing 60, 44, and 36 uranyl peroxide hydroxide polyhedra (see picture) adopt fullerene topologies of maximum symmetry. The largest of these, denoted U60, is topologically ...identical to C60 with no pentagonal adjacencies and the highest possible symmetry. U44 adopts the topology with maximum symmetry rather than that with the lowest number of pentagonal adjacencies.
C U soon: Clusters containing 60, 44, and 36 uranyl peroxide hydroxide polyhedra (see picture) adopt fullerene topologies of maximum symmetry. The largest of these, denoted U60, is topologically identical to C60 with no pentagonal adjacencies and the highest possible symmetry. U44 adopts the topology with maximum symmetry rather than that with the lowest number of pentagonal adjacencies.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The polyhedral oligosilsesquioxane complexes, {(isobutyl) 7 Si 7 O 12 ZrOPr i ·(HOPr i )} 2 ( I ), {(cyclohexyl) 7 Si 7 O 12 ZrOPr i ·(HOPr i )} 2 ( II ), {(isobutyl) 7 Si 7 O 12 HfOPr i ·(HOPr i )} ...2 ( III ) and {(cyclohexyl) 7 Si 7 O 12 HfOPr i ·(HOPr i )} 2 ( IV ), were synthesized in good yields from the reactions of M(OPr i ) 4 (M = Zr, Hf) with R-POSS(OH) 3 (R = isobutyl, cyclohexyl), resp. I – IV were characterized by 1 H, 13 C and 29 Si NMR spectroscopy and their dimeric solid-state structures were confirmed by X-ray analysis. I – IV catalyze the reductive etherification of 2-hydroxy- and 4-hydroxy and 2-methoxy and 4-methoxybenzaldehyde and vanillin to their respective isopropyl ethers in isopropanol as a “green” solvent and reagent. I – IV are durable and robust homogeneous catalysts operating at temperatures of 100–160 °C for days without significant loss of catalytic activity. Likewise, I – IV selectively catalyze the conversion of 5-hydroxymethylfurfural (HMF) into 2,5-bis(isopropoxymethyl)furane (BPMF), a potentially high-performance fuel additive. Similar results were achieved by using a combination of M(OPr i ) 4 and ligand R-POSS(OH) 3 as a catalyst system demonstrating the potential of this “ in situ ” approach for applications in biomass transformations. A tentative reaction mechanism for the reductive etherification of aldehydes catalysed by I – IV is proposed.
A new approach to intermolecular frustrated Lewis pairs (FLPs) that combines readily available bulky organosuperbases with moderate to weak boron-containing Lewis acids is reported. These so-called ...“inverse” FLPs are demonstrated to heterolytically cleave dihydrogen, allowing for the isolation and structural characterization of various phosphonium borohydride salts; two FLPs proved to be efficient catalyst’s in the metal-free hydrogenation of N-benzylidenaniline.
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Trimethoprim (TMP) is an active pharmaceutical ingredient with poor aqueous solubility. Here, we describe the cocrystallization of TMP with five co-formers, three bipyridines and two monopyridines. ...Cocrystallization of TMP with the bipyridines yields neutral cocrystals, and while both components are present in the lattice, they do not interact with each other via any strong intermolecular forces. Conversely, cocrystallization of TMP with the monopyridines yields salts, and included water or solvent molecules that play a key role in supramolecular assembly. Moreover, the salts exhibit significantly higher aqueous solubility than that of TMP. Two solids feature a compound on the Generally Recognized as Safe list, making them promising as ionic pharmaceutical cocrystals.
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The BINOL supported chiral boronate ester C
10
H
12
O
2
BC
6
F
5
(THF) (
R
)-
1
, C
10
H
12
O
2
BC
6
F
5
(O&z.dbd;PEt
3
) (
R
)-
3
and C
10
H
12
O
2
BC
6
F
5
2
(
R
,
R
)-
2
as well as the chiral ...boronium salts C
10
H
12
O
2
B(O&z.dbd;PEt
3
)
2
+
B(O
2
C
10
H
12
)
2
−
, (
R
)-
6
and C
10
H
12
O
2
B(O&z.dbd;SMe
2
)
2
+
B(O
2
C
10
H
12
)
2
−
(
R
)-
7
have been synthesized, characterized by NMR spectroscopy, and the solid state structures of (
R
)-
1
, (
R
,
R
)-
2
and (
R
)-
3
determined. Chiral ester (
R
)-
1
was found to be a potent Lewis acid, similar to B(C
6
F
5
)
3
, and capable of rapidly catalyzing the annulation of (
R
)-, (
S
)- and
rac
-styrene oxide with nitrone PhCH&z.dbd;N(O)Me to
trans
-2-methyl-3,6-diphenyl-1,4,2-dioxazine (
trans
-
11
) with high regio- and diastereoslectivities.
The BINOL supported 1,3,2-dioxaborepines C
10
H
12
O
2
BC
6
F
5
(THF) (
R
)-
1
, C
10
H
12
O
2
BC
6
F
5
(O&z.dbd;PEt
3
) (
R
)-
3
, C
10
H
12
O
2
BC
6
F
5
2
(
R
,
R
)-
2
, C
10
H
12
O
2
B(O&z.dbd;PEt
3
)
2
+
B(O
2
C
10
H
12
)
2
−
(
R
)-
6
, and C
10
H
12
O
2
B(O&z.dbd;SMe
2
)
2
+
B(O
2
C
10
H
12
)
2
−
(
R
)-
7
have been prepared.
Salification of the drug trimethoprim with enantiopure
d
- or
l
-lactic acid afforded salts with up to five times improved solubility. Both salts are polymorphic and we demonstrate fully reversible ...interconversion (cycling) between the drug polymorphs using mechanochemistry and slurry methods. We show drug polymorph interconversion requires both solvent contact and mechanical force, revealing strategies for cycling between solid material forms.
Fully reversible interconversion of pharmaceutical salt polymorphs is facilitated by a combination of mechanical force and solvent contact.
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