Synthesis of Chiral Starburst Dendrimers from PHB-Derived Triols as Central Cores

Chiral triols 1 – 3 (‘tris(hydroxymethyl)methane’ derivatives), prepared from (R)-3-hydroxybutanoic acid and aldehydes, are used as center pieces of dendrimers. The triols may be employed as such or after attachment of spacers containing alkyl or aryl moieties (see 5 and 7 ). The branches combined with the original or elongated triols are those first reported by Fréchet ( 9 – 12 , benzyl ethers of 3,5-dihydroxybenzyl alcohol and bromide). In this way, 1st-, 2nd-, and 3rd-generation chiral dendrimers without ( 13 – 15 ), or with aliphatic ( 16 – 18 ) or aromatic ( 19 – 21 ) spacers are prepared. The molecular weights range from 447 to 2716 Dalton. Two of the chiral triols, i.e., 2 and 3 , are used as center pieces for chiral dendrimers containing 6 NH₂, or 6 and 12 NO₂ groups on the periphery ( 22 – 27 ), with 3,5-dinitrobenzoyl chloride as the branching unit. All compounds thus synthesized are of course monodisperse and are fully characterized. In some cases, the optical activity of the dendrimers indicates that conformationally chiral substructures might be present. The NH₂- and NO₂-substituted compounds avidly clathrate smaller molecules; they are sorbents exchanging host molecules through the gas phase.     

Toward Creation of a Universal NMR Database for Stereochemical Assignment: The Case of 1,3,5‐Trisubstituted Acyclic Systems

Using the diastereoisomeric triols 1a – d (Fig. 1) and examples summarized in Fig. 2, the central C‐atom of acyclic 1,3,5‐triols is demonstrated to exhibit a distinctive chemical shift that is dependent on the 1,3‐ and 3,5‐relative configuration, but is independent of the functionalities present outside of this structural motif. These NMR characteristics are then used to predict the relative configuration of several natural products (Fig. 7). In addition, an example is given to show the possibility of assembling an NMR database for a larger array of functional groups from NMR databases of smaller arrays of functional groups.     

A Simple Procedure for the Preparation of Chiral ‘Tris(hydroxymethyl)methane’ Derivatives

The aldol adducts 1a – 13a of R,R-2(tertbutyl)-6-methyl-1,3-dioxan-4-one (from 3-hydroxybutanoic acid) to aldehydes, single diastereoisomers obtained as described previously, are acetylated or benzoylated to the corresponding esters 1b – 5b and 6c – 13c , respectively, which in turn are reduced with LiAlH₄ to the title compounds 14 – 24 . The enantiomerically pure triols thus available may be useful as chiral building blocks, as auxiliaries for enantioselective reactions, and as center pieces for chiral dendrimers.     

Regioselectivity Control in Lipase‐Catalyzed Polymerization of Divinyl Sebacate and Triols

Lipase‐catalyzed regioselective polymerization of divinyl sebacate and triols has been performed in bulk. NMR analysis of the product obtained by the polymerization of divinyl sebacate and glycerol using Candida antarctica lipase at 60°C showed that 1,3‐diglyceride was a main unit and a small amount of the branching unit (triglyceride) was contained. The polymerization of divinyl sebacate with 1,2,4‐butanetriol or 1,2,6‐hexanetriol at 60°C produced a branched polymer. In polymerization at a lower temperature, the regioselectivity was perfectly controlled to give a linear polymer consisting of the α,ω‐disubstituted unit exclusively. The lipase origin and feed ratio of monomers greatly affected the microstructure of the polymer; under selected conditions, regiospecific polymerization was achieved.     

