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Metathesis of Functionalized Alkane: Understanding the Unsolved Story

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Metathesis of Functionalized Alkane: Understanding the Unsolved Story catalysts Communication Metathesis of Functionalized Alkane: Understanding the Unsolved Story Mykyta Tretiakov, Yury Lebedev, Manoja K. Samantaray *, Aya Saidi , Magnus Rueping * and Jean-Marie Basset * KAUST Catalysis Center (KCC), King Abdullah University of Science & Technology, Thuwal 23955-6900, Saudi Arabia; [email protected] (M.T.); [email protected] (Y.L.); [email protected] (A.S.) * Correspondence: [email protected] (M.K.S.); [email protected] (M.R.); [email protected] (J.-M.B.) Received: 6 February 2019; Accepted: 23 February 2019; Published: 4 March 2019 Abstract: For the first time, we developed a method which enables a functionalized alkane to be metathesized to its lower and higher homologues. For this metathesis reaction, we used [(≡Si-O-)W(CH3)5] as a catalyst precursor and 9-hexyl-9H-carbazole as a reactant. Keywords: metathesis; functionalized alkane; tungsten; carbazole; reaction mechanism; pyrrole 1. Introduction Since the discovery of alkane metathesis reaction by well-defined silica supported [Ta]-H catalyst [1], many groups, including us, have been continuously working on the development of well-defined catalysts for “functionalized” alkanes metathesis reaction or alkyl chains bearing any functionality, along with improved reactivity in alkane metathesis reaction [2–8]. While significant progress was achieved in improving the catalyst activity (e.g., by changing catalyst support and by employing a tandem system, where two catalysts act back to back for cascade reaction), metathesis of a functionalized alkane remains elusive in the scientific community [7,9–12]. To metathesize a functionalized alkane, the reactant should undergo dehydrogenation to a functionalized olefin, functionalized olefin metathesis, and finally reduction of the newly-formed, functionalized olefins to new functionalized alkanes. The main difficulty in functionalized alkane metathesis reaction is the functional moiety, which is expected to poison the catalyst by coordinating with the metal, and hence deactivating the system for further reaction. Therefore, it is a tremendous challenge to metathesize an alkane having a functional group. Herein we present the first example of functionalized alkane metathesis using silica supported W(CH3)5 1 as a catalyst precursor and 9-Hexyl-9H-carbazole 6 as a reactant. 2. Results and Discussion From previous publications it was known that ligands that have a lone pair of electrons coordinate with electron-deficient early-transition metals, making them electron-rich and blocking the coordination site for C-H bond activation [13]. As the metal is more electron-rich and coordinatively more saturated, its affinity towards alkane decreases [14]. Considering, that alkane metathesis occurs first by sigma-bond metathesis with a d0 system, this becomes increasingly important [15]. To avoid this problem, while carrying out the functionalized alkane metathesis reaction, our first aim was to focus on the protection of the functional group. To implement this idea, we chose N-Alkyl pyrroles, with the assumption that the lone pair of electrons on the nitrogen atom is involved in the formation of an aromatic system and thus is poorly available for coordination. To our surprise, after many attempts, we only observed a starting material along with its decomposition products. Catalysts 2019, 9, 238; doi:10.3390/catal9030238 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 238 2 of 10 Catalysts 2019, 9, 238 2 of 10 manyCatalysts attempts,2019, 9 , 238we only observed a starting material along with its decomposition products.2 of 10 A manyliterature attempts, report showswe only that observed pyrroles ainteract starting with ma Lewisterial alongacids towith produce its decomposition pyrrole oligomers products. through A literature report shows that pyrroles interact with Lewis acids to produce pyrrole oligomers through the activationA literature of report its α showsposition that [16]. pyrroles To avoid interact this, with and Lewis to carry acids out to produce the reaction, pyrrole we oligomers thought through to protect the activation of its α position [16]. To avoid this, and to carry out the reaction, we thought to protect the mostthe activation reactive of α its positionα position of[ 16the]. Topyrrole avoid this,ring and by tometh carryyl out groups, the reaction, in order we thoughtto avoid to protectunwanted thereactions mostthe most reactiveof reactivethe pyrroleαα positionposition ring ofof thewhile the pyrrole pyrrole carrying ring ring by ou methyl byt the meth groups, metathesis.yl groups, in order We toin avoid ordersynthesized unwanted to avoid reactions1-Hexyl-2,5- unwanted reactionsDimethyl-1H-pyrroleof the pyrroleof the ringpyrrole while 2 and ring carrying 1-Propyl-2,5-Dimethyl-1H-pyrrole while out thecarrying metathesis. out Wethe synthesized metathesis. 