Zinc-Catalyzed Hydrosilylation and Hydroboration of N-Heterocycles

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Zinc-Catalyzed Hydrosilylation and Hydroboration of N-Heterocycles John L. Lortie, Travis Dudding, Bulat Gabidullin, and Georgii I. Nikonov ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02811 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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Zinc-Catalyzed Hydrosilylation and Hydroboration of N-Heterocycles John L. Lortie,a Travis Dudding,a Bulat M. Gabidullin,b and Georgii I. Nikonov*a a

Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, L2S

3A1 Ontario, Canada; b X-Ray Core Facility, Faculty of Science, University of Ottawa, 150 Louis Pasteur, Ottawa, K1N 6N5 Ontario, Canada

ABSTRACT: The zinc hydride NacNacZnH (2; NacNac = [Ar’NC(Me)CHC(Me)NAr’]−, Ar’ = 2,6-Me2C6H3) catalyzes regioselective hydrosilylation and hydroboration of pyridines, including the unprecedented hydroboration of phenanthroline. Mechanistic studies of hydrosilylation, including labelling, kinetic analysis, and DFT calculations, suggest the possibility of a novel reaction pathway based on hydride transfer from an out-of-sphere activated silane.

KEYWORDS: zinc • hydrosilylation • hydroboration • N-heterocycles • DFT calculation

Reduced nitrogen heterocycles constitute an integral part of numerous pharmaceutical and agrochemical products.1 For example, 1,4-dihydropyridines find applications as Ca2+ channel blockers2 and also play an important role as reducing agents both in biological systems (e.g. NADH)3 and in laboratory applications, such as organocatalytic4 and metal catalyzed reactions.5 Traditional approaches to convert pyridines into dihydropyridines are based on a two-stage

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synthesis starting with electrophilic activation (e.g. acylation) and followed by reduction by a reactive metal hydride, alkali metal, or by addition of a nucleophile.6 The main drawbacks of these methods are that expensive and pyrophoric reagents are used and that difficult-to-separate mixtures of 1,2- and 1,4-dihydropyridines are usually formed.6,7 In this regard, catalytic regioand/or chemoselective reduction provides a powerful alternative. From the economical point of view, reduction by hydrogen gas is most desirable. But as most hydrogenation methods require elevated temperatures and pressure, overreduction becomes an issue.8 Our group9 and Oestreich et al.10 have recently reported a highly regiospecific Ru-catalyzed hydrosilylation of pyridines to the synthetically important 1,4-dihydropyridines.11 A complimentary catalytic process based on selective 1,2-hydroboration of pyridines has been developed by the groups of Suginome,12 Marks,13 and Gunanathan.14 Suginome et al. also reported a related Pd-catalyzed silaboration of pyridines leading to silylated dihydropyridines.15 There is a significant current interest in replacing the usual transition metal catalysts by more earth abundant and less toxic main group surrogates.16,17 In the field of catalytic reduction of pyridines, Wang et al. showed that the electrophilic borane ArF2BMe (ArF = 2,4,6tris(trifluoromethyl)phenyl) mediates 1,4- regioselective hydroboration of pyridines.18 The Crudden group reported on the hydroboration of acridine catalyzed by a borenium ion.19 Selective main-group catalyzed hydrosilylation and hydroboration of pyridines happened to be more challenging. In 2010, Stephan et al. reported an example of 1,4-regioselectivity in the B(C6F5)3-catalyzed reduction of 2-phenyl-quinoline by HSiEt3 to 1,4-N-silyl dihydroquinoline.20 Chang et al. then extended this work to a wide series of quinolines but observed double hydrosilylation leading to 3-silyl tetrahydroquinolines.21 In contrast, the Harder group described a very selective 1,2-hydrosilylation of pyridines catalyzed by NacNacCaH, where NacNac is a


