Syntheses of 2-silicon-substituted 1,3-dienes

Journal of Organometallic Chemistry 754 (2014) 88e93

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Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem

Syntheses of 2-silicon-substituted 1,3-dienes Partha P. Choudhury, Christopher S. Junker, Ramakrishna R. Pidaparthi, Mark E. Welker* Department of Chemistry, Wake Forest University, P.O. Box 7486, Winston-Salem, NC 27109, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 October 2013 Received in revised form 23 December 2013 Accepted 24 December 2013

A number of 2-silicon substituted 1,3-dienes have been prepared by one of three routes: 1) Reactions of 1,3-dienyl Grignard reagents with silyl electrophiles or silyl Grignard reagents with 1,3-dienyl electrophiles; 2) Hydrosilylation of enynes; 3) Enyne cross metathesis. The strengths and limitations of each preparative method are discussed. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: 1,3-Dienes Metal dienyls Grignard reactions Hydrosilylation Enyne cross metathesis

1. Introduction We have been interested in the preparation and reaction chemistry of metal substituted dienes for over 15 years. Initially, we prepared a number of transition metal substituted dienes [1] for these studies but more recently we have been interested in the investigation of silicon and boron substituted dienes [2]. We have reported the preparation of 2-silicon substituted 1,3-butadienes by one of three different routes and demonstrated that they could be used in sequential DielseAlder/cross coupling reactions but we have never discussed the strengths and limitations of these different silicon substituted diene preparative routes [3]. Here we report the preparation of new 2-silicon substituted 1,3-dienes by one of three different routes and focus our discussion on a comparison of those routes. 2. Results and discussion 2.1. Preparation of 2-silicon substituted 1,3-dienes via Grignard chemistry Preparation of 2-silicon substituted 1,3 dienes via chemistry can be accomplished by one of two methods: ration of a dienyl Grignard reagent and its addition to electrophile or ii) preparation of a silicon containing

Grignard i) prepaa silicon Grignard

* Corresponding author. Fax: þ1 336 758 4656. E-mail address: [email protected] (M.E. Welker). 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.12.046

reagent and its addition to a dienyl electrophile (Scheme 1). In our prior work, we have been successful in preparing Grignard reagents from simple halodienes like chloroprene (1) followed by treatment of those reagents with boron substituted electrophiles [4] or alkoxy silyl electrophiles [3a], but had always encountered some difficulties in attempting to use Grignard reagents prepared from more highly substituted 1,3-dienes [5]. We have found the same trends to hold for silicon substituted diene synthesis with alkyl or aryl silyl reagents. So while a Grignard reagent prepared from chloroprene (1) reacts with dimethylphenylsilyl chloride or dichlorodimethylsilane (followed by hydrolysis) [6] to provide new dienes (2 & 3), we found it most convenient to use trimethylsilylmagnesium chloride addition to the more highly substituted halodienes (4 and 6) to produce the trimethylsilyl substituted dienes 5 [7] and 7. Attempts to prepare Grignard reagents from dienes 4 and 6 and react them with TMSCl produced diene dimerization and decomposition products rather than 5 and 7. Diene 2 participated in a DielseAlder reaction cleanly with N-phenylmaleimide in high yield to produce 8.

2.2. Preparation of 2-silicon substituted 1,3-dienes via hydrosilylation of enynes Our next attempts at the synthesis of 4-substituted 2-silyl dienes involved the cis-hydrosilylation of 1,3-enynes (Scheme 2). (E)4-Phenyl-3-buten-1-yne (9) was synthesized from trans-cinnamaldehyde via the Colvin rearrangement [8]. Ruthenium-catalyzed hydrosilylation of enyne (9) with triethoxysilane (10) was

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Scheme 1. Preparation of silyl dienes via Grignard chemistry.

