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Reactions of Alkenes

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Reactions of Alkenes Alkenes generally react in an addition mechanism (addition – two new species add to a molecule and none leave) R X Y X Y H R R R H Have already observed using a H+ electrophile (HBr or H+/H2O) that a carbocation intermediate is generated which directs the regiochemistry Whenever a free carbocation intermediate is generated there will not be a stereopreference due to the nucleophile being able to react on either lobe of the carbocation (already observed this with SN1 and E1 reactions) Br Br H+ H H3C Br CH2CH3 Obtain racemic mixture of this regioisomer Reactions of Alkenes There are three questions to ask for any addition reaction R X Y X Y H R R R H 1) What is being added? (what is the electrophile?) 2) What is the regiochemistry? (do the reagents add with the X group to the left or right?) 3) What is the stereochemistry? (do both the X and Y groups add to the same side of the double bond or opposite sides?) All of these questions can be answered if the intermediate structure is known Reactions of Alkenes Dihalogen compounds can also react as electrophiles in reactions with alkenes Possible partial bond structures + !+ Br Br ! Br Br Br !+ or Br !+ More stable partial positive charge Experimentally it is known, however, that rearrangements do nor occur with Br2 addition -therefore free carbocations must not be present The large size and polarizability of the halogen can stabilize the unstable carbocation With an unsymmetrical alkene, however, both bonds to the bromine need not be equivalent Called a “Bromonium” ion -this structure will direct further reactions Br Br Br Does not rearrange, therefore this carbocation must not be present Dihalogen Addition The bromonium ion thus forms a partial bond to the carbon that can best stabilize a positive charge which will then react with the bromide nucleophile !+ Br Br Br Br Br !+ Br Due to the 3-centered intermediate, dihalogen additions occur with an anti addition H Br H Br H3C H3C CH Br Br H C 3 CH3 H 3 Br H Br CH3 Obtained product Not obtained Formation of Halohydrins When water is present when a dihalogen is added to a double bond, then water can react as the nucleophile with the halonium (e.g. bromonium) ion !+ Br Br OH Br !+ Br Br H2O Favored product While water is a weaker nucleophile than bromide, because it is the solvent there is a much greater concentration present The halonium ion thus directs both the regiochemistry (oxygen adds to the carbon that can best stabilize the partial positive charge) and the stereochemistry (due to the three membered ring the oxygen must add anti to the the bromine already present) + ! CH3 Br + H ! Br CH Br Br H2O OH 3 H CH3 D D H D The halohydrin is named according to which halogen is present (chlorohydrin, bromohydrin, iodohydrin) Halogenation of Alkynes Dihalogen can be added to alkynes in addition to alkenes The reaction is similar to alkenes with the main difference being the presence of two π bonds thus allowing reaction to occur twice for a total of 4 halogens adding to the compound H C Br Br Br Br Br 3 Br Br H3C CH3 CH3 H3C Br CH3 Br Br With one addition, obtain Second addition is favored, trans vicinal dihalogen hard to stop at alkene stage as alkene is more reactive than alkyne Due to difference in reactivity, it is possible to selectively add to an alkene in the presence of an alkyne Br Br Br 1 equiv. Br Oxymercuration An alkene can also be hydrated using mercury salts (called oxymercuration) Mercury diacetate [Hg(OAc)2] is a common reagent which loses one acetate to generate an electrophilic source of mercury H CH3 !+ O O O AcO Hg H !+ O O O H CH3 Hg Hg H The electrophilic mercury reacts with an alkene to form a mercurinium ion which is similar to bromonium ions in that a three membered ring is formed with a partial bond to the carbon that can best handle the partial positive charge Water can then react (which is typically the solvent for these reactions) in an anti addition !+ AcO CH3 Hg !+ H O NaBH OH 2 AcOHg 4 H CH3 OH H H H The mercury can subsequently be removed with sodium borohydride to form the alcohol Routes to Hydrate an Alkene Different routes have been seen to hydrate an alkene, each route though offers different advantages and often an entirely different product CH 3 Markovnikov product CH3 H+/H2O CH3 H3C Generate free carbocation that H3C CH3 HO CH3 rearranges to more stable 3˚ cation 1) BH3•THF CH3 HO CH3 2) H2O2, NaOH Anti-Markovnikov H3C CH3 H3C CH3 OH 1) Hg(OAc) , H O CH 2 2 CH Markovnikov product 3 2) NaBH 3 4 Do not generate free carbocation H3C CH3 H3C CH3 therefore no rearrangements occur Epoxidation To form an epoxide from an alkene, need to generate an electrophilic source of oxygen Previously we have observed oxygen acting as a nucleophile and reacting with carbocation sites A peroxy acid (or peracid) is a source of electrophilic oxygen - - ! O ! O !