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diketiminate of acetylacetone.22 In a closely related Mg chemistry, the groups of Hill, Harder and Okuda reported hydroboration of pyridine derivatives leading to mixtures of 1,2- and 1,4dihydropyridines, but usually 1,4 isomers are produced from 1,2-products upon heating.23,24,25 To the best of our knowledge, there have been no reports on the application of zinc catalysts in the reduction of heterocycles. Zinc is an earth abundant, inexpensive and biocompatible metal, which makes it very promising for application in catalysis.26,27 Here we report the first example of zinc catalyzed hydrosilylation and hydroboration of pyridine substrates and provide an experimental and computational insight into the mechanism of this reaction. We have recently reported the ability of the zinc NacNac complex 1 (Chart 1) to catalyze chemoselective hydrosilylation of nitriles to imines.28 In light of our interest in developing catalytic methods for reduction of heterocycles,9,29 we attempted to use catalyst 1 in the hydrosilylation of pyridines. Albeit some catalytic activity was observed, the efficiency was much lower in comparison with nitriles. We therefore reckoned that the more sterically accessible compound 2 will be a more active catalyst.30 To this end, a variety of silanes were probed in combination with complex 2, using quinoline as the test substrate. The tertiary silane HSiMe2Ph was inactive even upon heating to 70 °C (Table 1, entry 1). In contrast, the more Lewis acidic alkoxysilanes HSi(OEt)3 and HSiMe(OEt)2 were active but furnished multiple products, which can be attributed to silane redistribution. H2SiMePh worked well, affording a mixture of the 1,2- and 1,4- dihydroquinolines in approximately 4:1 ratio (entry 4). A mixture of excess PMHS and tBuOK is very active at room temperature but it produces a lot of difficult-toseparate silicon byproducts (entry 5). Gaseous silane H3SiMe, pre-made by the treatment of PMHS with tBuOK followed by vacuum transfer,31 was very active (entry 6) but operationally more difficult. Finally, the liquid primary silane H3SiPh (entry 7) showed a comparable activity


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and was chosen as the principal reagent due to the ease of manipulation, producing selectively the 1,2-dihydroquinoline derivative.

Chart 1. Zinc catalysts 1 and 2. Table 1. Optimization of the catalytic system for hydrosilylation of quinoline

Entrya

Silane

Temp

Time

Yield, % (a:b)

1

HSiMe2Ph

70

6d

6

2

HSi(OEt)3

70

102h

53

3b

HSiMe(OEt)2

70

16h

44 (78:22)d

4

H2SiMePh

70

22h

47 (88:12)

2w

98 (81:19)

5c

PMHS/tBuOK

RT

17h

99

6

H3SiMe

50

1.5h

99e

7a

H3SiPh

RT

0.6h

93 [95]f

a

5% catalyst load unless stated otherwise; b 8% catalyst load; c 10% catalyst load. d Mixture of products due to silane redistribution. e A 9:90 mixture of mono- and bis(hydrosilylation) products formed. f Preparative scale reaction: 0.44 mmol, 8% catalyst, RT, 40 min.


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A series of N-heterocyclic substrates were then reduced with H3SiPh in the presence of catalytic amount of 2 (Table 2). Interestingly, although quinaldine has an increased steric bulk at the 2-position, it too can be selectively reduced to 1,2-dihydroquinaldine. 1,5-Naphthyridine was completely hydrosilylated after 3 days at 70 ˚C with high selectivity, whereas acridine was unexpectedly slowly reduced to the 1,4-derivative (entry 3). We attribute the sluggishness of this reaction to the steric shielding of the 2 position (vide infra). Isoquinoline is a more challenging substrate for catalytic hydrosilylation, but rewardingly, this substrate can be also reduced with phenylsilane to give bis(dihydroisoquinoline) with a reduced catalyst load of 5 mol% albeit at a longer reaction time (entry 4). Interestingly, the bis(isoquinolyl) substituted product is formed in a greater extent than the mono(isoquinolyl) silane. Reduction of the more aromatically stabilized single ring molecules was more problematic. Attempted hydrosilylation of pyrazine proceeded only up to 10% after heating at 90 ˚C for 8 days (entry 5). Isomeric pyrimidine was not reduced even upon prolonged heating (entry 6), whereas pyridine was reduced to N-silyl-1,2dihydropyridine in only 3% yield after 2 days at 70 ˚C. Attempted catalytic hydrosilylation of sterically encumbered 1,10-phenanthroline also failed (entry 8), but a careful inspection of the NMR spectrum revealed the formation of hydrozincation product NacNacZn(PhenH). Complete conversion of NacNacZnH to the insertion product was quickly achieved within 15 minutes to give both the 1,2- and 1,4-isomers in the 1:3 ratio. Notably, an earlier work on hydrozincation of heterocycles with ZnH2 reported the formation of a mixture of 1,2-dihydro-1,10-phenanthroline and 1,4-dihydro-1,10-phenanthroline in the 3:2 ratio.32


