Photoinitiated Multicomponent Anti-Markovnikov Alkoxylation over Graphene Oxide

The development of graphene oxide–based heterogeneous materials with an economical and environmentally–friendly manner has the potential to facilitate many important organic transformations but proves to have few relevant reported reactions. Herein, we explore the synergistic role of catalytic systems driven by graphene oxide and visible light that form nucleophilic alkoxyl radical intermediates, which enable an anti-Markovnikov addition exclusively to the terminal alkenes, and then the produced benzyl radicals are subsequently added with N–methylquinoxalones. This photoinduced cascade radical difunctionalization of olefins offers a concise and applicable protocol for constructing alkoxyl–substituted N–methylquinoxalones.

Catalytic Polymerization of Phthalonitrile Resins by Carborane with Enhanced Thermal Oxidation Resistance: Experimental and Molecular Simulation

Biphenyl phthalonitrile (BPh) resins with good thermal and thermo-oxidative stability demonstrate great application potential in aerospace and national defense industries. However, BPh monomer has a high melting point, poor solubility, slow curing speed and high curing temperature. It is difficult to control the polymerization process to obtain the resins with high performance. Here, a BPh prepolymer (BPh-Q) was prepared by reacting 1,7-bis(hydroxymethyl)-m-carborane (QCB) with BPh monomers. The BPh-Q exhibited much better solubility, faster curing speed and lower curing temperature compared with pure BPh and BPh modified with bisphenol A (BPh-B, a common prepolymer of BPh). Thus, the polymerization process of BPh was greatly accelerated at a low temperature, resulting in a BPh resin with enhanced thermostability and oxidation resistance. The experimental and theoretical models revealed the promotion effect of B-H bond on the curing reaction of phthalonitrile via Markovnikov addition reaction due to the special steric structure of carborane. This study provided an efficient method to obtain low-temperature curing phthalonitrile resins with high thermal and thermo-oxidative resistance, which would be potentially useful for the preparation of high-performance cyanide resin-based composites.

Platinum Complexes of Bicyclopropylidene and Related Ligands

<p>The coordination chemistry of the cyclopropyl-substituted alkenes, bicyclopropylidene (BCP) and methylenecyclopropane (MCP), with platinum was explored. A range of complexes with ŋ²-alkene ligands were synthesised by the displacement of a ligand, typically ethene, from a precursor complex. These complexes are [Pt(L)(P—P)] (L = BCP, MCP; P—P = Ph₂P(CH₂)₃PPh₂, Cy₂P(CH₂)₂PCy₂, ᵗBu₂P(CH₂)₂PᵗBu₂, ᵗBu₂PCH₂(o-C₆H₄)₂PᵗBu₂), [Pt(L)(P—S)] (L = BCP, MCP; P—S = ᵗBu₂PCH₂(o-₆H4)CH₂SᵗBu), [Pt(C₂H4)(L)(PR₃)] (L = BCP, MCP; PR₃ = PPh₃, PCy₃), [Pt(MCP)₂(PR₃)] (PR₃ = PPh₃, PCy₃) and [PtCl₂(L)(L′)] (L = BCP, MCP; L′ = Py, DMSO). These were the first examples of platinum complexes with ŋ²-BCP ligands, and the first bis-MCP Pt complexes. BCP underwent ring-opening reactions with both Pt(0) and Pt(II) complexes to form the 1,3-diene allylidenecyclopropane (ACP). The first transition metal complexes of ACP [Pt(ACP)(P—P)] (P—P = Ph₂P(CH₂)₃PPh₂, Cy₂P(CH₂)₂PCy₂, ᵗBu₂P(CH₂)₂PᵗBu₂) were synthesised. Some of these complexes rearranged to form ŋ²:σ²-metallacyclopentene complexes, the first instances of the formation of ŋ²:σ²-metallacyclopentene complexes from ŋ²:π-diene complexes. With MCP, the ring-opening reaction only occurred with [₂(COD)], as a result of the anti-Markovnikov addition of Pt–H, generated by the β-hydride elimination of an Et group, across the double-bond. The major products of this reaction were the 1-methylcyclopropyl complexes [Pt(C(CH₂)₂CH₃)Et(COD)] and [Pt(C(CH₂)₂CH₃)₂(COD)], the first examples of such complexes. Protonation of [Pt(L)(P—P)] resulted in a ring-opening reaction to form both the 2-substituted and 1-methyl allyl complexes, [Pt(ŋ³-CH₂CRCH₂)(P—P)]⁺ (R = ᶜPr, Me; P—P = Ph₂P(CH₂)₃PPh₂, ᵗBu₂PCH₂(o-C₆H₄)CH₂PᵗBu₂) and [Pt(ŋ³-CR₂CHCHMe)(P—P)]⁺ (R = cPr, Me; P—P = Ph₂P(CH₂)₃PPh₂, ᵗBuPCH₂(o-C₆H₄)CH₂PᵗBu₂). The analogous 1-methyl complexes were also formed from [Pt(L)(P—S)], wherein the alkene reacted with a hydride formed by the ortho-metallation of the P—S ligand. Computational models were used to investigate the formation of the allyl structures and it was found that the activation energy had a more significant effect than complex stability on product distributions. Complexes with β-chloroalkyl ligands [Pt(C(CH₂)₂CR₂Cl)Cl(L)₂] (R = CH₂, H, L = SEt₂, NCᵗBu, Py) were formed by the addition of Pt–Cl across the alkene double bond. Phosphine complexes were formed by the displacement of a ligand from cis–[Pt(C(CH₂)₂CR₂Cl)Cl(Py)₂] (R = CH₂, H). These are the first examples of stable Pt(II) β-haloalkyl complexes. It was found using computational models that the presence of cyclopropyl rings had a stabilising effect on these complexes.</p>

