Non-Selective Dimerization of Vinyl Silanes by the Putative (Phenanthroline)PdMe Cation to 1,4-Bis(trialkoxysilyl)butenes

: Activation of the dialkylpalladium complex (phen)Pd(CH 3 ) 2 (phen = 1,10-phenanthroline) with B(C 6 F 5 ) 3 affords a competent catalyst for the dimerization of vinyl silanes. All organic products of the catalytic dimerization of trialkoxyvinylsilanes were characterized by in situ NMR spectroscopy and GC–MS. The putative palladium cation was characterized by NMR spectroscopy. Upon activation, the palladium complex generated products in moderate yield (60–70%) and selectivity (~60:40, dimer:disproportionation products).


Results and Discussion
The cationic system (Phen)PdMe is readily prepared by mixing the known dimethyl precursor, (Phen)PdMe 2 [58], with one equivalent of B(C 6 F 5 ) 3 (Scheme 2). Upon mixing, the newly formed compound is readily soluble in methylene chloride. When carried out in CD 2 Cl 2 at room temperature, formation of a new ionic complex is clear from NMR analysis; the 11 B NMR of the reaction mixture shows a new sharp peak at −16 ppm, while, in the 1 H NMR of the same mixture, a new singlet shows up at 0.2 ppm. These new NMR signals represent the tetra-coordinated anionic boron species [MeB(C 6 F 5 ) 3 ] − [59,60]. From these observations, the formation of the ionic species [(Phen)PdMe] + [MeB(C 6 F 5 ) 3 ] − is proposed.
Subsequently, we applied this complex as the catalyst (1 mol %) in the homodimerization of trimethoxy(vinyl)silane (1a) to furnish 1,4-bis(trimethoxysilyl)butene isomers (2aa and 3aa), 1,2-bis(triethoxy(silane))ethylene (4aa), and 1,1′-bis(trimethoxysilylmethyl)ethylene (5aa) (Scheme 3). Organic reaction products were identified based upon GC-MS and NMR analysis (see Supplementary Materials). There is moderate selectivity for homodimers 2aa and 3aa over disproportionation products 4aa and 5aa. Notably, control experiments that employed (Phen)PdMe2 or B(C6F5)3 alone, under otherwise identical conditions, did not affect the dimerization reaction. We believe this strongly implicates the cationic, coordinatively unsaturated [(Phen)PdMe] + to be responsible for catalytic activity. Under these non-optimized reaction conditions triethoxy(vinyl)silane also dimerizes to form the corresponding dimers with 70% combined yield as a mixture of isomers (Scheme 4). Disproportionation of (vinyl)silanes was somewhat minimized in the reaction of the bulkier substrate 1b compared to 1a; 1,2-and 1,1-bis(trialkoxylsilyl)ethene isomers produced in combined yields of 38% (4aa and 5aa, Scheme 4) and 24% (4bb and 5bb, Scheme 4) with trimethoxy-and triethoxy(vinyl)silane, respectively. In competing reactions of 1a with styrene (1c), under similar reaction conditions, the cross-dimer product 6 was obtained as the major product after a 96-h reaction period (Scheme 5, top). At 96 h, the conversion of trimethoxy(vinyl)silane was only 76%. Homodimerization and disproportionation of trimethoxy(vinyl)silane also took place, forming corresponding products in trace amounts (as evidenced by GC-MS). In a similar competition experiment between trimethoxy(vinyl)silane and ethylene (1d), no cross-dimer product between alkene monomers or homodimerization product of trimethoxy(vinyl)silane was observed (Scheme 5, bottom). Taken together, our results lead us to propose the catalytic cycle shown in Scheme 6. Of particular significance, the work of Elsby and Johnson, in closely related C-H silylation chemistry employing a nickel catalyst, lends support to our proposed mechanism [61]. Under these non-optimized reaction conditions triethoxy(vinyl)silane also dimerizes to form the corresponding dimers with 70% combined yield as a mixture of isomers (Scheme 4). Disproportionation of (vinyl)silanes was somewhat minimized in the reaction of the bulkier substrate 1b compared to 1a; 1,2-and 1,1-bis(trialkoxylsilyl)ethene isomers produced in combined yields of 38% (4aa and 5aa, Scheme 4) and 24% (4bb and 5bb, Scheme 4) with trimethoxy-and triethoxy(vinyl)silane, respectively. Subsequently, we applied this complex as the catalyst (1 mol %) in the homodimerization of trimethoxy(vinyl)silane (1a) to furnish 1,4-bis(trimethoxysilyl)butene isomers (2aa and 3aa), 1,2-bis(triethoxy(silane))ethylene (4aa), and 1,1′-bis(trimethoxysilylmethyl)ethylene (5aa) (Scheme 3). Organic reaction products were identified based upon GC-MS and NMR analysis (see Supplementary Materials). There is moderate selectivity for homodimers 2aa and 3aa over disproportionation products 4aa and 5aa. Notably, control experiments that employed (Phen)PdMe2 or B(C6F5)3 alone, under otherwise identical conditions, did not affect the dimerization reaction. We believe this strongly implicates the cationic, coordinatively unsaturated [(Phen)PdMe] + to be responsible for catalytic activity. Under these non-optimized reaction conditions triethoxy(vinyl)silane also dimerizes to form the corresponding dimers with 70% combined yield as a mixture of isomers (Scheme 4). Disproportionation of (vinyl)silanes was somewhat minimized in the reaction of the bulkier substrate 1b compared to 1a; 1,2-and 1,1-bis(trialkoxylsilyl)ethene isomers produced in combined yields of 38% (4aa and 5aa, Scheme 4) and 24% (4bb and 5bb, Scheme 4) with trimethoxy-and triethoxy(vinyl)silane, respectively. In competing reactions of 1a with styrene (1c), under similar reaction conditions, the cross-dimer product 6 was obtained as the major product after a 96-h reaction period (Scheme 5, top). At 96 h, the conversion of trimethoxy(vinyl)silane was only 76%. Homodimerization and disproportionation of trimethoxy(vinyl)silane also took place, forming corresponding products in trace amounts (as evidenced by GC-MS). In a similar competition experiment between trimethoxy(vinyl)silane and ethylene (1d), no cross-dimer product between alkene monomers or homodimerization product of trimethoxy(vinyl)silane was observed (Scheme 5, bottom). Taken together, our results lead us to propose the catalytic cycle shown in Scheme 6. Of particular significance, the work of Elsby and Johnson, in closely related C-H silylation chemistry employing a nickel catalyst, lends support to our proposed mechanism [61]. In competing reactions of 1a with styrene (1c), under similar reaction conditions, the cross-dimer product 6 was obtained as the major product after a 96-h reaction period (Scheme 5, top). At 96 h, the conversion of trimethoxy(vinyl)silane was only 76%. Homodimerization and disproportionation of trimethoxy(vinyl)silane also took place, forming corresponding products in trace amounts (as evidenced by GC-MS). In a similar competition experiment between trimethoxy(vinyl)silane and ethylene (1d), no cross-dimer product between alkene monomers or homodimerization product of trimethoxy(vinyl)silane was observed (Scheme 5, bottom). Taken together, our results lead us to propose the catalytic cycle shown in Scheme 6. Of particular significance, the work of Elsby and Johnson, in closely related C-H silylation chemistry employing a nickel catalyst, lends support to our proposed mechanism [61].

