16 - WITTING REACTION AND REAGENTS Flashcards

1
Q

why are witting reactions used?

A

make an alkene from an aldehyde or a ketone and control the stereochemistry (E or Z)

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2
Q

what is an ylide?

A

a species with opposite formal charges on adjacent atoms

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3
Q

what drives wittig reactions forward?

A

the highly stable P=O bond formed as a side product drives the reaction forward

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4
Q

how were alkenes formed before wittig?

A
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5
Q

how are simple ylides prepared?

A
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6
Q

what is the ylide form and what is the phosphorane form?

A
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7
Q

what are the characteristics of non stabilized vs stabilized ylides?

A

non stabilized ylides:
R=simple alkyl
reacts with aldehydes and ketones
good nucleophile

stabilized ylides:
R=aryl, alkenyl, CO2R, or anything that stabilizes the anion (EWG)
react with aldehydes only
not reactive enough for ketones
lower pKa

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8
Q

what are the characteristics of phosphonate stabilized anions

A

react with aldehydes and ketones
W= CO2- CO2R, CN, aryl, vinyl, SO2R, SR, OR, NR2
phosphonate stabilized carbanions are more nucleophilic and more basic than the corresponding stabilized phosphonium ylides

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9
Q

why are aldehydes more reactive than ketones?

A
  1. sterics: ketone has more alkyl groups, gives hindrance
  2. inductive effect: delta + on C is greater in aldehydes which makes them more reactive to nucleophiles
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10
Q

what is the mechanism of an ylide reaction?

A

the arrows go to make the P=O bond

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11
Q

how is wittig a thermal concerted [2+2] cycloaddition that allows to form?

A

the ylide and carbonyl approach approach each other perpendicularly
the large substituents are kept apart, and the P-C bond is long enough so that the ring can form
when we break the ring, we make a Z alkene

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12
Q

what happens if you use the sulfur ylide and how does the S=O bond compare to the P=O bond?

A

if you use a sulfur ylide you get an epoxide, if you use a phosphorus ylide you get the alkene
the P=O bond is is stronger than the S=O bond
the driving force is weaker in the sulfur analogue

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13
Q

what stereochemistry does a non stabilized ylide give?

A

Z stereochemistry

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14
Q

if there is a ketone and an aldehyde on the same molecule, how can you select for one?

A

get one equivalent only and put the ylide into the reactant not the opposite

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15
Q

why is the puckered ring conformation preferred for non stabilized ylides

A

minimizes angle strain and ecliping bond interactions that occur if the ring was planar
this transition state will lead to the Z alkene

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16
Q

why is the E alkene not favored for non stabilized ylides?

A

more steric interaction, when it’s puckered
when it’s planar, less sterics but more angular strain

17
Q

what do stabilized ylides give?

A

more E than Z

18
Q

what do CisTS and transTS yield?

A
19
Q

what is (Z)-tricos-9-ene?

A

commonly available pheromone for control of the housefly in indoor areas

20
Q

what are the two ways you can make this pheromone with a wittig reaction?

A
21
Q

how is vitamin A industrially synthesized with a wittig reaction?

A
22
Q

why is a normal ylide not used for the synthesis of juvenile hormone?

A

gives a very poor yield

23
Q

what kind of ylide can be used for the synthesis of juvenile hormone?

A

Horner-Wadsworth-Emmons modification
phosphonate stabilized carbanion
more nucleophilic and more basic

24
Q

mechanism to make juvenile hormone

A

makes the E alkene so that there is the least amount of steric effects between the CO2Me and the largest group

25
Q

how does Horner-Wadsworth-Emmons work and what does it favor?

A

allows for E alkene on ketones
favours the E alkenes
the more equilibration amongst the intermediates, the higher the selectivity for the (E) alkene formation

26
Q

with Horner-Wadsworth-Emmons, what determines the ratio between E and Z alkenes?

A

depends on the stereochemical outcome of the initial carbanion addition and the ability of the intermediates to equilibrate

27
Q

how is the phosphonate reagent made for the synthesis of the juvenile hormone? (Arbusov reaction)

A
28
Q

summary table

A