Epoxide ring opening by alcohols generates molecules that are used in a wide range of applications, including as polyols that are components of polyurethanes. The effects of nucleophile structure on regioselectivity and initial reaction rates were compared for base-catalyzed and coordination-catalyzed 1,2-epoxyoctane ring opening by 1-propanol (1PrOH) and tert-butanol (tBuOH) using potassium tert-butoxide and tris(pentafluorophenyl)borane as case studies. Experimentally measured initial rates of the tris(pentafluorophenyl)borane-catalyzed system were deconvoluted using a microkinetic model built upon a triple-pathway (Lewis acid, water-mediated, and alcohol-mediated) reaction scheme we previously developed. Benchmark, base-catalyzed systems consistently showed slow rates and low selectivities to the primary alcohol regioisomer (P1), especially for the bulkier nucleophile. In contrast, aryl boranes achieved high P1 selectivity and relatively fast epoxide ring-opening rates. Interestingly, low water levels led to tBuOH being the faster nucleophile, while at high water levels, 1PrOH had faster rates. Analyses of reactive flux and catalyst speciation using the model showed that the unexpectedly high relative rate of tBuOH at low water levels was due to higher concentrations of catalytically active species from faster pathways and lower flux through the slow, low-selectivity alcohol-mediated pathway for tBuOH.

Epoxide ring-opening is a key reaction in organic chemistry. We have previously shown that B(C6F5)(3), a strongly Lewis acidic arylborane, exhibited high rates and unusual selectivities for catalyzing the ring-opening of aliphatic epoxides with alcohols. Here we compare catalysts of the form B(C6H5_XFX)(3 )(x = 5, 4, 3, and 0) and determine that moderately Lewis acidic arylboranes have higher regioselectivity, but slower rates in this reaction. At high temperatures, these arylboranes can also hydrolyze into inactive species. However, deactivation is suppressed in the presence of co-catalytic amounts of 1,2-propanediol, and DFT calculations suggest a role for arylborane-H2O-diol complexes. Thermal stabilization and regioselectivity enhancement by diol are both more pronounced for B (C6F5)(3) than for less Lewis acidic B(C6HF4)(3) and B(C6H2F3)3 catalysts. These results further demonstrate the catalytic relevance of H-bound networks of arylboranes and diols and enable their use at higher temperatures and greatly increased rates.

Silanol polycondensation is an important industrial process that is used to produce high-molecular-weight silicones for a wide array of applications. This technology is practiced commercially using strong Bronsted acids or bases that produce cyclosiloxane byproducts that can compromise performance properties and are also under increased regulatory scrutiny. We have found that Lewis acids are active silanol polycondensation catalysts that limit the formation of cyclosiloxane byproducts. Ligand donicity was found to have a dramatic effect on the activity of metal salts, as polycondensation catalysts with more weakly coordinating ligands-i.e., trifluoromethanesulfonate-displaying higher activity. Similarly, the Lewis acidities of the metal cations contribute strongly to the activity of these catalysts. Finally, the steric environment around the metal center can be tuned by coordination to a bis imine ligand to further suppress the formation of cyclosiloxanes.

Polycondensation of α,ω-disilanols is a foundational technology for silicones producers. Commercially, this process is carried out with strong Brønsted acids and bases, which generates cyclosiloxane byproducts. Homoconjugated acids (a 2:1 complex of acid:base or a 1:1 complex of acid:salt), a seldom used class of silanol polycondensation catalysts, were evaluated for their ability to polymerize α,ω-disilanols while forming low levels of cyclosiloxane byproducts. Homoconjugated acid catalysts were highly active for silanol polycondensation, even when made from relatively mild acids such as acetic acid. Both the acid and base (or cation) component of the homoconjugated species was important for activity and avoiding cyclosiloxane byproduct formation. Stronger acids and bases were found to positively affect reactivity, and the pKa of the acid was found to correlate with cyclosiloxane byproduct formation. The individual components of the homoconjugated species (the acid and base) were ineffective as catalysts by themselves, and compositions with fewer than 2 mol of acid to 1 mol of base were much less reactive. Homoconjugated trifluoroacetic acid tetramethylguanidinium and tetrabutylphosphonium complexes were found to be privileged catalysts, able to give high-molecular-weight siloxanes (Mn > 60 kDa) while generating less than 100 ppm of octamethylcyclotetrasiloxane byproduct. Finally, a mechanism has been proposed where silanols are electrophilically and nucleophilically activated by the homoconjugated species, leading to silanol polycondensation.

