to a CN Triple Bond of Organonitriles

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Selective Double Addition Reaction of an E-H Bond (E = Si, B) to a C≡N Triple Bond of Organonitriles Masumi Itazaki * and Hiroshi Nakazawa * Department of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan * Correspondence: [email protected] (M.I.); [email protected] (H.N.); Tel.: +81-6-6605-3123 (M.I.); +81-6-6605-2547 (H.N.) Academic Editors: Akio Baba and Makoto Yasuda Received: 21 August 2018; Accepted: 23 October 2018; Published: 25 October 2018

 

Abstract: The catalytic double hydrometalation such as hydrosilylation and hydroborylation of organonitriles has attracted considerable attention because the obtained products are widely used in organic synthesis and it is thought to be one of the effective methods for reduction of organonitriles. However, the examples of these reactions are quite limited to date. This paper summarizes the development of selective double hydrosilylation, double hydroborylation, and dihydroborylsilylation of organonitriles, including their reaction mechanisms and the role of the metal species in the catalytic cycle. Keywords: hydrosilylation; hydroborylation; dihydroborylsilylation

1. Introduction The catalytic hydrosilylation and hydroborylation of the carbon-nitrogen triple bond (C≡N bond) in organonitriles is becoming important in the synthetic chemistry. Although the term “hydroboration” is also widely used, we use “hydroborylation” in this paper from comparison with hydrosilylation. There is an advantage that these reactions do not generate by-products theoretically and the compounds with an N−Si or N−B bond thus obtained are useful products for the synthetic intermediates in organic chemistry. For examples, disilylamines (double hydrosilylation product) act as precursors for the production of Si,N-containing polymers [1–4], amine ligands for organometallic compounds [5,6], and silylating [7] and coupling [8] agents. Borylamines (hydroborylation products) have been reported to show a unique reactivity as iminium ion generators [9]. In addition, it is known that hydrosilylation and hydroborylation of a C≡N bond are one of effective methods to reduce organonitriles [10–13]. However, these hydrometalations do not occur under typical reaction conditions for hydrosilylation [14] and hydroborylation [15] because of the strong C≡N bond dissociation energy (179.3 kcal/mol, 750.0 kJ/mol) [16]. Actually, examples of catalytic double hydrometalation of the carbon-nitrogen triple bond (C≡N bond) in organonitriles are limited: one example of Fe [12], Pt [17], Ir [18], and Ru [19], two examples of Co [20,21], four examples of Rh [22–25], and main group elements and fluoride [13,26–28] for double hydrosilylation and one example of Mg [29], Co [30], and Ni [31], and two examples of Ru [32,33] and Mo [34,35] for double hydroborylation have been reported to date. Those metal catalysts are depicted in Figure 1. Although it is known that borylsilylamines are advantageous precursors for obtaining B/Si/N/C ceramics having a highly heat-resistant property [36,37], catalytic dihydroborylsilylation of organonitriles has not been achieved yet. (Scheme 1). In addition, the dual catalyst having both hydrosilylation and hydroborylation activities has not been found.

Molecules 2018, 23, 2769; doi:10.3390/molecules23112769

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Molecules 2018, 23,1.2769 Scheme hydrosilylation, double hydroborylation, and dihydroborylsilylation of2 of2 18 Molecules 2018, 23, Double x FOR PEER REVIEW of 18 Molecules 2018, 23, x FOR PEER REVIEW 2 of 18

organonitriles.