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Table 2. Hydrosilylation of N-heterocycles catalyzed by compound 2

Time

Temp

Yield, %a

72h

80

44

3d

70

99 (11:1)b,c

4d

70

95 (4:1)

3d

50

11

22h

70

17

3d

80

45

4b,d

7d

70

99b

5

8d

90

10

2d

70

0

2d

70

3

1d

70

0

Entry

Substrate

1

2

3 N

N

6 N

7

8 a

NMR yield of the 1,2-isomer relative to substrate unless the ratio of isomers given in parenthases (1,2:1,4); 1,4-isomer for acridine. b Multiple products observed. c 4 equivalents of silane used relative to substrate. d 5 mol% catalyst used


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Given this limited success with hydrosilylation, we reckoned that application of the more Lewis acidic borane, HBpin (pin = pinacolato), will be beneficial. And indeed, quinoline was reduced in 96% yield already at room temperature after only 1.2 h (Table 3, entry 1). Reduction of sterically encumbered quinaldine (entry 2) was still low, but acridine gave a much increased yield (96%) in comparison with hydrosilylation (entry 3). Pyrazine, pyrimidine and even pyridine can be hydroborated in high yields at only 5% catalyst load (entries 4-6). Previously, Suginome et al. have showed that pyrazine undergoes hydroboration with 2 equivalents of pinacolborane to give a tetrahydropyrazine without any catalyst, but this reaction requires heating at 50˚C for 72 h to obtain 87% yield.33 For the zinc-catalyzed hydroboration, 93% reduction was achieved at room temperature after just 15 min. Interestingly, with one equivalent of HBpin and 10% zinc catalyst 2 complete hydroboration of phenanthroline was observed (Table 3, entry 7). Initially, monohydroborated N-boryl 1,2-dihydro-1,10-phenanthroline was formed in preference to the 1,4-reduced isomer, but addition of extra 1.5 equivalents of HBpin with heating at 70 °C for 4 days results in hydroboration of the second pyridine ring. Interestingly, the 1,4-hydrozincation product which was observed in the reaction of 2 with phenanthroline reacts with HBpin very slowly even upon heating at 70 °C for a week.


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Table 3. Hydroboration of N-heterocycles catalyzed by compound 2 [Zn] cat. 5 mol% HBpin

R N

C6D6

Entry

Substrate

1b

2

3

4d

R N Bpin

Time

Temp

Yield, %a

1.2h

RT

96 [67]c

9d

70

13

9d

70

90 93

15m RT 16h

5

6

7e

99

5.5h

RT

73

70h

70

93

23h

RT

70 (1.8:1)f

a

NMR yield of 1,2-isomer relative to substrate unless the ratio of (1,2:1,4) given in parentheses; 1,4-isomer for acridine. b 8% catalyst load. c Preparative scale reaction: 1.99 mmol load, 5% catalyst, 1.2h, RT. d 2 equivalents of HBpin used. e 10% catalyst load. f The reaction mixture also contained >1% 1,2-dihydroborated and >3% 1,4-dihydroborated products, plus 10% of product of 1,4-hydrozincation.


