A Sn atom-economical approach toward arylstannanes: Ni-catalysed stannylation of aryl halides using Bu3SnOMe†

Stannylation of carbon–halogen bonds is one of the most promising and straightforward approaches for the preparation of organostannane compounds. Although a wide variety of methods are now available, all protocols require the use of highly nucleophilic organometals or wasteful stannyl sources like distannanes. Here, we report a new nickel-catalysed stannylation of aryl and alkenyl-halides using Bu3SnOMe as a stannyl source to afford aryl and vinyl-stannanes, respectively. This method enables the stannylation of not only bromides, but also chlorides and triflates to furnish functionalized aryland alkenyl-stannanes without the release of wasteful and toxic stannyl byproducts.

Arylstannanes are useful synthetic intermediates because of their versatility in the construction of aryl-C, 1 -NR 2 , 2 -F, 3 and -OCF 3 4 bonds.The most promising route to afford arylstannanes relies on the trapping of arylmetal species (Mg, Li, and Zn) 3a,5 using trialkylstannyl electrophiles R 3 SnX′ (route a, Scheme 1).However, these protocols have some drawbacks: poor functional group tolerance and/or delicate conditions for the preparation of arylmetal species.In contrast, the Pd-catalysed stannylation of aryl halides (Scheme 1b) 6 and the recently proposed Sandmeyer-type reaction of anilines (Scheme 1c) 7 using hexaalkyl distannanes have been demon-strated as alternative procedures.Although both of the above methods are useful and powerful for functionalized arylstannane synthesis, the release of highly toxic stannyl byproducts is unavoidable.These disadvantages drastically reduce the efficiency of organostannane chemistry in both academia and industrial pursuits.
On the other hand, a few transition metal-catalysed reactions involve nucleophilic stannylation processes using trialkylstannyl alkoxides ROSn(alkyl) 3 as terminal electrophiles.For instance, the interception of alkynylzinc 8 and alkenylcopper intermediates 9 generated in situ using stannyl alkoxides quickly transforms into the corresponding alkynyl or alkenyl stannane compounds, respectively. 10However, these catalytic processes have never been applied to the stannylation of ubiquitous carbon (sp 2 )-halogen bonds because of their low reactivity for zinc and copper complexes.In this paper, we report a Ni-catalysed stannylation of aryl halides using Bu 3 SnOMe in the presence of manganese powder (Scheme 1d).This stannylation process could be an ideal and straightforward approach to afford aryl or vinyl stannanes from both organohalides and organotriflates.The proposed process possesses the following advantages: (1) available substrates, (2) a broad scope of functional groups, and (3) Sn atom-economy without the release of wasteful toxic inorganic stannyl residues.
With the optimized conditions in hand (entries 2 and 13, Table 1), we next investigated the substrate scope in the Nicatalysed stannylation by employing various aryl or vinyl halides with Bu 3 SnOMe (Table 2).Aryl bromides containing electron-donating (1d) and -withdrawing substituents (1e-1g) at the para-position were well tolerated, giving rise to the   corresponding stannylated products (2c-2g) in good yields.In addition, the stannylation was successfully carried out on a gram-scale synthesis, giving rise to 2e in 2.05 g (89% yield).For the efficient stannylation of 4-cyano bromobenzene (1h), bidentate phosphine ligands bearing large bite angles were effective (Table S2 in the ESI †).ortho and meta-substituents (1i-1o) also participated in the stannylation, leading to the corresponding stannylated products (2i-2o).Heteroaryl bromides (1p-1r) also underwent this transformation to afford the stannylation products in good-to-high yields.Slightly acidic N-H bonds did not prevent the reaction and afforded 4-amino-2-fluorophenyl stannane (2s), which is an intermediate in torezolid synthesis. 17In addition, the stereochemistry of (E)-and (Z)-olefinic moieties (1t) was maintained during the stannylations.Furthermore, the present stannylation is active not only for bromides, but also for chloride 1u and triflates 1o and 1v, yielding the corresponding stannylated products 2u, 2o, and 2v, respectively.
A stereocontrol study was conducted using (Z)-1-(bromovinyl)naphthalene (1w), as shown in eqn (1).The reaction of 1w with Bu 3 SnOMe exclusively yielded (Z)-vinyl stannane 2w with complete retention of the stereo-integrity. 18In addition, it is known that the aryl radical possessing a (dialkylamino) methyl group at the ortho-position, derived from the halogen atom abstraction of 1-(2-iodobenzyl)piperidine (1x), rapidly undergoes 1,5-hydrogen atom transfer to afford an α-amino alkyl radical. 19This radical might be converted to alkyl stannane 4x through the formation of alkyl nickel species via recombination between Ni and the alkyl radical. 20However, the reaction of 1w provided simple arylstannane 2x (eqn (2)).Furthermore, the addition of a hydrogen atom donor like 9,10dihydroanthracene 21 into the reaction media did not fully block the stannylation; 45-62% yields of 2a were obtained even if excess scavenger (2.0-3.0 equiv.) was present in the reaction media. 22These findings imply that the primary pathways for oxidative addition of Ni into organohalides 1 do not generate free organic radicals.
The stoichiometric reaction of NiBr 2 (bpy) with 1a and Bu 3 SnOMe in the presence of various amounts of Mn powder (Scheme 2) provided some important information about the mechanism.As the loading of Mn was increased, the yield of stannylated product 2a increased, while that of the homocoupling product 3a decreased.Particularly note that the lower loading (0.9-1.1 equiv.) of Mn powder mainly produced the homocoupling product 3a.would spontaneously occur to afford [Ni] II X 2 and [Ni] II Ar 2 C, 14a,23 which would lead to the homocoupling product 3 and A via reductive elimination.In contrast, in the presence of excess Mn, B could be preferentially reduced to [Ni] I -Ar D, which would be more active than B for interception because of the higher nucleophilicity of monovalent Ni. 15,16 This would afford the stannylated product 2 as well as [Ni] I -OMe E, followed by the regeneration of A with Mn.

Conclusions
In conclusion, we have demonstrated a simple and atom-economical stannylation using stannyl electrophiles catalysed by Ni complexes in the presence of an Mn reductant.This stannylation can be tolerated by a diverse set of functional groups on aryl halides and does not release wasteful stannyl residues.Preliminary mechanistic studies suggest that aryl Ni(I) species are intermediates in this transformation.Further mechanistic studies and synthetic applications of this transmetalation process of the C-Ni bond are underway.