Scheme 1. Double hydrosilylation, double hydroborylation, and dihydroborylsilylation of Scheme 1. Double hydrosilylation, double hydroborylation, and dihydroborylsilylation of organonitriles. organonitriles. Molecules 2018, 23, x FOR catalysts PEER REVIEW Figure 1. for Figure 1. Metal for double double hydrosilylation hydrosilylation and and double double hydroborylation hydroborylationof oforganonitriles. organonitriles. 2 of 18

2. Double Hydrosilylation of Organonitriles In 1982, Corriu and co-workers reported that the reaction of 1,4-dicyanobutane with 1,2-bis(dimethylsilyl)benzene in the presence of a catalytic amount of RhCl(PPh3)3 afforded a mixture of trans-N,N-disilylenamines (major product) and N,N-disilylamines (minor product) (Equation (1)) [23]. In the case of benzonitrile, only double hydrosilylation product was obtained, although the yield of the product was low (Equation (2)):

(1) 1. Double hydrosilylation, doubledouble hydroborylation, and dihydroborylsilylation of organonitriles. Scheme Double hydrosilylation, hydroborylation, and dihydroborylsilylation of Figure1.1. Metal catalysts for double hydrosilylation and double hydroborylation of organonitriles. Figure 1. Metal catalysts for double hydrosilylation and double hydroborylation of organonitriles. organonitriles.

2. Double Hydrosilylation of Organonitriles 2. Double Hydrosilylation of Organonitriles 2. Double Hydrosilylation of Organonitriles In 1982, Corriu and co-workers reported that the reaction of 1,4-dicyanobutane with In 1982, Corriu andin co-workers reported that the of reaction of )1,4-dicyanobutane with (2) 1,2-bis(dimethylsilyl)benzene the presence of a catalytic amount RhCl(PPh of In 1982, Corriu and co-workers reported that the reaction of 3 1,4-dicyanobutane 3 afforded a mixture with 1,2-bis(dimethylsilyl)benzene in the presence of a catalytic amount of RhCl(PPh 3)3 afforded a trans-N,N-disilylenamines (major product) and N,N-disilylamines (minor product) (Equation (1)) [23]. 1,2-bis(dimethylsilyl)benzene in the presence of a catalytic amount of RhCl(PPh3)3 afforded a of benzonitrile, trans-N,N-disilylenamines (major product) andwas N,N-disilylamines (minor product) In mixture theThe caseproposed of only double hydrosilylation product obtained, although the yield of mixture of trans-N,N-disilylenamines (major product) and N,N-disilylamines product) reaction mechanism of Rh-catalyzed double hydrosilylation of(minor organonitriles (Equation (1)) [23]. In the case of benzonitrile, only double hydrosilylation product was obtained, the product was low (Equation (2)): (Equation (1)) [23]. In the case of benzonitrile, only double hydrosilylation product was obtained, with 1,2-bis(dimethylsilyl)benzene shown in Scheme(2)): 2. Intermediates A and B are generated via although the yield of the product is was low (Equation although the yield of the product was low (Equation (2)): the first hydrosilylation and they may be in equilibrium. The second hydrosilylation of A takes place to give the N,N-disilylamine (double hydrosilylation product) C. On the other hand, intermediate B is converted into the N,N-disilylenamine D as a result of aminolysis. (1) (1) (1)

Figure 1. Metal catalysts for double hydrosilylation and double hydroborylation of organonitriles.

(2) (2) (2)

2. Double Hydrosilylation of Organonitriles proposed mechanism of Rh-catalyzed of organonitriles In The 1982, Corriu reaction and co-workers reported that the double reactionhydrosilylation of 1,4-dicyanobutane with The proposed reaction mechanism of Rh-catalyzed double hydrosilylation of organonitriles The proposed reaction mechanism of Rh-catalyzed double hydrosilylation of organonitriles with with 1,2-bis(dimethylsilyl)benzene shown in 2. Intermediates A and B are 1,2-bis(dimethylsilyl)benzene in the is presence of Scheme a catalytic amount of RhCl(PPh 3)3 generated afforded avia with 1,2-bis(dimethylsilyl)benzene is shown in Scheme 2. Intermediates andgenerated B are generated 1,2-bis(dimethylsilyl)benzene is shown in Scheme 2. Intermediates A and A B are via thevia the firstofhydrosilylation and they may(major be in equilibrium. The second hydrosilylation of A product) takes place mixture trans-N,N-disilylenamines product) and N,N-disilylamines (minor thehydrosilylation first hydrosilylation andmay theybe may be in equilibrium. The second hydrosilylation of A takes first and they in equilibrium. The second hydrosilylation of A takes placeplace to to give the N,N-disilylamine (double hydrosilylation product) C. On the other hand, intermediate B (Equation (1)) [23]. In the case of benzonitrile, only double hydrosilylation product was obtained, to the giveN,N-disilylamine the N,N-disilylamine (double hydrosilylation product) C.the Onother the other intermediate give (double hydrosilylation product) C. On hand,hand, intermediate B is B is converted intoof the N,N-disilylenamine D as a result aminolysis. although the yield the product was low (Equation (2)):of is converted intoN,N-disilylenamine the N,N-disilylenamine as a result of aminolysis. converted into the D as D a result of aminolysis.

