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 2 4 molozonide R R 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 similarity tetraoxide 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 .