Olefin Hydroborations with Diamidocarbene – BH 3 Adducts at Room Temperature

An isolable N,N’-diamidocarbene (DAC) was previously shown to promote the B–H bond activation of various BH3 complexes. The resultant DAC–BH3 adducts facilitated olefin hydroborations under mild conditions and in the absence of exogenous initiators. The substrate scope for such transformations was further explored and is described herein. While organoboranes were obtained in quantitative yields from various terminal and internal olefins, use of the latter substrates resulted in intramolecular ring-expansion of the newly formed DAC–borane adducts.

Unlike NHCs and CAACs, the DACs did not displace the datively bonded Lewis base, but instead facilitated B-H activation.The relative basicity of the coordinated ligand was found to directly correlate with the stability of the corresponding DAC-BH 3 adduct.For example, adduct 1a, which contains SMe 2 , was prone to intramolecular ring-expansion to 2 and de-coordination (Scheme 2).The use of a stronger Lewis base, such as pyridine, afforded an adduct (1b) that exhibited increased stability toward water and air; ring-expansion was not observed, even at elevated temperatures.To explore the hydroboration chemistry displayed by DAC-BH 3 adducts, 1a and 1b were independently treated with a series of unactivated olefins.While 1a was successfully used as a hydroboration reagent and operated in the absence of exogenous radical initiators at room temperature, adduct 1b was found to require relatively high reaction temperatures; as such, subsequent efforts focused on the former.
Unlike NHCs and CAACs, the DACs did not displace the datively bonded Lewis base, but instead facilitated B-H activation.The relative basicity of the coordinated ligand was found to directly correlate with the stability of the corresponding DAC-BH3 adduct.For example, adduct 1a, which contains SMe2, was prone to intramolecular ring-expansion to 2 and de-coordination (Scheme 2).The use of a stronger Lewis base, such as pyridine, afforded an adduct (1b) that exhibited increased stability toward water and air; ring-expansion was not observed, even at elevated temperatures.To explore the hydroboration chemistry displayed by DAC-BH3 adducts, 1a and 1b were independently treated with a series of unactivated olefins.While 1a was successfully used as a hydroboration reagent and operated in the absence of exogenous radical initiators at room temperature, adduct 1b was found to require relatively high reaction temperatures; as such, subsequent efforts focused on the former.

Results and Discussion
As summarized in Scheme 3, the addition of an excess of a terminal olefin, such as 1-hexene, to a solution of 1a in CH2Cl2 resulted in the formation of 3, as determined by 1 H, 13 C, and 11 B NMR spectroscopy as well as high resolution mass spectrometry.Inspection of the 1 H NMR data revealed a diagnostic singlet at δ 6.09 ppm (C6D6), which was assigned to the methine group of 3. When an excess of 1,5-hexadiene was added to 1a, both double bonds underwent hydroboration, and 4 was obtained as the exclusive product (Scheme 4).The ability of 1a to facilitate the hydroboration of internal olefins was also explored.When 1a was treated with an excess of cyclohexene, a product (6) containing only one cyclohexyl moiety was

Results and Discussion
As summarized in Scheme 3, the addition of an excess of a terminal olefin, such as 1-hexene, to a solution of 1a in CH 2 Cl 2 resulted in the formation of 3, as determined by 1 H, 13 C, and 11 B NMR spectroscopy as well as high resolution mass spectrometry.Inspection of the 1 H NMR data revealed a diagnostic singlet at δ 6.09 ppm (C 6 D 6 ), which was assigned to the methine group of 3. When an excess of 1,5-hexadiene was added to 1a, both double bonds underwent hydroboration, and 4 was obtained as the exclusive product (Scheme 4).
Unlike NHCs and CAACs, the DACs did not displace the datively bonded Lewis base, but instead facilitated B-H activation.The relative basicity of the coordinated ligand was found to directly correlate with the stability of the corresponding DAC-BH3 adduct.For example, adduct 1a, which contains SMe2, was prone to intramolecular ring-expansion to 2 and de-coordination (Scheme 2).The use of a stronger Lewis base, such as pyridine, afforded an adduct (1b) that exhibited increased stability toward water and air; ring-expansion was not observed, even at elevated temperatures.To explore the hydroboration chemistry displayed by DAC-BH3 adducts, 1a and 1b were independently treated with a series of unactivated olefins.While 1a was successfully used as a hydroboration reagent and operated in the absence of exogenous radical initiators at room temperature, adduct 1b was found to require relatively high reaction temperatures; as such, subsequent efforts focused on the former.
