Daniel Holmes

Michigan State University, Ист-Лансинг, Michigan, United States

Are you Daniel Holmes?

Claim your profile

Publications (15)121.92 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Catalytic C–H borylation using the five-coordinate tris-boryl complex (dippe)Ir(Bpin)3 (5a, dippe = 1,2-bis(diisopropylphosphino)ethane) has been examined using 31P{1H} and 1H NMR spectroscopy. Compound 5a was shown to react rapidly and reversibly with HBpin to generate a six-coordinate borylene complex, (dippe)Ir(H)(Bpin)2(BOCMe2CMe2OBpin) (6), whose structure was confirmed by X-ray crystallography. Under catalytic conditions, the H2 generated from C–H borylation converted compound 6 to a series of intermediates. The first is tentatively assigned from 31P{1H} and 1H NMR spectra as (dippe)Ir(H2B3pin3) (7), which is the product of formal H2 addition to compound 5a. As catalysis progressed, compound 7 was converted to a new species with the formula (dippe)Ir(H3B2pin2) (8), which arose from H2 addition to compound 7 with loss of HBpin. Compound 8 was characterized by 31P{1H} and 1H NMR spectroscopy, and its structure was confirmed by X-ray crystallography, where two molecules with different ligand orientations were found in the unit cell. DFT calculations support the formulation of compound 8 as an IrIII agostic borane complex, (dippe)IrH2(Bpin)(η2-HBpin). Compound 8 was gradually converted to (dippe)Ir(H4Bpin) (9), which was characterized by 31P{1H} and 1H NMR spectroscopy and X-ray crystallography. DFT calculations favor its formulation as an agostic borane complex of IrIII with the formula (dippe)IrH3(η2-HBpin). Compound 9 reacted further with H2 to afford the dimeric structure [(dippe)IrH2(μ2-H)]2 (10), which was characterized by 1H NMR and X-ray crystallography. Compounds 7–10 are in equilibrium when H2 and HBpin are present.
    No preview · Article · Aug 2015 · Organometallics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We introduce a previously unexplored parameter -Halenium Affinity (HalA)- as a quantitative descriptor of the bond strengths of various functional groups to halenium ions. The HalA scale ranks potential halenium ion acceptors based on their ability to stabilize a 'free halenium ion'. Alkenes in particular but other Lewis bases as well, such as amines, amides, carbonyls and ether oxygen atoms, etc. have been classified on the HalA scale. This indirect approach enables a rapid and straightforward prediction of chemoselectivity for systems involved in halofunctionalization reactions that have multiple nucleophilic sites. The influences of subtle electronic and steric variations, as well as the less predictable anchimeric and stereoelectronic effects, are intrinsically accounted for by HalA computations, providing quantitative assessments beyond simple 'chemical intuition'. This combined theoretical-experimental approach offers an expeditious means of predicting and identifying unprecedented reactions.
    Preview · Article · Aug 2014 · Journal of the American Chemical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: For cellulosic biofuels processes, suitable characterization of the lignin remaining within the cell wall and correlation of quantified properties of lignin to cell wall polysaccharide enzymatic deconstruction is underrepresented in the literature. This is particularly true for grasses which represent a number of promising bioenergy feedstocks where quantification of grass lignins is particularly problematic due to the high fraction of p-hydroxycinnamates. The main focus of this work is to use grasses with a diverse range of lignin properties, and applying multiple lignin characterization platforms, attempt to correlate the differences in these lignin properties to the susceptibility to alkaline hydrogen peroxide (AHP) pretreatment and subsequent enzymatic deconstruction. We were able to determine that the enzymatic hydrolysis of cellulose to to glucose (i.e. digestibility) of four grasses with relatively diverse lignin phenotypes could be correlated to total lignin content and the content of p-hydroxycinnamates, while S/G ratios did not appear to contribute to the enzymatic digestibility or delignification. The lignins of the brown midrib corn stovers tested were significantly more condensed than a typical commercial corn stover and a significant finding was that pretreatment with alkaline hydrogen peroxide increases the fraction of lignins involved in condensed linkages from 88-95% to ~99% for all the corn stovers tested, which is much more than has been reported in the literature for other pretreatments. This indicates significant scission of β-O-4 bonds by pretreatment and/or induction of lignin condensation reactions. The S/G ratios in grasses determined by analytical pyrolysis are significantly lower than values obtained using either thioacidolysis or 2DHSQC NMR due to presumed interference by ferulates. It was found that grass cell wall polysaccharide hydrolysis by cellulolytic enzymes for grasses exhibiting a diversity of lignin structures and compositions could be linked to quantifiable changes in the composition of the cell wall and properties of the lignin including apparent content of the p-hydroxycinnamates while the limitations of S/G estimation in grasses is highlighted.
    Full-text · Article · Jun 2012 · Biotechnology for Biofuels
  • Source

