A General Rhodium Catalyst for the Deuteration of Boranes and Hy‐ drides of the Group 14 Elements

Pinacolborane, catecholborane, triethylsilane, triphenylsilane, dimethylphenylsilane, 1,1,1,3,5,5,5heptamethyltrisiloxane, triethylgermane, triphenylgermane, and triphenylstannane deuterated at the heteroatom position have been catalytically prepared in 50-70% isolated yield, through H/D exchange between the D2 molecule and the respective boranes and hydrides of the group 14 elements, in the presence of the rhodium(I)-monohydride catalyst precursor RhH{κ-P,O,P-[xant(PPr2)2]} (xant(PPr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene). Additions of B-H and E-H bonds of boranes (R2BH) and hydrides of the group 14 elements (R3EH; E = Si, Ge, Sn) to unsaturated organic molecules, the so called reactions of hydroboration, hydrosilylation, hydrogermylation, and hydrostannation, are powerful tools in the modern organic synthesis, which allow the access to a great number of handy synthetic intermediates. The use of isotopically-labeled reagents for these reactions is of paramount importance from two different points of view: they provide relevant mechanistic information, which is necessary for a rational improvement of the reactions, and generate products bearing heavy isotopes with a slower metabolism, facilitating lower pharmaceutical doses. Unfortunately, deuterated-boranes, -silanes, -germanes, and -stannanes are very expensive and the required one is not always commercially available. As a consequence, they are prepared by graduate students and postdoctoral researchers in the majority of the academic laboratories of organic chemistry, including organometallics. Preparation of such reagents by multistep procedures may be even more expensive than the commercial products, given the loss of yield in each step and the time required. So, catalytic one-pot synthesis promoted by transition metals is an attractive alternative. Benzene-d6, D2O, and D2 are usual sources of deuterium. The use of the former requires that the catalyst activates C-D bonds, whereas the utility of the second one needs of stable metal species toward the hydrolysis. So, only D2 has general applicability. Thus, reactions shown in Scheme 1 are the most clean and straightforward method to prepare these classes of deuterated compounds. Catalysts are also specific for each type of reagent. Some complexes of iron, cobalt, rhodium, and iridium stabilized by P,N,Pand N,N,N-pincer and chelate-diphosphines and NHC ligands have shown to be efficient for the exchange in boranes (a). Deuterated pinacolborane (pinBD) has been also synthetized in high yield, by deuterogenolysis of B2pin2, in the presence of iron and cobalt precursors. Catalysts for the deuteration of silanes (b) include ruthenium-polyhydrides, half-sandwich rhodium(III) derivatives, iridium(III)-NHC compounds, and platinum(0)-phosphine species. As far as we know, catalysts for the reactions with germanes (c) and stannanes (d) have not been reported so far. This specificity forces us to prepare or to buy a particular catalyst for the preparation of each class of deuterated reagent that is desired. In this note, we report and recommend the use of the square-planar complex RhH{κ-P,O,P-[xant(PPr2)2]} (xant(PPr2)2 = 9,9-dimethyl4,5-bis(diisopropylphosphino)xanthene) as catalyst precusor, which is efficient for the four reactions of Scheme 1, working under very mild conditions and with low catalyst loading. Scheme 1. Reactions with Deuterium This complex is a notable example of stable late-transitionmetal unsaturated monohydride. It catalyzes the direct borylation of arenes, the decianative borylation of nitriles, the dehydrogenation of ammonia borane, the dehydropolymerization of amine boranes, and the hydroboration of diphenylacetylene. Furthermore, it participates in the catalytic cycle of dehydrogenative borylation-hydroborylation of bis(alkyl)alkynes. So far, a handicap for its use was the preparation procedure, which afforded moderated yield. In order to solve this issue and to give it a more general use, we have significantly improved its preparation. Now, it is simple (see Experimental Section), little time is needed, and takes place in high yield (90%) in spite of involving two steps. It starts from the chloride derivative RhCl{κ-P,O,P-[xant(PPr2)2]}, which is almost quantitatively transformed into the dihydriderhodium(III) complex RhH2Cl{κ-P,O,P-[xant(PPr2)2]} by oxidative addition of H2 in pentane. The dechlorination of the latter with KOBu also in pentane yields the monohydride (Scheme 2). Scheme 2 Preparation of RhH{κ-P,O,P-[xant(PPr2)2]} The catalysis was carried out at room temperature, under 1.14 bar of D2, in diethyl ether as solvent, with concentrations of substrate and catalyst of 0.36 M and 3.6 x 10 M, respectively. Under these conditions, the boranes pinacolborane and catecholborane; the silanes triethylsilane, triphenylsilane, dimethylphenylsilane and 1,1,1,3,5,5,5-heptamethyltrisiloxane; and the germanes triethylgermane and triphenylgermane were transformed in the respective species monodeuterated at the element with conversions higher than 95%, in all cases, after 6 h (Table 1). The stannane triphenylstannane was also converted into the monodeuterated counterpart. However, in this case, the reaction had to be performed in the absence of light and in the presence of 5 mol% of hydroquinone, to prevent the formation of the radical Ph3Sn·. The latter dimerizes to afford Ph3Sn-SnPh3, which is a usual impurity of the reagent. The deuterated compounds were isolated in 50-70% yield after purification and were characterized by NMR spectroscopy. The purification of the deuterated triphenylsilane, triphenylgermane, and triphenylstannane was performed by column chromatography on silica gel, whereas the remaining compounds were distillated in a Kugelrohr glass oven. Table 1. H/D Isotopic Exchange of Boranes, Silanes, Germanes, and Stannanes Catalyzed by RhH{κ-P,O,P[xant(PPr2)2]}. Product Deuterium incorporation Isolated yield