Unlike NHCs and CAACs, the DACs did not displace the datively bonded Lewis base, but instead facilitated B-H activation.The relative basicity of the coordinated ligand was found to directly correlate with the stability of the corresponding DAC-BH3 adduct.For example, adduct 1a, which contains SMe2, was prone to intramolecular ring-expansion to 2 and de-coordination (Scheme 2).The use of a stronger Lewis base, such as pyridine, afforded an adduct (1b) that exhibited increased stability toward water and air; ring-expansion was not observed, even at elevated temperatures.To explore the hydroboration chemistry displayed by DAC-BH3 adducts, 1a and 1b were independently treated with a series of unactivated olefins.While 1a was successfully used as a hydroboration reagent and operated in the absence of exogenous radical initiators at room temperature, adduct 1b was found to require relatively high reaction temperatures; as such, subsequent efforts focused on the former.
The ability of 1a to facilitate the hydroboration of internal olefins was also explored.When 1a was treated with an excess of cyclohexene, a product (6) containing only one cyclohexyl moiety was obtained (Scheme 5).The 1 H NMR recorded for 6 showed a diagnostic singlet at δ 3.61 ppm (C 6 D 6 ), which corresponded to the two hydrogen atoms at the former carbenoid center.As no reaction was observed between 2 and cyclohexene, we hypothesized that the initial formation of 5 (not observed) was rapidly followed by intramolecular ring-expansion.The hydroboration chemistry of 1a with internal olefins was further explored by independently treating 1a with 1,3-or 1,4-cyclohexadiene.Similar to the results obtained when cyclohexene was used as a substrate, the formation of ring-expanded products was observed (Scheme 6).While the hydroboration of 1,4-cyclohexadiene readily produced 8 as a single product, as evidenced by the appearance of a doublet of triplets at δ 3.56 ppm (C 6 D 6 ), the hydroboration of 1,3-cyclohexadiene yielded an equimolar mixture of 7 and 8.The mixture of isomers was supported by two distinct 1 H NMR doublet of triplets at δ 3.56 ppm and δ 3.73 ppm (C 6 D 6 ), which were assigned to the two hydrogen atoms attached to the former carbenoid centers in the respective compounds.
Catalysts 2016, 6, 141 3 of 10 obtained (Scheme 5).The 1 H NMR recorded for 6 showed a diagnostic singlet at δ 3.61 ppm (C6D6), which corresponded to the two hydrogen atoms at the former carbenoid center.As no reaction was observed between 2 and cyclohexene, we hypothesized that the initial formation of 5 (not observed) was rapidly followed by intramolecular ring-expansion.The hydroboration chemistry of 1a with internal olefins was further explored by independently treating 1a with 1,3-or 1,4-cyclohexadiene.Similar to the results obtained when cyclohexene was used as a substrate, the formation of ringexpanded products was observed (Scheme 6).While the hydroboration of 1,4-cyclohexadiene readily produced 8 as a single product, as evidenced by the appearance of a doublet of triplets at δ 3.56 ppm (C6D6), the hydroboration of 1,3-cyclohexadiene yielded an equimolar mixture of 7 and 8.The mixture of isomers was supported by two distinct 1 H NMR doublet of triplets at δ 3.56 ppm and δ 3.73 ppm (C6D6), which were assigned to the two hydrogen atoms attached to the former carbenoid centers in the respective compounds.
Acyclic internal olefins were also studied as hydroboration substrates.Upon the addition of cisor trans-2-hexene to a CH2Cl2 solution of 1a, the appearance of overlapping multiplets at δ 3.67 ppm was observed in a 1:2 ratio in the corresponding 1 H NMR spectrum (C6D6) recorded for the respective products 9 and 10 (Scheme 7).Over time or upon heating the reaction mixture to 55 °C, the two multiplets converted to a single multiplet resonance at δ 3.67 ppm (see Supplementary Materials, Figure S17).Similar results were reported by Curran and co-workers, who attributed the phenomena to "chain walking" of a carbene-BH3 adduct [19][20][21][22][23][24][25][26].Additionally, when cis-or trans-3-hexene was introduced to 1a, compound 10 was obtained as the sole product, as indicated by the presence of a single multiplet at δ 3.67 ppm (C6D6).The structure of 10 was also unambiguously confirmed using single crystal X-ray crystallography (Figure 1).Scheme 5. Hydroboration of cyclohexene with 1a followed by intramolecular ring-expansion.