(1)

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Scheme 2. Proposed mechanism for double hydrosilylation of organonitriles with Scheme 2.2. Proposed Proposed mechanism mechanism for organonitriles with with 1,2Scheme for double double hydrosilylation hydrosilylationof of organonitriles 1,2-bis(dimethylsilyl)benzene. bis(dimethylsilyl)benzene. 1,2-bis(dimethylsilyl)benzene.

Murai’s group found the double hydrosilylation of aromatic and aliphatic nitriles catalyzed by Murai’s group group found found the the double double hydrosilylation hydrosilylation of of aromatic aromatic and and aliphatic aliphatic nitriles nitriles catalyzed catalyzed by by Murai’s a cobalt carbonyl Co 2(CO)8 in 1985 and 1990 [20,21]. The desired products were obtained in the cobalt carbonyl Co22(CO) (CO)8 in 1985 1985 and and 1990 [20,21]. The desired desired products products were were obtained obtained in in the the aa cobalt carbonyl 1990 [20,21]. The reaction of variousCo aromatic8 in nitriles with 10 equiv. of hydrosilane at 60 ◦°C for 20 h in the presence of reaction of various aromatic nitriles with 10 equiv. of hydrosilane at 60 C for 20 h in the presence of reaction of various aromatic nitriles with 10 equiv. of hydrosilane at 60 °C for 20 h in the presence of Co2(CO)8 (Table 1). The system possesses an excellent degree of functional group tolerance for the Co22(CO) (CO)88(Table (Table1). 1). The Thesystem systempossesses possessesan an excellent excellentdegree degree of of functional functional group group tolerance for for the Co functionalized benzonitriles with electron-withdrawing or -donating groups such tolerance as Me, OMe, the Cl, functionalized benzonitriles benzonitriles with with electron-withdrawing electron-withdrawingor or-donating -donatinggroups groupssuch suchas as Me, Me, OMe, OMe, Cl, Cl, functionalized NMe2, CN, and CO2Me in the para position on the aryl ring. A Me group in meta position shows good NMe , CN, and CO Me in the para position on the aryl ring. A Me group in meta position shows good NMe 22, CN, and CO22Me in the para position on the aryl ring. A Me group in meta position shows good reactivity, whereas that in the ortho position shows low reactivity. reactivity, reactivity, whereas whereas that that in in the the ortho ortho position position shows shows low low reactivity. reactivity. a,b. Table 1. Cobalt carbonyl catalyzed double hydrosilylation of aromatic nitrilesa,b Table 1. Cobalt carbonyl carbonyl catalyzed Table 1. Cobalt catalyzed double double hydrosilylation hydrosilylation of of aromatic aromatic nitriles nitriles a,b..

a

b GLC yields inb Reaction conditions: nitrile (2.5 mmol), HSiMeHSiMe Co2 (CO)8Co (0.2 mmol), toluene (10 mL). Reaction conditions: nitrile (2.5 mmol), 3 (25 mmol), 2(CO) 8 (0.2 mmol), toluene (10 mL). 3 (25 mmol), d e h. 48 h. Coc240 (CO) was used. Reaction nitrile (2.5 3 (25 mmol), Co2(CO) (0.2 mmol), toluene (10 mL). b eHSiMe 8 d(0.625 GLC yieldsconditions: in40parentheses. h.mmol), 48 h. mmol) Co2(CO) 8 (0.625 mmol) was8used. GLC yields in parentheses. c 40 h. d 48 h. e Co2(CO)8 (0.625 mmol) was used. aparentheses. c

