CHEM 203

Topics Discussed on Oct. 9

"Cycloaddition:" an addition reaction that forms a new ring, such as the formation of a molozonide from ozone + an alkene (or the formation of a halonium ion from a halogen molecule + an alkene)

Ozone as a "1,3-dipole:"

O O O O O O 1,3-dipole!

The addition of O3 to an alkene as a concerted "1,3-dipolar cycloaddition:"

O O O O O O 1,3-dipolar cycloaddtion R1 R3 1 C C R R3 R2 R4 C C R2 R4 molozonide

Drawing the above mechanism using the more accurate resonance form of O3:

O O O O O O R1 R3 R1 R3 C C C C R2 R4 R2 R4 molozonide

Pericyclic reaction: one that can be described as a "circular" movement of electrons

The above 1,3-dipolar cycloaddition as a syn addition reactions: new bonds form from the same face of the π system"

The breakup of the molozonide as an inverse 1,3-dipolar cycloaddition; i.e., a cycloreversion:

O O O O O O C + 1 3 R1 R2 R C C R C R3 R4 R2 R4

Concerted nature (multiple bonds broken/formed simultaneously) of the above cycloaddition and cycloreversion reactions Lecture of Oct 9 p. 2

note: examples of concerted / non concerted reactions seen in CHEM 203

concerted reactions

• the formation of a halonium ion from an olefin + a molecular halogen • the formation of a molozonide upon reaction of ozone with an alkene • the reverse cycloaddition (=cycloreversion) of a molozonide to a carbonyl oxide + a carbonyl compound • the formation of an ozonide upon reaction of a carbonyl oxide with a carbonyl compound

non-concerted reactions

• the addition of HBr to alkenes (first a carbocation forms, then Br – adds to the carbocation) • the hydration of alkenes (first a carbocation forms, then H2O adds to the carbocation) • the halogenation of alkenes (first a halonium ion forms, then a halide ion reacts with it)

Absence of rearrangements during the ozonolysis of alkenes (= no cationic intermediates)

Peroxide: a molecule incorporating an O–O linkage

Instability and explosive properties of peroxidic compounds, including ozonides, due to the presence of O atoms at the oxidation state of –1

– Agents containing metals in a high oxidation state: MnO4 (+7), RuO4, OsO4 (both +8), …

Analogy between the electronic distribution in the molecule of O3 and in that of OsO4:

Os (VIII): extremely electrophilic

O O O O O O O Os Os O O O O ozone notice osmium tetraoxide similarity

Ability of OsO4 to add to olefinic π bonds through a concerted mechanism similar to a 1,3- dipolar cycloaddition:

O VIII O O VI O Os Os O O concerted O O an osmate ester R1 R3 syn addition R1 R3 C C C C R2 R4 R2 R4

Driving force for the above reaction: reduction of Os(VIII) to Os(VI):

— Strictly syn course of the addition of OsO4 (but also of MnO4 and RuO4) to alkenes

Absence of rearrangements in reactions involving OsO4 (= no cationic intermediates)

Lecture of Oct 9 p. 3

Stable character of osmate esters (unlike molozonides …)

Treatment of osmate esters with aqueous NaHSO3 leading to the release of Os and formation of vicinal diols (also described as 1,2-diols. Diol = double alcohol):

O VI O Os HO OH O O aqueous R1 R3 R1 R3 C C + reduced forms of osmium C C NaHSO3 R2 R4 R2 R4 a 1,2 diol

osmate ester (vicinal diol) (stable )

Dihydroxylation (osmylation) reaction: the conversion of an alkene to a 1,2-diol by reaction with OsO4 followed by reduction of the intermediate osmate ester, e.g., with NaHSO3

Strictly syn course of the dihydroxylation (osmylation) reaction

Representative examples of osmylation reactions:

1. OsO OH 4 H OH (chiral: formed as the racemate) 2. Aq. NaHSO 3

OH 1. OsO4 H (chiral: formed as the racemate) 2. Aq. NaHSO H 3 OH

1. OsO OH OH 4 H H OH (meso compound: achiral) 2. Aq. NaHSO3 H H HO

H OH 1. OsO4 (meso compound: achiral) 2. Aq. NaHSO3 OH H

CH3 1. OsO4 OH (chiral: formed as the racemate) 2. Aq. NaHSO3 OH H

Chemically and technologically important reactions of alkenes initiated by the interaction of the π system with molecules containing a strongly Lewis acidic site: the case of borane, BH3 Lecture of Oct 9 p. 4

Strongly Lewis acidic character of borane, BH3

the molecule is H vacant atomic H B 2 p-type orbital: trigonal: B is sp H Lewis acid

Probable initial interaction of the π electrons of the alkene with the vacant p orbital of BH3

Development of positive character on the C atoms of the alkene as the BH3 molecule draws electronic density away from the π system, resulting in the ultimate transfer of one of the H atoms (as a formal hydride) from boron to carbon:

H H H B δ H B BH H H H 2 H C C H H H H H C C C C H H H H H H δ δ