Alkene carboamination
Alkene carboamination is the simultaneous formation of C–N and C–C bonds across an alkene. This method represents a powerful strategy to build molecular complexity with up to two stereocenters in a single operation. Generally, there are four categories of reaction modes for alkene carboamination.{{Cite journal|last1=Coldham|first1=I.|last2=Hufton|first2=R.|date=2005|title=Intramolecular dipolar cycloaddition reactions of azomethine ylides|url=https://www.researchgate.net/publication/231619031|journal=Chem. Rev.|volume=105|issue=7|pages=2765–2810|doi=10.1021/cr040004c|pmid=16011324}}{{Cite journal|last1=Nakamura|first1=I.|last2=Yamamoto|first2=Y.|s2cid=39798353|date=2004|title=Transition-metal-catalyzed reactions in heterocyclic synthesis|journal=Chem. Rev.|volume=104|issue=5|pages=2127–2198|doi=10.1021/cr020095i|pmid=15137788}}{{Cite journal|last=Wolfe|first=J. P.|date=2006|title=Palladium-catalyzed carboetherification and carboamination reactions of γ-hydroxy- and γ-aminoalkenes for the synthesis of tetrahydrofurans and pyrrolidines|journal=Eur. J. Org. Chem.|volume=2007|issue=4|pages=571–582|doi=10.1002/ejoc.200600767|pmid=19183704|pmc=2633094}}{{Cite journal|last1=Piou|first1=T.|last2=Rovis|first2=T.|date=2015|title=Rhodium-catalysed syn-carboamination of alkenes via a transient directing group|journal=Nature|volume=527|issue=7576|pages=86–90|doi=10.1038/nature15691|pmid=26503048|bibcode=2015Natur.527...86P|pmc=4636455}} The first class is cyclization reactions, which will form a N-heterocycle as a result. The second class has been well established in the last decade. Alkene substrates with a tethered nitrogen nucleophile have been used in these transformations to promote intramolecular aminocyclization.{{Cite journal|last1=Ney|first1=J. E.|last2=Wolfe|first2=J. P.|date=2004|title=Palladium-catalyzed synthesis of N-aryl pyrrolidines from γ-(N-arylamino) alkenes: evidence for chemoselective alkene insertion into Pd–N bonds|url=http://www.nature.com/nature/journal/v527/n7576/abs/nature15691.html|journal=Angew. Chem. Int. Ed.|volume=43|issue=27|pages=3605–3608|doi=10.1002/anie.200460060|pmid=15293259|url-access=subscription}}{{Cite journal|last1=Lira|first1=R.|last2=Wolfe|first2=J. P.|date=2004|title=Palladium-catalyzed synthesis of N-aryl-2-benzylindolines via tandem arylation of 2-allylaniline: control of selectivity through in situ catalyst modification|journal=J. Am. Chem. Soc.|volume=126|issue=43|pages=13906–13907|doi=10.1021/ja0460920|pmid=15506735}} While intermolecular carboamination is extremely hard, people have developed a strategy to combine the nitrogen and carbon part, which is known as the third class. The most general carboamination, which takes three individual parts and couples them together is still underdeveloped.
Reaction mechanisms
Different transition metals have been used to catalyze carboamination reactions, including palladium,{{Cite journal|last1=Sibbald|first1=P. A.|last2=Rosewell|first2=C. F.|last3=Swartz|first3=R. D.|last4=Michael|first4=F. E.|date=2009|title=Mechanism of N-fluorobenzenesulfonimide promoted deamination and carboamination reactions: divergent reactivity of a Pd(IV) species|journal=J. Am. Chem. Soc.|volume=131|issue=43|pages=15945–15951|doi=10.1021/ja906915w|pmid=19824646}} copper,{{Cite journal|last1=Zeng|first1=W.|last2=Chemler|first2=S. R.|date=2007|title=Copper(II)-catalyzed enantioselective intramolecular carboamination of alkenes|journal=J. Am. Chem. Soc.|volume=129|issue=43|pages=12948–12949|doi=10.1021/ja0762240|pmid=17918850|pmc=2531285}}{{Cite journal|last1=Liwosz|first1=T. W.|last2=Chemler|first2=S. R.|date=2012|title=Copper-catalyzed enantioselective intramolecular alkene amination/intermolecular heck-type coupling cascade|journal=J. Am. Chem. Soc.|volume=134|issue=4|pages=2020–2023|doi=10.1021/ja211272v|pmid=22257169|pmc=3314298}} and rhodium etc. The reaction mechanism varies with different transition metals. For palladium-catalyzed carboamination reactions, Pd(0)/Pd(II) and Pd(II)/Pd(IV) catalytic cycles are the most common mechanisms that have been proposed.