Epoxide alcoholysis is extensively employed in the synthesis of polymers and chemical intermediates, and it generally requires an acid catalyst for high rates and selectivity. Tris(pentafluorophenyl)borane [B(C6F5)(3)] is among few catalysts that are selective to primary alcohol products of terminal aliphatic epoxides that do not possess any directing groups. We have previously observed that under many conditions, the reaction regioselectivity increases with conversion. Here, we confirm a prediction from our earlier computational model, and we experimentally demonstrate that this increase is due to a selectivity-enhancing role of the reaction products. We then show that deliberate addition of catalytic amounts of certain diols increases the reaction regioselectivity. Cis-1,2 or 1,3-diols are required to enhance selectivity, consistent with a mechanism where extended hydrogen-bonding networks preferentially organize the reactants. This work presents a route to tune regioselectivity without altering the catalyst backbone and provides another example of the role of H-bonding networks in reactions taking place in protic media.

Density functional theory (DFT) calculations, experimental data, and microkinetic modeling are used to extend a triple-pathway (Lewis acid, water-mediated, and alcohol-mediated) mechanism for tris(pentafluorophenyl)borane-catalyzed ring opening of 1,2-epoxyoctane by alkyl alcohol nucleophiles previously applied to 2-propanol to 1-propanol. Although simpler models may capture overall rates, the reaction schemes proposed here are required to explain the increasing regioselectivity to the primary product with conversion and the dependence of the overall regioselectivity on residual water concentration and additives as a function of reaction conditions. The model indicates that the different reaction conditions (nucleophile, water concentration, temperature, and conversion) lead to different amounts of flux through alcohol-mediated pathways, different speciation of tris(pentafluorophenyl)borane adducts, and differences among the inherent selectivities of water-mediated mechanisms.

We report the synthesis and characterization of catechol-functionalized film-forming latexes that display excellent adhesion to low-surface-energy polyolefin-based substrates. The aromatic 1,2-diol functional group in catechol derivatives is believed to be responsible for enhancing the adhesion of a variety of polymers to a range of substrates. Here, we describe a postpolymerization modification approach to the design of emulsion polymers with catechol-functionalized side chains. A series of analogous small-molecule reactions, together with latex characterization by infrared (IR) spectroscopy and liquid chromatography (LC) methods, provides evidence for polymer functionalization. Films prepared from catechol-containing latexes displayed remarkable adhesion to challenging, commercially-available thermoplastic polyolefin (TPO) (as determined by a standard ASTM cross-hatch method). We provide evidence that covalent bonding and the unique catechol structure are required to promote adhesion. The catechol-functionalized emulsion polymers reported here represent a new class of functional latex, and this postpolymerization modification approach will present further opportunities to improve, modulate, and control the adhesion of water-borne coatings to a variety of polyolefin-based substrates.

The controlled self-assembly of well-defined and spatially ordered π-systems has attracted considerable interest because of their potential applications in organic electronics. An important contemporary pursuit relates to the investigation of charge transport across noncovalently coupled components in a stepwise fashion. Dynamic oligorotaxanes, prepared by template-directed methods, provide a scaffold for directing the construction of monodisperse one-dimensional assemblies in which the functional units communicate electronically through-space by way of π-orbital interactions. Reported herein is a series of oligorotaxanes containing one, two, three and four naphthalene diimide (NDI) redox-active units, which have been shown by cyclic voltammetry, and by EPR and ENDOR spectroscopies, to share electrons across the NDI stacks. Thermally driven motions between the neighboring NDI units in the oligorotaxanes influence the passage of electrons through the NDI stacks in a manner reminiscent of the conformationally gated charge transfer observed in DNA.

A general method for the gram scale resolution of 2-substituted and 2,8-disubstituted Tröger's base (TB) derivatives in 63-91% yield has been achieved through the application of crystallization-induced asymmetric transformation (CIAT). Enantiomeric ratios of the resolved TB derivatives range from 99.1:0.9 to >99.5:0.5. Among the Tröger's base compounds resolved are four synthetically valuable bromo and iodo derivatives.

