anti-Markovnikov Products Research Articles

ChemInform Abstract: Chemoselective Alkene Hydrosilylation Catalyzed by Nickel Pincer Complexes.

AbstractTerminal alkenes bearing amino, ester, amido, ketone, and formyl groups are selectively hydrosilylated using a bis(amino)amide nickel pincer complex affording the corresponding anti—Markovnikov products.

Iodoazidation of Alkenes by Using Iodine Pentafluoride–Pyridine–Hydrogen Fluoride and Trimethylsilyl Azide

Iodoazidation of alkenes was carried out by using iodine pentafluoride–pyridine–hydrogen fluoride and trimethylsilyl azide. In the reactions of terminal alkenes, anti-Markovnikov products were formed selectively. Cyclohexene gave a mixture of <i>cis</i>- and <i>trans</i>-adducts. These results suggest the involvement of radical species in the reaction.

Amido Ln(II) Complexes Coordinated by Bi- and Tridentate Amidinate Ligands: Nonconventional Coordination Modes of Amidinate Ligands and Catalytic Activity in Intermolecular Hydrophosphination of Styrenes and Tolane.

Heteroleptic Ln(II) and Ca(II) amides [tBuC(NC6H3-iPr2-2,6)2]MN(SiMe3)2(THF) (M = Yb (1Yb), Ca (1Ca)), [2-MeOC6H4NC(tBu)N(C6H3-iPr2-2,6)]LnN(SiMe3)2(THF) (Ln = Sm (2Sm), Yb (2Yb)), and [2-Ph2P(O)C6H4NC(tBu)N(C6H3-Me2-2,6)]YbN(SiMe3)2(THF) (3Yb) coordinated by bi- and tridentate amidinate ligands were obtained by the amine elimination reactions of M[N(SiMe3)2](THF)2 (M = Yb, Sm, Ca) with parent amidines in good yields. Complex [tBuC(NC6H3-iPr2-2,6)2]SmN(SiMe3)2 can be obtained only by a salt metathesis reaction of [tBuC(NC6H3-2,6-iPr2)2]SmI(THF)2 with NaN(SiMe3)2. Unlike 1Yb and 1Ca in 1Sm the amidinate ligand is coordinated to metal ion in κ(1)-amido:η(6)-arene fashion preventing THF coordination. The derivatives of tridentate amidinate ligands bearing pendant donor 2-MeOC6H4 or 2-Ph2P(O)C6H4N groups feature nonconventional κ(1)-N,κ(2)-O,η(6)-arene coordination mode. Complexes 1Ca, 1Sm, 1Yb, 2Sm, 2Yb, and 3Yb proved to be efficient catalysts for styrene hydrophosphination with PhPH2 and Ph2PH. In styrene hydrophosphination with PhPH2 all the catalysts perform excellent chemoselectivity and afford a monoaddition product-secondary phosphine (PhCH2CH2)PhPH. Moreover, all the catalysts perform hydrophosphination reactions regioselectively with exclusive formation of the anti-Markovnikov addition product. Within the series of complexes coordinated by the same amidinate ligand catalytic activity decreases in the following order 1Ca ≥ 1Sm>1Yb. The turnover frequencies were in the range of TOF ≈ 0.3-0.7 h(-1). However, application of tridentate amidinate ligand allowed one to increase catalytic activity significantly: for 2Sm TOF was found to be 8.3 h(-1). For the addition of PhPH2 to para-substituted styrenes catalyzed by 2Sm it was found that electron-withdrawing substituents (Cl, F) do not affect the reaction rate while electron-donating groups (tBu, OMe) noticeably slow down the reaction.

Deviation from the anti-Markovnikov rule: a computational study of the regio- and stereoselectivity of diene hydroboration reactions