Additions of B-H and E-H bonds of boranes (R 2 BH) and hydrides of the group 14 elements (R 3 EH; E = Si, Ge, Sn) to unsaturated organic molecules, the so called reactions of hydroboration, 1 hydrosilylation, 2 hydrogermylation, 3 and hydrostannation, 4 are powerful tools in the modern organic synthesis, which allow the access to a great number of handy synthetic intermediates. The use of isotopically-labeled reagents for these reactions is of paramount importance from two different points of view: they provide relevant mechanistic information, 5 which is necessary for a rational improvement of the reactions, and generate products bearing heavy isotopes with a slower metabolism, 6 facilitating lower pharmaceutical doses. Unfortunately, deuterated-boranes, -silanes, -germanes, and -stannanes are very expensive and the required one is not always commercially available. As a consequence, they are prepared by graduate students and postdoctoral researchers in the majority of the academic laboratories of organic chemistry, including organometallics. Preparation of such reagents by multistep procedures may be even more expensive than the commercial products, given the loss of yield in each step and the time required. So, catalytic one-pot synthesis promoted by transition metals is an attractive alternative. Benzene-d 6 , 7 D 2 O, 8 and D 2 are usual sources of deuterium. The use of the former requires that the catalyst activates C-D bonds, whereas the utility of the second one needs of stable metal species toward the hydrolysis. So, only D 2 has general applicability. Thus, reactions shown in Scheme 1 are the most clean and straightforward method to prepare these classes of deuterated compounds. Catalysts are also specific for each type of reagent. Some complexes of iron, 9 cobalt, 10 rhodium, 11 and iridium 12 stabilized by P,N,P-and N,N,N-pincer and chelate-diphosphines and NHC ligands have shown to be efficient for the exchange in boranes (a). Deuterated pinacolborane (pinBD) has been also synthetized in high yield, by deuterogenolysis of B 2 pin 2 , in the presence of iron and cobalt precursors. 9,10b Catalysts for the deuteration of silanes (b) include ruthenium-polyhydrides, 13 half-sandwich rhodium(III) derivatives, 14 iridium(III)-NHC compounds, 15 and platinum(0)-phosphine species. 16 As far as we know, catalysts for the reactions with germanes (c) and stannanes (d) have not been reported so far. This specificity forces us to prepare or to buy a particular catalyst for the preparation of each class of deuterated reagent that is desired. In this note, we report and recommend the use of the square-planar complex RhH{κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} (xant(P i Pr 2 ) 2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) as catalyst precusor, which is efficient for the four reactions of Scheme 1, working under very mild conditions and with low catalyst loading.

