Transition metal vinylidene complex
A transition metal vinylidene complex is an organometallic compound containing a metal bound to a vinylidene group (i.e. bearing the motif M=C=CRR').[1][2][3] Free vinylidenes (:C=CRR') are the less thermodynamically stable valence tautomers of alkynes, and interconversion between the two species typically requires extremely harsh conditions.[4] However, the presence of a coordinated metal can greatly decrease the kinetic barrier between alkyne-vinylidene interconversion. Furthermore, the equilibrium can be shifted from metal π-alkyne complex to metal vinylidene complex, depending on the identity of the metal, the nature of the ligands, and the alkyne substituents. Since metal vinylidenes have a broad range of reactivities and the conditions for their formation are generally very mild, they have found use in a variety of synthetic organic and organometallic contexts.
Structure
[edit]According to the covalent bond classification method, vinylidenes are neutral L-type ligands donating two electrons. From a molecular orbital perspective, there are two primary bonding interactions involved between the vinylidene carbon and the metal: (1) the filled vinylidene carbon sp orbital donating into an unfilled metal d orbital in a σ-fashion, and (2) a filled metal d orbital donating into the empty vinylidene carbon p orbital in a π-fashion. As a result of these two interactions, most texts represent the metal-vinylidene bond as a metal-carbon double bond.
Some vinylidenes are bridging ligands.[5] In this case, the vinylidene carbon rests as a bridging atom between two metal centers. From an electron counting perspective, the vinylidene counts for only one electron per metal center. From a molecular orbital perspective, the vinylidene carbon is sp2 hybridized, with singly-filled sp2 orbitals overlapping with singly-filled metal orbitals.
Synthesis
[edit]From alkynes
[edit]Commonly, transition metal vinylidene complexes are synthesized by treating metal electrophiles with terminal alkynes.[6]
The mechanism of the isomerization process begins with formation of a metal-alkyne complex. It is proposed that "slippage" converts this species to a transient σ complex involving binding of the metal to the C–H bond.[7] For metals with a d6 electron count, such as Mn(I) or Ru(II), a subsequent 1,2-H shift gives the vinylidene complex.[8][9] For some electron-rich transition metals, such as Co(I), Rh(I), or Ir(I), oxidative addition of the C-H bong give a σ-alkynyl-hydrido metal complex. A subsequent 1,3-H shift (which also reduces the metal center to its starting oxidation state) then gives the transition metal vinylidene complex.
Other methods
[edit]Vinylidene complexes can be prepared by many routes aside from the acetylide route. These more specialized methods include deprotonation of [[[transition metal carbyne complexes|carbyne complex]]es, dehydration of [[[transition metal acyl complexes|metal acyl complex]]es, and rearrangement of vinyl complexes.[6] An example of the latter case is the α-elimination of a σ-bound alkenyl ligand leads to a vinylidene complex.This path way is exemplified by an molybenum complex of chlorodicyanovinyl.[10]
Reactions
[edit]Nucleophilic attack
[edit]Metal vinylidenes are usually electrophilic species at the vinylidene carbon, analogous to the electrophilic nature of Fischer carbenes. This can be predicted a priori from the empty p orbital on the vinylidene carbon, which is primed to accept electron density from a nucleophile. Illustrative of this reactivity is the behavior of homo-propargylic alcohols with various mid transition metal carbonyl complexes.[11] The observed products can be justified by the initial formation of a metal vinylidene, followed by nucleophilic attack by the pendent alcohol onto the vinylidene carbon. Finally, proton transfer gives the observed Fischer carbene products.
Cycloaddition
[edit]Cycloadditions can occur on metal vinylidenes at either the M=Cα or Cα=Cβ bonds. These can serve in both [2+2] and [4+2] cycloadditions with a variety of cycloaddition partners. For example, Takanori Matsuda and coworkers reported the transformation of 2,2'-ethynyl-biphenyls into ethynyl-phenanthrenes using ruthenium catalysis.[12] This transformation can be rationalized by the initial formation of a ruthenium vinylidenes, followed by [2+2] cycloaddition onto the pendent alkyne. The formed cyclobutene then undergoes retro-[2+2] to the phenanthrene-substituted ruthenium vinylidene. Lastly, 1,2-H migration and de-coordination of the ruthenium complex (i.e. the steps of ruthenium vinylidene formation in reverse) gives the observed product.
Electrocyclization
[edit]Metal vinylidenes may undergo electrocyclization if they are present within a suitable π-system. For example, Sakae Uemura and coworkers showed that conjugated eneyne-esters react with mid transition metal carbonyl complexes to give cyclized metal pyranylidene products.[13] This can be rationalized by the initial formation of the metal vinylidene, followed by 6π-electrocyclization to form the observed products.
Use in total synthesis
[edit]Vinylidene complexes have not proven of any commercial value,[14] but they have been exploited in the total synthesis of various natural products. The route to acetylaranotin involves a vinylidene complex.[15] Specifically, the seven-membered oxepin ring was constructed using a rhodium-catalyzed cycloisomerization between a terminal alkyne and a pendent alcohol. The mechanism starts with formation of the rhodium vinylidene (likely through oxidative addition to the alkyne C–H and 1,3-H shift as above). The alcohol then attacks the electrophilic vinylidene to form an alkenyl rhodium species. Protodemetallation gives the cycloisomerization product, as well as regenerating the catalyst.
In the synthesis of 3-demethoxyerythratidinone, the angularly fused 6-5-6 A/B/C ring system was constructed using a tandem rhodium-catalyzed alkylation/cycloisomerization. The mechanism of this cascade sequence,[16] starts with formation of the σ-alkynyl hydrido rhodium complex. In the presence of a pendent alkyl iodide and triethylamine as a base, the alkynyl rhodium undergoes β-alkylation and deprotonation to give a rhodium vinylidene. Subsequently, the Rh=Cα bond undergoes [2+2] cycloaddition to the pendent alkene. The formed metallacyclobutene then undergoes β-hydride elimination and reductive elimination to give the final product.
