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01 Making Alcohols Leave Ans.Cdx Making Alcohols into Leaving Groups 1. The OH from an alcohol is not a good leaving group. Why? Good leaving groups are weak bases, but OH- is a strong base! One way of making it a good leaving group is to protonate it. This often creates problems with carbocations, rearrangements, etc. It can be used in limited cases: 2. Provide curved-arrow mechanisms for the following transformations. Why are the mechanisms different? HBrOH HBr Br OH Br HBr HBr OH2 :Br OH2 :Br The substrate can f orm a stabilized tertiary Here our substrate could not form a stabilized carbocation, so the only available pathway is carbocation, so SN1 is a viable pathway. It is too direct displacement by S 2. sterically hindered to undergo SN2 displacement. N The best ways of turning alcohols into leaving groups are to use SOCl2, PBr3, or to form a sulfonate ester (most often a "tosylate" or "triflate"): 3. Provide the intermediates and products in the following synthetic sequences. Watch stereochemistry! OH Cl CN SOCl2, pyridine KCN OH OTs N3 TsCl, pyridine NaN3 OH Br SH PBr3 NaSH Making Alcohols into Leaving Groups: Mechanisms 1. Provide a complete curved-arrow mechanism for each of the following transformations. Be sure to pay attention to stereochemistry! OH OTs TsCl, pyridine O O TsCl = Cl S CH3 =ClSO2R R S Cl O O O R S H N O O OH Br PBr3 Br P Br Br Br H P (stable leaving O Br O group after P rearrangement H Br to phosphonic Br :Br dibromide) (SN2 displacement inverts stereochem!) Making Alcohols into Leaving Groups: More Mechanisms 1. You learned in lecture that treatment of a primary or secondary alcohol with thionyl chloride (SOCl2)in pyridine as solvent provides access to alkyl chlorides, with accompanying inversion of stereochemistry. a. Provide a complete curved-arrow mechanism for this transformation. Clearly show the stereochemistry of all intermediates. SOCl2, pyridine OH Cl O O H S S Cl O Cl Cl O Cl H N S 2 N Cl inversion O Cl + O S O + Cl S O Cl b. When the same reaction is run using 1,4-dioxane (structure shown below) as the solvent instead, retention of stereochemistry is observed in the product. Provide a complete curved-arrow mechanism for this observation, clearly showing the stereochemistry of all intermediates. Be sure your mechanism accounts for the stereochemical result observed. SOCl2, pyridine OH Cl OO (1,4-dioxane) O O H S S Cl O Cl Cl O Cl H N O O H Cl SN2 O inversion O S O S O + O Cl O S 2 The participation of 1,4-dioxane in the N H inversion mechanism results in two SN2 inversions, leading to overall retention. Cl Making Alcohols into Leaving Groups: Even More Mechanisms 1. Treatment of primary alcohols with phosphoryl chloride (POCl3) in pyridine results in formation of alkyl chlorides, similarly to treatment with thionyl chloride. However, treatment of secondary alcohols more often leads to formation of alkenes instead, as shown in the example below. Provide a complete curved-arrow mechanism that accounts for this result. Cl P Cl OH O Cl (phosphoryl chloride) O O + P pyridine Cl Cl O O N H P P Cl OH Cl Cl O Cl Cl O P Cl N O Cl O O E2 H P + Cl Cl Making Alcohols into Leaving Groups: Challenging Mechanisms 1. You have learned that alkyl iodides may be prepared by SN2 replacement of other leaving groups, or by treatment of alcohols or ethers with concentrated hydriodic acid (HI). A milder method for preparation of alkyl iodides from alcohols is shown below: Ph Ph 1. I2 + Ph P I P Ph Ph 2. imidazole, MeO O Ph OH MeO a. Imidazole is an organic base that functions similarly to pyridine. Its structure is shown below; determine the most basic atom of imidazole, and explain your choice. N N H (imidazole) N H N HN H+ H+ N N N N H H H N H H no resonance! resonance with not basic! full octets! stabilized and basic! b. Taking into account your answer to part (a), provide a complete curved-arrow mechanism for the transformation shown at the top of the page. It may help to consider first the mechanism of bromination using PBr3. Ph Ph II P P I Ph Ph Ph Ph I Ph Ph P N Ph I H OH O Ph + MeO MeO P N Ph H Ph I MeO O Ph I + MeO P + Ph O Ph Ph P Ph Ph Making Alcohols into Leaving Groups: Stereochemistry Practice 1. The following three reactions are superficially similar, yet each has a very different stereochemical result. For each reaction, provide a complete-curved arrow mechanism that