3 [17,18] 1-Hexyl-2,5-Dimethyl-1 We (Scheme synthesized 1) and H1-Hexyl-2,5- -pyrrolecarried out Dimethyl-1H-pyrrole 2 and 1-Propyl-2,5-Dimethyl-1H-pyrrole 3 [17,18] (Scheme 1) and carried out the metathesis2 and 1-Propyl-2,5-Dimethyl-1 reaction using [(≡HSi-O-)W(CH-pyrrole 3 [173),518] 1] (Schemeas a catalyst1) and precursor carried out (Figure the metathesis 1). reaction the metathesisusing [(≡Si-O-)W(CH reaction using3)5] 1 [(as≡ aSi-O-)W(CH catalyst precursor3)5] 1 as (Figure a catalyst1). precursor (Figure 1). Figure 1. Well-defined silica-supported [(≡Si-O-)W(CH3)5] 1. Figure 1. Well-defined silica-supported [(≡Si-O-)W(CH3)5] 1. Figure 1. Well-defined silica-supported [(≡Si-O-)W(CH3)5] 1. Scheme 1. Synthesis of N-Alkyl pyrroles. Scheme 1. Synthesis of N-Alkyl pyrroles. Scheme 1. Synthesis of N-Alkyl pyrroles. It has been already observed that 1 leads to [(≡Si-O-)W(H)3(=CH2)] during catalyst activation [19]. WhileIt has testingbeen already this family observed of molecules, that 1 leads expecting to [( the≡Si-O-)W(H) metathesis3(=CH products2)] during of the reactants catalyst 2activationand 3, we [19]. WhileendedIt testinghas been up this with already family unreacted observed of molecules, starting that material 1 expect leads along ingto [( the≡ withSi-O-)W(H) metathesis traces of3(=CH decomposition, products2)] during of the andcatalyst reactants isomerization activation 2 and of 3 [19]., we Whileendedboth testingup the with substrates this unreacted family without of starting molecules, any metathesismaterial expect along product.ing the with metathesis traces of decomposition,products of the reactantsand isomerization 2 and 3, we of endedboth the up substratesThe with alkane unreacted without metathesis starting any consists metathesis material of mainly alongproduct. threewith traces subsequent of decomposition, paths—dehydrogenation, and isomerization olefin of both metathesis,Thethe substratesalkane and metathesis without hydrogenation—as any consists metathesis weof mainly did product. not observethree subsequent any expected paths—dehydrogenation, metathesis products from the olefin above two reactants, we decided to investigate if the olefin metathesis step could be performed in our metathesis,The alkane and hydrogenation—asmetathesis consists we of didmainly not obsethreerve subsequent any expected paths—dehydrogenation, metathesis products from olefin the catalytic system. We synthesized 1-Allyl-2,5-Dimethyl-1H-pyrrole 4, which is the olefinic analogue of metathesis,above two reactants,and hydrogenation—as we decided to weinvestigate did not obseif therve olefin any expectedmetathesis metathesis step could products be performed from the in abovethe two 1-Propyl-2,5-Dimethyl-1 reactants, we decidedH-pyrrole to investigate2, for olefin if metathesisthe olefin reactionmetathesis using step catalyst could1 (Figure be performed2). in our catalyticThe productsystem. was We analyzed synthesized by gas 1-Allyl-2,5-Dimethyl-1H-pyrrole chromatography (GC), gas chromatography 4, which coupled is the witholefinic our catalytic system. We synthesized 1-Allyl-2,5-Dimethyl-1H-pyrrole 4, which is the olefinic analoguemass spectrometryof the 1-Propyl (GC-MS),-2,5-Dimethyl-1H-pyrrole and nuclear magnetic 2, resonance for olefin (NMR) metathesis spectroscopy, reaction confirming using catalyst the 1 analogue of the 1-Propyl-2,5-Dimethyl-1H-pyrrole 2, for olefin metathesis reaction using catalyst 1 (Figureformation 2). of expected product 1,4-bis-(2,5-Dimethyl-1H-pyrrole-1yl)but-2-ene, with 85% conversion (Figure 2). and a turnover number (TON) of 850 (Figure2)[ 20–22]. These results clearly indicated that our catalyst was capable of carrying out the olefin metathesis reaction for this particular substrate, whereas in the case of 1-Propyl-2,5-Dimethyl-1H-pyrrole 2 or 1-Hexyl-2,5-Dimethyl-1H-pyrrole 3, no metathesis was observed. The limiting step in the latter case was the alkyl chain C-H activation (as an entry into the alkane metathesis productive cycle). During the catalytic reaction, we understood that isomerization of the substrates occurs because of walking of α-methyl groups on the ring [23]. To avoid that, we thought to synthesize 9-Hexyl-9H-carbazole (a rigid molecule) to carry out the metathesis reaction. 9-Hexyl-9H-carbazole was obtained by the reaction of carbazole and hexyl bromide in the presence of KOH, by following a literature procedure (Scheme2)[24]. Catalysts 2019, 9, 238 3 of 10 100 90 80 70 60 50 40 Conversion (%) 30 20 10 0 Catalysts 2019, 9, 238 0 20406080100120 3 of 10 Catalysts 2019, 9, 238 Time (h) 3 of 10 100 Figure 2. Metathesis of 1-Alkyl-2,5-Dimethyl-1H-pyrrole, conversion
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