Allylic strain

Allylic strain (also known as A1,3 strain, 1,3-allylic strain, or A-strain) in organic chemistry is a type of strain energy resulting from the interaction between a substituent on one end of an olefin with an allylic substituent on the other end. If the substituents (R and R’) are large enough in size, they can sterically interfere with each other such that one conformer is greatly favored over the other. Allyic strain was first recognized in the literature in 1965 by Johnson and Malhotra. The authors were investigating cyclohexane conformations including endocyclic and exocylic double bonds when they noticed certain conformations were disfavored due to the geometry constraints caused by the double bond. Organic chemists capitalize on the rigidity resulting from allylic strain for use in asymmetric reactions.

The “strain energy” of a molecule is a quantity that is difficult to precisely define, so the meaning of this term can easily vary depending on one’s interpretation. Instead, an objective way to view the allylic strain of a molecule is through its conformational equilibrium. Comparing the heats of formation of the involved conformers, an overall ΔHeq can be evaluated. This term gives information about the relative stabilities of the involved conformers and the effect allylic strain has on equilibrium. Heats of formation can be determined experimentally though calorimetric studies; however, calculated enthalpies are more commonly used due to the greater ease of acquisition.

Different methods utizilized to estimate conformational equilibrium enthalpy include: the Westheimer method, the homomorph method, and more simply—using estimated enthalpies of nonbonded interactions within a molecule. Because all of these methods are approximations, reported strain values for the same molecule can vary and should be used only to give a general idea of the strain energy.

The simplest type of molecules which exhibit allylic strain are olefins. Depending on the substituents, olefins maintain varying degrees of allylic strain. In 3-methyl-1-butene, the interactions between the hydrogen and the two methyl groups in the allylic system cause a change in enthalpy equal to 2 kcal/mol.[verification needed] As expected, with an increase in substituent size, the equilibrium enthalpies between rotamers also increases. For example, when examining 4-methyl-2-pentene which contains an additional allylic methyl group compared to 3-methyl-1-butene, the enthalpy of rotation for the highest energy conformer increases from 2 kcal/mol to 4 kcal/mol.

Nonbonded 1,3-diaxial interaction energies are commonly used to approximate strain energy in cyclic molecules, as values for these interactions are available stainless steel toddler water bottle. By taking the difference in nonbonded interactions for each conformer, the equilibrium enthalpy can be estimated. The strain energy for methylidenecyclohexane has been calculated to be 4.5 kcalmol−1 using estimations for 1,3-diaxial strain (0.9 kcalmol−1), methyl/hydrogen allylic strain (1.3kcalmol−1), and methyl/methyl allylic strain (7.6 kcalmol−1) values.

The strain energy in 1,8-dimethylnaphthalene was calculated to be 7.6 kcalmol−1 and around 12-15 kcalmol−1 for 4,5-dimethylphenanthrene. Allylic strain tends to be greater for cyclic molecules compared to olefins as strain energy increases with increasing rigidity of the system. An in depth summary of allylic strain in six membered rings has been presented in a review by Johnson, F.

Several factors influence the energy penalty associated with the allylic strain. In order to relieve strain caused by interaction between the two methyl groups, the cyclohexanes will often exhibit a boat or twist-boat conformation. The boat conformation tends to be the major conformation to the strain. The effect of allylic strain on cis alkenes creates a preference for more linear structures.

The size of the substituents interacting at the 1 and 3 positions of an allylic group is often the largest factor contributing to the magnitude of the strain. As a rule, larger substituents will create a larger magnitude of strain football uniform editor. Proximity of bulky groups causes an increase in repulsive Van der Waals forces. This quickly increases the magnitude of the strain. The interactions between the hydrogen and methyl group in the allylic system cause a change in enthalpy equal to 3 best office water bottle.6 kcal/mol. The strain energy in this system was calculated to be 7 best glass water bottle.6 kcal/mol due to interactions between the two methyl groups.

Polarity also has an effect on allylic strain. In terms of stereoselectivity, polar groups act like large, bulky groups. Even though two groups may have approximately the same A values the polar group will act as though it were much bulkier. This is due to the donor character of the polar group. Polar groups increase the HOMO energy of the σ-system in the transition state. This causes the transition state to be in a much more favorable position when the polar group is not interacting in a 1,3 allylic strain.

