A four-coordinate organoboron compound B(ppy)Mes(2) (1, ppy=2-phenylpyridyl, Mes=mesityl) was previously found to undergo reversible photochromic switching through the formation/breaking of a C-C bond, accompanied by a dramatic color change from colorless to dark blue. To understand this unusual phenomenon, a series of new four-coordinate boron compounds based on the ppy-chelate ligand and its derivatives have been synthesized. In addition, new N,C-chelate ligands based on benzo[b]thiophenylpyridine and indolylpyridine have also been synthesized and their boron compounds were investigated. The crystal structures of most of the new compounds were determined by X-ray diffraction analysis. UV/Vis, NMR, and electrochemical methods were used to monitor the photoisomerization process. DFT calculations were performed for all compounds to understand the photophysical and electronic properties of this class of molecules. The results of our study showed that the bulky mesityl group is necessary for photochromic switching. Electron-donating and electron-withdrawing groups on the ppy chelate have a distinct impact on the photoisomerization rate and the photochemical stability of the molecule. Furthermore, we have found that the non-ppy-based N,C-chelate ligands such as benzo[b]thiophenepyridyl can also promote photoisomerization of the boron chromophore in the same manner as the ppy chelate, but the product is thermally unstable.

The dehydroformylation of aldehydes to generate olefins occurs during the biosynthesis of various sterols, including cholesterol in humans. Here, we implement a synthetic version that features the transfer of a formyl group and hydride from an aldehyde substrate to a strained olefin acceptor. A Rh(Xantphos)(benzoate) catalyst activates aldehyde C–H bonds with high chemoselectivity to trigger C–C bond cleavage and generate olefins at low loadings (0.3 to 2 mol%) and temperatures (22 to 80 °C). This mild protocol can be applied to various natural products and was used to achieve a three step synthesis of (+)-yohimbenone. A study of the mechanism reveals that the benzoate counterion acts as a proton-shuttle to enable transfer hydroformylation.

Rh-catalysed hydroacylation allows the construction of chiral ketones from olefins and aldehydes. Since James' and Young's serendipitous discovery of the enantioselective 4-pentenal cyclisation, both intra and intermolecular variants have emerged that enable broader applications.

Many materials consisting of a p-conjugated organic backbone attached to either a tetrahedral or trigonal-planar boron center display enhanced fluorescence and charge-transport properties. This phenomenon can be attributed to either chelation-enhanced p-conjugation of the organic backbone by a tetrahedral boron center [1][2] or to the strong electronaccepting ability of a trigonal-planar boron center.[3] Hence, boron-containing molecules have found applications in OLEDs, organic transistors, and nonlinear optical materials. [1][2][3] We recently reported an unusual photochromic switch involving tetrahedral N,C-chelating boron chromophores such as B(ppy)Mes 2 (ppy = 2-phenylpyridine, Mes = mesityl).[4] These compounds undergo a thermally reversible photoisomerization from a colorless or light colored state to a dark colored state upon irradiation with 350-450 nm light (Scheme 1). These compounds are highly fluorescent in the light-colored state with tunable emission colors, whereas the dark-colored isomers are nonemissive.We have recently shown that this photoisomerization can be suppressed when the system is conjugated to an olefinic bond, which can dissipate the excitation energy through trans-cis isomerization.[4b] While this conjugation has the benefit of stabilizing the system towards photodegradation and preserving its luminescence properties, it also renders the system photochromically inert. To elucidate the impact of incorporating multiple photochromic boron centers into these materials, we prepared a series of new p-conjugated molecules as shown in Scheme 2. Our study has shown that isomerization of one chromophore prevents isomerization of the others, leading to amplified and reversible fluorescence quenching of the entire molecule. The details of our study are presented herein.The synthesis of the monoboryl B1 was reported previously.[4b] Diboryl B2 and B2', triboryl B3, and hexaboryl B6 were synthesized from a common intermediate 1 (see the Supporting Information). B2' was prepared by Hay coupling, [5] and B2, B3, and B6 were prepared by Sonogashira coupling [6] with the appropriate aryl halides. The crystal structures of B2, B2', and B6 (Figure 1) [7] reveal several important features of these materials. B2' and B2 have good coplanarity between the two boron chromophores and dihedral angles of 08 between the two ppy rings for B2 ' and 23.98 between the ppy and the central phenyl ring for B2, which is consistent with strong p conjugation and electronic communication between the boron centers in these molecules. In contrast, the electronic communication between the boron centers in B6 appears to be much weaker as a result of the high degree of steric congestion, as evidenced by the large dihedral angles of 22.38, 59.58, and 82.88 between the ppy chelates and adjacent phenyl rings.These trends in electronic communication are manifested in the reduction potentials and absorption spectra of these materials. The polyboryl compounds except B3 all show two well-resolved reduction peaks, and the first redu...

An enantioselective ketone hydroacylation enables the direct preparation of lactones from keto alcohols. The alcohol is oxidized in situ to an aldehyde, obviating the need to prepare sensitive keto aldehyde substrates. Noyori's asymmetric transfer hydrogenation catalyst was applied to address challenges of reactivity, chemoselectivity, and enantioselectivity.

The tricyclic diterpene fungal metabolite (+)-pleuromutilin has served as a starting point for antibiotic development. Semisynthetic modification of its glycolic acid subunit at C14 provided the first analogs fit for human use, and derivatization at C12 led to 12--pleuromutilins with extended-spectrum antibacterial activity, including activity against Gram-negative pathogens. Given the inherent limitations of semisynthesis, however, accessing derivatives of (+)-pleuromutilin with full control over their structure presents an opportunity to develop derivatives with improved antibacterial activities. Here we disclose a modular synthesis of pleuromutilins by the convergent union of an enimide with a bifunctional iodoether. We illustrate our approach through synthesis of (+)-12--mutilin, (+)-11,12-di--mutilin, (+)-12--pleuromutilin, (+)-11,12-di--pleuromutilin, and (+)-pleuromutilin itself in 17 to 20 steps.

We report a protocol for branched-selective hydroacylation of vinylphenols with aryl, alkenyl and alkyl aldehydes. This cross-coupling yields α-aryl ketones that can be cyclized to benzofurans, and it enables access to eupomatenoid natural products in four steps or less from eugenol. Excellent reactivity and high levels of branched regioselectivity are obtained. We propose that aldehyde decarbonylation is overcome by using an anionic directing group on the olefin and a small bite-angle diphosphine ligand.

Rhodium catalysts with small-bite-angle diphosphines enable branched-selective hydroacylation of 2-vinylphenols via C–H activation of non-chelating aldehydes.