Hydroboration and subsequent use of boron compounds in novel organic synthesis have been flourishing in recent years largely due to its amiability in producing asymmetric stereo- and regioselective products. Direct products of diene hydroboration, however, have received little attention, with most substrates being assumed to produce the anti-Markovnikov product expected from textbook organic chemistry understanding. Previous experimental studies have observed the presence of a plethora of hydroboration products, and a significant progress has been made in assigning species to experimental data—though often with contradicting results. This study has used a computational approach employing quantum chemical calculations to determine atomic charges of cyclic and acyclic dienes and correlate these with calculated activation energy barriers in order to predict the regio- and stereoselective outcome of hydroboration reactions. Results indicated a strong correlation between the most polarized atomic charges of double-bonded carbons and the lowest energy transition states as expected. Intriguingly, we identified 1,3-cyclohexadiene as the main example that does not follow the anti-Markovnikov rule. We proceed to show that in addition to the polarity of the double bonds within a molecule, in this case, the conjugation with the allyl double bond and the specific geometric features of the cyclohexane ring were key stabilizing factors for the unexpected transition state preference, resulting in a regioselectivity that is in quantitative agreement with previous experimental data. Our results further indicated that Re-face attacks and steric factors due to substituents of the substrate influenced mainly the stereoselective outcome of the reaction, also affecting the pathways available to proceed through to complete the hydroboration process.

ChemInform Abstract: N,O‐Chelating Four‐Membered Metallacyclic Titanium(IV) Complexes for Atom‐Economic Catalytic Reactions

AbstractReview: 66 refs.

N,O-Chelating Four-Membered Metallacyclic Titanium(IV) Complexes for Atom-Economic Catalytic Reactions.

Titanium, as the second most abundant transition metal in the earth's crust, lends itself as a sustainable and inexpensive resource in catalysis. Its nontoxicity and biocompatibility are also attractive features for handling and disposal. Titanium has excelled as a catalyst for a broad range of transformations, including ethylene and α-olefin polymerizations. However, many reactions relevant to fine chemical synthesis have preferrentially employed late transition metals, and reactive, inexpensive early transition metals have been largely overlooked. In addition to promising reactivity, titanium complexes feature more robust character compared with some other highly Lewis-acidic metals such as those found in the lanthanide series. Since the advent of modulating ligand scaffolds, titanium has found use in a growing variety of reactions as a versatile homogeneous catalyst. These catalytic transformations include hydrofunctionalization reactions (adding an element-hydrogen (E-H) bond across a C-C multiple bond), as well as the ring-opening polymerization of cyclic esters, all of which are atom-economic transformations. Our investigations have focused on tight bite angle monoanionic N,O-chelating ligands, forming four-membered metallacycles. These ligand sets, including amidates, ureates, pyridonates, and sulfonamidates, have flexible binding modes offering a range of stable and reactive intermediates necessary for catalytic activity. Additionally, the simple form of these ligands leads to easily prepared proligands, along with facile tuning of steric and electronic factors. A sterically bulky titanium amidate complex has proven to be a leading catalyst for the selective formation of anti-Markovnikov addition products via intermolecular hydroamination of terminal alkynes, while sterically less demanding titanium pyridonates have opened the path to the selective formation of amine substituted cycloalkanes via the intramolecular hydroaminoalkylation of aminoalkenes over the competing hydroamination pathway. Sulfonamidates have boosted reactivity for hydrofunctionalization and polymerization reactions compared with amide ligands not bearing a sulfonyl group. N,O-Chelated titanium complexes have been used to synthesize ultrahigh molecular weight polyethylene and have been utilized in the challenging task of realizing equal incorporation of two different cyclic esters in a random ring-opening copolymerization. These discrete complexes have allowed for careful study of fundamental coordination chemistry and stoichiometric organometallic investigations. With inexpensive starting materials and modular ligands, titanium N,O-chelated complexes are well-suited to address the challenges of achieving greener chemical processes while accessing useful reaction manifolds for sustainable synthesis.

ChemInform Abstract: Room Temperature Hydrophosphination Using a Simple Iron Salen Pre‐Catalyst.

Phosphines are fundamentally important to the fine chemicals, pharmaceutical and agrochemical industries. Reported is the first example of alkene hydrophosphination using a designed iron pre-catalyst which yields the anti-Markovnikov products in high yield at room temperature. The phosphine products are excellent pro-ligands for Fe-catalyzed Negishi cross-coupling.