Preparation of Chiral Building Blocks for Starburst Dendrimer Synthesis

Double aldols, formally derived from acetic acid and two different aldehydes, as obtained by addition of the enolate of (R,R)-2-(tert-butyl)-6-methyl-1,3-dioxan-4-one ( A ) to various aldehydes, are reduced to triols which are actually substituted chiral ‘tris(hydroxymethyl)methanes’ (see B and 3–8 ). Etherifications of the three OH groups of these triols with functionalized halides (allyl, 4-(silyloxy)but-2-en-l-yl, 4-substituted benzyl) and esterifications with pent-4-enoic and 3,5-dinitrobenzyl chlorides, followed by functional group manipulations, lead to the potential center pieces 14–30 for the construction of chiral dendrimers: the building blocks prepared contain the required ‘spacers’ between the core unit, as well as three vinyl groups, three aryl bromide groups, three alcoholic or phenolic OH groups, three mesylate groups, three ester groups, or six arylamino groups at the terminus of their branches. The new compounds are all obtained on a preparative scale and are fully characterized (including elemental analysis).     

The stereoselective syntheses and reactions of 5,6-epoxy-perhydroisoindolines

Stereo-selective syntheses of 2,5,6-trisubstituted perhydroisoindolines are achieved by preparation of the three diastereoisomeric 5,6-epoxy-perhydroisoindolines and their reactions with nucleophiles, e.g. amines. The transformation of trans-amino alcohols obtained in this way into cis-amino alcohols is described.     

C3‐Symmetric Tricyclo[2.2.1.02,6]heptane‐3,5,7‐triol

A straightforward access to a hitherto unknown C ₃‐symmetric tricyclic triol both in racemic and enantiopure forms has been developed. Treatment of 7‐tert ‐butoxynorbornadiene with peroxycarboxylic acids provided mixtures of C ₁‐ and C ₃‐symmetric 3,5,7‐triacyloxynortricyclenes via transannular π‐cyclization and replacement of the tert ‐butoxy group. By refluxing in formic acid, the C ₁‐symmetric esters were converted to the C ₃‐symmetric formate. Hydrolysis gave diastereoisomeric triols, which were separated by recrystallization. Enantiomer resolution via diastereoisomeric tri(O ‐methylmandelates) delivered the target triols on a gram scale. The pure enantiomers are useful as core units of dopants for liquid crystals.     

Hetero type III-B rotaxane dendrimers

Type III-B rotaxane dendrimer is a member of the rotaxane dendrimer family, but it is defined as dendritic polyrotaxane, branched through rings. Such rotaxane dendrimers have been synthesized through versatile copper-catalyzed azide-alkyne cycloaddition reactions. In order to incorporate more functionalities into type III-B rotaxane dendrimers, three different potential functionalities—azobenzene toward light switch, daisy chain toward higher degree of contraction, and extension and cyclobis (paraquat-p-phenylene) (CBPQT) toward redox chemistry—were combined into the original design of type III-B rotaxane dendrimers. Two azo-based G1/G2 and four new G1 and G2 hetero type III-B rotaxane dendrimers were synthesized and characterized by ¹H NMR spectroscopy and electrospray ionization mass spectrometry.     

Convergent synthesis of internally branched PAMAM dendrimers

[reaction: see text] A series of aliphatic internally branched poly(amido amine) dendrons and dendrimers has been synthesized. The internal branching unit was 1,2-propanediamine and a series of PAMAM-type dendrons of the types AB2, AB4, and AB8 were built. These were anchored on a core molecule containing four carboxylic acid moieties and the 1,2-propanediamine unit resulted in PAMAM dendrimers with 4, 8, 16, and 32 end groups.     

Chiral Acylsilanes in Organic Synthesis. Diastereoselective 1,2-Additions to Racemic Alkoxymethyl-Substituted Acylsilanes

The synthesis of the three alkoxymethyl-substituted acyisilanes 1 – 3 is described (Schemes 1 and 2). Their reactions with NaBH₄ as well as PhLi gave the corresponding alcohols with moderate to good diastereoisomeric induction (up to 78% de; see Table), depending upon the solvent used (Scheme 3). The results indicate that in Et₂O, the reactions with PhLi proceed via 6-membered chelates (see C in Scheme 4) leading to the products with high de's (74–78%). In THF, these chelates are not formed, and as a consequence, the additions take place with reversed and lower stereoselectivities (34–50% de).     