performed with Grubbs 1st generation catalyst and cyclopentadienyl ruthenium salt (11). These reactions produced mixtures of 2- and 1-siloxy substituted dienes (12 and 13). We also investigated a copper(I)-catalyzed hydrosilylation protocol employing silyl boranes and this method also leads to a mixture of 2-silicon and 1-silicon substituted 1,3 diene products (15:16) [9]. The authors who first reported this method hypothesized a mechanism involving transmetallation to a cupric silane species and subsequent silylcupration of an alkyne. Under these conditions we were able to effect hydrosilylation using (dimethylphenylsilyl) boronic acid pinacol ester (14) to yield dienes (15 & 16) in moderate yield and regioselectivity. In addition to these enyne hydrosilylation reactions, we also executed new examples of Lee’s protocol [10] which utilizes an initial silylation of a terminal alkyne followed by a carbonyl condensation (Scheme 3). The overall reaction is an example of an unusual trans hydrosilylation of an alkyne. With commercially

available 3-methyl-3-buten-1-yne (17 R1 ¼ H, R2 ¼ Me) the yield for 2 steps (condensation with diisopropylchlorosilane to yield 18a followed by condensation with acetone to yield 19a) was an acceptable 72%. Silylation of that same alkyne (17 R1 ¼ H, R2 ¼ Me) with dimethylchlorosilane also yields an alkyne (18b) in good yield. The crude product after condensation with acetone contains 19b (by 1H NMR) but this compound did not survive chromatographic purification on silica or alumina. 4-Phenyl-3-buten-1-yne (9) also condenses with diisopropylchlorosilane to yield 18c in acceptable yield and that alkyne condenses with formaldehyde to produce another example of a siloxycyclopentene (19c) in good yield. 2.3. Preparation of 2-silicon substituted-1,3-dienes via enyne metathesis In the past we had also reported the use of intermolecular enyne cross metathesis to prepare 2-trialkoxysilyl substituted 1,3 dienes

Scheme 2. Enyne cis-hydrosilylation.

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Scheme 3. Enyne trans-hydrosilylation.

[3d,e] but had not tried this method for triaryl or trialkylsilyl substituted alkynes or for any intramolecular enyne cross metathesis. We first tried this reaction with triphenylsilylacetylene (20) and styrene (21), however we only isolated 2-triphenylsilyl-1,3butadiene (23) in good yield (69%) along with trans-stilbene (24) (Scheme 4). We hypothesized that the ethylene atmosphere typically used in these reactions contributed to the formation of 23 however, in the absence of ethylene the reaction still resulted in formation of 23. Given that observation, it appears that ruthenium benzylidene catalyst must react much faster with styrene (21) to produce trans-stilbene (24) and methylidene catalyst (25) than it reacts with triphenylsilylacetylene (20) so it is the methylidene catalyst (25) that reacts with 20 and leads to the isolated silyl substituted diene product (23). We also tried enyne metathesis on a commercially available internal alkyne (26) which bears a trialkylsilyl group on the alkyne (Scheme 5) [11]. This reaction resulted in low yield and the use of an internal alkyne changed the regiochemistry of the metathesis reaction to yield 1,2,3-trisubstituted diene (27). We had never attempted an intramolecular version of this enyne cross metathesis reaction but we found that it could be effected for a benzyldimethyl substituted heptenyne (29) to produce silyl substituted diene (30) (Scheme 6). However, when we went to the next higher homolog, i.e. an octenyne (32), we could prepare the enyne (32) in good yield but we noted only enyne decomposition with different ruthenium catalysts, catalyst loadings, and solvents in attempts to produce 33 by enyne metathesis. The observed product 30 is most easily rationalized through an yne then ene cross metathesis mechanism rather than an ene then yne mechanism [12].

Scheme 4. Enyne metathesis of triphenylsilylacetylene.

3. Conclusions In cases where 2-halo-1,3 dienes are commercially available or synthetically accessed easily using Grignard chemistry to make 2silyl-1,3 dienes is the preferred method of synthesis. When the 2halo-1,3-dienes are not readily available, cis-hydrosilylation of enynes containing terminal alkynes leads to mixtures of regioisomeric products. Compare, for instance, the preparation of diene 5 using Grignard chemistry to the preparation of structurally related diene 15 by hydrosilylation to note the advantages of the Grignard approach when feasible. Trans-hydrosilylation of enynes in conjunction with carbonyl compound trapping is a viable synthetic route to siloxy substituted alkenyl cyclopentenes (19). Intermolecular enyne cross metathesis can be used to make 4-alkyl or 4aryl-2-silicon substituted dienes if the terminal alkynes used for cross metathesis bear small substituents on silicon [3d]. The reaction fails as a route to make 4-substituted-2-silicon dienes if the terminal alkyne used bears silicon with large substituents such as 20. Intramolecular enyne cross metathesis can be used to make additional silyl substituted alkenyl cyclopentenes such as 30, but fails as a route to make a silyl substituted alkenyl cyclohexenes such as 33.