Role of ligands in controlling the regioselectivity in ruthenium-catalysed addition of carboxylic acids to terminal alkynes: A DFT study

Density functional studies are performed to understand the role of chelating bi-phosphine ligands [(Ph 2P(CH 2) m PPh 2); m = 1–4] in modulating the regio-selectivity of benzoic acid addition to 1-hexyne, in presence of ruthenium(II) catalyst [(Ph 2P(CH 2)mPPh 2)Ru(methallyl) 2]. The Markovnikov addition to 1-hexyne is observed when catalyst 1 a [(Ph 2P(CH 2)PPh 2)Ru(methallyl) 2] is employed, whereas a reverse regio-selectivity is witnessed in presence of 1 d [(Ph 2P(CH 2)4PPh 2)Ru(methallyl) 2]. Anti-Markovnikov addition occurs via the neutral vinylidene intermediates (5 a / d ) formed after 1,2-hydrogen shift in hexyne coordinated ruthenium(II) complexes 3 a / d . The energy profile shows clear preference for Markovnikov addition by 15.0 kcal/mol (\({\Delta } G_{\mathrm {L}}^{S})\) in case of catalyst system 1 a . In contrast, anti-Markovnikov pathway following neutral vinylidenes are more favourable by 9.1 kcal/mol (\({\Delta } G_{\mathrm {L}}^{S})\) for catalyst system 1 d . The Z-enol ester formation is more predominant in the anti-Markovnikov pathway since the activation barrier for this step requires less energy (5.9 kcal/mol, \({\Delta } G_{\mathrm {L}}^{S})\) than the one furnishing the E-product. The calculated results are in good agreement with the reported experimental findings. Preliminary DFT studies are performed to explore the mechanistic pathway of the benzoic acid addition to 1-hexyne in presence of Ru(II) catalyst. Our calculations emphasized on the understanding of the regio-selectivity controlled by the chelating bi-phosphine ligand present in catalysts. Markovnikov product formation is the lower energy pathway in case of least spacer group (CH2) whereas it is anti-Markovnikov product for the highest spacer group, {(CH2)4} in bi-phosphine ligands.

Brønsted acid catalyzed PhSe transfer versus radical aryl transfer: linear codimerization of styrenes and internal olefins.

Brønsted acid p-TsOH·H2O-catalyzed hydrovinylation of styrenes with internal olefins α-oxo ketene dithioacetals was efficiently achieved in the presence of N-phenylselenophthalimide (N-PSP), regioselectively affording Markovnikov phenylselenative hydrovinylation products through PhSe transfer of the phenylseleno ketene dithioacetal intermediates. Radical reduction of the resultant PhSe-alkyl-substituted ketene dithioacetals with AIBN/n-Bu3SnH gave the corresponding anti-Markovnikov hydrovinylation products formally from the linear codimerization of styrenes and the internal olefins.

Palladium-Catalyzed Anti-Markovnikov Oxidation of Allylic Amides to Protected β-Amino Aldehydes.

A general method for the preparation of N-protected β-amino aldehydes from allylic amines or linear allylic alcohols is described. Here the Pd(II)-catalyzed oxidation of N-protected allylic amines with benzoquinone is achieved in tBuOH under ambient conditions with excellent selectivity toward the anti-Markovnikov aldehyde products and full retention of configuration at the allylic carbon. The method shows a wide substrate scope and is tolerant of a range of protecting groups. Furthermore, β-amino aldehydes can be obtained directly from protected allylic alcohols via palladium-catalyzed autotandem reactions, and the application of this method to the synthesis of β-peptide aldehydes is described. From a mechanistic perspective, we demonstrate that tBuOH acts as a nucleophile in the reaction and that the initially formed tert-butyl ether undergoes spontaneous loss of isobutene to yield the aldehyde product. Furthermore, tBuOH can be used stoichiometrically, thereby broadening the solvent scope of the reaction. Primary and secondary alcohols do not undergo elimination, allowing the isolation of acetals, which subsequently can be hydrolyzed to their corresponding aldehyde products.

Cover Picture: Titania Nanoparticles Stabilized HPA in SBA-15 for the Intermolecular Hydroamination of Activated Olefins (ChemCatChem 12/2014)

The front cover picture shows the enhanced activity and selectivity of the titanic nanoparticles stabilized heteropolyacids (HPA) in nanoporous hexagonally ordered catalytic supports for the hydroamination of activated olefins. In their Full Paper A. Vinu and co-workers describe the preparation of SBA-15 nanocomposites with HPA and TiO2 nanoparticles with tunable textural parameters and acidity. These catalysts were found to be very active for the hydroamination of ethyl acrylate with both aliphatic and aromatic amines, offering a high regio- and chemoselectivity to anti-Markovnikov products. The catalytic performance of these nanocomposites could be finely tuned with simple adjustment of HPA in the nanoporous matrix and the calcination temperature. The article highlighted by this cover can be found on p. 3347 ff. of Issue 12, 2014.