    Full-text · Article · Jan 2012 · Biotechnology for Biofuels
  • [Show abstract] [Hide abstract]
    ABSTRACT: Good to excellent stereoselectivity has been found in the addition reactions of Grignard and organozinc reagents to N-protected aziridine-2-carboxaldehydes. Specifically, high syn selectivity was obtained with benzyl-protected cis, tert-butyloxycarbonyl-protected trans, and tosyl-protected 2,3-disubstituted aziridine-2-carboxaldehydes. Furthermore, rate and selectivity effects of ring substituents, temperature, solvent, and Lewis acid and base modifiers were studied. The diastereomeric preference of addition is dominated by the substrate aziridines' substitution pattern and especially the electronic character and conformational preferences of the nitrogen protecting groups. To help rationalize the observed stereochemical outcomes, conformational and electronic structural analyses of a series of model systems representing the various substitution patterns have been explored by density functional calculations at the B3LYP/6-31G* level of theory with the SM8 solvation model to account for solvent effects.
    No preview · Article · Oct 2011 · Chemistry - A European Journal
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Glandular secreting trichomes of cultivated tomato (Solanum lycopersicum) and close relatives produce a variety of structurally diverse volatile and non-volatile specialized ('secondary') metabolites, including terpenes, flavonoids and acyl sugars. A genetic screen is described here to profile leaf trichome and surface metabolite extracts of nearly isogenic chromosomal substitution lines covering the tomato genome. These lines contain specific regions of the Solanum pennellii LA0716 genome in an otherwise 'wild-type' M82 tomato genetic background. Regions that have an impact on the total amount of extractable mono- and sesquiterpenes (IL2-2) or only sesquiterpenes (IL10-3) or specifically influence accumulation of the monoterpene alpha-thujene (IL1-3 and IL1-4) were identified using GC-MS. A rapid LC-TOF-MS method was developed and used to identify changes in non-volatile metabolites through non-targeted analysis. Metabolite profiles generated using this approach led to the discovery of introgression lines producing different acyl chain substitutions on acyl sugar metabolites (IL1-3/1-4 and IL8-1/8-1-1), as well as two regions that influence the quantity of acyl sugars (IL5-3 and IL11-3). Chromosomal region 1-1/1-1-3 was found to influence the types of glycoalkaloids that are detected in leaf surface extracts. These results show that direct chemical screening is a powerful way to characterize genetic diversity in trichome specialized metabolism.
    Preview · Article · May 2010 · The Plant Journal
  • Source
    Gang Hu · Daniel Holmes · Brina Fay Gendhar · William D Wulff
    [Show abstract] [Hide abstract]
    ABSTRACT: The copper-mediated deracemization of the C2-symmetric vaulted biaryl ligands VANOL and VAPOL has been investigated. In the course of the studies that have led to a more reliable procedure for this process, an unprecedented oxidative dimerization of these ligands has been uncovered. The structures of these oxidative dimerization products were elucidated by a series of NMR experiments, and these assignments were supported by other spectroscopic techniques as well as their chemical reactivity. This oxidative dimerization process was not observed for the linear biaryl ligands BANOL and BINOL, although the new deracemization procedure was effective for the generation of BINOL with high optical purity. The (aS)-enantiomers of BINOL, VANOL and VAPOL were accessible with a copper complex of (-)-sparteine, and the (aR)-enantiomeric series were accessible with a copper complex of O'Brien's diamine. Both (-)-sparteine and O'Brien's diamine give higher optical purities with VANOL and VAPOL than with BINOL, and this is consistent with the steric congestion present in the matched and mismatched copper complexes of these diamines with the biaryl ligands.
    Preview · Article · Sep 2009 · Journal of the American Chemical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Investigation of the methanol extract of Aswagandha (Withania somnifera) roots for bioactive constituents yielded a novel withanolide sulfoxide compound (1) along with a known withanolide dimer ashwagandhanolide (2) with an S-linkage. The structure of compound 1 was established by extensive NMR and MS experiments. Compound 1 was highly selective in inhibiting cyclooxygenase-2 (COX-2) enzyme by 60% at 100 microm with no activity against COX-1 enzyme. The IC(50) values of compound 1 against human gastric (AGS), breast (MCF-7), central nervous system (SF-268) and colon (HCT-116) cancer cell lines were in the range 0.74-3.63 microm. Both S-containing dimeric withanolides, 1 and 2, completely suppressed TNF-induced NF-kappaB activation when tested at 100 microm. The isolation of a withanolide sulfoxide from W. somnifera roots and its ability to inhibit COX-2 enzyme and to suppress human tumor cell proliferation are reported here for the first time. In addition, this is the first report on the abrogation of TNF-induced NF-kappaB activation for compounds 1 and 2.
    Full-text · Article · Jul 2009 · Phytotherapy Research
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Glycyrrhiza uralensis (Gan-Cao), commonly called "licorice", is one of the most commonly used herbs in traditional Chinese medicine (TCM). In the United States, licorice products are most often consumed as flavoring and sweetening agents in food products. The licorice triterpenoid glycyrrhizin has several biological activities, including anti-inflammatory activity. Other potential anti-inflammatory constituents in G. uralensis have not been fully investigated. Airway eosinophilic inflammation is a major feature of allergic asthma. Eotaxin-1 (eotaxin) is involved in the recruitment of eosinophils to sites of antigen-induced inflammation in asthmatic airways. Because human lung fibroblasts are the major source of eotaxin, inhibition of eosinophil recruitment by suppression of fibroblast eotaxin production is a potentially valuable approach for the pharmacological intervention in asthma. A systematic bioassay-guided purification of G. uralensis yielded five flavonoids: liquiritin, liquiritigenin, isoliquiritigenin, 7,4'-dihydroxyflavone, and isoononin. The structures of the compounds were established by (1)H and (13)C nuclear magnetic resonance (NMR) and liquid chromatography-mass spectrometry (LC-MS) studies. The potential ability of these isolated pure compounds and glycyrrhizin to inhibit secretion of eotaxin-1 by human fetal lung fibroblasts (HFL-1) was tested. Liquiritigenin, isoliquiritigenin, and 7,4'-dihydroxyflavone were more effective than liquiritin, isoononin, and glycyrrhizin in suppressing eotaxin secretion. A concentration-response study showed the IC(50) concentrations of liquiritigenin, isoliquiritigenin, and 7,4'-dihydroxyflavone were 4.2, 0.92, and 0.21 microg/mL, respectively, demonstrating that Glycyrrhiza flavonoids inhibit eotaxin-1 secretion in vitro.
    Full-text · Article · Feb 2009 · Journal of Agricultural and Food Chemistry
  • [Show abstract] [Hide abstract]
    ABSTRACT: One way to build complex molecules, such as pharmaceuticals and pesticides, is with a Suzuki coupling reaction. This versatile coupling reaction requires precursors with a carbonboron bond. Making these precursors, however, typically requires harsh conditions and generates significant amounts of hazardous waste. Professors Maleczka and Smith developed a new catalytic method to make these compounds under mild conditions and with minimal waste and hazard. Their discovery allows the rapid, green manufacture of chemical building blocks, including some that had been commercially unavailable or environmentally unattractive.
    No preview · Conference Paper · Jun 2008
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ir-catalyzed borylation of 2-substituted indoles selectively yields 7-borylated products in good yields. N-Protection, required for previous functionalizations of 2-substituted indoles, is unnecessary.
    No preview · Article · Apr 2007 · Journal of the American Chemical Society
  • S. Paul · G. Chotana · D. Holmes · R. Reichle · R. Jr · M. III