4. Experimental 4.1. General procedures are part of the Supplementary material 4.1.1. Synthesis of (buta-1,3-dien-2-yl)dimethyl(phenyl)silane (2) Chloroprene (4.58 g, 51.7 mmol) and phenyldimethylchlorosilane (8.03 g, 47.1 mmol) were used according to the general procedure to yield a light yellow colored crude product (14.4 g) as a mixture of diene and xylenes. The crude product was subjected to fractional distillation at reduced pressure (20 mm, 45  C) which resulted in a brown colored liquid, which was further purified by flash chromatography (100% pentanes) to yield the title compound (2) as a light yellow liquid (8.36 g, 44.4 mmol, 97%): Rf 0.63 (100% pentanes); 1H NMR (500 MHz, CDCl3) d 7.49e7.55 (m, 2H), 7.31e7.37 (m, 3H), 6.46 (dd, J ¼ 17.7, 10.9 Hz, 1H), 5.88 (d, J ¼ 3.2 Hz, 1H), 5.51 (d, J ¼ 3.2 Hz, 1H), 5.10 (d, J ¼ 17.7 Hz, 1H), 5.00 (d, J ¼ 10.9 Hz, 1H), 0.43 (s, 6H); 13C

Scheme 5. Enyne metathesis of an internal trimethylsilyl substituted alkyne.

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Scheme 6. Intramolecular silyl enyne cross metathesis.

NMR (75 MHz, CDCl3) d 147.6, 141.1, 138.2, 133.9, 130.4, 129.0, 127.8, 116.7, 2.3; HRMS calcd for C12H16Si (Mþ) 188.1021, found 188.1020. 4.1.2. Synthesis of (buta-1,3-dien-2-yl) dimethylsilanol (3) Chloroprene in 50% xylenes (1.0 mL, 5.15 mmol) and dichlorodimethylsilane (0.645 g, 5 mmol) were used according to the general procedure above (1 M acetate buffer of pH 5 was used instead of 0.5 M HCl for the quench) to yield a light yellow colored liquid. 1H NMR of the crude product (1.1 g) showed a mixture of xylenes, diene and the corresponding disiloxane dimer. Kugelrohr distillation at 60  C (4 mm Hg) of the crude product yielded the target compound (3) (0.300 g, 2.3 mmol, 46%). 1H NMR (300 MHz CDCl3) d 6.46 (dd, J ¼ 17.8, 10.8 Hz, 1H), 5.77 (d, J ¼ 3.0 Hz, 1H), 5.56 (d, J ¼ 2.9 Hz, 1H), 5.37 (d, J ¼ 18.2 Hz, 1H), 5.12 (d, J ¼ 10.8 Hz, 1H), 2.25 (bs, 1H), 0.30 (s, 6H); 13C NMR (75 MHz, CDCl3) d 148.65, 140.88, 129.58, 116.56, 0.32; EIMS calcd for C6H13OSi (M þ H)þ 129.06, found 129.0. 4.1.3. Synthesis of trimethyl[(E)-4-phenyl-1,3-butadien-2-yl]silane (5) 1-[(E)-3-Bromobuta-1,3-dienyl]benzene [13] (4) (1.99 g, 9.49 mmol) and trimethylsilylchloride (1.37 g, 12.6 mmol) were used according to the general method, producing the crude compound as a dark brown liquid. Purification by flash chromatography (hexanes/Et2O, 9:1) yielded compound 5 as a brown-yellow oil (1.56 g, 7.73 mmol, 85%). This compound has been reported previously by a different route but no characterization data were provided [7]: Rf 0.15 (hexanes/Et2O, 9:1); 1H NMR (500 MHz, CDCl3) d 7.43 (d, J ¼ 7.7 Hz, 2H), 7.34 (t, J ¼ 7.7 Hz, 2H), 7.24 (t, J ¼ 7.4 Hz, 1H), 6.94 (d, J ¼ 16.5 Hz, 1H), 6.64 (d, J ¼ 16.5 Hz, 1H), 5.89 (d, J ¼ 3.0 Hz, 1H), 5.53 (d, J ¼ 3.0 Hz, 1H), 0.27 (s, 9H); 13C NMR (75 MHz, CDCl3) d 148.9, 137.7, 134.0, 130.5, 128.6, 128.5, 127.3, 126.3, 0.8. 4.1.4. Synthesis of (1-cyclohexylideneprop-2-en-2-yl) trimethylsilane (7) Chlorotrimethylsilane (0.685 g, 6.30 mmol) and (2bromoallylidene)cyclohexene [14] (6) (0.929 g, 4.64 mmol) were used according to the general procedure. The resulting dark brown crude residue after purification by flash chromatography (hexanes/ Et2O, 15:1 / 9:1) yielded compound 7 as a browneyellow oil (0.483 g, 2.49 mmol, 47%): Rf 0.84 (hexanes/Et2O, 15:1); 1H NMR (500 MHz, CDCl3) d 5.71 (s, 1H), 5.40e5.49 (m, 2H), 2.10e2.23 (m,