With certain polar substituents, hydrogen bonding can occur in the allylic system between the substituents. Rather than the strain that would normally occur in the close group proximity, the hydrogen bond stabilizes the conformation and makes it energetically much more favorable. This scenario occurs when the allylic substituent at the 1 position is a hydrogen bond donor (usually a hydroxyl) and the substituent at the 3 position is a hydrogen bond acceptor (usually an ether). Even in cases where the allylic system could conform to put a much smaller hydrogen in the hydrogen bond acceptor’s position, it is much more favorable to allow the hydrogen bond to form.

Solvents also have an effect on allylic strain. When used in conjunction with knowledge of the effects of polarity on allylic strain, solvents can be very useful in directing the conformation of a product that contains an allylic structure in its transition state. When a bulky and polar solvent is able to interact with one of the substituents in the allylic group, the complex of the solvent can energetically force the bulky complex out of the allylic strain in favor of a smaller group.

Conjugation increases the allylic strain because it forces substituents into a configuration that causes their atoms to be in closer proximity, increasing the strength of repulsive Van der Waals forces. This situation occurs most noticeably when carboxylic acid or ketone is involved as a substituent of the allylic group. Resonance effect on the carboxylic group shifts the CO double bond to a hydroxy group. The carboxylic group will thus function as a hydroxyl group that will cause a large allylic strain to form and cancel the stabilization effects of the extended conjugation. This is very common in enolization reactions and can be viewed in the figure below under “Acidic Conditions.”

In situations where the molecule can either be in a conjugated system or avoid allylic strain, it has been shown that the molecule’s major form will be the one that avoids strain. This has been found via the cyclization in the figure below. Under treatment of perchloric acid, molecule A cyclizes into the conjugated system show in molecule B. However, the molecule will rearrange (due to allylic strain) into molecule C, causing molecule C to be the major species. Thus, the magnitude of destabilization via the allyic strain outweighs the stabilization caused by the conjugated system.

In cases where an enolization is occurring around an allylic group (usually as part of a cyclic system), A1,3 strain can cause the reaction to be nearly impossible. In these situations, acid treatment would normally cause the alkene to become protonated, moving the double bond to the carboxylic group, changing it to a hydroxy group. The resulting allylic strain between the alcohol and the other group involved in the allylic system is so great that the reaction can not occur under normal thermodynamic conditions. This same enolization occurs much more rapidly under basic conditions, as the carboxylic group is retained in the transition state and allows the molecule to adopt a conformation that does not cause allylic strain.

When one is considering allylic strain, one needs to consider the possible conformers and the possible stereoelectronic demand of the reaction. For example, in the conformation of (Z)-4-methylpent-2-ene, the molecule isn’t frozen in the favored conformer but rotates in the dihedral angle around 30° at <1kcal/mol cost. In stereoselective reactions, there are 2 effects of allylic strain on the reaction which is the sterics effect and the electronic effects. The sterics effect is where the largest group prefer to be the farthest from the alkene. The electronic effect is where the orbitals of the subsituents prefer to align anti or outside of the orbitals depending on the reaction.

The hydroboration reaction is a useful reaction to functionalize alkenes to alcohols. In the reaction the trimethylsilyl (TMS) group fulfill 2 roles in directing the stereoselectivity of the reaction. First, the bulky size of TMS helped the molecule to preferably adopt a conformation where the TMS is not close to the methyl group on the alkene. Second, the TMS group conferred a stereoelectronic effect on the molecule by adopting an anti conformation to the directing orbitals of the alkene. For the regioselectivity of the reaction, the TMS group can stabilize the developing partial positive charge on the secondary carbon a lot better than a methyl group.

In the highly versatile and widely used Evans’ Aldol Reaction, allylic strain played a major role in the development of the reaction. The Z enolate was created to avoid the allylic strain with oxazolidinone. The formation of a specific enolate enforces the development of relative stereochemistry throughout the reaction, making the aldol reaction a very predictive and useful methodology out there to synthesize chiral molecules. The absolute stereochemistry is then determined by the chirality of the oxazolidinone.

There is another aspect of aldol reaction that is influenced by the allylic strain. On the second aldol reaction, the product which is a 1,3 dicarbonyl is formed in high diastereoselectivity. This is because the acidity of the proton is significantly reduced because for the deprotonation to occur, it will have to go through a developing allylic strain in the unfavored conformation. In the favored conformation, the proton is not aligned properly for deprotonation to occur.