The reaction mode for the key aminopalladation step is different in these two cases. In Wolfe’s chemistry,{{Cite web|url=http://www.umich.edu/~wolfelab/|title=wolfe research group webpage}} which is known as the Pd(0)/Pd(II) catalytic system, syn-aminopalladation is observed. While in the Pd(II)/Pd(IV) catalytic system, which was developed by Forrest Michael,{{Cite web|url=https://depts.washington.edu/chem/people/faculty/michael.html|title=forrest michael webpage}} anti-aminopalladation was observed. It is believed that the pH of the reaction will affect the existing form of the amine nucleophile, which will determine whether the nitrogen coordinates with palladium center or not during the aminopalladation step. For the C–H activation step in Pd(II)/Pd(IV) chemistry, since there is no directing effect on the aromatic ring, large excess of arenes are required.
In 2015, Rovis and coworkers reported a rhodium-catalyzed intermolecular carboamination. In this reaction, enoxyphthalimide was used to serve as both the nitrogen and carbon source. The reaction mechanism is proposed in the paper (vide infra).
File:Wiki3carbo.pngIn 2017, Liu and coworkers reported a copper-catalyzed three component carboamination reaction of styrenes.{{Cite journal|last1=Wang|first1=D.|last2=Wu|first2=L.|last3=Wang|first3=F.|last4=Wan|first4=X.|last5=Chen|first5=P.|last6=Lin|first6=Z.|last7=Liu|first7=G.|title=Asymmetric Copper-Catalyzed Intermolecular Aminoarylation of Styrenes: Efficient Access to Optical 2,2-Diarylethylamines|journal=J. Am. Chem. Soc.|volume=139|issue=20|pages=6811–6814|doi=10.1021/jacs.7b02455|pmid=28471665|year=2017}} In the meantime, Engle and coworkers published a palladium-catalyzed three component carboamination reaction using directing group strategy.{{Cite journal|last1=Liu|first1=Z.|last2=Wang|first2=Y.|last3=Wang|first3=Z.|last4=Zeng|first4=T.|last5=Liu|first5=P.|last6=Engle|first6=K.|title=Catalytic Intermolecular Carboamination of Unactivated Alkenes via Directed Aminopalladation|journal=J. Am. Chem. Soc.|volume=139|issue=32|pages=11261–11270|doi=10.1021/jacs.7b06520|pmid=28727452|year=2017}} These two works are the very rare examples of three component carboamination reactions.
Applications
Carboamination is an efficient method to access nitrogen-containing molecules, especially N-heterocycles. (+)-Preussin, a pyrrolidine alkaloid, can be easily prepared via this methodology.{{Cite journal|last1=Bertrand|first1=M. B.|last2=Wolfe|first2=J. P.|date=2006|title=A concise stereoselective synthesis of preussin, 3-epi-preussin, and analogues|journal=Org. Lett.|volume=8|issue=11|pages=2253–2356|doi=10.1021/ol0606435|pmid=16706524|pmc=2613785}}
(−)-Tylophorine is another example,{{Cite journal|last1=Mai|first1=D. N.|last2=Wolfe|first2=J. P.|date=2010|title=Asymmetric palladium-catalyzed carboamination reactions for the synthesis of enantiomerically enriched 2-(arylmethyl)- and 2-(alkenylmethyl)pyrrolidines|journal=J. Am. Chem. Soc.|volume=132|issue=35|pages=12157–12159|doi=10.1021/ja106989h|pmid=20718417|pmc=2932842}} which can be synthesized using carboamination reaction.