Predicting MWD and Branching Distribution of Terminally Branched Polymers Undergoing Random Scission

A new semi‐analytical approach to model simultaneous chain scission and branching is described that assumes the separation of the scission and the branching problem. The required properties of the linear segments or primary polymers forming the branched architectures are found by a kinetic model. The general rules for the construction of branched architectures from populations of linear segments then lead to an analytical expression for the branching distribution and a semi‐analytical expression for the bivariate length/branching distribution. The method is applied to the scission of an initially branched polymer and subsequent terminal branching on scission points and activated terminal double bonds. Exact agreement is found with Monte Carlo sampling results.

     

Analysis of branched polymers by high resolution multidetector size exclusion chromatography: Separation of the effects of branching and molecular weight distribution

This article presents the SEC analysis of branched polyisobutylene PIB and polystyrene PS with high molecular weight and broad multimodal molecular weight distribution. Both polymers were synthesized using an inimer technique, which results in long‐chain branched polymers with statistical branching and broad multimodal distributions. Using high resolution multidetector Size Exclusion Chromatography SEC the polymers were analyzed based on three branching factors: g = (R_z,br/R_z,lin)_Mw; h = (〈R_h〉_z,br/〈R_h〉_z,lin)_{Mw ;} and ρ = (R ^1/2/〈R_h〉_z). It is generally accepted that for monodisperse branched polymers g and h < 1. In the case of our polydisperse PIB and PS, it was seen that g and h > 1, and ρ increases with molar mass and the number of chain ends as predicted earlier. The multidetector SEC system allowed for the separation of branching and polydispersity, reported here for the first time experimentally. The g parameter as a function of DP_i was compared to the theory developed by Zimm and Stockmayer. The plots followed a similar trend, but were shifted by a factor related to the average chain length between branching points. The ρ parameter decreased with increasing DP_i, as predicted theoretically by Kajiwara. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Synthesis and electrochemical properties of dendrimers containing meta-terphenyl peripheral groups and a 4,4′-bipyridinium core

Fréchet-type poly(arylether) dendrons carrying m-terphenyl peripheral groups were synthesized up to second generation by convergent methodology. Simple quarternisation of 4,4′-bipyridine with the dendritic bromides afforded the corresponding dendrimers containing a 4,4′-bipyridine core. The electrochemical parameters were obtained for all the dendrimers and the half-wave potentials of both the first and second redox processes shift to less-negative values as the dendrimer generation increases.     

Branched polymers through redox emulsion polymerization using peroxide monomer as the branching agent

Redox emulsion polymerization to branched vinyl polymers in the presence of 2-(tert-butylperoxy)ethyl methacrylate (BPEMA), ferrous sulfate, and sodium formaldehyde sulfoxylate (SFS) is reported in this paper. The peroxide monomer BPEMA containing alkyl peroxide was designed for high stability during preparation and storage. Nuclear magnetic resonance spectroscopy (NMR), Raman, and triple-detection size-exclusion chromatography (TD-SEC) measurements were used to reveal the polymerization procedure and provide evidence of branching structure. In the case of polymerization at St₁₀₀-BPEMA_1.0-FeSO_{4 0.5}-SFS_0.2, the molecular weight increased and decreased with conversion below and above 75% monomer conversion, respectively. The decreasing of molecular weight with monomer conversion came from the increased viscosity of the micelle, which makes it difficult for the formed macromolecules containing vinyl group to participate into polymerization. Finally, the molecular weight reached a value of M_{n. SEC} = 439,200 g/mol at 92.2% conversion. In addition, the Zimm branching factor, g', also decreased and increased with conversion below and above 60% conversion, respectively, and then the g' finally attends a value of 0.41, showing high degree of branching. Branched poly(methyl methacrylate) was also prepared through this strategy, showing a versatile approach to branched vinyl polymers.     

