Addition eactions of arbon-arbon Pi Bonds Part 3 2 Ni 2 ( 2 ) 14 ( 2 ) 7 =( 2 ) 7 3 2 P 2 2 N( ) 3 Miscellaneous Pi Bond Addition eactions Do all carbon-carbon pi bond addition reactions......involve electrophiles and carbocations?...follow Markovnikov's rule? } Examples ydroboration-xidation 1. B 3 2. Na, atalytic ydrogenation Anti-Markovnikov addition of 2 2 No electrophile Pt Markovnikov's rule irrelevant hem 14D Lecture Supplement: Addition eactions of arbon-arbon Pi Bonds Part 3 -- Page 1
atalytic ydrogenation atalytic hydrogenation: Addition of 2 in the presence of a catalyst. Pt atalyst usually = Pt, Pd, Ni Why catalyst needed? Bonds lost: - pi and - sigma Bonds gained: 2 x - sigma Bonds strengths: Sigma > pi Therefore ΔG < 0 (K eq > 1)... but ΔG (one or more mechanism steps) too high atalyst alters mechanism but not K eq atalytic ydrogenation 2 Pt Syn addition New bonds on same alkene face Why syn addition? You are not responsible for this mechanism. 2 2 is not an electrophile 2 symmetrical so Markovnikov's rule irrelevant hem 14D Lecture Supplement: Addition eactions of arbon-arbon Pi Bonds Part 3 -- Page 2
atalytic ydrogenation - Application Partially ydrogenated Vegetable il and Margarine 2, Ni Polyunsaturated fat Lower melting point (oil) Susceptible to air oxidation 2, Ni Monounsaturated fat igher melting point (margarine) Less susceptible to air oxidation and/or Saturated fat ealth concerns Trans fat ealth concerns Addition of 2 and l 2 Anti addition New bonds on opposite alkene face 2 has no so Markovnikov's ule does not apply Mechanism: You are not responsible for this mechanism δ + δ Similar reaction for l 2 2 polarized by pi electrons Backside attack hem 14D Lecture Supplement: Addition eactions of arbon-arbon Pi Bonds Part 3 -- Page 3
ydroboration-xidation B 3 ydroboration - B 2 xidation Same as: 1. B 3 2., - Syn, anti-markovnikov addition of water ompare with 3 +, which gives Markovnikov addition Electrophile = B 3 pen octet on boron ydroboration-xidation ydroboration-oxidation used broadly in laboratory, industrial synthesis 3 1. B 3 2., - Additional steps Artemisinin (anti-malarial) hem 14D Lecture Supplement: Addition eactions of arbon-arbon Pi Bonds Part 3 -- Page 4
FAQ: Addition eaction Stereochemistry Syn or anti or mixture? Addition reaction stereochemistry: atalytic hydrogenation syn anti 2 or l 2 syn anti ydroboration/oxidation syn anti Explore via mechanism..., 3 +, etc. syn anti??? 3 Anti addition 3 Syn addition onclusion: - + alkene gives... Anti addition Syn addition A mixture is formed zonolysis ozone destruction leavage of = or by reaction with ozone ( 3 ). Ph 1. 3 + 2. ( 3 ) 2 S Ph Mechanism: You are not responsible for this mechanism Ph Ph Ph ( ) 2 S Ph + Addition stereochemistry: Syn Anti But none at the end rientation: Markovnikov addition Anti-Markovnikov addition Both Neither Electrophile: Alkene 3 Both Neither hem 14D Lecture Supplement: Addition eactions of arbon-arbon Pi Bonds Part 3 -- Page 5
FAQ: What Must I Know About These eactions? You should know... eactants and products Stereochemistry: Syn addition, anti addition, mixture, etc. rientation: Markovnikov versus anti-markovnikov addition You are not responsible for mechanisms of... atalytic hydrogenation Addition of 2 /l 2 ydroboration-oxidation zonolysis Familiarity with mechanisms = easier to remember stereochemistry, etc. Biological Examples: zonolysis of Pulmonary Surfactant 2 ( 2 ) 14 2 ( 2 ) 14 ( 2 ) 7 =( 2 ) 7 3 ( 2 ) 7 + ( 2 ) 7 2 P 2 2 N( ) 3 2 P 2 2 N( ) 3 Pulmonary surfactant assists diffusion of 2 from air into bloodstream Surfactant = surface active agent equires both polar and nonpolar regions nonpolar portion = surfactant effect = less efficient 2 diffusion hem 14D Lecture Supplement: Addition eactions of arbon-arbon Pi Bonds Part 3 -- Page 6
Biological Examples: Steroid Biosynthesis Steroid skeleton: Biosynthetic pathway: Start by gathering the necessary number of carbons... 15 x 3 SoA Many enzymatic steps Acetyl oa SoA = Good leaving group 2, squalene epoxidase Squalene Squalene oxide Biological Examples: Steroid Biosynthesis Form the rings... Enzymatic protonation hem 14D Lecture Supplement: Addition eactions of arbon-arbon Pi Bonds Part 3 -- Page 7
Biological Examples: Steroid Biosynthesis Biological Examples: Steroid Biosynthesis Now for some carbocation fates... Enz hem 14D Lecture Supplement: Addition eactions of arbon-arbon Pi Bonds Part 3 -- Page 8
Biological Examples: Steroid Biosynthesis Almost done... Lanosterol 19 more enzymatic steps holesterol Precursor to all other steroids hem 14D Lecture Supplement: Addition eactions of arbon-arbon Pi Bonds Part 3 -- Page 9