The 6-membered geometry for transferring the R2 ligand from the metal to the C=O is far less strained.

Transcription

1 549 M Williams Addition eactions to arbonyl Groups 1 The uncatalyzed addition of to 2 = has been carefully studied theoretically T 1 It was found that the entropic liability of employing a second moleclule to assist in the addition process through the formation of bridging -bonds was more than offset by the release of strain in the resulting transition structure 2 T 2 The resulting transiton structure T 2 was determined to be ~40 kcal/mol more stable than transition structure T 1 2 Grignard additions 2 Mg Br 2 MgBr 3 fast MgBr The molecularity and transition structure for this reaction have not been carefully elucidated The fact that the Grignard reagent is not a single species in solution greatly complicates the kinetic analysis a Monometallic mechanism: Mg Br slow 2 fast Mg Br Mg 2 2 Br solvent () Mg 2 Br MgBr b The bimetallic (binuclear) mechanism is more probable: Br Br 2 2 fast Mg Mg Mg Mg Br Br 2 break bridge Br 2 Mg Mg Br 2 slow 2 Br Mg Br Mg 2 2 solvent () 2 MgBr Mg 2 The 6-membered geometry for transferring the 2 ligand from the metal to the = is far less strained *Drawings modified from renderings of DA Evans, arvard University

2 arbonyl Addition eactions: General onsiderations : teric effects: As increases in size, = reactivity decreases Inductive effects: As increases in ability to withdraw electrons, = reactivity increases (The p* LUM elergy is lowered) tereoelectronic effects: The M-LUM interaction dictates the following reaction geometry: p* d ~107 The Dunitz-Bürgi Angle d LUM p Models for predicitng the facial selectivity of additions to carbonyl groups: The first model (empirical) for predicting the stereochemical outcome for the addition of nucleophiles to a- substituted ketones and aldehydes was proposed by D J ram (ULA, 1956): L = large ligand (sterically dominant); M = medium ligand; = small ligand : = Grignard and ydride reagents : L L L M M M "ram Product" "Anti-ram Product" major diastereomer minor diastereomer : L M The ram model has been replaced in recent years with a succession of more refined models The widely used and generally accepted Felkin-Anh Model is described below The Felkin-Anh Model: M : Felkin L M L M favored L M : Anti-Felkin : L M : L Destabilizing interaction M disfavored The rules governing the Felkin-Anh addition model: 1 The preferred transition state orients the sterically dominant group, L, anti- to incoming : and perpendicular to the sigma plane of the = group to minimize nonbonding interactions and transition state torsional effects (the eclipsing of substituents during rehybridization of the carbonyl carbon) 2 The preferentially attacks the = along the Dunitz-Burgi path via that conformation which minimizes nonbonding interactions with those substituents at the adjacent carbon (M or )

3 3 The destabilizing interaction in the disfavored transition state is the steric interaction between the : and the M substituent B 3 atio 22(Felkin): 1(anti-Felkin) To synthesize the other diastereomer, methyl Grignard addition to the corresponding aldehyde is predicted to provide, via a Felkin-Anh transition state, the anti-felkin isomer from the above hydride addition reaction to the ketone: MgBr predicted major diastereomer arbonyl Addition eactions onsider the general class of carbonyl compounds undergoing the net addition of the species El- across the carbonyl p-system: General eaction to onsider 1 2 El 1 2 For the case where El= : chanistically, there are two basic pathways by which this addition reaction can occur: (A) by initial protonation of the carbonyl oxygen atom followed by addition of the nucleophile to the carbonyl carbon; and (B) by initial addition of the nucleophile followed by protonation of the carbonyl oxygen atom chanistic ptions for case where El = A B ( ) Path A: Electrophilic Initiation: Path B: cleophilic Initiation: Both pathways can be catalyzed by Bronsted acids and bases: Acid atalysis: ( ) X El Both mechanistic options observed atalysis by Bronsted Acids and Bases Base atalysis: : better electrophile better nucleophile

4 Bronsted acid catalysis: A 2 Bronsted base catalysis: : B onsider the simple case of hydration of an aldehyde to geminal-diol: catalyst DG ~ 0 kcal/mol Keq = 1 for Proton Activation pka = -174 pka = -6 This species is ca 10 5 ~10 6 times as reactive as its unprotonated counterpart ' these oxocarbenium ions are similarly reactive intermediates in numerous reactions of aldehydes and ketones teric Effects For the standard battery of nucleophilic carbonyl addition reactions, the now familiar trends for steric effects may now be applied to predict the hierarchy of chemical reactivity For example: more reactive than (>) more reactive than (>) = arbon ligand For ketones, the accumulated steric effects of the two carbon ligands,, will also dictate the reactivity hierarchy For example: (>) (>) (>) Electronic Effects Electronic, or inductive effects also operate to modulate the reactivity of the = functional group For example: (<) (<) (<) l 2 l 2 l 3

5 Electron withdrawing groups will increase carbonyl reactivity as in the trend illustrated above This is due to removing electron density away from an already electron-deficient functional group if 2 is or contains an electron-withdrawing group Electronic Effects of the econd Kind Lets now consider the more generalized functional group, =X, where X may be either an oxygen or nitrogen derivative, either charged or uncharged ee if you can rationalize the reactivity hierarchy illustrated below: 2 (>) N (>) (>) N tereoelectronic onsiderations p* LUM is p* ; M Provided by : p* LUM Dunitz-Burgi trajectory M d ~107 p d 1 Increased steric effects will retard the reaction 2 ine principle of least motion: the nucleophile approaches the carbonyl from a trajectory (~107 o ) that resembles the product geometry 3 arbonyl protonation (or Lewis acid complexation) lowers the LUM For Example: p* add p* p p Acetals & Ketals Acetals and ketals are common protecting groups in synthetic organic chemistry Aldehydes and ketones undergo both acid- and base-catalyzed additions of oxygen nucleophiles including water and alcohols The addition of water to an aldehyde or ketone is a net hydration reaction that