Platinum(II) olefin hydroarylation catalysts: tuning selectivity for the anti-Markovnikov product.

Pt(II) complexes containing unsymmetrical (pyridyl)pyrrolide ligands are shown to catalyze the hydroarylation of unactivated alkenes with selectivity for the anti-Markovnikov product. Substitution on the pyrrolide portion of the ligand allows effective tuning of the selectivity to anti-Markovnikov alkylarene products, whereas substitution on the pyridyl portion can promote competitive alkenylarene production.

Research Update: A hafnium-based metal-organic framework as a catalyst for regioselective ring-opening of epoxides with a mild hydride source

Reaction of styrene oxide with sodium cyanoborohydride and a catalytic amount of Hf-NU-1000 yields the anti-Markovnikov product, 2-phenylethanol, with over 98% regioselectivity. On the other hand, propylene oxide is ring opened in a Markovnikov fashion to form 2-propanol with 95% regioselectivity. Both styrene oxide and propylene oxide failed to react with sodium cyanoborohydride without the addition of Hf-NU-1000 indicative of the crucial role of Hf-NU-1000 as a catalyst in this reaction. To the best of our knowledge, this is the first report of the use of a metal-organic framework material as a catalyst for ring-opening of epoxides with hydrides.

Linear-selective hydroarylation of unactivated terminal and internal olefins with trifluoromethyl-substituted arenes.

We report a series of hydroarylations of unactivated olefins with trifluoromethyl-substituted arenes that occur with high selectivity for the linear product without directing groups on the arene. We also show that hydroarylations occur with internal, acyclic olefins to yield linear alkylarene products. Experimental mechanistic data provide evidence for reversible formation of an alkylnickel-aryl intermediate and rate-determining reductive elimination to form the carbon-carbon bond. Labeling studies show that formation of terminal alkylarenes from internal alkenes occurs by initial establishment of an equilibrating mixture of alkene isomers, followed by addition of the arene to the terminal alkene. Computational (DFT) studies imply that the aryl C-H bond transfers to a coordinated alkene without oxidative addition and support the conclusion from experiment that reductive elimination is rate-determining and forms the anti-Markovnikov product. The reactions are inverse order in α-olefin; thus the catalytic reaction occurs, in part, because isomerization creates a low concentration of the reactant α-olefin.

Ligand-controlled reactivity, selectivity, and mechanism of cationic ruthenium-catalyzed hydrosilylations of alkynes, ketones, and nitriles: a theoretical study.

Density functional theory calculations with the M06 functional have been performed on the reactivity, selectivity, and mechanism of hydrosilylations of alkynes, ketones, and nitriles catalyzed by cationic ruthenium complexes [CpRu(L)(MeCN)2](+), with L = P(i)Pr3 or MeCN. The hydrosilylation of alkynes with L = P(i)Pr3 involves an initial silyl migration mechanism to generate the anti-Markovnikov product, in contrast to the Markovnikov product obtained with L = MeCN. The bulky phosphine ligand directs the silyl group to migrate to Cβ of the alkyne. This explains the anti-Markovnikov selectivity of the catalyst with L = P(i)Pr3. By contrast, the silane additions to either ketone or nitrile proceed through an ionic SN2-Si outer-sphere mechanism, in which the substrate attacks the Si center. The P(i)Pr3 ligand facilitates the activation of the Si-H bond to furnish a η(2)-silane complex, whereas a η(1)-silane complex is formed for the MeCN ligand. This property of the phosphine ligand enables the catalytic hydrosilylation of ketones and nitriles in addition to that of alkynes.