Formation of an imine--from an amine and an aldehyde--is a reversible reaction which operates under thermodynamic control such that the formation of kinetically competitive intermediates are, in the fullness of time, replaced by the thermodynamically most stable product(s). For this fundamental reason, the imine bond has emerged as an extraordinarily diverse and useful one in the hands of synthetic chemists. Imine bond formation is one of a handful of reactions which define a discipline known as dynamic covalent chemistry (DCC), which is now employed widely in the construction of exotic molecules and extended structures on account of the inherent 'proof-reading' and 'error-checking' associated with these reversible reactions. While both supramolecular chemistry and DCC operate under the regime of reversibility, DCC has the added advantage of constructing robust molecules on account of the formation of covalent bonds rather than fragile supermolecules resulting from noncovalent bonding interactions. On the other hand, these products tend to require more time to form--sometimes days or even months--but their formation can often be catalysed. In this manner, highly symmetrical molecules and extended structures can be prepared from relatively simple precursors. When DCC is utilised in conjunction with template-directed protocols--which rely on the use of noncovalent bonding interactions between molecular building blocks in order to preorganise them into certain relative geometries as a prelude to the formation of covalent bonds under equilibrium control--an additional level of control of structure and topology arises which offers a disarmingly simple way of constructing mechanically-interlocked molecules, such as rotaxanes, catenanes, Borromean rings, and Solomon knots. This tutorial review focuses on the use of dynamic imine bonds in the construction of compounds and products formed with and without the aid of additional templates. While synthesis under thermodynamic control is giving the field of chemical topology a new lease of life, it is also providing access to an endless array of new materials that are, in many circumstances, simply not accessible using more traditional synthetic methodologies where kinetic control rules the roost. One of the most endearing qualities of chemistry is its ability to reinvent itself in order to create its own object, as Berthelot first pointed out a century and a half ago.

A couple of [c2]daisy chains have been assembled in each case from four components in quantitative yields at room temperature in acetonitrile as a result of the self-templated clippings of their [24]crown-8 rings by reversible imine bond formation around secondary dialkylammonium recognition sites in their stalks.

The quest to construct mechanically interlocked polymers, which present precise monodisperse primary structures that are produced both consistently and with high efficiencies, has been a daunting goal for synthetic chemists for many years. Our ability to realise this goal has been limited, until recently, by the need to develop synthetic strategies that can direct the formation of the desired covalent bonds in a precise and concise fashion while avoiding the formation of unwanted kinetic by-products. The challenge, however, is a timely and welcome one, as a consequence of, primarily, the potential for mechanically interlocked polymers to act as dynamic (noncovalent) yet robust (covalent) new materials for a wide array of applications. One such strategy which has been employed widely in recent years to address this issue, known as Dynamic Covalent Chemistry (DCC), is a strategy in which reactions operate under equilibrium and so offer elements of "proof-reading" and "error-checking" to the bond forming and breaking processes such that the final product distribution always reflects the thermodynamically most favourable compound. By coupling DCC with template-directed protocols, which utilise multiple weak noncovalent interactions to pre-organise and self-assemble simpler small molecular precursors into their desired geometries prior to covalent bond formation, we are able to produce compounds with highly symmetric, robust and complex topologies that are otherwise simply unobtainable by more traditional methods. Harnessing these strategies in an iterative, step-wise fashion brings us ever so much closer towards perfecting the controlled synthesis of high order main-chain mechanically interlocked polymers. This tutorial review focuses (i) on the development of DCC-namely, the formation of dynamic imine bonds-used in conjunction with template-directed protocols to afford a variety of mechanically interlocked molecules (MIMs) and ultimately (ii) on the synthesis of highly ordered poly[n]rotaxanes with high conversion efficiencies.

