Structural and theoretical studies of stannylenes, a class of stable, divalent tin carbene analogues, [1][2][3] have the general formula of SnR 2 where usually, R = bulky organic or related ligand. Stannylenes feature a vacant p-orbital, as well as an orbital occupied by a non-bonding pair of electrons. [4][5][6][7] These frontier orbitals define their electron acceptor and donor characteristics. Like other heavier group 14 carbene analogues, stannylenes are highly colored and typically display high reactivity towards small molecules due to their relatively modest HOMO-LUMO gap (ca. 2.0-2.5 eV) 8,9 and often well-defined Sn(I) radical character. 10,11 It has been shown that stannylenes react with H 2 , 9,12,13 CO 2 , [14][15][16] RNH 2 , 9 and ROH (R = Me or H) 17 and ethylene 6 under mild conditions.

In contrast to these studies, investigations of stannylene reactivity with alkynes remain scarce. In late 1980s, Sita and coworkers 18 reported a reversible complexation of a stannylene with a strained cycloheptyne species, which afforded a 1 : 1 complex (Fig. 1a). It is noteworthy that the resulting addition product dissociated to regenerate the corresponding stannylene and free alkyne at elevated temperature, suggesting that this coordination can be understood in terms of a weak p-complex binding. 19 Veith's group 20 reported the synthesis of the first distannacyclobutene, which was prepared from a formal [2+2] cycloaddition reaction of a diamidostannylene (equilibrated with the corresponding distannene) with a strained cyclic alkyne under ambient conditions (Fig. 1b).

In contrast to simple coordination/cycloaddition products mentioned above, Kira et al. reported a more complicated reaction of a diaryl stannylene and methyl/ethyl propynoates, which gave a 1 : 2 complex (Fig. 1c). 21 However, treatment of the Fig. 1 Previously reported reactions of stannylenes (a-d) with alkynes and the known homolysis exploited for arylstannylation, reported here. 18,[20][21][22] same starting reagents with non-terminal alkynes afforded no reaction. A double insertion reaction of bis(boryl)stannylenes and alkynes was reported by Aldridge and co-workers, showing that the treatment of bis(boryl)stannylene with diphenylacetylene yielded a borane-appended (vinyl) Sn(II) compound (Fig. 1d). 22 Previously, we showed that the diarylstannylene Sn(Ar iPr4 ) 2 underwent a facile migratory insertion reaction with ethylene, to afford two stannylene species with slightly different terphenyl substituents (Ar iPr4 and Ar iPr6 , Ar iPr6 = C 6 H 3 -2,6-(C 6 H 2 -2,4,6-iPr 3 ) 2 ). 6,23 While the use of tin compounds is widespread in organic chemistry 24,25 (e.g. Stille coupling), 26 the tin species employed usually have the Sn(IV) oxidation state. However, recent work on radical-based borylstannylations of alkynes using Sn(IV)-B containing compounds [27][28][29] has provided evidence that a Sn(II) stannylene can accomplish an arylstannylation. By accessing a one-coordinate Sn(I) radical-terphenyl radical pair by thermal homolysis of Sn(Ar iPr4 ) 2 at elevated temperatures, 23,30 similar reactivity of this radical pair with alkynes could be envisioned. We herein describe the insertion reactions of the diarylstannylene Sn(Ar iPr4 ) 2 and a series of alkynes (Scheme 1).