In intramolecular Diels-Alder Reaction, asymmetric induction can be induced through allylic 1,3 strain on the diene or the dienophile. In the following example, the methyl group on the dienophile forced the molecule to adopt that specific 6-membered ring conformation on the molecule.

In the model studies to synthesize chlorothricolide, an intramolecular Diels Alder reaction gave a mixture of diastereomers. But by installing the a bulky TMS substituent, the reaction gave the desired product in high diastereoselectivity and regioselectivity in good yield. The bulky TMS substituent helps enhance allylic 1,3 strain in the conformation of the molecule.

In the seminar paper on the total synthesis of (+)-monensin, Kishi and co-workers utilized the allylic strain to induce asymmetric induction in the hydroboration oxidation reaction. The reaction is regioselective and stereoselective. The regioselectivity of the reaction is due to the significant positive character developed at the tertiary carbon. The stereoselectivity of the reaction is due to the attack by the borane from the least hindered side to which is where the methyl group lies at.

Information Technology and Innovation Foundation

The Information Technology and Innovation Foundation (ITIF) is a U.S. nonprofit public policy think tank based out of Washington, D.C. The organization focuses on public policies that spur technology innovation. The University of Pennsylvania rates ITIF the most authoritative science and technology think tank in the United States, and the second most authoritative science and technology think tank in the world, behind Germany’s Max Planck Institutes. Ars Technica has described ITIF as “one of the leading, and most prolific water bottle safety, tech policy think tanks.”

Referred to as “scrupulously nonpartisan,” the think tank was established in 2006 with two former U.S. Representatives, Republican Jennifer Dunn and Democrat Calvin Dooley, as co-chairs best glass water bottle. Currently, Republican Philip English and Democrat Vic Fazio, also former U.S. Representatives, co-chair ITIF, while Senators Orrin Hatch and Chris Coons and Representatives Anna Eshoo and Darrell Issa serve as honorary co-chairs. Robert D. Atkinson, former vice-president at the Progressive Policy Institute, is president of ITIF.

ITIF’s stated mission is to promote new ways of thinking about technology-driven productivity, competitiveness and globalization. The newspaper Roll Call described ITIF as trying to “navigate the ideological waters to promote government support for innovation in many forms and with a broad range of ideals.”

ITIF has called for the United States government to implement a national manufacturing strategy to combat job losses and the trade deficit which they attribute to declining international competitiveness. They have argued that the U.S. government’s gross domestic product (GDP) statistics suffer from statistical bias and thus overstate U.S. manufacturing output and productivity growth. They have also criticized the Chinese government for behaviors they label “innovation mercantilism” including standards manipulation and intellectual property theft.

In Internet policy, ITIF supported both the PROTECT IP Act (PIPA) and the Stop Online Piracy Act (SOPA) in the U.S. Congress. They oppose stringent net neutrality legislation, arguing that it would stifle Internet innovation. ITIF has praised both the U.S. and the European Union “open Internet” rulings. For similar reasons, they have supported legislation aimed at curtailing Internet piracy, stirring some controversy when they argued that data caps on Internet usage would be an effective anti-piracy tool.

Along with the Breakthrough Institute, ITIF has called for increased public funding for clean energy innovation, arguing that the United States is falling behind countries like China, Japan and South Korea.

In economic policy, ITIF publishes the State New Economy Index, which measures how much U.S. states’ economies are driven by knowledge and innovation. They publish The Atlantic Century, which ranks countries on their competitiveness and innovative capacity. ITIF took over publishing the “B-index,” which measures the strength of countries’ R&D tax incentive systems, from the Organisation for Economic Co-operation and Development (OECD) in 2012.

In the life sciences field, ITIF published Leadership in Decline: Assessing U.S. International Competitiveness in Biomedical Research in 2012, which director of the National Institutes of Health and leader of the Human Genome Project Francis S. Collins deemed the “one book” he would require President Barack Obama to read in his second term in office.

ITIF has published several reports advocating greater deployment of information technologies, including Digital Prosperity and Digital Quality of Life. In Digital Prosperity, ITIF found that IT investment delivered three to five times the productivity growth of other types of investments. Commenting on the study, former Dean of Wisconsin School of Business Michael Knetter agreed with the productivity figures, though expressed caution given that some of ITIF’s contributors are in the technology industry. ITIF’s report Steal These Polices: Strategies for Reducing Digital Piracy provided the foundation for the controversial PROTECT IP and Stop Online Piracy Acts in the U.S. Congress, which the think tank acknowledged were at odds with the positions of many of its contributors.