Scheme 1. Reactions with Deuterium
This complex is a notable example of stable late-transitionmetal unsaturated monohydride. 17 It catalyzes the direct borylation of arenes, 18 the decianative borylation of nitriles, 19 the dehydrogenation of ammonia borane, 20 the dehydropolymerization of amine boranes, 21 and the hydroboration of diphenylacetylene. 22 Furthermore, it participates in the catalytic cycle of dehydrogenative borylation-hydroborylation of bis(alkyl)alkynes. 23 So far, a handicap for its use was the preparation procedure, which afforded moderated yield. In order to solve this issue and to give it a more general use, we have significantly improved its preparation. Now, it is simple (see Experimental Section), little time is needed, and takes place in high yield (90%) in spite of involving two steps. It starts from the chloride derivative RhCl{κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]}, which is almost quantitatively transformed into the dihydriderhodium(III) complex RhH 2 Cl{κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} by oxidative addition of H 2 in pentane. The dechlorination of the latter with KO t Bu also in pentane yields the monohydride (Scheme 2).

Scheme 2 Preparation of RhH{κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]}
The catalysis was carried out at room temperature, under 1.14 bar of D 2, in diethyl ether as solvent, with concentrations of substrate and catalyst of 0.36 M and 3.6 x 10 -3 M, respectively. Under these conditions, the boranes pinacolborane and catecholborane; the silanes triethylsilane, triphenylsilane, dimethylphenylsilane and 1,1,1,3,5,5,5-heptamethyltrisiloxane; and the germanes triethylgermane and triphenylgermane were transformed in the respective species monodeuterated at the element with conversions higher than 95%, in all cases, after 6 h ( Table 1). The stannane triphenylstannane was also converted into the monodeuterated counterpart. However, in this case, the reaction had to be performed in the absence of light and in the presence of 5 mol% of hydroquinone, to prevent the formation of the radical Ph 3 Sn·. The latter dimerizes to afford Ph 3 Sn-SnPh 3 , which is a usual impurity of the reagent. The deuterated compounds were isolated in 50-70% yield after purification and were characterized by NMR spectroscopy. The purification of the deuterated triphenylsilane, triphenylgermane, and triphenylstannane was performed by column chromatography on silica gel, whereas the remaining compounds were distillated in a Kugelrohr glass oven.

Product
Deuterium incorporation , and triphenylsilane (Ph 3 SiH). 24 As is well known, there is a marked diagonal relationship between the elements of rows 2 and 3, which is particularly pronounced for boron and silicon and evident in the chemistry of the platinum group metals. 25 In accordance with it, the square-planar monohydride reacts with the four compounds of the same manner, to give the related trans- . 24 The ether-diphosphine xant(P i Pr 2 ) 2 is flexible and the ether function displays hemilabile character. As results, transitory and stable species bearing the ligand coordinated in fashions κ 3 -mer, 26 κ 3 -fac, 27 κ 2 -cis, 28 2 ]}, which should yield the wished R 2 BD and R 3 ED products through the respective reductive eliminations, regenerating the catalyst. The behavior of the catalyst during the deuteration of Et 3 SiH was followed by 31 P{ 1 H} NMR spectroscopy ( Figure S30). In accordance with Scheme 3, the hydride-deuteride-rhodium(III) complex RhHD(SiEt 3 ){κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]}(δ 31P , 61.8; δ 1H , -5.87; δ 2H , -5.9) is the key species of the catalysis. It is rapidly formed and the unique detected species while Et 3 SiH is present in the solution. Once the deuteration is completed the deuteride-rhodium(I) catalyst is quantitatively regenerated. These observations confirm the cycle proposed on the basis of the previously mentioned stoichiometric reactions and points out the reductive elimination of H-D from RhHD(SiEt 3 ){κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} as the rate determining step of the catalysis.
In conclusion, the rhodium(I)-monohydride RhH{κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} is an efficient catalyst precursor to perform the H/D exchange between the D 2 molecule and the heteroatom-H bond of boranes and hydrides of the group 14 elements. The deuterated products are obtained in high yields and deuterium incorporation at the wished position higher than 95%. The preparation of this precursor is simple, little time is needed, and takes place in very high yield.