Furthermore, aliphatic are adaptable to this reaction andsystem gave theand corresponding aliphaticnitriles nitriles are adaptable to this system reaction gave the Furthermore, aliphatic nitriles adaptable towhen thisto reaction system gave the products in moderate to in excellent yields when PPh the reaction system (Table 2). It is corresponding products moderate toare excellent yields PPh 3 is added to the and reaction system 3 is added corresponding productsthat in moderate to complex excellent yields when PPh 3by is added to the reaction system thought silylcobalt complex R3 SiCo(CO) , which is prepared the reaction of Co (CO) with (Table 2).that It isa thought a silylcobalt R 3 SiCo(CO) 4 , which is prepared by the reaction of 4 2 8 (Table 2). It isimportant thought a silylcobalt complex R3SiCo(CO) which is prepared by the reaction of R is8 an catalytic activecatalytic spices inactive this system. Co 2(CO) with R3SiH, isthat an important spices in4,this system. 3 SiH, Co2(CO)8 with R3SiH, is an important catalytic active spices in this system.

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a,b

Table 2. Cobalt carbonyl catalyzed double hydrosilylation of aliphatic nitriles a,b . Table 2. Cobalt carbonyl catalyzed double hydrosilylation of aliphatic nitriles a,b. Table 2. Cobalt carbonyl catalyzed double hydrosilylation of aliphatic nitriles a,b .

Reaction conditions: nitrile (2.5 mmol), HSiMe3 (25 mmol), Co2(CO)8 (0.2 mmol), PPh3 (0.4 mmol), toluene (10 mL). b The yields of enamine in parentheses. c HSi(OEt)3 (10 mmol) and toluene (5 mL) a Reaction conditions: nitrile (2.5 mmol), HSiMe3 (25 mmol), Co2(CO)8 (0.2 mmol), PPh3 (0.4 mmol), d P(OEt)3 (0.4 mmol) was used. awere used. a Reaction conditions: nitrile (2.5 mmol), HSiMe3HSiMe (25 mmol), PPh3mmol), (0.4 mmol), (10 mL). Reaction conditions: nitrile (2.5 mmol), 3 (25Co mmol), Co2mmol), (CO)8 (0.2 PPhtoluene 3 (0.4 mmol), 2 (CO) 8 (0.2 b The yields of enamine c HSi(OEt)3 (10 mmol) and b toluene (10ofmL). in parentheses. toluene (5 mL) The yields enamine in parentheses. c HSi(OEt) (10 mmol) andc toluene (5 mL) were used. d P(OEt) 3 (0.4 mmol) b The toluene (10dmL). yields of enamine in3parentheses. HSi(OEt) 3 (10 mmol) and toluene (5 mL) was used. were used. d P(OEt)3 (0.4 mmol) was used. A platinum-catalyzed reaction of various nitriles with 1,2-bis(dimethylsilyl)benzene was were used. P(OEt)3 (0.4 mmol) was used. a