    No preview · Article · Apr 2007 · Synfacts
  • [Show abstract] [Hide abstract]
    ABSTRACT: [reaction: see text] A one-pot protocol for converting 1,3- and 1,4-substituted aryl halides to arylamine boronate esters is described. This is achieved by sequential Ir-catalyzed aromatic borylation at the least hindered C-H bond of the aryl halide and subsequent Pd-catalyzed C-N coupling at the halide position of the crude arylboronic ester.
    No preview · Article · Apr 2006 · Organic Letters
  • [Show abstract] [Hide abstract]
    ABSTRACT: An efficient one-pot C-H activation/borylation/oxidation protocol for the preparation of phenols is described. This method is particularly attractive for the generation of meta-substituted phenols bearing ortho-/para-directing groups, as such substrates are difficult to access by other phenol syntheses.
    No preview · Article · Aug 2003 · Journal of the American Chemical Society
  • [Show abstract] [Hide abstract]
    ABSTRACT: Arylboron compounds have intriguing properties and are important building blocks for chemical synthesis. A family of Ir catalysts now enables the direct synthesis of arylboron compounds from aromatic hydrocarbons and boranes under "solventless" conditions. The Ir catalysts are highly selective for C-H activation and do not interfere with subsequent in situ transformations, including Pd-mediated cross-couplings with aryl halides. By virtue of their favorable activities and exceptional selectivities, these Ir catalysts impart the synthetic versatility of arylboron reagents to C-H bonds in aromatic and heteroaromatic hydrocarbons.
    No preview · Article · Feb 2002 · Science