See also
[edit]References
[edit]- ^ Hartwig, John F. (2010). Organotransition metal chemistry: from bonding to catalysis. Sausalito, Calif: University Science Books. ISBN 978-1-891389-53-5. OCLC 310401036.
- ^ Trost, Barry M.; McClory, Andrew (February 2008). "Metal Vinylidenes as Catalytic Species in Organic Reactions". Chemistry – an Asian Journal. 3 (2): 164–194. doi:10.1002/asia.200700247. ISSN 1861-4728. PMC 2483426. PMID 18172846.
- ^ Roh, Sang Weon; Choi, Kyoungmin; Lee, Chulbom (2019-03-27). "Transition Metal Vinylidene- and Allenylidene-Mediated Catalysis in Organic Synthesis". Chemical Reviews. 119 (6): 4293–4356. doi:10.1021/acs.chemrev.8b00568. ISSN 0009-2665. PMID 30768261.
- ^ Huguet, Joan; Karpf, Martin; Dreiding, André S. (1983). "Synthesis of a stereoisomer of ptychanolide". Tetrahedron Letters. 24 (39): 4177–4180. doi:10.1016/S0040-4039(00)88292-1.
- ^ Mills, O. S.; Redhouse, A. D. (1966). "The structure of diphenylvinylideneoctacarbonyldi-iron". Chemical Communications (London) (14): 444. doi:10.1039/c19660000444. ISSN 0009-241X.
- ^ a b Bruce, Michael I. (1991). "Organometallic Chemistry of Vinylidene and Related Unsaturated Carbenes". Chemical Reviews. 91 (2): 197–257. doi:10.1021/cr00002a005.
- ^ Silvestre, Jérôme; Hoffmann, Roald (1985-09-25). "Hydrogen Migration in Transition Metal Alkyne and Related Complexes". Helvetica Chimica Acta. 68 (6): 1461–1506. doi:10.1002/hlca.19850680602. ISSN 0018-019X.
- ^ De Angelis, Filippo; Sgamellotti, Antonio; Re, Nazzareno (2002-12-01). "Density Functional Study of Alkyne to Vinylidene Rearrangements in [(Cp)(PMe 3 ) 2 Ru(HC⋮CR)] + (R = H, Me)". Organometallics. 21 (26): 5944–5950. doi:10.1021/om020723m. ISSN 0276-7333.
- ^ De Angelis, Filippo; Sgamellotti, Antonio; Re, Nazzareno (2004). "Acetylene to vinylidene rearrangements on electron rich d6 metal centers: a density functional study". Dalton Transactions (20): 3225–3230. doi:10.1039/b408452g. ISSN 1477-9226. PMID 15483705.
- ^ King, R. B.; Saran, Mohan Singh (1972). "Metal complexes with terminal dicyanomethylenecarbene ligands formed by chlorine migration reactions". Journal of the Chemical Society, Chemical Communications (19): 1053. doi:10.1039/c39720001053. ISSN 0022-4936.
- ^ McDonald, Frank E.; Connolly, Colleen B.; Gleason, Mark M.; Towne, Timothy B.; Treiber, Karl D. (December 1993). "A new synthesis of 2,3-dihydrofurans: cycloisomerization of alkynyl alcohols to endocyclic enol ethers". The Journal of Organic Chemistry. 58 (25): 6952–6953. doi:10.1021/jo00077a006. ISSN 0022-3263.
- ^ Matsuda, Takanori; Kato, Kotaro; Goya, Tsuyoshi; Shimada, Shingo; Murakami, Masahiro (2016-02-05). "Ruthenium-Catalyzed Cycloisomerization of 2,2′-Diethynyl- biphenyls Involving Cleavage of a Carbon–Carbon Triple Bond". Chemistry – A European Journal. 22 (6): 1941–1943. doi:10.1002/chem.201504937. ISSN 0947-6539. PMID 26660656.
- ^ Ohe, Kouichi; Miki, Koji; Yokoi, Tomomi; Nishino, Fumiaki; Uemura, Sakae (2000-12-01). "Novel Pyranylidene Complexes from Group 6 Transition Metals and β-Ethynyl α,β-Unsaturated Carbonyl Compounds". Organometallics. 19 (25): 5525–5528. doi:10.1021/om0006763. ISSN 0276-7333.
- ^ Roh, Sang Weon; Choi, Kyoungmin; Lee, Chulbom (2019). "Transition Metal Vinylidene- and Allenylidene-Mediated Catalysis in Organic Synthesis". Chemical Reviews. 119 (6): 4293–4356. doi:10.1021/acs.chemrev.8b00568. PMID 30768261.
- ^ Codelli, Julian A.; Puchlopek, Angela L. A.; Reisman, Sarah E. (2012-02-01). "Enantioselective Total Synthesis of (−)-Acetylaranotin, a Dihydrooxepine Epidithiodiketopiperazine". Journal of the American Chemical Society. 134 (4): 1930–1933. Bibcode:2012JAChS.134.1930C. doi:10.1021/ja209354e. ISSN 0002-7863. PMC 3271125. PMID 22023250.
- ^ Joo, Jung Min; Yuan, Yu; Lee, Chulbom (2006-11-01). "Tandem Cyclization of Alkynes via Rhodium Alkynyl and Alkenylidene Catalysis". Journal of the American Chemical Society. 128 (46): 14818–14819. Bibcode:2006JAChS.12814818J. doi:10.1021/ja066374v. ISSN 0002-7863. PMC 1762102. PMID 17105287.