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Synthesis of the Cobalt-Alkyne Complex

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Synthesis of the Cobalt-Alkyne Complex UC San Diego UC San Diego Previously Published Works Title Synthesis of the cobalt-alkyne complex (η5-C 5H5)(PPh3)Co{η2-(Me 3Si)CC(CO2Et)} and structural characterization of trimethylsilyl substituted cobaltacyclopentadiene complexes derived therefrom Permalink https://escholarship.org/uc/item/1p6184kv Authors Bunker, KD Rheingold, AL Moore, CE et al. Publication Date 2014 DOI 10.1016/j.jorganchem.2013.09.002 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Journal of Organometallic Chemistry 749 (2014) 100e105 Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem 5 Synthesis of the cobaltealkyne complex (h -C5H5)(PPh3)Co 2 {h -(Me3Si)C^C(CO2Et)} and structural characterization of trimethylsilyl substituted cobaltacyclopentadiene complexes derived therefrom Kevin D. Bunker, Arnold L. Rheingold, Curtis E. Moore, Marissa Aubrey, Joseph M. O’Connor* Department of Chemistry and Biochemistry (0358), University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0358, USA article info abstract 5 Article history: The reaction of (h -C5H5)Co(PPh3)2 with TMSC^C(CO2Et) leads to the formation of the cobaltealkyne Received 5 August 2013 5 2 complex (h -C5H5)(PPh3)Co{h -(Me3Si)C^C(CO2Et)} (6). In benzene-d6 solution containing PPh3, 6 un- Received in revised form 5 2 dergoes slow conversion to the cobaltacyclopentadiene complex (h -C5H5)(PPh3)Co{k -(TMS)C] 2 September 2013 C(CO2Et)C(TMS)]C(CO2Et)} (7, TMS ¼ SiMe3). Alternatively, reaction of 6 and PhC^CPh forms the Accepted 2 September 2013 5 2 1 2 ¼ metallacyclopentadiene regioisomers (h -C5H5)(PPh3)Co{k -(R )C]C(R )C(Ph) C(Ph)} [9-major, 1 2 1 2 R ¼ SiMe3,R ¼ CO2Et; 9-minor, R ¼ CO2Et, R ¼ SiMe3). The metallacycle substitution pattern in 9- Keywords: major and 9-minor is readily deduced from the 1H NMR spectral resonances of the diastereotopic Cobaltacycle ethoxycarbonyl hydrogens. When the diastereotopic hydrogens of the ethoxycarbonyl have similar Cobaltacyclopentadiene b Cobaltealkyne chemical shifts the ester is situated on the -carbon of the metallacycle. When the methylene hydrogens Metallacyclopentadiene give rise to well-separated resonances the ethoxycarbonyl is situated on the a-carbon of the metallacycle. Trimethylsilyl alkyne The solid state structures of 7 and 9-major were determined by X-ray crystallography. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction characterized cobaltacyclopentadienes bearing trimethylsilyl ring substituents. Here we report the synthesis of the cobaltealkyne 5 2 Trimethylsilyl substituted alkynes play a key role in many re- complex, (h -C5H5)(PPh3)Co[h -(Me3Si)C^C(CO2Et)] (6), and its actions that proceed via cobaltacycle intermediates [1e5]. Voll- conversion to the first trimethylsilyl-substituted cobaltacyclo- hardt pioneered the use of trimethylsilyl alkynes for regio- and pentadiene complexes to be characterized by X-ray crystallography. chemoselective reactions involving metallacyclopentadiene in- The substitution pattern in chiral cobaltacyclopentadienes bearing termediates [1,2], as highlighted by the cyclization of 1 and (TMS) an ethoxycarbonyl ring substituent is readily deduced from the 1H C^C(R) to give 2,3-dihydro-5(1H)-indolizinone derivatives 2 NMR resonances of the ester ring substituent. When the diaster- (Scheme 1) [5]. In the cobaltacyclobutene area, we previously found eotopic hydrogens of the ethoxycarbonyl have similar chemical that alkyne complex 3 undergoes regio- and diastereoselective shifts the ester is situated on the b-carbon of the metallacycle. cyclization to cobaltacyclobutene 4 [4a,b], which in turn undergoes When the methylene hydrogens give rise to well-separated reso- regioselective coupling reactions with diazo compounds [4c], al- nances the ethoxycarbonyl is situated on the a-carbon of the kenes [4d], and alkynes [4e], as well as oxidation to trisubstituted metallacycle. furan 5 [4f] (Scheme 1). The trimethylsilyl alkyne substituent plays an important role in 2. Results and discussion the high degree of selectivity observed in the 1 to 2 and 3 to 5 cyclizations. In the course of our studies on the conversion of co- In order to prepare an analog of cobaltealkyne 3,weempl- e fi 5 balt alkyne complexes to cobaltacycles, we were surprised to nd oyed the room temperature reaction of (h -C5H5)Co(PPh3)2 and that there have been no previous examples of structurally 1-(ethoxycarbonyl)-2-(trimethylsilyl)acetylene in dry tetrahy- drofuran. The resultant air stable cobaltealkyne complex,(h5- 2 C5H5)(PPh3)Co[h -(Me3Si)C^C(CO2Et)] (6), was isolated as or- e * Corresponding author. ange brown crystals in 83% yield (Scheme 2). In the IR spectrum À1 À1 E-mail address: [email protected] (J.M. O’Connor). (neat, NaCl) of 6, n(C^C) is observed at 1797 cm ; a 385 cm 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.09.002 K.D. Bunker et al. / Journal of Organometallic Chemistry 749 (2014) 100e105 101 Scheme 1. Regioselective cyclizations of TMS substituted alkynes. 