+ !- O H3C !+OH H3C !+O H !- Acetic acid Peracetic acid (called peracid or peroxy acid) Due to the high electronegativity for oxygen, typically the oxygen atoms in an organic compound have a partial negative charge (therefore nucleophilic) In a peracid, however, the terminal oxygen is already adjacent to an oxygen with a partial negative charge The terminal oxygen thus has a partial positive charge and thus is electrophilic Epoxidation When an alkene reacts with a peracid, an electrophilic reaction occurs where the π bond reacts with the electrophilic oxygen O CH O CH 3 H 3 H O O O O CH3 CH3 CH3 CH3 The reaction forms an epoxide (oxirane) with a carboxylic acid leaving group Due to the cyclic transition state for this reaction, the two new bonds to oxygen form SYN O RCO H CH3 3 CH3 CH3 CH3 O CH RCO3H 3 H3C CH3 CH3 Epoxides Selectivity in Epoxide Formation When synthesizing an epoxide from an alkene with peracid the peracid is acting as a source of an electron deficient oxygen, therefore the most electron rich double bond will react preferentially O RCO3H 1 equivalent More alkyl substituents, If more equivalents are added, therefore more electron rich the remaining double bonds double bond can still react Reaction of Epoxides Unlike straight chain ethers, epoxides react readily with good nucleophiles Reason is release of ring strain in 3-membered ring (even with poor alkoxide leaving group) O O CH3O O Same reaction would never occur with straight chain ether CH3O O No reaction Reaction of Epoxides Most GOOD nucleophiles will react through a basic mechanism where the nucleophile reacts in a SN2 reaction at the least hindered carbon of the epoxide OH O All products CH3MgBr CH3 H3C H3C after work-up Grignard reagents are a source of nucleophilic carbon based anions “R-” OH O NH3 NH2 H3C H3C Neutral amines also are good nucleophiles OH O LiAlH 4 H H C H C 3 "LAH" 3 "H-" Lithium aluminum hydride is a source of “H-” which also reacts in a SN2 type reaction Reaction of Epoxides Epoxides will also react under acidic conditions The oxygen is first protonated which then allows the positive charge to be placed selectively on the carbon that is most stable with a partial positive charge similar to bromonium or mercurinium ions H !+ OH O H+ O !+ O H H2O H C H C H C OH 3 3 3 H3C Vicinal diol (glycol) Can use weaker nucleophiles in this manner since we have a better leaving group Common examples of nucleophiles include water or alcohols Reaction of Epoxides Differences in Regiochemistry The base catalyzed opening of epoxides goes through a common SN2 mechanism, therefore the nucleophile attacks the least hindered carbon of the epoxide O O CH3MgBr In the acid catalyzed opening of epoxides, the reaction first protonates the oxygen This protonated oxygen can equilibrate to an open form that places more partial positive charge on more substituted carbon, therefore the more substituted carbon is the preferred reaction site for the nucleophile H O H+ O HO CH3OH OCH3 Reaction of Epoxides Grignard and Organolithium compounds are good nucleophiles which can react with an epoxide in a basic mechanism OH O CH3MgBr CH3 H3C H3C These reagents can sometimes cause problems due to their very strong base strength -side reactions can occur and also they are very reactive and thus not selective (they will react with any carbonyl present in the compound for example) To overcome these drawbacks organocuprates can also deliver an R- source as a nucleophile They will not react, however, with carbonyl compounds CH3Li CuCN OH O (CH3)2Cu(CN)Li2 CH3 H3C H3C Asymmetric Epoxidation Epoxides are thus a very versatile functional group that can react with a variety of nucleophiles to allow synthesis of a wide selection of products When an achiral alkene and an achiral peracid react, however, the epoxide formed would not be chiral Many targeted compounds are chiral and their chirality is critical for the properties A tremendous advantage was obtained when a simple and convenient method was developed to synthesize chiral epoxides Sharpless epoxidation OH Ti[OCH(CH3)2]4 O EtO2C (CH3)3CO3H R OH CO2Et R OH OH Glycol Formation We have observed glycols (vicinal diols) being formed by reacting epoxides with either basic or acidic water OH O NaOH OH H3C H3C This reaction generates an ANTI glycol OH RCO3H NaOH O OH Would need another method to generate a SYN glycol Glycol Formation There are two common reagents for SYN dihydroxy addition to alkenes Both involve transition metals that deliver both oxygens from the same face CH3 O O H3C O O H2O2 HO OH Os Os O O O O or H3C H3C Na2SO3 H2O CH3 H3C O O O O H O HO OH Mn Mn 2 O O H3C O O H3C NaOH Contrast this stereochemistry with glycols formed by reacting
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