Chem 230-001 Lecture 23 Page- 1 - ydroboration/ oxidation - acemic trans-2-methylcyclohexanol may be neatly synthesized from 1-methyl- 1-cyclohexene by using a reagent called borane, 3. 0.33 equivs. 3, (C 2 C 3 ) 2 2 2 2 - () 3 Et 2 - Let s discuss the details of this reaction. - Take heart! There is nothing new to remember. You can predict the products of the borane reagent with any alkene based on what you already know! - 3, orane exists as 2 6, diborane, which is a gas that readily dissolves in ether 2 6 (C 2 C 3 ) 2 3 -(C 2 C 3 ) 2 - If you draw the Lewis structure you have to put a negative formal charge on and a positive formal charge on. - in ether the equilibrium below exists - - thus releasing a little bit of borane at a time in solution - borane is a powerful Lewis acid, borane is trigonal - ***why? - It reacts with the alkene in the following manner. Why? C 3 2 C 3 transition state - The first usage of the first - bond in borane generates another Lewis acid which can react with two more alkenes.
Chem 230-001 Lecture 23 Page- 2 - - The addition is anti-markovnikov. Why? - It is anti-markovnikov for the same reason that the addition of X to the double bond is Markovnikov! - The addition reaction is syn. o Syn is the opposite of anti. o This means that the and the go on the same side of the double bond. o Syn addition has stereochemical consequences. Details of the oxidation reaction Et 2 - - - The last intermediate hydrolyzes. Instructor will show you how in class. Asymmetric synthesis of trans-2-methylcyclohexanol -. C. rown discovered the following reagent α-pinene 2 α-pinene is chiral and optically pure. - It reacts with 1-methyl-1-hexene, followed by oxidation to make chiral and optically pure 2-methylcyclohexanol (one enantiomer). ***Why? Cationic polymerization - Poly means many. - Mer means body. - Polymers are long chains. o We discussed these in when we talked about long-chain alkanes
Chem 230-001 Lecture 23 Page- 3 - C 2 - Think about the feasibility of forming long chains by the following cationic reaction, ***what conditions would you need? Would you need a high or a low concentration of styrene? What would the product be? F 3 C S (cat) Cl 2? r 2 / C 3 C 2 r C 3 Ktu tu C 3 The better nucleophile attacks the most substituted carbon of the epibromonium ion. - do you know why the r(-) was eliminated instead of the (-)C 2 C 3 group? After all both are nucleophiles and both could be leaving groups. r - Can you come up with a reason for the selectivity of the last substitution reaction? Mercuration - This reaction is never run on a large scale. - This reaction is not environmentally friendly. - I am presenting this reaction primarily for historical reasons. You need to know the organic cannon to take the MCAT and other pre-professional entrance examinations. 1) g(ac) 2, C 3 C 2 2) Na 4, Et
Chem 230-001 Lecture 23 Page- 4 - - You may equate Na 4 to the borane reagents we discussed previously. This would be a mistake. They are very different. - The borane reagents 3(-)2() are Lewis acids. - Na 4 is a base. - It is usually used in alcoholic solvent to perform reduction - It reacts with the alcohol in following manner g(ac) 2 2 o 4 (-) C2C3 2 3 -C 2 C 3 o This goes to a point but does not consume all the 4 (-) reagent. There are still reactive - sites to perform reduction. g 2 2 g g(0) Na 4 Et Net hydration across the double bond. egiochemistry is Markovikov. g Addition of other reagents across the double bond. -- --- A C D E F A: ydrogen oxide, AKA water, : hydrogen peroxide C: a carboxylic acid, D: a peroxyacid E: Trifluoroacetic acid, F: peroxytrifluoroacetic acid, trifluoroperacetic acid - Like the r-r bond the - bond is weak also. o r (-) is a good leaving group.
Chem 230-001 Lecture 23 Page- 5 - - The reagents above have the tendency to react in the following manner - Which one of the reagents shown above would you suspect would be more likely to undergo heterolytic bond cleavage? F F F F F F r r r r - ecause () is so high in energy these dissociations do not occur. - ut you can think about the these reagents in terms of their reactivity with the C-C double in these terms for example: r r r r epoxide - The double bonds in the phenyl ring are exceptionally stable. More on this later. - The product epoxide is strained and hence high in energy, but the weak - bond in the starting material provide enough of an energetic advantage to make the three membered ring. - There is not an open cation on the reaction path - We don t detect products from the rearrangement of the putative naked cation.
Chem 230-001 Lecture 23 Page- 6 - C 2 leic acid 20 C C 2 C 2 no free carbocation C 2 trans-product is not observed Asymmetric addition of ::: - Sharpless epoxidation TiiPr 4 diethyltartrate tu C 2 Cl 2 one enantiomer - need an allylic alcohol - C 2 =C C 2 - - The reagent that accomplishes the epoxidation is optically pure and chiral - arry Sharpless recently was awarded the Nobel Prize for the development of this chemistry and chemistry similar to it. - http://www.nobel.se/chemistry/laureates/2001/public.html