reported by Tanaka’s group in 1992 (Scheme 3) [17]. In the presence of Pt(H2C=CH2)(PPh3)2 catalyst, platinum-catalyzed reaction of various nitriles with 1,2-bis(dimethylsilyl)benzene was A platinum-catalyzed reaction of various nitriles with 1,2-bis(dimethylsilyl)benzene waswhile reported reactions of aliphatic nitriles with 1,2-bis(dimethylsilyl)benzene the N-silyl enamies, aryl A platinum-catalyzed reaction of various nitriles with gave 1,2-bis(dimethylsilyl)benzene was reported by Tanaka’s group in 1992 (Scheme 3) [17]. In the presence of Pt(H 2 C=CH 2 )(PPh 3 ) 2 catalyst, by Tanaka’s group in 1992 (Scheme 3) [17]. In the presence of Pt(H C=CH )(PPh ) catalyst, reactions 2 2 3 2 nitriles converted intointhe imines high toofexcellent yields. The double reportedwere by Tanaka’s group 1992corresponding (Scheme 3) [17]. In theinpresence Pt(H2C=CH 2)(PPh 3)2 catalyst, reactions of nitriles aliphatic nitriles with 1,2-bis(dimethylsilyl)benzene theused. N-silyl enamies, while aryl of aliphatic with 1,2-bis(dimethylsilyl)benzene gave the gave N-silyl enamies, while aryl nitriles hydrosilylation product was yielded in 64% when 9-anthroylnitrile was reactions of aliphatic nitriles with 1,2-bis(dimethylsilyl)benzene gave the N-silyl enamies, while aryl nitriles were converted into the corresponding imines in highyields. to excellent yields. The double were converted into the corresponding imines in high to excellent The double hydrosilylation nitriles were converted into the corresponding imines in high to excellent yields. The double hydrosilylation product waswhen yielded in 64% when 9-anthroylnitrile was used. product was yielded in 64% 9-anthroylnitrile was used. hydrosilylation product was yielded in 64% when 9-anthroylnitrile was used.

Scheme 3. A platinum-catalyzed reaction of various nitriles with 1,2-bis(dimethylsilyl)benzene. 3. A platinum-catalyzed of various catalyzed nitriles with 1,2-bis(dimethylsilyl)benzene. In Scheme 1999, double hydrosilylationreaction of arylnitriles heterogenous Rh powder and Scheme 3. A platinum-catalyzed reaction of various nitriles withby 1,2-bis(dimethylsilyl)benzene. rhodium on γ-alumina was achieved by Pertici and co-workers (Table 3) [22]. The tendency of the In double hydrosilylation of arylnitriles catalyzed by heterogenous Rh powder rhodium In 1999, 1999, double of arylnitriles catalyzed by heterogenous Rh and powder reaction is similar to hydrosilylation the Murai’s report [20]. The desired product was not obtained when and the In 1999, double hydrosilylation of arylnitriles catalyzed by heterogenous Rh powder and on γ-alumina was achieved by Pertici by andPertici co-workers (Table 3) [22]. The3)tendency oftendency the reaction is rhodium on γ-alumina was in achieved and co-workers (Table [22]. The of the substrate with a Me group the ortho position on the aryl ring was used. In addition, the yields rhodium on γ-alumina was [20]. achieved by Pertici and was co-workers (Tablewhen 3) [22]. The tendency of the similar to the Murai’s report The desired product not obtained the substrate with a Me reaction is when similar to the 3Murai’s report [20]. desired product was not obtained when was the decreased HSi(OEt) as a hydrosilane or The rhodium on γ-alumina instead of Rh powder reaction is similar to theon Murai’s report [20]. TheIndesired product was not obtained when the group in the ortho position thethe aryl ringposition was used. addition, thewas yields decreased when the HSi(OEt) 3 substrate with a Me group in ortho on the aryl ring used. In addition, yields used. substrate with aor Me group in ortho position on the powder aryl ringwas was used. In addition, the yields as a hydrosilane rhodium onathe γ-alumina instead of Rh used. decreased when HSi(OEt) 3 as hydrosilane or rhodium on γ-alumina instead of Rh powder was decreased when HSi(OEt)3 as a hydrosilane or rhodium on γ-alumina instead of Rh powder was used. Table 3. Double hydrosilylation of aromatic nitriles promoted by rhodium metal particlesa,b a,b used. Table 3. Double hydrosilylation of aromatic nitriles promoted by rhodium metal particles .. Table 3. Double hydrosilylation of aromatic nitriles promoted by rhodium metal particles a,b . Table 3. Double hydrosilylation of aromatic nitriles promoted by rhodium metal particles a,b.

