White catalyst: Difference between revisions

| verifiedrevid = 425211585

| IUPACName =

| Section1 = {{Chembox Identifiers

| PubChem =

| SMILES = }}

| Section2 = {{Chembox Properties

| C=18|H=20|O=6|Pd=1|S=2

| Section3 = {{Chembox Hazards

| Autoignition = }}

This compound is commercially available. It may be prepared by oxidation of 1,2-bis(phenylthio)ethane to the [[sulfoxide]], followed by reaction with [[palladium acetate]].<ref>{{cite journal | title = syn-1,2-Amino Alcohols via Diastereoselective Allylic C-H Amination | author = Kenneth J. Fraunhoffer and M. Christina White | journal = [[J. Am. Chem. Soc.]] | year = 2007 | volume = 129 | pages = 7274–7276 | doi = 10.1021/ja071905g | issue = 23}}</ref>

The [[reaction mechanism]] of [[allylic]] C-H acetoxylation has been studied.<ref name=chen>{{cite journal |title=Serial Ligand Catalysis: A Highly Selective Allylic C-H Oxidation |author=Chen, M. S.; Prabagaran, N.; Labenz, N. A.; White, M. C. |journal=[[J. Am. Chem. Soc.]] |year=2005 | volume=127 |issue=19 | pages=6970–6971 | doi=10.1021/ja0500198}}</ref> The first step in the [[catalytic cycle]] is cleavage of the allylic C-H bond. The [[sulfoxide]] [[ligand]] is thought to promote this step by generating a highly [[electrophilic]], possibly [[cationic]] palladium species ''in situ''. This species coordinates to the alkene and acidifies the adjacent C-H bond, which allows [[acetate]] to abstract the [[proton]] and forms a π-allyl palladium complex ('''II'''). Subsequently, a π-acid such as [[benzoquinone]] coordinates to the palladium, activating the π-allyl complex to nucleophilic attack ('''III'''). A [[nucleophile]], in this case acetate, attacks to [[reductive elimination|reductively eliminate]] palladium, generating the product and palladium(0) ('''IV'''). The palladium(0) is reoxidized to palladium(II) by benzoquinone and the sulfoxide ligand reassociates, closing the catalytic cycle.

The White catalyst was originally developed for use in a branched allylic acetoxylation reaction.<ref name=chen /> An [[enantioselective]] version of this reaction was subsequently reported, using chromium(III) [[salen ligand|salen]] fluoride as a chiral cocatalyst.<ref name=covell>{{cite journal |title=A Chiral Lewis Acid Strategy for Enantioselective Allylic C-H Oxidation |author=Covell, D. J.; White, M. C. |journal=[[Angewandte Chemie International Edition]] |year=2008 | volume=47 |issue=34 | pages=6448–6451 | doi=10.1002/anie.200802106}}</ref> A [[macrolactonization]] reaction based on the branched allylic esterification was developed for the preparation of 14- to 19-membered [[macrolides]].<ref name=fraunhoffer>{{cite journal |title=Macrolactonization via Hydrocarbon Oxidation |author=Fraunhoffer, K. J.; Prabagaran, N.; Sirois, L. E.; White, M. C. |journal=[[J. Am. Chem. Soc.]] |year=2006 | volume=128 |issue=28 | pages=9032–9033 | doi=10.1021/ja063096r}}</ref> This method was applied to the total synthesis of [[6-deoxyerythronolide B]].<ref name=stang>{{cite journal |title=Total synthesis and study of 6-deoxyerythronolide B by late-stage C–H oxidation |author=Stang, E. M.; White, M. C. |journal=[[Nature Chemistry]] |year=2009 | volume=1 |issue=7 | pages=547–551 | doi=10.1038/NCHEM.351}}</ref> In addition to acetate, a wide variety of [[carboxylic acids]] may be employed as nucleophiles in the branch allylic esterification reaction. As the first step in an esterification/Heck sequence, [[aliphatic]] and [[aromatic]] carboxylates were demonstrated, including [[amino acids]].<ref name=delcamp1>{{cite journal |title=Sequential Hydrocarbon Functionalization: Allylic C-H Oxidation/Vinylic C-H Arylation |author=Delcamp, J. H.; White, M. C. |journal=[[J. Am. Chem. Soc.]] |year=2006 | volume=128 |issue=47 | pages=15076–15077 | doi=10.1021/ja066563d}}</ref>