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Hill et al. suggested that NacNacMg catalyzed reduction of N-heterocycles occurs via heterocycle insertion into the M-H bond followed by bond metathesis with borane to furnish the product and regenerate the hydride catalyst. 23 For a N,N-tethered bis(NacNac) complex of magnesium, the Harder group also proposed an alternative mechanism based on direct transfer of hydride from a borane-dihydropyridide magnesium to pyridine without formation of an intermediate magnesium hydride.24 Related NacNacCa system is inactive in hydroboration due to fast degradation of HBpin to B2pin3 but catalyzes very 1,2-selective hydrosilylation of Nheterocycles.22 In this case, Harder et al. suggested that insertion of pyridine into the Ca-H bond is followed by pyridide transfer to silane to give the ion pair [NacNacCa(THF)n][PhH3Si(1,2NC5H6)] featuring a tris(hydrido)-amido silicate anion. This suggestion is supported by the observation of formation of a silicate complex in the reaction of a calcium hydride complex with Ph2SiH2.34 In contrast, our previous kinetic study on the 1-mediated hydrosilylation of nitrile was consistent with silane coordination to zinc as the key mechanistic event.28 Intrigued by these divergent scenarios, we decided to study the mechanism of 2-catalyzed hydrosilylation of quinoline. A stoichiometric reaction between the zinc hydride 2 and quinoline afforded the insertion product 3. NMR monitoring showed that a fast 1,2 insertion takes place followed by a slow rearrangement into the 1,4-derivative, but only the latter crystallizes from the reaction mixture in the form of a quinoline adduct (Figure 1).35. Interestingly, although the initial insertion is fast, a 1:1 reaction slows down at approximately 39% conversion, so that an 8-fold excess of quinoline is required to drive this reaction to completion. Further suggesting the reversibility of heterocycle insertion was the observation that 2 undergoes facile H/D scrambling when treated with pyridine-d5, resulting in a 2-labelled pyridine and NacNacZnD. No insertion product was identified in the latter process. Note that pyridine cannot be hydrosilylated under


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these conditions. Unlike complex 1, the hydride 2 undergoes facile Zn-H/Si-D exchange with PhSiD3, and interestingly, addition of silane to a solution of 2 results in a noticeable shift of the Zn-H resonance, further suggesting the formation of a transient silane adduct 4 (Scheme 1). When (m-Tol)SiH3 was used in catalytic hydrosilylation of quinoline, (m-Tol)H2Si-substituted N-silyl dihydroquinoline was produced, but after addition of excess PhSiH3, we observed release of free (m-Tol)SiH3 and formation of a PhH2Si-substituted product. Likewise, addition of quinoline to a solution of N-silyl-dihydro(6-methylquinoline) results in liberation of 6methylquinoline and formation of N-silyl dihydroquinoline.36 These labelling experiments establish conclusively that both the heterocycle insertion into the Zn-H bond and Zn-catalyzed hydrosilylation are reversible processes. Addition of HBipin to a solution of 3 in C6D6 provides N-boryl-dihydroquinoline and the starting hydride 2.

Figure 1. Top: Molecular structure of adduct 3 (thermal ellipsoids are shown at 30%; hydrogen atoms except those on quinoline and quinolide ligands are omitted for clarity). Selected bond lengths (Å) and angles (deg): Zn1-N1 1.9991(18), Zn1-N2 2.003(2), Zn1-N3 1.949(2), Zn- N4


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2.177(2), N1-Zn1-N2 96.30(8), N3-Zn1-N4 100.61(8), N3-Zn1-N2 119.01(8), N3-Zn1-N1 123.48(8), N1-Zn1-N4 111.25(8), N2-Zn1-N4 105.40(8). Molecular structure of complex 3 is presented in Figure 1. The structure can be better described as trigonal pyramidal, with the three amido ligands, one from dihydroquinoline (DHQ) and two from NacNac, forming the base of the pyramid with the sum of bond angles around zinc equal to 338.81(8)°, whereas the bond angles formed by the apical quinoline and each of the amido groups are 100.61(8)°, 111.25(8)° and 105.40(8)°. The Zn-amido bond to DHQ (1.949(2) Å) is comparable to the Zn-N distances to NacNac and longer than the Zn-amino bond to coordinated quinoline (2.177(2) Å). The metrics of the NacNacZn fragment are unexceptional. Albeit the hydrogen atoms of the CH2 unit of DHQ could be seen from difference maps, all hydrogen atoms were placed at calculated positions and refined in the riding model. The C-C distances to the carbon in the 4 position of the DHQ ligand are 1.494(5) and 1.422(5) Å, and expectedly longer than the corresponding distances in the unreduced quinoline (1.400(5) and 1.355(5) Å). Kinetic studies were performed next. H2SiMePh was chosen as the reducing reagent to minimize complications arising from multiple hydrosilylations. Monitoring the progress of catalysis under the pseudo-first-order conditions (excess of silane) revealed that the reaction is first order in quinoline.37 Variation of the effective reaction constant as a function of silane amount showed that the reaction is also first order in silane. Altogether, these labelling and kinetic experiments prove the reversibility of hydrosilylation but do not establish which reagent, the silane or heterocycle, reacts with the zinc hydride in the first step of catalysis.


