23 - Acetals

23 - Acetals - Recap: Addition Organometallic Reagents to...

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Unformatted text preview: Recap: Addition Organometallic Reagents to the Carbonyl '4 ‘ocloelt $3.2 (om/S Guide to organometallic reactions ° note: organolithium and Grignard reagents are strong nucleophiles, C-C bond formation is irreversible , . Y IS a bad leavmg group, tetrahedral intermediate is stable H 69 no)?2 e l R1 Y M = Li or MgBr typ _ , g, , r r Y = H (starting material was aldehyde) WM 0 Fl2 CnH"1 (starting material was ketone) a , R1 Y RX“ R1 = c H tetrahedral " m intermediate ” o 1- “2‘” Ho R2 (e.g. R = CH3 or Ph or tYPe JL 2. H30* other alkyl, aryl, alkenyl Y is an okay leaving R1 R2 type I F!1 R2 or alkynyl groups) group, tetrahedral not (starting material was intermediate collapses to isolated ester. acid chloride. 6 g ‘D 'f. 8 regenerate carbonyl “ ' “hydride: em) I». ' ._ Y = CI OZCR ma ... J; 3 OH Recap: Nucleophilic Substitution at the Carbonyl Guide to addition/elimination reactions 0 note: HOR, HNR2 are relatively weak nucleophiles Nu = HOR, HNF:2 Y is a better f . leaving group (9%.. Nu 9° N“ than Nu o R1 v RXY —Y R‘JLNu 1 _ tetrahedral n -anm intermediate (e.g. R = CH3 or Ph or other alkyl, aryl, alkenyl reactions proceed down the hierarcy or alkynyl groups) of carboxylic acid derivatives . . . . o ‘ Must meet the followmg 3 conditions for reaction to JL most reaa've occur: R‘ Cl . o 0 ° Y must be a better leavmg group than Nu L /[L (otherwise the reverse reaction will dominate) 3‘ ° “2 0 ° Nu must be nucleophilic enough to attack JL RIC(O)Y R1 OF!2 0 . I a o R C(O)Y must be electrophilic enough to 1JLNH22 least reactive . R react With Nu Follow Up Slide: Why are Ketones More Electrophilic than Esters? Recall from Lecture l9: influences that effect the electrophilicity of carbonyl compounds - resonance donation ° inductive effects 7 TIM; l‘CllS d M mm“. Ma Q cul- vootgi’ \lAOL'NleA-Q LgfldT/ SMLQ he {AU/cw ales/Mn albums able m Qm/Lumsl “Jun MlceJ that; ell—8’ l S le 'lml‘llla. K* M) \Mfi om‘ir 04 wlobk +13 ed 6 c t3 as male deem“ 1S MIe daybrbe ll c . Recap: Acetal Formation Acetal formation: a reversible process 1 HOR3’H+ I10><0R3 HOR3’H'.’ 330 OBS a a n1 R2 ‘ R R2 ‘ R R2 hemiacetal acetal ° mechanism involves a series of proton transfers: protonate on oxygen to make better leavin rou .9," . 8 8, P "-9 4- Mno-u- r._ h 3' all steps are reversnble I“ ° only catalytic acid is required *3“ a x, { 7“. ° follow the principle of microscopic reversibility! It... ' ‘ 1°“? F0“ “'" 11k“ ‘—’e— "W ‘F-** ‘ =55: "' "‘ u. . . .7,“ 5m!- EKJUS 5m? 9 a “19‘. r- , . * u on. C...” o:qu L 41g: \uulq lilce & umpllcm‘leel INK—kaer M, cam . K3 , l \/ l L Mkltej larch ll alum Ml’o ox scnca cl wwk “rafter waging Lel‘weea KKCLeQPLulJZJ cwxol 66 ‘ a ‘MC You-V9 ‘l\‘~di ‘l’l’tlj 'lwvflkl lwl- $1 $2; alk You“ l? «l’ {ac/ls Sltp chL is. l’k-e meleoo'a‘le ouqi mule ES fie CleL/Ilflfihll€ ? The Principle of Microscopic Reversibility There is only one least-energy pathway between two inter-converting compounds (e.g. between two intermediates). Thus, the low energy pathway in the forward direction is the same as the low energy pathway in the reverse direction. / Me Is Owe! [mask WWW 5M4. ./\ f with curl-3* ° your mechanism from ketone to ketal should follow the reverse path of your mechanism from ketal to ketone Looking Deeper at Acetal Formation Hemiacetal formation occurs under acid or base catalysis, but acetal formation requires acid catalysis. H“ or B" o HORS HO 0R3 H033, H“ 330 033 ni’u‘nz " n n2 ‘ n n2 hemlacetal acetal formation formation occurs hemiacemi reguireg acid acetal GD) /_g p, 3 with acid or base catalysis ° catalysis /l.l\ ,2 I ’l’ln‘xs 0’le (3J5. m‘ ow.‘ “' _€ 50:: / add. 5‘3 -’ k ‘ ' a a r ' ' . me ~ _ a .a ' . u 0 , -ua. M u WV mi. await (:ljlqsis: :"‘3* > n-é'fi T M N ' . . ‘- ‘R I MED-N Oak“ ‘I—a fl ‘ r ‘ z ‘ ‘ mwmawl 7 eg Hu—vz we flux! WLW m louc waif: can}, 6mm ‘ Xfi‘m Wulm, .* 1. __5 ,1 3: —:D r- 73- sd 4: esk'l- m Relax loalc '- t t h e: *. m 3“ +3 (5 c dalilslx m * ‘ufl .— 0M9. Ax \{OMe mall: - in general: aldehydes form acetals faster than ketones “Jr 41% “as”: 0J3 W ouu‘mL QA—H‘SSU rho Mi .ProMeJV} Cyclic Acetals: Five and Six-Membered Ring Formation is Favorable i. f O OH O OCH H\n/\/\/OH ‘—l U HI}! CHSOH . 3 0‘1 ' , at- l said» "8“ “ ér.“ .g. a.» - high effective concentration of the nucleophile and electrophile since they are constrained to close proximity [R Or NME Molecule of the Day: Sucrose ‘33 o ,3 More commonly referred to as sugar 1 txn'l ° l50,000,000 tons of sucrose are produced annually ' most sugar comes from sugar cane or sugar beet, but it can come from oth r plant sources as well ° h olysis of sucrose give fructose and glucose (this is process that is used to make high fructose corn syrup) Acetals from Cyclic Diols: A Ketone Protecting Group Consider the following reaction: 0 1) M90, THF 0 Ph H --------- --> m. H Br cu3 In. it, iii a}? Ll. v fl fl’ 0/ m , r 0 ED L0H p ' MUM? H} C M Br dioxcl‘we ‘- lcel‘ml Ll H7910“ \9H H 6m 1 l PM?“ Wscex Ccowx who Cl‘IQN AS 'lnltrwbe tux LCM) ...
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This note was uploaded on 11/10/2011 for the course CH 1b taught by Professor Natelewis during the Winter '09 term at Caltech.

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23 - Acetals - Recap: Addition Organometallic Reagents to...

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