4H), 1.54e1.60 (m, 4H), 1.41e1.50 (m, 2H), 0.07 (s, 9H); 13C NMR (75 MHz, CDCl3) d 150.3, 140.5, 125.1, 123.5, 37.4, 29.3, 29.0, 28.3, 26.9, 1.95; HRMS calcd for C12H22Si (Mþ) 194.1491, found 194.1489. 4.2. DielseAlder reactions 4.2.1. Synthesis of 3a,4,7,7a-tetrahydro-5-[dimethyl(phenyl)silyl]2-phenyl-2H-isoindole-1,3-dione (8) Diene (2) (0.302 g, 1.60 mmol) and N-phenylmaleimide (0.103 g, 0.595 mmol) were dissolved in THF (5 mL) and degassed in a sealed tube. After heating for 4 h at 90  C, the reaction mixture was filtered through a cotton plug and volatiles were removed by rotary evaporation. The resulting yellow colored residue was purified by flash chromatography with the excess diene (2) eluting (0.151 g, 0.802 mmol, 78% recovery: Rf 0.84, pentane/diethyl ether, 1:1) followed by the cycloadduct 8 as a colorless, clear viscous liquid (0.204 g, 0.564 mmol, 98%): Rf 0.29 (pentane/diethyl ether, 1:1); 1H NMR (500 MHz, CDCl3) d 7.41e7.54 (m, 4H), 7.29e7.41 (m, 4H), 7.09e7.20 (m, 2H), 6.34 (m, 1H), 3.18e3.30 (m, 2H), 2.71e2.88 (m, 2H), 2.22e2.36 (m, 2H), 0.36 (s, 3H), 0.35 (s, 3H); 13C NMR (75 MHz, CDCl3) d 179.1, 178.7, 140.7, 138.4, 137.0, 133.9, 132.0, 129.2, 129.0, 128.4, 127.8, 126.3, 39.3, 39.2, 26.4, 25.0, 3.88, 3.89; HRMS calcd for C22H23NO2Si (Mþ) 361.1498, found 361.1490. Anal. calcd for C22H23NO2Si: C, 73.10; H, 6.42. Found: C, 72.63; H, 6.38. 4.3. Cis-hydrosilylation reactions 4.3.1. Copper-catalyzed hydrosilylation of (E)-4-phenyl-3-buten-1yne (9) To a 5-mL sealable microwave tube equipped with a stir bar, copper(I) chloride (0.031 g, 0.31 mmol), JohnPhos (0.037 g, 0.12 mmol), and sodium tert-butoxide (0.015 g, 0.16 mmol) were suspended in THF (2.6 mL) and degassed with argon. (Dimethylphenylsilyl)boronic acid pinacol ester (14) (234 mL, 0.86 mmol) was added to the reaction vessel, the tube was sealed with a crimp cap, and placed in a circulating chiller at 0  C. After 30 min (E)-4-phenyl3-buten-1-yne (9) (0.108 g, 0.84 mmol) and MeOH (65 mL, 1.61 mmol) were injected into the reaction vessel. After 24 h at 0  C the reaction mixture was filtered through a plug of celite with DCM (100 mL). The organic solution was then condensed by rotary evaporation and dried under reduced pressure to yield a greenish brown liquid (0.386 g). The crude product was purified on silica gel