Catalysts 2016, 6, 141 3 of 10 obtained (Scheme 5).The 1 H NMR recorded for 6 showed a diagnostic singlet at δ 3.61 ppm (C6D6), which corresponded to the two hydrogen atoms at the former carbenoid center.As no reaction was observed between 2 and cyclohexene, we hypothesized that the initial formation of 5 (not observed) was rapidly followed by intramolecular ring-expansion.The hydroboration chemistry of 1a with internal olefins was further explored by independently treating 1a with 1,3-or 1,4-cyclohexadiene.Similar to the results obtained when cyclohexene was used as a substrate, the formation of ringexpanded products was observed (Scheme 6).While the hydroboration of 1,4-cyclohexadiene readily produced 8 as a single product, as evidenced by the appearance of a doublet of triplets at δ 3.56 ppm (C6D6), the hydroboration of 1,3-cyclohexadiene yielded an equimolar mixture of 7 and 8.The mixture of isomers was supported by two distinct 1 H NMR doublet of triplets at δ 3.56 ppm and δ 3.73 ppm (C6D6), which were assigned to the two hydrogen atoms attached to the former carbenoid centers in the respective compounds.
Acyclic internal olefins were also studied as hydroboration substrates.Upon the addition of cisor trans-2-hexene to a CH2Cl2 solution of 1a, the appearance of overlapping multiplets at δ 3.67 ppm was observed in a 1:2 ratio in the corresponding 1 H NMR spectrum (C6D6) recorded for the respective products 9 and 10 (Scheme 7).Over time or upon heating the reaction mixture to 55 °C, the two multiplets converted to a single multiplet resonance at δ 3.67 ppm (see Supplementary Materials, Figure S17).Similar results were reported by Curran and co-workers, who attributed the phenomena to "chain walking" of a carbene-BH3 adduct [19][20][21][22][23][24][25][26].Additionally, when cis-or trans-3-hexene was introduced to 1a, compound 10 was obtained as the sole product, as indicated by the presence of a single multiplet at δ 3.67 ppm (C6D6).The structure of 10 was also unambiguously confirmed using single crystal X-ray crystallography (Figure 1).Acyclic internal olefins were also studied as hydroboration substrates.Upon the addition of cisor trans-2-hexene to a CH 2 Cl 2 solution of 1a, the appearance of overlapping multiplets at δ 3.67 ppm was observed in a 1:2 ratio in the corresponding 1 H NMR spectrum (C 6 D 6 ) recorded for the respective products 9 and 10 (Scheme 7).Over time or upon heating the reaction mixture to 55 • C, the two multiplets converted to a single multiplet resonance at δ 3.67 ppm (see Supplementary Materials, Figure S17).Similar results were reported by Curran and co-workers, who attributed the phenomena to "chain walking" of a carbene-BH 3 adduct [19][20][21][22][23][24][25][26].Additionally, when cisor trans-3-hexene was introduced to 1a, compound 10 was obtained as the sole product, as indicated by the presence of a single multiplet at δ 3.67 ppm (C 6 D 6 ).The structure of 10 was also unambiguously confirmed using single crystal X-ray crystallography (Figure 1).Finally, efforts were directed toward the determination of conditions which facilitate a stepwise hydroboration of an internal olefin and a terminal olefin to obtain the corresponding mixed product.Upon the initial addition of excess cyclohexene to a benzene solution of 1a, the ring-expanded compound 6 was observed as the exclusive product via 1 H NMR spectroscopy analysis of the crude reaction mixture.No reaction was observed upon the subsequent addition of 1-octene.In a separate experiment, a stoichiometric mixture of cyclohexene and 1-octene was added in excess to a benzene solution of 1a.The product of this reaction was determined by 1 H NMR spectroscopy and mass spectrometry to contain a nearly equimolar mixture of the mixed product 11, the ring-expanded product 6, and 12 [29] (Scheme 8).We surmise that after the initial hydroboration of cyclohexene, the respective product did not enable the hydroboration of 1-octene, but instead underwent intramolecular ring-expansion to yield 6. These results indicated that 1a facilitated the hydroboration of two olefins, but only when the first reaction was with a terminal olefin.Furthermore, a 1 H NMR spectrum recorded after the addition of 0.5 equiv of 1-hexene to a CD2Cl2 solution of 1a exhibited singlets at δ 6.08 ppm and δ 5.69 ppm in a 1:3 ratio, which were assigned to the methine groups of 3 and residual 1a, respectively.Based on these results, we concluded that hydroboration preceded intramolecular ring-expansion.Finally, efforts were directed toward the determination of conditions which facilitate a stepwise hydroboration of an internal olefin and a terminal olefin to obtain the corresponding mixed product.Upon the initial addition of excess cyclohexene to a benzene solution of 1a, the ring-expanded compound 6 was observed as the exclusive product via 1 H NMR spectroscopy analysis of the crude reaction mixture.No reaction was observed upon the subsequent addition of 1-octene.In a separate experiment, a stoichiometric mixture of cyclohexene and 1-octene was added in excess to a benzene solution of 1a.The product of this reaction was determined by 1 H NMR spectroscopy and mass spectrometry to contain a nearly equimolar mixture of the mixed product 11, the ring-expanded product 6, and 12 [29] (Scheme 8).We surmise that after the initial hydroboration of cyclohexene, the respective product did not enable the hydroboration of 1-octene, but instead underwent intramolecular ring-expansion to yield 6. These results indicated that 1a facilitated the hydroboration of two olefins, but only when the first reaction was with a terminal olefin.Furthermore, a 1 H NMR spectrum recorded after the addition of 0.5 equiv of 1-hexene to a CD2Cl2 solution of 1a exhibited singlets at δ 6.08 ppm and δ 5.69 ppm in a 1:3 ratio, which were assigned to the methine groups of 3 and residual 1a, respectively.Based on these results, we concluded that hydroboration preceded intramolecular ring-expansion.Finally, efforts were directed toward the determination of conditions which facilitate a stepwise hydroboration of an internal olefin and a terminal olefin to obtain the corresponding mixed product.Upon the initial addition of excess cyclohexene to a benzene solution of 1a, the ring-expanded compound 6 was observed as the exclusive product via 1 H NMR spectroscopy analysis of the crude reaction mixture.No reaction was observed upon the subsequent addition of 1-octene.In a separate experiment, a stoichiometric mixture of cyclohexene and 1-octene was added in excess to a benzene solution of 1a.The product of this reaction was determined by 1 H NMR spectroscopy and mass spectrometry to contain a nearly equimolar mixture of the mixed product 11, the ring-expanded product 6, and 12 [29] (Scheme 8).We surmise that after the initial hydroboration of cyclohexene, the respective product did not enable the hydroboration of 1-octene, but instead underwent intramolecular ring-expansion to yield 6. These results indicated that 1a facilitated the hydroboration of two olefins, but only when the first reaction was with a terminal olefin.Furthermore, a 1 H NMR spectrum recorded after the addition of 0.5 equiv of 1-hexene to a CD 2 Cl 2 solution of 1a exhibited singlets at δ 6.08 ppm and δ 5.69 ppm in a 1:3 ratio, which were assigned to the methine groups of 3 and residual 1a, respectively.Based on these results, we concluded that hydroboration preceded intramolecular ring-expansion.Scheme 8. Competitive hydroboration reactions between terminal and internal olefins.A mixture of the three products shown was observed in a 1:1:1 ratio.