6 is reversible Addition of water to an aldehyde gives an aldehyde hydrate (also called a geminal diol) and that to a ketone gives a ketone hydrate The hydrates are generally unstable readily reverting back to the carbonyl derivative It is usually not possible to isolate hydrates of aldehydes and ketones since they are unstable to dehydration (an entropically favored process) back to the carbonyl derivative ydrates are formed in basic, neutral and acidic aqueous solutions and the relative proportions of the carbonyl form relative to the hydrated form will be a specific function of the individual substrate molecule and conditions employed The hydration of aldehydes and ketones has been a very powerful tool in studying the mechanisms of various chemical and enzymatic reactions since, the reversible hydration reaction makes it possible to exchange the abundant isotope of oxygen ( 16 ) for the stable, heavier atom isotope ( 18 ) from the solvent water The mechanism is shown below Acidic onditions 2, 2 : 2 : Aldehyde ydrate Basic onditions: 2, _ 2 - Aldehyde ydrate chanism for hydration of an aldehyde When alcohols are used instead of water, aldehydes and ketones form hemi-acetals and hemi-ketals, respectively Again, the mechanism is the same as that shown above for hydration and is also fully reversible Acyclic hemi-acetals and hemi-ketals are also generally unstable to isolation Acidic onditions, emi-acetal Basic onditions:, _ - emi-acetal chanism for the nucleophilic addition of alcohols to an aldehyde yclic hemi-acetals and cyclic hemi-ketals are, on the other, usually quite stable and, if five- or sixmembered rings are involved, the cyclic hemi-acetal (or ketal) is often the favored form at equilibrium Examples of cyclic hemi-acetals and cyclic hemi-ketals abound in carbohydrate chemistry Three examples are

7 shown below Glucose and ribose exist in the cyclic hemi-acetal form and fructose exists in the cyclic hemiketal form Glucose Fructose ibose arbohydrates (glucose, ribose and fructose) are cyclic hemi-acetals and hemi-ketals Under acidic conditions and with the use of an appropriate dehydrating reagent, it is possible to convert aldehydes and ketones into acetals and ketals, respectively Acetals and ketals, unlike hemi-acetals and hemi-ketals, are stable compounds that can be isolated and used in a variety of ways ne important application of acetals and ketals is their use as protecting groups for aldehydes and ketones The general mechanism for acetal formation is shown below Both acetals and ketals are formed under acidic / dehydrating conditions and their formation proceeds through the intermediate hemi-acetals and hemi-ketals, respectively Formation of Acetals / Ketals, emi-acetal - 2 ENANE-TABILIZED XNIUM IN AETAL General mechanism for acetal (or ketal) formation Acetals and ketals are stable under basic conditions and are unstable under Lewis acidic or Brønsted acidic conditions Thus, the formation of acetals and ketals is performed with an acid catalyst in the presence of an alcohol (usually in large excess) with a dehydrating reagent n the other hand, acetals and ketals can be hydrolyzed back to the starting carbonyl derivative by treatment with aqueous acid The mechanism for hydrolytic cleavage is essentially the reverse of the mechanism for formation In both cases, formation and cleavage, the equilibrium is driven in the direction of product through mass action: the solvent for formation of acetals and ketals is generally the alcohol and the solvent for hydrolysis is water In the formation mechanism, the molecule of water lost from the hemi-acetal / hemi-ketal intermediate is diluted into a large molar volume of the solvent alcohol In the hydrolysis mechanism, the two molecules of alcohol are diluted into a large molar volume of the solvent water Acetals and ketals are stable to many reducing agents, basic nucleophiles, bases, oxidizing agents and organometallic reagents We shall examine their use in synthesis as a protecting group shortly

8 ydrolysis of Acetals / Ketals AETAL 2, 2 : - ENANE-TABILIZED XNIUM IN - - Acetals and Ketals in Natural Products frontalin aggregating pheromone outhern Pine Beetle chanism for the acidic hydrolysis of acetals (or ketals) multistriatin aggregating pheromone European Elm Bark Beetle xygens ( l) participating in ketalization zaragozic acid 2 l l Ac Ac Ph Acetals and Ketals in Natural Products Ph 2 Ac 2 N N alborixin Et Ph N 3 3 palytoxin acetal 3 3 hemi-ketal glycosidic linkages: acetals & ketals cellobiose (b-1,4-linked disaccharide of glucose) N _ P P _ N P uridine triphosphate (UTP) NA & DNA nucleotides hemi-amino acetal

9 2 2 2 monensin narasin salinomycin Br aplysiatoxin model study for monensin spiroketal formation: ketals in natural products Bn 2, Pd-, Ac kinetic control ~1:1 Ac N N halichondrin B N rifamycin calcimycin A 2 l 2, rt N 2 thermodynamic control ~20:1 1 2 no anomeric effect stabilization LP 1 2 næ s* overlap: the anomeric effect s* (-) 2 Bn 2 Et 1 2, Pd-, /Ac 2 A, 2, 2 l 2 3 Na- 2 Et monensin Kishi, et al, JAmhemoc, 1979, 101, 262

10 halichondrin synthesis: 2 spiro ketals 1 tricyclic ketal TB TB TB TB TB 1 TBAF 2 p-ts-py TB TB TB TB TB 1 Dess-Martin periodinane 2 then, with the vinyl iodide: Nil 2, rl 2, DMF 3 Dess-Martin periodinane 4 TBAF, DMF 5 DDQ 6 A, 2 l 2 MPM I halichondrin B Kishi, et al, JAmhemoc, 1992, 114, 3162