aa

b GLC conversionb of the Reaction nitrile (9.8 mmol), hydrosilane (49 mmol),(49 rhodium (0.1rhodium mg atom).(0.1 Reactionconditions: conditions: nitrile (9.8 mmol), hydrosilane mmol), mg atom). GLC c d nitriles in parentheses. 4 h. in7parentheses. h. c 4 h. d 7 h. conversion of the nitriles

Reaction conditions: nitrile (9.8 mmol), hydrosilane (49 mmol), rhodium (0.1 mg atom). Reaction conditions: nitrile (9.8 mmol), hydrosilane (49 mmol), rhodium (0.1 mg atom). conversion of the nitriles in parentheses. cc 4 h. dd 7 h. conversion of the nitriles in parentheses. 4 h. 7 h. a a

b b

GLC GLC

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Selective catalytic hydrosilylation of nitriles was found by Nikonov and Gutsulyak in 2010 [19]. Selective catalytic hydrosilylation of nitriles was found found by by Nikonov Nikonov and and Gutsulyak Gutsulyak in in 2010 2010 [19]. [19]. The reaction of organonitriles with HSiMe 22Ph in a 1:1 molar ratio afforded the corresponding imines. organonitriles with with HSiMe HSiMe22Ph in a 1:1 molar ratio afforded the corresponding imines. The reaction of organonitriles In the N,N-disilylamines were produced by the reaction of organonitriles with 2.5 equiv. of In addition, addition, reaction of of organonitriles with 2.52.5 equiv. of addition, the theN,N-disilylamines N,N-disilylamineswere wereproduced producedbybythe the reaction organonitriles with equiv. HSiMe 22Ph although a long reaction time was required (Table 4). In the case of isobutyronitrile, the HSiMe 2Ph Ph a long reaction timetime waswas required (Table 4). In of isobutyronitrile, the of HSiMe although a long reaction required (Table 4).the In case the case of isobutyronitrile, 2 although mixture of (57%) and N,Ndisilylamines (43%) were yielded. mixture of N,N-disilyenamine N,N-disilyenamine (57%) andand N,Ndisilylamines (43%) were yielded. the mixture of N,N-disilyenamine (57%) N,Ndisilylamines (43%) were yielded. Table 4. double hydrosilylation of nitriles Table 4. 4. Ru-catalyzed Ru-catalyzed double double hydrosilylation hydrosilylation of of nitriles nitriles aaaa... Table 4. Ru-catalyzed double hydrosilylation of nitriles Table

aaa a

b Nitrile (0.08 mmol), Reaction nitrile (0.13 mmol), HSiMe2 Ph (0.33 22mmol), Ru mmol), cat. (7.0 mg). HSiMe Reaction conditions: nitrile (0.13 HSiMe Ph Ru (0.08 2 Ph Reactionconditions: conditions: nitrile (0.13 mmol), mmol), HSiMe Ph (0.33 (0.33 mmol), Ru cat. cat. (7.0 (7.0 mg). mg). bbb Nitrile Nitrile (0.08 Reaction conditions: nitrile (0.13 mmol), HSiMe 2Ph (0.33 mmol), Ru cat. (7.0 mg). Nitrile (0.08 (0.19 mmol), Ru cat. (3.0 mg), and CD3 Cl instead of CD2 Cl2 were used.

mmol), mmol), HSiMe HSiMe222Ph Ph (0.19 (0.19 mmol), mmol), Ru Ru cat. cat. (3.0 (3.0 mg), mg), and and CD CD333Cl Cl instead instead of of CD CD222Cl Cl222 were were used. used. mmol), HSiMe Ph (0.19 mmol), Ru cat. (3.0 mg), and CD Cl instead of CD Cl were used.

Rhodium-catalyzed hydrosilylation of α,β-unsaturated nitriles into vinylamines was achieved by Rhodium-catalyzed Rhodium-catalyzed hydrosilylation hydrosilylation of of α,β-unsaturated α,β-unsaturated nitriles nitriles into into vinylamines vinylamines was was achieved achieved Carmona’s group in 2011 (Scheme 4) [24]. Acetonitrile showed low activity (