Multiplurifunctionalized Phosphorus Dendrimers: Selective Functionalization of P(X)CL2 Terminal Groups

Abstract

Dendrimers are hyperbranched macromolecules constituted of repetitive branched units. They are synthesized step-by-step, a method which ensures a perfectly defined structure. This mini-review reports examples of rare “multiplurifunctionalized” dendrimers that have several types of functional groups precisely located on each terminal branching point. They are obtained thanks to the specificity of the reactivity of each Cl of P(X)Cl₂ (X = generally S, but also O) terminal groups on the surface of poly(phosphorhydrazone) dendrimers.     

Liquid-crystalline carbosilane dendrimers of different molecular architecture with photochromic mesogenic and aliphatic terminal groups

Second-generation (G-2) liquid-crystalline carbosilane block and statistical dendrimers with aliphatic (decyl) and photochromic (azobenzene-containing) mesogenic terminal groups and a G-2 homo-dendrimer containing the same mesogenic terminal groups were synthesized for the first time. The influence of dendritic architecture on the phase behavior of the dendrimers and on photoinduced Z-E-isomerization of the azobenzene fragments in mesogenic terminal groups in dendrimer solutions are discussed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2325–2331, December, 2007.     

N-Alkyl-2-pyrrolidones as Derivatives for the Structure Elucidation of Long-chain Primary Alcohols by Mass Spectrometry

One unsaturated and three branched long-chain primary alcohols have been converted into N-alkyl-2-pyrrolidones for investigation by mass spectrometry. The EI. mass spectra of these derivatives have been found to exhibit unambiguously the branching points and, albeit less clearly, the position of a double bond in the chain.     

DETERMINATION OF BRANCHING DISTRIBUTION IN HIGH cis-1,4-POLYBUTADIENE BY GPC AND AUTOMATIC VISCOMETRY

Based on the methods reported by Ambler and Kraus, a method has been developed for the determination of long-chain branching distribution in polymers by the combined use of GPC and intrinsic viscosity data of polymer fractions. In this method, g_i, λ_i, G_i, m_i, the weight percentage of polymer that is branched, etc. can be used simultaneously to characterize the distribution, degree and content of branching in polymers. Some relations between molecular weight polydispersity and branching polydispersity in Nickel-based high cis-1,4-polybutadiene samples are discussed. It was found that the number of long branches λ. per unit molecular weight is a function of molecular weight and all of the samples are highly branched at a molecular weight of about 10~6.     

A Novel One‐Pot Conversion of Allyl Alcohols into Primary Allyl Halides Mediated by Acetyl Halide

A new and simple method for the synthesis of the primary allyl chlorides and bromides 9 – 16 from the secondary or tertiary allyl alcohols 3 – 8 and acyl halide was developed (Scheme 2, Table 1). Non‐commercially available secondary and tertiary allyl alcohols were synthesized from the related ketones and aldehydes via the addition of vinylmagnesium chloride. Mechanistic studies indicate that the alcohols were first acetylated by the acetyl halide and then protonated prior to substitution by the halide, Cl^? or Br^?, via an S_N2′ reaction, to yield the primary halides (Scheme 5).     

Competition of Dehydrochlorination and Dehydrobromination in Reactions of N,N,N-Trialkyl-N-(2,3-dibromo-3-chlorobutyl)ammonium Bromides with Alcoholic Solution of KOH

Reactions of N,N,N-trialkyl-N-(2,3-dibromo-3-chlorobutyl)ammonium bromides with alcoholic solution of KOH mainly follow the dehydrochlorination pathway, yielding N,N,N-trialkyl-N-(2,3-dibromo-2-butenyl)ammonium bromides as major products.