General Information
All procedures were performed in a nitrogen-filled glove box unless otherwise noted.Solvents were dried and degassed by a Vacuum Atmospheres Company solvent purification system (Vacuum Atmosphere Co., Hawthorne, CA, USA) and stored over 4 Å molecular sieves in a nitrogen-filled glove box.Unless otherwise specified, reagents were purchased from commercial sources and used without further purification.N,N'-dimesityl-4,6-diketo-5,5-dimethylpyrimidin-2-ylidene, as well as adducts 1a and 1b, were synthesized according to previously-reported procedures [29,30].The hydroboration reactions described herein are unoptimized.Melting points were obtained using a MPA100 OptiMelt automated melting point apparatus (Stanford Research Systems Inc., Sunnyvale, CA, USA) and are uncorrected.NMR spectra were recorded on a MR 400, Inova 500, or DirectDrive 600 MHz spectrometer (Varian, Inc., Palo Alto, CA, USA), MR 400 MHz spectrometer (Agilent Technologies, Santa Clara, CA, USA) or an Ascend™ 400 MHz spectrometer (Bruker Co., Fällanden, Switzerland via Bruker Korea Co., Ltd., Seongnam-si, Korea).Chemical shifts (δ) are given in ppm and are referenced to the residual solvent ( 1 H: C6D6, 7.16 ppm; 13 C: C6D6, 128.06 ppm).Linear predictions were applied to all 11 B NMR spectra to remove the signals that corresponded to the boron found in the glass of the NMR tubes used in the corresponding experiments; the 11 B NMR spectrum of C6D6 as a "blank" was also collected as a reference.Infrared (IR) spectra were recorded on a Nicolet iS5 system (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an iD3 attenuated total reflectance (ATR) attachment (diamond crystal) or in a KBr pellet.High resolution mass spectra (HRMS) were obtained with a Autospec-Ultima mass spectrometer (Waters Co., Milford, MA, USA) using chemical ionization (CI) or a 6530 QTOF mass spectrometer (Agilent Technologies, Santa Clara, CA, USA) using electrospray ionization (ESI).Low resolution mass spectra (LRMS) were obtained with a 6130 single quadrupole mass spectrometer equipped with an 1200 LC system (Agilent Technologies, Santa Clara, CA, USA).Elemental analyses were performed with a 2000 Organic Elemental Analyzer (Thermo Fisher Scientific, Waltham, MA, USA).

General Information
All procedures were performed in a nitrogen-filled glove box unless otherwise noted.Solvents were dried and degassed by a Vacuum Atmospheres Company solvent purification system (Vacuum Atmosphere Co., Hawthorne, CA, USA) and stored over 4 Å molecular sieves in a nitrogen-filled glove box.Unless otherwise specified, reagents were purchased from commercial sources and used without further purification.N,N'-dimesityl-4,6-diketo-5,5-dimethylpyrimidin-2-ylidene, as well as adducts 1a and 1b, were synthesized according to previously-reported procedures [29,30].The hydroboration reactions described herein are unoptimized.Melting points were obtained using a MPA100 OptiMelt automated melting point apparatus (Stanford Research Systems Inc., Sunnyvale, CA, USA) and are uncorrected.NMR spectra were recorded on a MR 400, Inova 500, or DirectDrive 600 MHz spectrometer (Varian, Inc., Palo Alto, CA, USA), MR 400 MHz spectrometer (Agilent Technologies, Santa Clara, CA, USA) or an Ascend™ 400 MHz spectrometer (Bruker Co., Fällanden, Switzerland via Bruker Korea Co., Ltd., Seongnam-si, Korea).Chemical shifts (δ) are given in ppm and are referenced to the residual solvent ( 1 H: C 6 D 6 , 7.16 ppm; 13 C: C 6 D 6 , 128.06 ppm).Linear predictions were applied to all 11 B NMR spectra to remove the signals that corresponded to the boron found in the glass of the NMR tubes used in the corresponding experiments; the 11 B NMR spectrum of C 6 D 6 as a "blank" was also collected as a reference.Infrared (IR) spectra were recorded on a Nicolet iS5 system (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an iD3 attenuated total reflectance (ATR) attachment (diamond crystal) or in a KBr pellet.High resolution mass spectra (HRMS) were obtained with a Autospec-Ultima mass spectrometer (Waters Co., Milford, MA, USA) using chemical ionization (CI) or a 6530 QTOF mass spectrometer (Agilent Technologies, Santa Clara, CA, USA) using electrospray ionization (ESI).Low resolution mass spectra (LRMS) were obtained with a 6130 single quadrupole mass spectrometer equipped with an 1200 LC system (Agilent Technologies, Santa Clara, CA, USA).Elemental analyses were performed with a 2000 Organic Elemental Analyzer (Thermo Fisher Scientific, Waltham, MA, USA).

Conclusion
In conclusion, we have demonstrated that the DAC-BH 3 adduct 1a facilitated the hydroboration of a range of olefins.The outcomes of these reactions were found to depend on the substrate employed and have provided additional insight into the underlying mechanism.While terminal olefins underwent hydroboration and afforded the expected organoboranes, the use of internal olefins typically resulted in rapid intramolecular ring-expansion of the putative products.The ring-expansion was modulated through the inclusion of terminal olefins in the corresponding reaction mixtures and afforded organoboranes that contained primary and secondary alkyl groups.

Scheme 8 .
Scheme 8. Competitive hydroboration reactions between terminal and internal olefins.A mixture of the three products shown was observed in a 1:1:1 ratio.