Improved Method of Synthesis of RhH{κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]}.
A suspension of RhH 2 Cl{κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} (1000 mg, 1.71 mmol) in pentane (30 mL) was treated with K t BuO (250 mg, 2.23 mmol). The resulting mixture was stirred at room temperature for 12 h, and during this time the gradual darkening of the color was observed. The resulting suspension was extracted with pentane (5 x 30 mL) and filtered through a porous frit to remove the potassium salts, getting a very dark solution, that was dried to vacum to afford a very dark solid. Yield: 850 mg (91%). NMR data agree with those reported previously. 17 General Procedure for the Deuteration Experiments. In an argon-filled glovebox a screw cap Schlenk flask (volume ~ 160 mL) was charged with RhH{κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} (10 mg, 0.018 mmol), the corresponding substrate (1.83 mmol) and diethyl ether (5 mL). The resulting solution was freezepump-thaw degassed and filled with D 2 (0.14 bar over the atmospheric pressure) while immersed in liquid N 2 . The solution was thawed and stirred for 3 hours at room temperature. Then, the flask atmosphere was removed and it was charged again with 1.14 bar of D 2 . After 3 additional hours, the deuterium atmosphere was replaced by argon and two different purification methods were used: flash chromatography over silica gel using diethyl ether as eluent (DSiPh 3 , DGePh 3 and DSnPh 3 ) or by evaporation to dryness and vacuum distillation (DBpin, DBcat, DSiEt 3 , DSiMe 2 Ph, DSiMe(OSiMe 3 ) 2 , and DGeEt 3 ). The deuteration of HSnPh 3 was performed following the general procedure but in the presence of hydroquinone (0.09 mmol) and in the absence of light. Pinacolborane-d 1 . Colorless liquid. Yield: 153 mg (65%) . NMR Spectra (Figures S1-S3  In the glovebox, two J Young NMR tubes were charged with a solution of RhH{κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} (10 mg, 0.02 mmol), triethylsilane (30 µL, 0.2 mmol) in diethyl ether (0.40 mL) or diethyl ether-d 10 (0.40 mL), respectively. The argon atmosphere was replaced by a deuterium atmosphere (1.14 bar) and 31 P{ 1 H}, 1 H, and 2 H NMR spectra were recorded periodically. Gram-Scale Preparation of Triphenylsilane-d 1 . In an argonfilled glovebox a screw cap Schlenk flask (volume ~ 160 mL) was charged with RhH{κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} (54.6 mg, 0.1 mmol), HSiPh 3 (2.604 g, 10 mmol) and diethyl ether (27.5 mL). The resulting solution was freeze-pump-thaw degassed and filled with D 2 (0.14 bar over the atmospheric pressure) while immersed in liquid N 2 . The solution was thawed and stirred for 1 hour at room temperature. After this time, the flask atmosphere was removed and it was charged again with 1.14 bar of D 2 . This procedure was repeated each hour, up to four times. Then the deuterium atmosphere was replaced by argon and triphenylsilane-d 1 was purified by flash chromatography over silica gel using diethyl ether as eluent. Isolated yield: 2.28 g (87%).

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Notes
The authors declare no competing financial interest.