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with 20:1 hexanes/ethyl acetate resulting in dienes 15:16 as a yellow liquid (0.091 g, 0.34 mmol, 41%): Rf 0.5 (hexanes/ethyl acetate 20:1); 15 (major product): 1H NMR (300 MHz, CDCl3) d 7.59e 7.14 (m, 10H), 6.92 (d, J ¼ 16.4 Hz, 1H), 6.48 (d, J ¼ 16.2 Hz, 1H), 6.00 (d, J ¼ 2.9 Hz, 1H), 5.57 (d, J ¼ 2.9 Hz, 1H), 0.49 (s, 6H). 13C NMR (75.5 MHz, CDCl3) d 146.8 (C), 138.1 (C), 137.5 (C), 133.9 (CH), 133.5 (CH), 131.5 (CH), 130.5 (CH2), 129.1 (CH), 128.5 (CH), 127.9 (CH), 127.3 (CH), 126.2 (CH), 2.1 (CH3). GC/MS: m/z (relative, %): 265.2 (2) [M þ 1], 264.1 (8) [Mþ], 204.1 (2), 173.1 (3), 171.0 (3), 145.0 (4), 137.1 (4), 136.1 (14), 135.1 (100), 128.1 (27), 121.0 (4), 107.0 (6), 105.0 (8), 91.1 (4), 78,1 (7), 51.1 (3). 2:1 ratio of a:b regioisomers was determined by the integrations of vinyl resonances in the 1H NMR spectrum. 4.4. Trans-hydrosilylation reactions 4.4.1. Trans-hydrosilylation/carbonyl condensation of enynes to prepare siloxycyclopentenes (19) In a 100 mL flame dried flask equipped with a magnetic stir bar, 3-methyl-3-buten-1-yne (1.39 g, 20.9 mmol) was added followed by anhydrous THF (20 mL). The solution was cooled to 78  C and n BuLi (15.0 mL of a 1.6 M soln in hexanes, 23.0 mmol) was added slowly and stirred for 20 min. Chlorodiisopropylsilane (3.44 g, 23.0 mmol) in THF (10 mL) was added drop wise to the above reaction mixture. After stirring for 30 min at 78  C it was allowed to warm and stirred 12 h at RT. The milky white reaction mixture was diluted with diethyl ether (100 mL) and washed with half saturated NH4Cl solution (100 mL). The organic layer was dried over MgSO4 and the solvent was evaporated under reduced pressure to give a light yellow colored liquid (18a) (3.279 g, 18.18 mmol, 87%): 1H NMR (300 MHz, CDCl3) d 5.37e5.38 (m, 1H), 5.27 (p, J ¼ 1.66 Hz, 1H), 3.75 (bs, 1H), 1.90 (t, J ¼ 1.14 Hz, 3H), 1.06 (m, 14H); 13C NMR (75 MHz, CDCl3) d 126.79, 123.04, 109.17, 86.68, 23.88, 18.49, 18.24, 10.86; HRMS calcd for C11H20Si (Mþ) 180.1333, found 180.1333. In a 20 mL flame dried flask, anhydrous THF (5 mL) was added followed by acetone (0.269 g, 4.64 mmol). To that solution, the silylation product (18a) (0.92 g, 5.11 mmol) was added followed by 95% KOtBu (0.052 g, 0.464 mmol). The solution was stirred for 40 min at RT and then washed with half saturated NH4Cl solution (20 mL) and diethyl ether (20 mL), and the organics dried over MgSO4. Upon removal of the solvent under reduced pressure and flash chromatography with 4% diethyl ether in pentane (Rf 0.44), the target compound (19a) was isolated as a clear liquid, (0.918 g, 3.85 mmol, 83%): 1H NMR (300 MHz, CDCl3) d 6.51 (s, 1H), 4.98 (bs, 1H), 4.79 (bs, 1H), 1.91 (s, 3H), 1.35 (s, 6H), 0.98 (m, 14H); 13C NMR (75 MHz, CDCl3) d 151.04, 142.12, 137.93, 116.31, 82.43, 29.73, 20.7, 17.84, 17.4, 13.19; HRMS calcd for C14H26OSi (Mþ) 238.1753, found 238.1755. 4.4.2. Preparation of 19c (E)-4-Phenyl-3-buten-1-yne (9) was synthesized from transcinnamaldehyde as described previously [8]. Enyne 9 (1.361 g, 10.61 mmol), nBu-Li (7.3 mL of a 1.6 M soln in hexanes, 11.7 mmol), and chlorodiisopropylsilane (1.76 g, 11.7 mmol) were combined and worked up as described above to yield crude 18c (2.343 g) as a dark brown liquid. Flash chromatography using 3% triethylamine in pentane (Rf 0.6) produced 18c as a light yellow colored oil (1.906 g, 7.86 mmol, 74%). 1H NMR (300 MHz, CDCl3) d 7.28e7.40 (m, 5H), 7.03 (d, J ¼ 16.3 Hz, 1H), 6.20 (dd, J ¼ 16.3, 1.04 Hz), 3.80 (bs, 1H), 1.08e1.12 (m, 14H). 13C NMR (75 MHz, CDCl3) d 142.75, 136.05, 128.82, 128.71, 126.31, 107.96, 107.93, 99.56, 18.53, 18.30, 10.93; HRMS calcd for C16H12Si (Mþ) 242.1490, found 242.1486. Compound 18c (0.05 g, 0.206 mmol), paraformaldehyde, (0.0051 g, 0.17 mmol), and 95% KOtBu (0.002 g, 0.17 mmol)) were combined and worked up as described above for 19a to yield a crude product