Scheme 6.
Proposed mechanism of dimerization and disproportionation to account for product distribution.

Conclusions
In summary, we disclosed a (vinyl)silane dimerization reaction leading to a mixture of 1,4-bis(trimethylsilyl)but-2-ene and 1,4-bis(trimethylsilyl)but-1-ene as major products. This reaction is believed to be mediated by the [(Phen)PdMe] + [MeB(C6F5)3] − complex generated in situ at a 1 mol % catalyst loading. The substrate scope of the reaction over various (vinyl)silanes and more detailed mechanistic studies are underway in our laboratories to elaborate on this preliminary communication.

Scheme 6.
Proposed mechanism of dimerization and disproportionation to account for product distribution.

Conclusions
In summary, we disclosed a (vinyl)silane dimerization reaction leading to a mixture of 1,4-bis(trimethylsilyl)but-2-ene and 1,4-bis(trimethylsilyl)but-1-ene as major products. This reaction is believed to be mediated by the [(Phen)PdMe] + [MeB(C6F5)3] − complex generated in situ at a 1 mol % catalyst loading. The substrate scope of the reaction over various (vinyl)silanes and more detailed mechanistic studies are underway in our laboratories to elaborate on this preliminary communication. Scheme 6. Proposed mechanism of dimerization and disproportionation to account for product distribution.

Conclusions
In summary, we disclosed a (vinyl)silane dimerization reaction leading to a mixture of 1,4-bis (trimethylsilyl)but-2-ene and 1,4-bis(trimethylsilyl)but-1-ene as major products. This reaction is believed to be mediated by the [(Phen)PdMe] + [MeB(C 6 F 5 ) 3 ] − complex generated in situ at a 1 mol % catalyst loading. The substrate scope of the reaction over various (vinyl)silanes and more detailed mechanistic studies are underway in our laboratories to elaborate on this preliminary communication.