Platinum Complexes of Bicyclopropylidene and Related Ligands

<p>The coordination chemistry of the cyclopropyl-substituted alkenes, bicyclopropylidene (BCP) and methylenecyclopropane (MCP), with platinum was explored. A range of complexes with ŋ²-alkene ligands were synthesised by the displacement of a ligand, typically ethene, from a precursor complex. These complexes are [Pt(L)(P—P)] (L = BCP, MCP; P—P = Ph₂P(CH₂)₃PPh₂, Cy₂P(CH₂)₂PCy₂, ᵗBu₂P(CH₂)₂PᵗBu₂, ᵗBu₂PCH₂(o-C₆H₄)₂PᵗBu₂), [Pt(L)(P—S)] (L = BCP, MCP; P—S = ᵗBu₂PCH₂(o-₆H4)CH₂SᵗBu), [Pt(C₂H4)(L)(PR₃)] (L = BCP, MCP; PR₃ = PPh₃, PCy₃), [Pt(MCP)₂(PR₃)] (PR₃ = PPh₃, PCy₃) and [PtCl₂(L)(L′)] (L = BCP, MCP; L′ = Py, DMSO). These were the first examples of platinum complexes with ŋ²-BCP ligands, and the first bis-MCP Pt complexes. BCP underwent ring-opening reactions with both Pt(0) and Pt(II) complexes to form the 1,3-diene allylidenecyclopropane (ACP). The first transition metal complexes of ACP [Pt(ACP)(P—P)] (P—P = Ph₂P(CH₂)₃PPh₂, Cy₂P(CH₂)₂PCy₂, ᵗBu₂P(CH₂)₂PᵗBu₂) were synthesised. Some of these complexes rearranged to form ŋ²:σ²-metallacyclopentene complexes, the first instances of the formation of ŋ²:σ²-metallacyclopentene complexes from ŋ²:π-diene complexes. With MCP, the ring-opening reaction only occurred with [₂(COD)], as a result of the anti-Markovnikov addition of Pt–H, generated by the β-hydride elimination of an Et group, across the double-bond. The major products of this reaction were the 1-methylcyclopropyl complexes [Pt(C(CH₂)₂CH₃)Et(COD)] and [Pt(C(CH₂)₂CH₃)₂(COD)], the first examples of such complexes. Protonation of [Pt(L)(P—P)] resulted in a ring-opening reaction to form both the 2-substituted and 1-methyl allyl complexes, [Pt(ŋ³-CH₂CRCH₂)(P—P)]⁺ (R = ᶜPr, Me; P—P = Ph₂P(CH₂)₃PPh₂, ᵗBu₂PCH₂(o-C₆H₄)CH₂PᵗBu₂) and [Pt(ŋ³-CR₂CHCHMe)(P—P)]⁺ (R = cPr, Me; P—P = Ph₂P(CH₂)₃PPh₂, ᵗBuPCH₂(o-C₆H₄)CH₂PᵗBu₂). The analogous 1-methyl complexes were also formed from [Pt(L)(P—S)], wherein the alkene reacted with a hydride formed by the ortho-metallation of the P—S ligand. Computational models were used to investigate the formation of the allyl structures and it was found that the activation energy had a more significant effect than complex stability on product distributions. Complexes with β-chloroalkyl ligands [Pt(C(CH₂)₂CR₂Cl)Cl(L)₂] (R = CH₂, H, L = SEt₂, NCᵗBu, Py) were formed by the addition of Pt–Cl across the alkene double bond. Phosphine complexes were formed by the displacement of a ligand from cis–[Pt(C(CH₂)₂CR₂Cl)Cl(Py)₂] (R = CH₂, H). These are the first examples of stable Pt(II) β-haloalkyl complexes. It was found using computational models that the presence of cyclopropyl rings had a stabilising effect on these complexes.</p>