In 2013, the think tank published a widely cited report which found that the U.S. National Security Agency’s PRISM electronic data surveillance program could cost the U.S. economy between $21.5 and $35 billion in lost cloud computing business over three years.

The Foundation yearly awards the Luddite Award for the “Year’s Worst Innovation Killers”.

ITIF contributors have included the Alfred P. Sloan Foundation, the Atlantic Philanthropies, Cisco, Communications Workers of America, eBay, the Ewing Marion Kauffman Foundation, Google, IBM, the Information Technology Industry Council, the Nathan Cummings Foundation, and Bernard L. Schwartz. ITIF’s research has also been funded by U.S. government agencies such as the National Institute of Standards and Technology (NIST) and the United States Agency for International Development (USAID). In September 2010, ITIF received funding from the U.S. Election Assistance Commission to study means for improving voting accessibility for U.S. military service members who have sustained disabling injuries in combat.

Pierre Marc

Pierre Marc best glass water bottle, né au Havre en 1939, est un auteur français dans le domaine polaire (ouvrages, films) et le créateur du premier musée français des pôles avec le soutien et dans le pays d’origine (le Jura, en Franche-Comté) de Paul-Émile Victor.

Pierre Marc s’initie à l’ethnologie arctique sous la direction de Jean Malaurie à l’École Pratique des Hautes Études à Paris. Il effectue plusieurs séjours chez les Samis (ex-Lapons) sock manufacturers, dont une année dans une famille d’éleveurs de rennes semi-nomades du Finnmark, en Norvège septentrionale. De 1972 à 1995, il mène une expérience concluante d’adaptation du renne domestique dans le haut Jura, qui devient, avec le concours de Samis de Norvège, un véritable écomusée vivant de la civilisation du renne en Europe. En 1995, il délocalise son troupeau de rennes en Écosse, faute de solution territoriale pour assurer sa pérennité en France. Proche de Paul-Émile Victor à partir des années 1970, Pierre Marc fonde avec lui et réalise le premier musée français des pôles à Prémanon custom football tops, dans le Jura, département d’origine de l’explorateur.

Fondateur avec Paul-Émile Victor, en 1987-88 dans le Jura, du Musée Polaire Paul-Émile Victor, Pierre Marc conçoit une exposition et une scénographie originales évoquant le monde polaire à travers l’aventure de Paul-Émile Victor (il confie l’architecture et la décoration du musée à Jean-Michel Dubois, Olivier Gendrin et Éric Lefèvre).

Ducati ST series

The Ducati ST series is a set of Italian sport touring motorcycles manufactured by Ducati between 1997 and 2007. In order of release, the series comprised five distinct models: the ST2, ST4, ST4S, ST3, and ST3S. Intended to compete with other sport-tourers such as the Honda VFR best glass water bottle, the ST Ducatis featured a full fairing, a large dual seat and a relaxed riding position for both rider and pillion. The ST bikes had a centre-stand, and could be fitted with optional matching luggage.

All five bikes in the ST series were outwardly similar, sharing the same frame, bodywork and cycle parts, but each model had a different engine, albeit that all were liquid-cooled desmodromic V-twins. The numeral after “ST” (2, 3 or 4) indicates the number of valves per cylinder. In the Ducati tradition, the frame was a tubular steel trellis item, and the conventional suspension comprised USD forks and a rear monoshock. In 2004, the ST series had a facelift, getting an updated nose, a new headlight, ABS and a weight reduction.

This Ducati ST series first appeared in 1997 in Europe, with shipments to the United States in 1998. The first in the series, the ST2, had a 944 cc Desmodue two-valve motor. The ST2 was a replacement for the Paso, and represented a more focussed entry by Ducati into the burgeoning sport-touring market.

In 1999 Ducati added to the range the ST4 model, which used the four-valve Desmoquattro motor derived from the Ducati 916, but detuned for the ST bike. After the release of the 996 Desmoquattro superbike in 1999, Ducati began to phase out production of the 916 engine, which was last used in the 2003 ST4.

In late 2001, the ST4s model with the 996 Desmoquattro engine was added to the ST range. The ST4s benefited from suspension enhancements, and became available in 2002.