accounts for the stereochemical consequences observed in each case. (Hint: the three mechanisms must be different from each other!) OH I conc. HI (enantiomerically pure) HI I This is an SN2 OH OH SN2 inversion 2 reaction, so it takes place with inversion. I OH I I conc. HI + HI (racemic mixture) OH OH2 I This is an S 1 reaction, because I I SN1 N the benzylic carbocation is extensively stabilized by + resonance. As a result, we get a racemic mixture. I OH conc. HI S S (enantiomerically pure) HI OH I OH2 SN2 inversion S S S SN2 inversion This is a "double SN2" reaction with sulf ur serving as an internal nucleophile. Two inversions gives overall I retention of stereochemistry! S Making Alcohols into Leaving Groups: More Stereochemistry Practice Consider two proposals to achieve the same synthetic transformation: H H OH 1. TsCl, pyridine Proposal A: 2. KOtBu, tBuOH H H H H OH 1. SOCl2,pyridine Proposal B: 2. KOtBu, tBuOH H H Only one of these proposals gave the desired product. Which one worked? Explain your answer with clear three-dimensional drawings. Proposal A worked. Treatment of an alcohol with TsCl in pyridine gives retention of stereochemistry. H H OTs H OTs H H H H OtBu The leaving group is axial, so there is an H that is periplanar, as required f or the E2 reaction. Proposal B does not work. Treatment of an alcohol with SOCl2 in pyridine gives inversion of stereochemistry. H H Cl Cl H No E2 reaction H H H The leaving group is equatorial, so there are no H's that are periplanar. Ether Syntheses 1. Devise efficient syntheses for each of the following types of ether. Primary—Primary example: O Choose either SN2 route (typical Williamson synthesis): NaH NaH OR CH OH CH O Na +EtBr OH O Na +CH3I 3 3 Primary—Secondary example: O Use the secondary alkoxide as the nucleophile, and a primary halide in an SN2 process: NaH (other way around will only give E2 + as the major product!) OH O Na Br Secondary—Secondary: tough! Either way gives a strong probability of elimination! Show SN2 and SN1. example: O +H + +H OH OH (E1 will compete, but (a large excess of the alcohol must + O Na Br gives SN1 reliably as be used in order for this route to be major product; this useful -- draw the mechanism f or (poor choice -- more practice!) likely to give E2 pdt.) is only possible here because the ether is symmetric) Tertiary—anything Use a tertiary alcohol or an alkene, catalytic acid, and lots of the other alcohol, in an SN1-type route: example: O + +Hcat. OH (a large excess of the primary alcohol must + +Hcat. be used in order for this route to be useful, OH OH but you can certainly use it for synthesis; draw the mechanism for practice!) (a large excess of the primary alcohol must be used in order f or this route to be usef ul; this would not be accepted as the correct answer f or synthesis!) More Ether Syntheses 1. a) The following ethers can be synthesized by an SN2 reaction from two different combinations of an alkoxide and an alkyl halide. For each molecule, show the combinations of alkyl halide and alkoxide that could combinetoformbond1 and the combination that could combine to form bond 2, and circle the pair that will react to provide a higher yield of the desired ether (in other words, the correct synthesis). SN2 Starting Materials: SN2 Starting Materials: 1 2 Br ONa 1 O Me 2 NaOEt Br SN2 Starting Materials: SN2 Starting Materials: Me Me 1 Me 2 Br OK O Me CH3I NaOMe 1 2 b) Consider using an SN1 reaction to form the same ethers. For each molecule, show the combination of alkyl halide and alcohol that could combine to form bond 1 and the combination that will combine to form bond 2, and circle the pair that will react to provide a higher yield of the desired ether (in other words, the correct synthesis). SN1 Starting Materials: SN1 Starting Materials: 1 2 Br OH 1 O Me 2 OH Br SN1 Starting Materials: SN1 Starting Materials: Me Me 1 2 Me Br OH O Me CH3I CH3OH 1 2 Neighboring-Group Participation 1. Provide a curved-arrow mechanism for the following. Be sure to observe the stereochemistry of each transformation! CH3 1. TfCl, pyr S S OH 2. NaOCH3 OCH3 O CH3 F3C S Cl O CH3 O O CH S S 3 O CF S 3 S CH OTf 3 H episulf onium intermediate CH3O: N Br H Br H HBr CH3 CH3 H C H C 3 H OH 3 H Br HBr Br H Br CH3 H C H C H 3 H OH2 3 H CH3 epibromonium :Br intermediate: analogous to episulf onium More Neighboring-Group Participation 1.
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