Two series of oligorotaxanes R and R' that contain -CH(2)NH(2)(+)CH(2)- recognition sites in their dumbbell components have been synthesized employing template-directed protocols. [24]Crown-8 rings self-assemble by a clipping strategy around each and every recognition site using equimolar amounts of 2,6-pyridinedicarboxaldehyde and tetraethyleneglycol bis(2-aminophenyl) ether to efficiently provide up to a [20]rotaxane. In the R series, the -NH(2)(+)- recognition sites are separated by trismethylene bridges, whereas in the R' series the spacers are p-phenylene linkers. The underpinning idea here is that in the former series, the recognition sites are strategically positioned 3.5 Å apart from one another so as to facilitate efficient [π···π] stacking between the aromatic residues in contiguous rings in the rotaxanes and consequently, a discrete rigid and rod-like conformation is realized; these noncovalent interactions are absent in the latter series rendering them conformationally flexible/nondiscrete. Although in the R' series, the [3]-, [4]-, [8]-, and [12]rotaxanes were isolated after reaction times of <5-30 min in yields of 72-85%, in the R series, the [3]-, [4]-, [5]-, [8]-, [12]-, [16]-, and [20]rotaxanes were isolated in <5 min to 14 h in 88-98% yields. It follows that while in the R' series the higher order oligorotaxanes are formed in lower yields more rapidly, in the R series, the higher order oligorotaxanes are formed in higher yields more slowly. In the R series, the high percentage yields are sustained throughout, despite the fact that up to 39 components are participating in the template-directed self-assembly process. Simple arithmetic reveals that the conversion efficiency for each imine bond formation peaks at 99.9% in the R series and 99.3% in the R' series. This maintenance of reaction efficiency in the R series can be ascribed to positive cooperativity, that is, when one ring is formed it aids and abets the formation of subsequent rings presumably because of stabilizing extended [π···π] stacking interactions between the arene units. Experiments have been performed wherein the dumbbell is starved of the macrocyclic components, and up to five times more of the fully saturated rotaxane is formed than is predicted based on a purely statistical outcome, providing a clear indication that positive cooperativity is operative. Moreover, it would appear that as the R series is traversed from the [3]- to the [4]- to the [5]rotaxane, the cooperativity becomes increasingly positive. This kind of cooperative behavior is not observed for the analogous oligorotaxanes in the R' series. The conventional bevy of analytical techniques (e.g., HR-MS (ESI) and both (1)H and (13)C NMR spectroscopy) help establish the fact that all the oligorotaxanes are pure and monodisperse. Evidence of efficient [π···π] stacking between contiguous arene units in the rings in the R series is revealed by (1)H NMR spectroscopy. Ion-mobility mass spectrometry performed on the R and R' series yielded the collisional cross sections (CCSs), confirming the rigidity of the R oligorotaxanes and the flexibility of the R' ones. The extended [π···π] stacking interactions are found to be present in the solid-state structures of the [3]- and [4]rotaxanes in the R series and also on the basis of molecular mechanics calculations performed on the entire series of oligomers. The collective data presented herein supports our original design in that the extended [π···π] stacking between contiguous arene units in the rings of the R series of oligorotaxanes facilitate an essentially rigid rod-like conformation with evidence that positive cooperativity improves the efficiency of their formation. This situation stands in sharp contrast to the conformationally flexible R' series where the oligorotaxanes form with no cooperativity.

An overview of Pd-PEPPSI complexes in the Negishi cross-coupling reaction is presented Preliminary evaluations of differentially substituted imidazolium salts that generate highly active cross-coupling catalysts in situ provide the foundation for the development of an air- and moisture-table NHC-based precatalyst Pd-PEPPSI-IPr The application of Pd-PEPPSI-IPr in sp(3)-sp(3), sp(3)-sp(2) (and vice versa), and sp(2)-sp(2) Negishi cross-couplings is reviewed. This allowed the systematic development of Pd-PEPPSI-IPent, a more sterially demanding, second-generation catalyst that outperforms the IPr analogue in a variety of sp(2)-sp(2) Negishi cross-couplings General routes for the preparation of organozinc reagents are also examined, and an additives study reveals the probable transmetallating species that is operative in these cross-couplings. Mechanistic considerations of the Negishi cross-coupling reaction based on experimental and computational studies are summarized and evaluated

Linking struts containing Cram-like bisbinaphthyl[22]crown-6 with Zn(4)O(CO(2))(6) joints affords metal-organic frameworks with chiral recognition sites that are highly designed, ordered and placed in a precise manner throughout the entire crystal.