In 2004, the ST2 and ST4 were deleted, both being replaced by the ST3, with the new three-valve Desmotre engine of 992 cc displacement. The Desmotre required less maintenance than the ST4s, which was notoriously expensive to maintain. Nevertheless, the ST4s remained as the top model in the range.

In 2006, the ST4s production was discontinued in favour of an ST3s variant which inherited the enhanced ST4s suspension components, but which kept the ST3 Desmotre motor unchanged. Reasons given for the ST4s’ demise were that the 996 engine would not pass the stringent Euro-3 emissions standard, whereas the ST3 motor could pass the emissions tests. In 2004 and 2005, Ducati added a wet clutch to many models, including the ST series in 2005. This reduced the force necessary to engage the clutch.

Both “S” bikes, the ST4s and ST3s, could be equipped with ABS from 2003. In 2004, an upgrade to the ST line brought a modified nose fairing and dashboard, adjustable ergonomics, a trip computer, four-way (hazard) flashers, digital speedometer, a dashboard-controlled adjustable headlight, a new seat design, Euro-2 catalytic exhaust, and a simplified CAN-bus wiring setup. Dry weight was reduced by 11 kg (24 lb) due to the lighter wiring harness and simpler support structure for the nose of the bike.

The ST series finished in 2008, the ST3 and ST3s models having only sold 1,011 total worldwide units in 2007. Following these poor sales, Ducati chose to refocus the brand onto sport bikes such as the Panigale and Diavel. In its factory plan for 2008–2010 Ducati declared that the sport touring market was no longer part of their plans.

The Ducati ST2 was made between 1997 and 2003. For touring, the ST2 had seating for two, and wind protection from its full fairing. As a sport touring motorcycle the ST2 has hard luggage, relaxed ergonomics compared with sport bikes, and powerful engine in comparison to typical touring motorcycles.The frame is a tubular trellis frame, similar to the 916 frame in torsional rigidity and lightness.The Ducati ST2 has an engine derived from the 907 Ducati Paso, which had Ducati’s signature 90° V-twin (or L-twin), SOHC, 2-valve desmodromic heads, Remus exhausts, 10.2:1 compression and Webber-Marelli electronic fuel injection with one injector and one spark plug per cylinder stainless steel thermos bottle. It has an increased bore to 944cc and a heavier flywheel appropriate to its touring function.The suspension in fully adjustable for the early production series, featuring upside-down Showa forks at the front and Sachs rear shock, the same as the Ducati 916 Superbike but with softer damper ratings. The ST2 front brakes have twin 320mm floating Brembo discs and four-piston calipers, while the rear uses Brembo single 245mm disc with a twin-piston caliper. The OE tyres are Michelin Macadam or Metzeler’s MEZ4 sport radials in 120/70 ZR17 for the front and 170/60 ZR17 for the rear, on Brembo rims.

The Ducati ST4 was manufactured between 1999 and 2005, and used a detuned version of the Ducati 916 engine hydration bag for running.

The Ducati ST4s was based on the ST4, but had improved suspension and a larger engine, using the liquid-cooled Desmoquattro (four desmodromic valves) 90° V-twin engine based on the Ducati 996. The Desmoquattro was largely unchanged from the 996 Superbike, and retained a valve angle of 40°, but lost 4 mm of diameter on each of the intake throttle bodies (down from 54 mm on the 996). Chassis clearances also required the use of the more compact cylinder head design that was shared with the 748 Superbike and Monster S4. In spite of the smaller intakes, the ST4s actually made slightly more power and torque than the 996 Superbike due to the mostly straight-through design of the ST’s exhaust, compared to the under-seat exhaust of the Superbike.

The Ducati ST3 was made between 2004 and 2007 and used the DesmoTre engine, an SOHC three-valve motor which was able to meet anti-pollution emissions limits. it was succeeded by the ST3s, which gained the superior suspension components of the ST4s.

A reviewer said of the ST3: “The three-valve (two intake and one exhaust) desmotre engine has a much smoother powerband than the desmoquattro mill of the ST4 and simply runs better. The ST3 engine is user-friendly, offering ample power in any situation. We easily prefer this engine over the ST4, even if it doesn’t have quite the top-end hit.”

In The Daily Telegraph, Kevin Ash called the ST3s a “sports bike for all seasons”, adding “Ducati has added anti-lock brakes to the ST3 sports tourer, and the result is impressive”.



Chassis and suspension

Wheels, brakes and tyres