which was purified by preparative silica gel chromatography and eluted with (3:3:96) triethylamine:diethyl ether:pentane mixture (Rf 0.4). An orange colored liquid (19c) was isolated (0.030 g, 0.11 mmol, 53%). 1H NMR (300 MHz, CDCl3) d 7.23e7.43 (m, 5H), 7.13 (d, J ¼ 15.98 Hz, 1H), 6.85 (bs, 1H), 6.40 (d, J ¼ 15.98 Hz, 1H), 1.06e1.12 (m, 14H); 13C NMR (75 MHz, CDCl3) d 146.44, 137.45, 137.40, 132.71, 128.61, 127.48, 126.24, 72.63, 17. 36, 16.96, 13.27. 4.5. Enyne cross metathesis 4.5.1. (Z)-3-Methylene-5-phenyl-4-(trimethylsilyl)pent-4-en-2-one (27) To a dried 50-mL round bottom flask equipped with a stir bar, HoveydaeGrubbs 2nd generation catalyst (0.165 g, 0.26 mmol) was added, dissolved in DCM (36.5 mL), and degassed with argon. 4(Trimethylsilyl)-3-butyn-2-one (26) (292 mL, 1.78 mmol) and styrene (21) (1.0 mL, 8.7 mmol) were then added successively to the reaction flask. The flask was then fitted with a condenser and heated to 40  C. After 24 h the reaction was cooled to room temperature, condensed by rotary evaporation, and dried under reduced pressure to yield a brown liquid (0.42 g). The crude product was then purified on silica gel with 5:1 hexanes/ethyl acetate as eluent resulting in a yellow oil 27 (0.041 g, 0.17 mmol, 9%): Rf 0.3 (hexanes/ethyl acetate, 5:1); 1H NMR (300 MHz, CDCl3) d 7.30 (m, 5H), 7.16 (s, 1H), 5.94 (d, J ¼ 0.9 Hz, 1H), 5.75 (d, J ¼ 0.9 Hz, 1H), 2.39 (s, 3H), 0.05 (s, 9H). 13C NMR (75.5 MHz, CDCl3) d 200.0 (C]O), 155.7 (C), 144.8 (C), 144.5 (CH), 139.4 (C), 128.5 (CH), 127.8 (CH), 127.3 (CH), 123.0 (CH2), 26.4 (CH3), 0.48 (CH3); HRMS [M þ Na]þ calcd for C15H20NaOSi, 267.1181; found, 267.1194. Regiochemistry supported by HMBC spectroscopy. Double bond isomer supported by NOE spectroscopy. 4.5.2. Intramolecular enyne cross metathesis 6-Hept-1-yne (28) was synthesized from 5-bromo-1-pentene as described previously [15]. The enyne (28) (0.6 g, 6.37 mmol) was treated with nBu-Li (4.5 mL of a 1.6 M soln in hexanes, 7 mmol) and benzylchlorodimethylsilane (1.404 g, 7.5 mmol) as described above for 18aec. A light yellow colored liquid (29) was obtained (1.377 g, 5.68 mmol, 89%). 