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Scheme 1. Interconversion of 4 and 4’ by transition state TS(4,4’). To gain additional mechanistic insight, we performed density functional theory (DFT) calculations at the wB97/6-31g(d)/def2sv38 level of theory, using the (IEFPCM) implicit solvation model (benzene, ε = 2.27).39,40 To this end, three distinct reaction pathways were considered corresponding to: (1) 1,2-insertion of quinoline into the Zn-H bond of catalyst 2 via a four-membered transition subassembly (highlighted in red) followed by PhSiH3 mediated Zn-N bond metathesis; notably, this pathway is analogous to that suggested by Hill et al. for related magnesium systems;23 (2) silane coordination to zinc followed by quinoline attack at silicon with concerted formation of the 1,2-dihydro-product via a six-membered hydride transfer transition state geometry (highlighted in red) - this pathway is analogous to the cyclic mechanism suggested for 1-catalyzed hydrosilylation of nitriles;28 and (3) a mechanism related to the pathways 1 and 2, but different, in that it involves quinoline coordination to zinc followed by silane attack via a six-membered hydride transfer transition state geometry (highlighted in red) with subsequent PhSiH3 mediated Zn-N bond metathesis (Scheme 2). Associated with these pathways was an off-cycle van der Waals complex NacNacZnH*H3SiPh) (4 and 4’), the presence of which accounts for the observed deuterium scrambling in the reaction of 2 and D3SiPh. Further, calculations revealed that exchange of the terminal hydride with the Zn…H-Si


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proton proceeds by a 4c,4e transition state TS(4,4’) having a barrier of ~20.7 kcal mol−1 (see Supporting information SI1). Pathway 1 initiates with N-coordination of quinoline to catalyst 2 resulting in the exergonic -2.8 kcal mol−1 formation of intermediate 5 having a N…Zn bond length of 2.17 Å (Scheme 2). Rate determining 1,2-insertion transition state TS1 having C···H and Zn···H hydride distances of 1.64 Å and 1.73 Å, respectively, with an activation barrier of 32.1 kcal mol−1 then follows to generate the 1,2-insertion product 6 which is -5.2 kcal mol −1 more stable than the starting reagents and catalyst. Next, in the presence of H3SiPh intermediate 6 undergoes Zn-N bond metathesis via TS2 with the activation free energy of only 22.2 kcal mol−1. Catalyst regeneration and formation of N-silylated product 7 follow, affording a near thermoneutral twostep process that, overall, was exergonic by only 0.1 kcal mol−1. 1.64 Å

1.73 Å


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1.78 Å 1.66 Å 1.70 Å 1.73 Å

1.70Å 1.81 Å 1.64 Å 1.63 Å

Scheme 2. Mechanistic pathways for 2-catalyzed hydrosilylation of quinoline In contrast, pathway 2 commences with the endergonic formation of pre-complex 8, which is 12.5 kcal mol −1 less stable than the starting reagents and catalyst. 1,2-dihydro-product


