OChem Chapter 13 and 10

Epoxides, Esters, Thiols, Sulfides, and Radicals

Alkoxymercuration Demercuration

Does the same as oxymercuration demercuration, but adds an H and RO rather than an H and OH.
Reagents: 1. Hg(AcO)2, ROH; 2. NaBH4 (rather than H2O in the first step)

Williamson Ether synthesis

used to generate asymmetric ethers
Reagents: 1. NaH; 2. RX
adds the R group in RX to a R-OH
uses an Sn2 mechanism, so you must consider the sterics of the reagents

aryl

an aromatic R group

vinyl

an alkene R group

Acid-Promoted cleavage

Uses substitution reactions to turn R-O-R’ into R-X and R’-X.
Reagents: H-X (strong acids)

does not occur for aryl or vinyl groups
Mostly uses Sn2, unless it is a tertiary R group

Sharpless Catalyst

DET; a chiral catalyst, makes it so that you can choose which epoxide enantiomer you want

(+)-DET forms the epoxide above the plane
(-)-DET forms the epoxide below the plane

Ring Opening Reactions

Epoxides are very unstable, so it is easy to open them. Anything that can act as a Nu can open an epoxides. Mostly strong Nu’s, but weak ones can do it too (with acidic conditions)

Add in an anti fashion

Sn2, so the Nu attacks the least substituted carbon, unless there is a tertiary carbon (only applies for acidic conditions), in which case it attacks there (3>1>2)

Reagents: 1. Nu; 2. H2O
(ex: grignard reagents, hydride reagents, alcohols, alkoxides)

Hydrosulfide Ion

-SH-

a strong Nu and weak base; favors Sn2 over E2

Thiol Oxidation

Joins two molecules with HS- groups using basic conditions and Br2 (makes a disulfide/ R-S-S-R)

reagents: NaOH/H2O, Br2

Can also make a sulfoxide or a sulfone from a sulfide
To make a sulfoxide: NaIO4 (S=O)
To make a sulfone: 2H2O2 (O=S=O)

Thiol Reduction

Turns a disulfide into two separate molecules with thiol groups (R-SH)

Reagents: Zn, HCl

Radical Arrow Pushing Patterns (6):

1. Homolytic Cleavage

2. Addition to a Pi Bond

3. Hydrogen Abstraction

4. Halogen Abstraction

5. Elimination

6. Coupling

All of these except for the first and last use three fish-hook arrows for arrow pushing mechanism

Steps of radical reactions:

Initiation, Propagation, Termination

Radical initiators

Possess a weak bond the cleaves homolytically with heat or light

dihalides, alkyl peroxides, and acyl peroxides

Radical inhibitors

react with radicals to prevent a chain reaction (ex: O2, Hydroquinone)
Good radical inhibitors are compounds with stable radicals or can easily form stable radicals

Halogenation (using radicals)

Done using Cl2 or Br2 and hv or heat
bromination is much more selective than chlorination: affects regioselectivity, chlorination will form all possible while bromination will only form the most stable

If there is an alkene in the substrate, this reaction would favor adding 2X’s across the double bond, so you can use NBS and HBr to keep the concentration of Br low and make sure it does not do the double replacement across the pi bond. For this, major/minor is only based on the stability of the final resulting pi bond