1H NMR (500 MHz, CDCl3) d 7.22 (t, J ¼ 8.04 Hz, 2H), 7.09 (m, 3H), 5.8 (ddt, J ¼ 17, 10.25, 6.7 Hz, 1H), 5.045 (dd, J ¼ 17.1, 1.72 Hz, 1H), 5.00 (m, 1H), 2.24 (t, J ¼ 7.1 Hz, 2H), 2.19 (s, 2H), 2.14 (q, J ¼ 7.44 Hz, 2H), 1.61 (p, J ¼ 8.04 Hz, 2H), 0.11 (s, 6H); 13C NMR (75 MHz, CDCl3) d 139.22, 137.74, 128.33, 128.06, 124.21, 115.19, 108.55, 83.10, 32.68, 27.68, 26.51, 19.23, 1.91; HRMS calcd for C16H22Si (Mþ) 242.1491, found 242.1492. 4.5.3. Preparation of diene 30 HoveydaeGrubbs second generation catalyst (0.045 g, 0.053 mmol) was added to a flame dried flask equipped with stir bar and 4  A molecular sieves (40% w/w). DCM (3 mL) was added and the solution was degassed. Silyl enyne (29) (0.080 g, 0.353 mmol) was added and refluxed for 12 h under Ar. The reaction was monitored by 1H NMR spectroscopy. Upon completion of the reaction, solvent was removed under vacuum and the crude product was purified by preparative silica gel TLC (Rf 0.66 (triethylamine/Et2O/pentane, 1:1:50)) to yield 30 (0.034 g, 0.140 mmol, 40%). 1H NMR (500 MHz, CDCl3) d 7.19 (t, J ¼ 7.54 Hz, 2H), 7.06 (t, J ¼ 7.38 Hz, 1H), 6.98 (d, J ¼ 8.1 Hz, 2H), 5.8 (bs, 1H), 5.66 (d, J ¼ 2.66 Hz, 1H), 5.36 (d, J ¼ 2.65 Hz, 1H), 2.48e2.51 (m, 4H), 2.26 (s, 2H), 1.9 (p, J ¼ 7.64 Hz, 2H), 0.133 (s, 6H); 13C NMR (75 MHz, CDCl3) d 145.15, 144.19, 140.11, 129.40, 128.27, 128.00, 125.66, 123.96, 33.68, 33.07, 29.7, 25.96, 22.44; HRMS calcd for C16H22Si (Mþ) 242.1491, found 242.1494.

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Acknowledgments We thank the National Science Foundation for their support of this work (CHE-0749759) and the NMR and X-ray instrumentation used to characterize the compounds reported here. The UNC Mass Spectrometry facility and Mass Spectrometry Lab, School of Chemical Sciences, University of Illinois at Urbana-Champaign performed high resolution mass spectral analyses. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jorganchem.2013.12.046. References [1] (a) M.E. Welker, Adv. Cycloaddit. 4 (1997) 149; (b) M.E. Welker, Curr. Org. Chem. 5 (2001) 785. [2] (a) M.E. Welker, Tetrahedron 64 (2008) 11529; (b) D.S.W. Lim, E.A. Anderson, Synthesis-Stuttgart 44 (2012) 983; (c) J.W. Herndon, Coord. Chem. Rev. 256 (2012) 1281; (d) H.F. Sore, W. Galloway, D.R. Spring, Chem. Soc. Rev. 41 (2012) 1845; (e) J.K. Puri, R. Singh, V.K. Chahal, Chem. Soc. Rev. 40 (2011) 1791.

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