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(7) and catalyst 2 formation via TS3 then occurs with an activation barrier of 24.8 kcal mol −1 relative to 8, or, more accurately, with an overall free energy of activation of 37.3 kcal mol −1 with respect to H3SiPh, quinoline, and the catalyst. The salient feature of TS3 is a six-membered cyclic hydride transfer subassembly with the bridging Zn···H···Si distances of 1.70 Å and 1.73 Å along with the Zn···H···C distances of 1.66 Å and 1.77 Å. Exergonic product formation concomitant with catalyst regeneration then results. Related to pathway 2 is a more favorable pathway 3 proceeding with the initial generation of a 3.6 kcal mol −1 less stable pre-complex 9 derived from H3SiPh, quinoline, and the catalyst. Six-membered cyclic hydride transfer transition state TS4 with a free energy of 30.4 kcal mol−1 then follows, wherein the quinoline nitrogen is N-coordinated to zinc. The defining metrics of TS4 relative to the starting reagents and catalyst are the bridging Zn···H···Si hydride bond distances of 1.81 Å and 1.63 Å, as well as Si···H···C bond lengths of 1.64 Å and 1.70 Å. Following from TS4 is intermediate 6 and H3SiPh that subsequently convert to van der Waals complex 10. Metathesis transition state TS2 with a free energy of activation of 9.4 kcal mol−1 then provides the N-silylated product 7 and catalyst 2. Of the mechanistic possibilities considered above, pathway 3 is slightly preferred, albeit all three activation barriers are likely systematically overestimated due to limitations of the existing computational methods. This new pathway 3 can be classified as an out-of-sphere41 activation of silane. In conclusion, the zinc hydride NacNacZnH catalyzes regioselective 1,2-hydrosilylation and hydroboration of heterocycles, including the high yield hydroboration of pyridine and phenanthroline. Labelling experiments show that hydrosilylation is a reversible process allowing for transfer hydrosilylation. DFT studies suggest that the reaction preferably proceeds via a novel concerted six-membered transition state stemming from silane attack at the initially formed


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zinc*heterocycle (quinoline) adduct, albeit the conventional insertion/metathesis pathway may also realize. ASSOCIATED CONTENT Supporting Information. Experimental and computational details. The following files are available free of charge. Supporting Infiormation.pdf (file type, PDF) gn219_fin.cif (file type, CIF) checkcif.pdf (file type, PDF) AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This research was supported by Petroleum Research Fund administered by the American Chemical Society (grant 53349-ND3 to G.I.N.) and NSERC (Discovery Grant 2014-04410 to T.D.) ACKNOWLEDGMENT


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We thank Sharcnet for computing resources. J.L. is grateful to the Dean of Graduate Studies for a research fellowship. ABBREVIATIONS NMR, Nuclear Magnetic Resonance; DFT, Density Functional Theory. REFERENCES

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(30) Complex 2 was previously prepared by an alternative route by Roesky et al. and was reported to be a hydride-bridged dimer in the solid state: Hao, H.; Cui, C.; Roesky, H.W.; Bai, G.; Schmidt, H.G.; Noltemeyer, M. Chem. Commun. 2001, 1118–1119. (31) Revunova, K.; Nikonov, G.I. Chem. Eur. J. 2014, 20, 839–845. (32) De Koning, A.J.; Budzelaar, P.H.M.; Boersma, J.; van der Kerk, G.J.M. J. Organomet. Chem. 1980, 199, 153-169. (33) Oshima, K.; Ohmura, T.; Suginome, M. Chem. Commun. 2012, 48, 8571-8573. (34) Jochmann, P.; Davin, J. P.; Spaniol, T. P.; Maron, L.; Okuda, J. Angew. Chem. Int. Ed. 2012, 51, 4452 –4455.

(35) Hill and Harder reported related adducts with pyridine for NacNac compounds of magnesium and calcium: (a) Hill, M. S.; MacDougall, D. J.; Mahon, M. F. Dalton Trans. 2010, 39, 11129−11131. (b) refs 22 and 24. (36) For an authoritative review of transfer hydrosilylation, see: Oestreich, M. Angew. Chem. Int. Ed. 2016, 55, 494–499. (37) See Supporting information for details. (38) Chai, J.D.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2008, 10, 6615. (39) Gaussian 09, Revision C.01, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.;


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Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian, Inc., Wallingford CT, 2009 (40) Tomasi, J.; Mennucci, B.; Cance, E. J. Mol. Struct. 1999, 464, 211. (41) Eisenstein, O.; Crabtree, R.H. New. J. Chem. 2013, 37, 21-27. SYNOPSIS Zinc hydride NacNacZnH (NacNac = [Ar’NC(Me)CHC(Me)NAr]−, Ar’ = 2,6-Me2C6H3) catalyzes regioselective hydrosilylation and hydroboration of pyridines which in the case of hydrosilylation proceeds by an unusual outer-sphere mechanism.


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Zinc-Catalyzed Hydrosilylation and Hydroboration of N-Heterocycles

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