Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Palladium-catalyzed Cross
Document related concepts
no text concepts found
Transcript
Section 15.11 PALLADIUM-CATALYZED CROSS-COUPLING Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Extending Cross-coupling • If we consider the reaction of RLi with CuX, Gilman crosscoupling appears to have three steps: – Transmetalation of R from lithium to copper – Oxidative addition of R’X to copper – Reductive elimination of R–R’ • Overall process is stoichiometric, not catalytic, because a full equivalent of Cu is required • When palladium (Pd) is used with more mild R–M than organolithiums, all three steps can be carried out catalytically! Oxidative addition of the electrophile happens first, followed by transmetallation from R’M and reductive elimination. Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Palladium-catalyzed Cross-coupling • Different Pd-catalyzed cross-couplings involve different nucleophiles R’M; electrophiles are organohalides or pseudohalides (usually sp2 or sp) Stille coupling: organotin nucleophiles Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Palladium-catalyzed Cross-coupling • Different Pd-catalyzed cross-couplings involve different nucleophiles R’M; electrophiles are organohalides or pseudohalides (usually sp2 or sp) Negishi coupling: organozinc nucleophiles Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Palladium-catalyzed Cross-coupling • Different Pd-catalyzed cross-couplings involve different nucleophiles R’M; electrophiles are organohalides or pseudohalides (usually sp2 or sp) Suzuki coupling: organoboron nucleophiles Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Palladium-catalyzed Cross-coupling • Different Pd-catalyzed cross-couplings involve different nucleophiles R’M; electrophiles are organohalides or pseudohalides (usually sp2 or sp) Heck coupling: alkenes + organohalides Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Other Nucleophiles • Oxygen-, nitrogen-, and sulfur-based nucleophiles can also be used in palladium-catalyzed cross-couplings Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Section 15.12 HOMOGENEOUS CATALYTIC HYDROGENATION Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Homogeneous Hydrogenation • Hydrogenations we have seen so far have been heterogeneous, involving a metal catalyst suspended in solvent • Homogeneous hydrogenation is also possible using a catalyst dissolved in the reaction solvent • Wilkinson’s catalyst, based on rhodium, is a popular choice for homogeneous hydrogenation Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Hydrogenation with Wilkinson’s Catalyst • Wilkinson’s catalyst is selective for unhindered C=C • Addition is stereospecifically syn, as in heterogeneneous hydrogenation reactions Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Hydrogenation with Wilkinson’s Catalyst • Mechanism involves oxidative addition, insertion of C=C into M–H, and reductive elimination to form second C–H Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Hydrogenation with Wilkinson’s Catalyst • Mechanism involves oxidative addition, insertion of C=C into M–H, and reductive elimination to form second C–H Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Enantioselective Hydrogenation • Chiral ligands make the catalyst chiral, leading to selective formation of one enantiomer in hydrogenations that establish a stereocenter • The biaryl ligand BINAP is highly selective in combination with rhodium and other metals Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Section 15.13 OLEFIN METATHESIS Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Olefin Metathesis • In an olefin metathesis reaction, the ends of two C=C’s “trade partners” (however, the reaction is thermodynamically controlled) • The reaction involves metal carbenoids M=CR2. Insertion of C=C into the M=C π bond is followed by de-insertion Complexes A and B react via the same steps, eventually generating products. Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Ring-closing Metathesis • Two terminal alkenes within the same molecule react intramolecularly to give C2H4 (gas) and a cyclic alkene • This reaction is known as ring-closing metathesis Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Ring-opening Metathesis • Metathesis can also be used to open cyclic alkenes to form alkenic polymers • This reaction is called ring-opening metathesis; when used for polymerization it’s called ROMP Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Section 15.14 ZIEGLER-NATTA CATALYSIS OF ALKENE POLYMERIZATION Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Coordination Polymerization • Coordination to a Lewis-acidic metal renders alkenes electrophilic (similar to cationic polymerization) • Addition of certain metals (Ziegler’s catalysts) leads to very long polymers • Furthermore, the polymers thus formed were isotactic Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Ziegler-Natta Polymerization • In modern times, Zr-based metallocenes are used together with methylalumoxane (MAO) as Lewis-acidic promoter Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Mechanism of Z-N Polymerization • Step 1: a methyl group from MAO displaces Cl– in the Zr metallocene; Cl– departs to form a cationic complex • Step 2: the alkene coordinates to Zr • Step 3: the alkene inserts into the Zr–CH3 bond • Step 4: another molecule of alkene inserts into the Zr– CH2CH2CH3 bond • Step 4 repeats to form a long linear alkyl chain Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.