Markovnikov-addition of H-phosphonates to terminal alkynes under metal- and solvent-free conditions

Synthesis of α-vinylphosphonates from terminal alkynes and H-phosphonates as only the Markovnikov-regioisomer under metal- and solvent-free conditions.

Rh(iii)-Catalyzed C–H allylation/annulative Markovnikov addition with 5-methylene-1,3-dioxan-2-one: formation of isoquinolinones containing a C3 quaternary centre

Rh(iii)-Catalyzed C–H allylation/annulative Markovnikov addition reaction was disclosed, offering isoquinolinones containing a C3 quaternary centre. By using this method as the key step, the US28 inverse agonist analogs were synthesized.

Mechanism Underlying Anti-Markovnikov Addition in the Reaction of Pentalenene Synthase

Most terpene synthase reactions follow Markovnikov rules for formation of high energy carbenium ion intermediates. However, there are notable exceptions. For example, pentalenene synthase (PS) undergoes an initial anti-Markovnikov cyclization reaction followed by a 1,2-hydride shift to form an intermediate humulyl cation with positive charge on the secondary carbon C9 of the farnesyl diphosphate substrate. The mechanism by which these enzymes stabilize and guide regioselectivity of secondary carbocations has not heretofore been elucidated. In an effort to better understand these reactions, we grew crystals of apo-PS, soaked them with the non-reactive substrate analog 12,13-difluorofarnesyl diphosphate, and solved the x-ray structure of the resulting complex at 2.2 Å resolution. The most striking feature of the active site structure is that C9 is positioned 3.5 Å above the center of the side chain benzene ring of residue F76, perfectly poised for stabilization of the charge through a cation-π interaction. In addition, the main chain carbonyl of I177 and neighboring intramolecular C6,C7-double bond are positioned to stabilize the carbocation by interaction with the face opposite that of F76. Mutagenesis experiments also support a role for residue 76 in cation-π interactions. Most interesting is the F76W mutant which gives a mixture of products that likely result from stabilizing a positive charge on the adjacent secondary carbon C10 in addition to C9 as in the wild-type enzyme. The crystal structure of the F76W mutant clearly shows carbons C9 and C10 centered above the fused benzene and pyrrole rings of the indole side chain, respectively, such that a carbocation at either position could be stabilized in this complex, and two anti-Markovnikov products, pentalenene and humulene, are formed. Finally, we show that there is a rough correlation (although not absolute) of an aromatic side chain (F or Y) at position 76 in related terpene synthases from Streptomyces that catalyze similar anti-Markovnikov addition reactions.