The White catalyst can effect both branched and linear [[regioselective]] allylic C-H aminations. In order to promote nucleophilic attack at the internal terminus of the π-allyl to generate branched product, a tethered ''N''-sulfonyl carbamate nucleophile is used. This strategy has been applied to the synthesis of 1,2 and 1,3-[[amino alcohols]].<ref name=fraunhoffer2>{{cite journal |title=''syn''-1,2-Amino Alcohols via Diastereoselective Allylic C-H Amination |author=Fraunhoffer, K. J.; White, M. C. |journal=[[J. Am. Chem. Soc.]] |year=2007 | volume=129 |issue=23 | pages=7274–7276 | doi=10.1021/ja071905g}}</ref><ref name=rice>{{cite journal |title=Allylic C-H Amination for the Preparation of ''syn''-1,3-Amino Alcohol Motifs |author=Rice, G. T.; White, M. C. |journal=[[J. Am. Chem. Soc.]] |year=2009 | volume=131 |issue=33 | pages=11707–11711 | doi=10.1021/ja9054959}}</ref> The amination proceeds with high yields and good [[diastereoselectivity]], and the products may be readily elaborated to [[amino acids]] and other synthetic intermediates and [[natural products]]. Key to the development of the reaction was identification of a very acidic nitrogen nucleophile with a [[pKa]] close to [[acetic acid]], as more basic nucleophiles divert reactivity to aminopalladation. The [[intermolecular]] version of the allylic C-H amination is also known.<ref name=reed>{{cite journal |title=Catalytic Intermolecular Linear Allylic C-H Amination via Heterobimetallic Catalysis |author=Reed, S. A.; White, M. C. |journal=[[J. Am. Chem. Soc.]] |year=2008 | volume=130 |issue=11 | pages=3316–3318 | doi=10.1021/ja710206u}}</ref> Using methyl ''N''-tosyl carbamate nucleophile, the linear E-allylic amine products are obtained from α-olefin substrates. It has been shown that functionalization of the π-allyl intermediate may be promoted by chromium(III) salen chloride activation of the [[electrophile]], or [[Hunig's base]] activation of the [[nucleophile]].<ref name=reed2>{{cite journal |title=A Catalytic, Brønsted Base Strategy for Intermolecular Allylic C-H Amination |author=Reed, S. A.; Mazzotti, A. R.; White, M. C. |journal=[[J. Am. Chem. Soc.]] |year=2009 | volume=131 |issue=33 | pages=11701–11706 | doi=10.1021/ja903939k}}</ref>

In 2008, simultaneous publications described the allylic C-H alkylation of allylarene substrates.<ref name=young>{{cite journal |title=Catalytic Intermolecular Allylic C-H Alkylation |author=Young, A. J.; White, M. C. |journal=[[J. Am. Chem. Soc.]] |year=2008 | volume=130 |issue=43 | pages=14090–14091 | doi=10.1021/ja806867p}}</ref><ref name=shi>{{cite journal |title=Intra/Intermolecular Direct Allylic Alkylation via Pd(II)-Catalyzed Allylic C-H Activation |author=Lin, S.; Song, C.-X.; Cai, G.-X.; Wang, W.-H.; Shi, Z.-J. |journal=[[J. Am. Chem. Soc.]] |year=2008 | volume=130 |issue=39 | pages=12901–12903 | doi=10.1021/ja803452p}}</ref> These reactions were catalyzed by the White catalyst or by an earlier version of the complex bearing benzyl substituents on the sulfoxide in place of phenyl. It was demonstrated that an additional sulfoxide ligand, [[dimethylsulfoxide]] (DMSO), was essential for promoting functionalization of the π-allyl intermediate; the bis-sulfoxide ligand alone was unable to complete the catalytic cycle.

The White catalyst has been found to be an effective catalyst for an oxidative version of the classic [[Heck reaction]]. Rather than performing allylic C-H cleavage—a relatively slow process—the catalyst quickly [[transmetalation|transmetallates]] with a [[boronic acid]]. This aryl palladium intermediate undergoes a [[1,2-addition]] across the alkene double bond. [[Beta-Hydride elimination|β-Hydride elimination]] releases the product. The oxidative Heck was originally reported as a sequential process following allylic C-H esterification.<ref name=delcamp1 /> It was subsequently demonstrated as a stand-alone method for a broad range of [[alpha-olefin|α-olefin]] substrates.<ref name=delcamp2>{{cite journal |title=A General and Highly Selective Chelate-Controlled Intermolecular Oxidative Heck Reaction |author=Delcamp, J. H.; Brucks, A. P.; White, M. C. |journal=[[J. Am. Chem. Soc.]] |year=2008 | volume=130 |issue=34 | pages=11270–11271 | doi=10.1021/ja804120r}}</ref> The [[regioselectivity]] of the reaction is controlled by directing groups such as [[carbonyl]]s, [[alcohols]] and [[amines]].