13 shift to lower wavenumber relative to the corresponding value In the C NMR spectrum (C6D6)of7, six downfield resonances À1 13 for the free alkyne (2182 cm ). In the CNMR(C6D6)spectrum between 150 and 182 ppm are attributed to the four metallacycle of 6, the alkyne “sp” hybridized carbon resonances are observed ring carbons and the two ester carbonyl carbons. Resonances at at d 106.5 (JPC ¼ 10.7 Hz) and 105.1 (JPC z 0Hz);11e13 ppm d 179.5 (JPC ¼ 29.0 Hz) and 181.5 (JPC ¼ 26.7 Hz) are readily assigned downfield of the corresponding carbon resonances for the free to the a-carbons of the metallacycle based on the magnitude of the alkyne. For comparison, the alkyne “sp” carbons in (h5- carbon-phosphorus coupling constants. The b-carbons of the 2 C5H5)(PPh3)Co[h -(Ph)C^C(Ph)] are observed at essentially the metallacycle are tentatively assigned to doublets at d 167.3 same chemical shift (d 90.4) as those in diphenylacetylene [6]. (JPC ¼ 5.3 Hz) and 167.7 (JPC ¼ 3.0 Hz), and resonances at 163.4 (s) The similar chemical shift values for the two alkyne carbons in 6 and 176.2 (s) are assigned to the ester carbonyl carbons. For com- may be contrasted to those observed for more polar cobalte parison, one of the few cobaltacyclopentadiene analogs of 7 for 13 5 2 alkyne complex 3 which are observed at d 105.5 (d, JPC ¼ 3.5 Hz) which C NMR data have been reported, (h -C5H5)(PPh3)Co[k - and 118.1 (d, JPC ¼ 11.6 Hz) [4a].Inthecaseof3,itisthecarbon (Ph)C]C(Ph)C(Ph)]C(Ph)], exhibits metallacycle a- and b-carbon bearing the electron-withdrawing sulfone substituent that ex- resonances at d 167.7 (JPC ¼ 28 Hz) and 158.5 (JPC ¼ 2 Hz), respec- hibits the larger carbonephosphorus coupling constant, leading tively [6,7]. us to tentatively assign the d 106.5 resonance in 6 to the alkyne The minor product observed in the conversion of 6 to 7 exhibits carbon bearing the electron-withdrawing ester substituent. a TMS resonance at d 0.26 (18H) in addition to the C5H5 resonance In examining the solution behavior of 6, we observed the for- at 5.04 (5H). Thus the minor product has incorporated two equiv- mation of a bis(trimethylsilyl) substituted cobaltacyclopentadiene alents of TMSC^CCO2Et into a symmetric structure. Precedent complex (Scheme 2). Thus, when a benzene-d6 solution of 6, con- for conversion of trimethylsilyl alkynes to cyclobutadiene com- 1 taining triphenylphosphine (0.2 equiv), was monitored by H NMR plexes is found in the reaction of (Cp)Co(PPh3)2 and TMSC^ 4 spectroscopy at room temperature, complex 7 was observed to CC6H4C6H4C^TMS to give h -cyclobutadiene complexes (8, form over the course of 11 days, as indicated by the appearance of a Scheme 3) [8]. The first example of an isolated cobaltacyclopenta- new C5H5 resonance at d 4.97. In addition to the C5H5 resonance for diene complex bearing trimethylsilyl substituents (9) was also 7, a singlet at d 5.04 indicated that a minor cyclopentadienyl cobalt formed from (Cp)Co(PPh3)2 and a diyne [8]. Because the minor product had formed in <3% yield. product observed in the conversion of 6 to 7 was not successfully 1 2 In the H NMR spectrum (C6D6) of isolated 7, resonances at d 7.04 isolated it is not possible to distinguish between k -cobaltacyclo- 4 (m, 9H) and 7.70 (m, 6H) were indicative of a PPh3 ligand on cobalt, pentadiene and h -cyclobutadiene structures. and two singlets at d 0.13 (9H, SiMe3) and 0.21 (9H, SiMe3) indicated The solid state structure of 7 was determined by X-ray crystal- that 7 had formed from a head-to-tail coupling of two alkynes. The lographic analysis of an orangeebrown single crystal (Fig. 2, methylene hydrogens for one of the ethoxycarbonyl substituents Tables 1 and 2). A comparison of the structure for 7 to that of the 5 2 are observed as a two-proton multiplet at d 4.22 whereas the other parent cobaltacyclopentadiene, (h -C5H5)(PPh3)Co[k -CH] ethoxycarbonyl exhibits two methylene hydrogen resonances at CHCH]CH] (8 [3o]) reveals a number of significant perturbations 3 2 3.86 (m, JHH ¼ 7.2 Hz, JHH ¼ 11 Hz, 1H) and 4.03 (m, 1H) (Fig. 1). in the bond distance and angle metrics that are due to the large size The d 3.86 and 4.03 resonances are assigned to the diastereotopic of the TMS substituent (cone angle ¼ 118 [9], cyclohexane A-value hydrogens of the ethoxycarbonyl substituent on the a-carbon of the 2.5 kcal/mol [10]).
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