Titania Nanoparticles Stabilized HPA in SBA-15 for the Intermolecular Hydroamination of Activated Olefins

A liquid phase hydroamination (HA) of α,β-ethylenic compounds with amines was investigated with TiO2 nanoparticles stabilized 12-tungstophosphoric acid (TPA) in SBA-15. The catalysts were prepared by wet impregnation of TPA/TiO2 nanoparticles into the SBA-15 and calcined at different temperatures. The characterization results reveal that the textural properties and the acidity of the prepared catalysts can be finely controlled with the simple adjustment of the calcination temperature and the structure of the support, decorated with the TiO2 and TPA nanoparticles, was intact even after the modification. The prepared catalysts were investigated for HA of ethyl acrylate with different aromatic and aliphatic amines over a wide range of reaction conditions to optimize the yield and the selectivity of product. It was found that this process is 100 % atom efficient and the catalytic performance depended significantly on the loading of TPA over the catalyst and the calcination temperature. Under optimized reaction conditions, the best catalyst, 15 wt %TPA/22.4 wt %TiO2/SBA-15 calcined at 1123 K, offered the highest conversion of p-ethylaniline (70 %) with 100 % chemo-selectivity to the anti-Markovnikov product, i.e., the mono-addition product. The reaction was heterogeneously catalyzed and no contribution from leached TPA into the reaction was observed.

ChemInform Abstract: A Convenient Approach Towards the 1‐Aminomethyl‐1‐fluorocycloalkane Scaffold.

Abstract A three-step synthesis towards 1-aminomethyl-1-fluorocycloalkanes was developed starting from methylenecycloalkanes. Methylenecyclobutane, methylenecyclopentane and methylenecyclohexane were first bromofluorinated to provide the corresponding Markovnikov products, 1-bromomethyl-1-fluorocycloalkanes, which were then converted towards the title compound via azide substitution and hydrogenation. The bromofluorination of methylenecyclopropane, however, led to both the Markovnikov and the anti-Markovnikov product.

A Cobalt‐Catalyzed Alkene Hydroboration with Pinacolborane

An extremely efficient cobalt catalyst for the hydroboration of both vinylarenes and aliphatic α-olefins with pinacolborane is described, providing the anti-Markovnikov products with excellent regio- and chemoselectivity, broad functional-group tolerance, and high turnover numbers (up to 19,800). The alkene hydroboration route is further extended to a two-step, one-pot hydroboration and cross-coupling of alkylboronates with aryl chlorides.

Divalent Heteroleptic Ytterbium Complexes – Effective Catalysts for Intermolecular Styrene Hydrophosphination and Hydroamination

New heteroleptic Yb(II)-amide species supported by amidinate and 1,3,6,8-tetra-tert-butylcarbazol-9-yl ligands [2-MeOC6H4NC(tBu)N(C6H3-iPr2-2,6)]YbN(SiMe3)2(THF) (6) and [1,3,6,8-tBu4C12H4N]Yb[N(SiMe3)2](THF)n (n = 1 (7), 2 (8)) were synthesized using the amine elimination approach. Complex 6 features an unusual κ(1)-N,κ(2)-O,η(6)-arene coordination mode of the amidinate ligand onto Yb(II). Complexes 7 and 8 represent the first examples of lanthanide complexes with π-coordination of carbazol-9-yl ligands. Complexes 6 and 7, as well as the amidinate-Yb(II)-amide [tBuC(NC6H3-iPr2-2,6)2]YbN(SiMe3)2(THF) (5), are efficient precatalysts for the intermolecular hydrophosphination and hydroamination of styrene with diphenylphosphine, phenylphosphine, and pyrrolidine to give exclusively the anti-Markovnikov monoaddition product. For both types of reaction, the best performances were observed with carbazol-9-yl complex 7 (TONs up to 92 and 48 mol/mol at 60 °C, respectively).

Hedycaryol Synthase in Complex with Nerolidol Reveals Terpene Cyclase Mechanism

The biosynthesis of terpenes is catalysed by class I and II terpene cyclases. Here we present structural data from a class I hedycaryol synthase in complex with nerolidol, serving as a surrogate for the reaction intermediate nerolidyl diphosphate. This prefolded ligand allows mapping of the active site and hence the identification of a key carbonyl oxygen of Val179, a highly conserved helix break (G1/2) and its corresponding helix dipole. Stabilising the carbocation at the substrate's C1 position, these elements act in concert to catalyse the 1,10 ring closure, thereby exclusively generating the anti-Markovnikov product. The delineation of a general mechanistic scaffold was confirmed by site-specific mutations. This work serves as a basis for understanding carbocation chemistry in enzymatic reactions and should contribute to future application of these enzymes in organic synthesis.