The reaction of Sn(Ar iPr4 ) 2 and 3 equiv. of phenylacetylene in benzene at 60 1C for 3 days resulted in a color change from dark blue to dark red. Benzene was removed under reduced pressure to afford the product, Ar iPr4 Sn{C(C 6 H 5 )-C(H)(Ar iPr4 )} ( 1), as a dark red residue. Recrystallization from hexane yielded red blocks of 1 which were suitable for single crystal X-ray crystallography (SCXRD). The SCXRD data for 1 (Fig. 2) showed that the Sn(Ar iPr4 ) 2 had added across the phenylacetylene molecule to afford monomeric aryl vinyl divalent tin complex (1). Complex 1 features a mononuclear two-coordinate tin atom, with an interligand angle of 110.22(15)1, which is significantly narrower than those in the previously reported diaryl stannylene complexes (cf. Sn(Ar iPr4 ) 2 : 117.56(8)1; 31 Sn(Ar iPr6 ) 2 (Ar iPr6 = C 6 H 3 -2,6-(C 6 H 2 -2,4,6-i Pr 3 ) 2 ):107.61(9)1; 32 Sn(Ar iPr4 -4-Cl) 2 : 115.12(8)1; 33 Sn(Ar iPr4 -3,5-i Pr 2 ) 2 : 123.4(2)1; 33 Sn(Ar iPr4 -4-SiMe 3 ) 2 : 115.37(9)1) 33 but considerably wider than those in the related aryl alkyl substituted stannylenes (cf. Ar iPr4 SnCH 2 CH 2 Ar iPr4 : 94.7(5)1; 6 Ar iPr6 SnCH 2 CH 2 Ar iPr6 : 99.22(1)1). 6 The Sn-C1 and Sn-C9 bond lengths in 1 span the range 2.217(4) to 2.225(16) Å, which slightly exceed the sum of the single bond radii of carbon (0.77 Å) and tin (1.4 Å) 34 2) and what is likely the corresponding distannene, [Ar iPr4 Sn{C(C 6 H 5 )-C(H)(Ar iPr4 )}] 2 (1 dimer ), which displays a more upfield shift of +389.55 ppm. A rapid association/dissociation likely occurs in the solution state leading to the formation of 1 dimer which is also evident by the broad peaks in the 1 H NMR spectrum of 1 indicating high fluxionality. Other distannene compounds also display signals in this region of the 119 Sn NMR spectrum. 35,36 The use of diphenylacetylene, a more sterically demanding alkyne yielded a monomeric organotin species Ar iPr4 Sn{C(C 6 H 5 )-C(H)(C 6 H 5 )} ( 2) that displays only one terphenyl ligand Ar iPr4 at the Sn(II) atom (Fig. 3). The tin atom in 2 (Fig. 3) has a bent twocoordinate configuration with an C1-Sn-C15 angle of 98.574(5)1, which is much narrower than that in 1 but is comparable to those in other alkyl substituted arylstannylenes (cf. Ar iPr4 SnCH 2- CH 2 Ar iPr4 : 98.59(15)1, 6 and AriPr6 SnCH 2 CH 2 AriPr6 : 99.23(7)1). 6 The H1, C2, C1 atoms in 2 lie essentially coplanar with the C15-Sn1-C1 plane with a twist angle of 7.04(11)1, while the two phenyl rings on the vinyl substituent are nearly orthogonal to each other (a twist angle of 86.07(6)1). The 119 Sn NMR spectrum of compound 2 revealed a single resonance at +1601 ppm, which falls just upfield of the observed monomeric divalent organotin species (cf. Ar iPr4 SnCH 2 CH 2 Ar iPr4 : Scheme 1 Overview of insertion reactions of diarylstannylene with alkynes reported here. 22 which also indicates that 2 is a monomer in solution. The UV-vis spectrum of 2 shows a broad absorption band at 518 nm that tails into visible region. The absorption signal is attributed to np transitions which are directly correlated to the HOMO-LUMO energy gap. This value is comparable to those of other monomeric alkyl/aryl stannylene species (cf. Ar iPr4 SnCH 2 C 6 H 5 : 486 nm; 23 Ar iPr4 SnCH 2 C 6 H 4 -3-Me: 490 nm; 23 Ar iPr4 SnCH 2 CH 2 tBu: 486 nm; 37 Ar iPr6 SnCH 2 CH 2 tBu: 484 nm; 37 and Ar iPr4 SnR (R = norbornyl, 494 nm; norbornenyl, 502 nm) and Ar iPr4 Sn(norbornyl)SnAr iPr4 (496 nm)). 38 We then tested reactive aliphatic alkynes with Sn(Ar iPr4 ) 2 , including 1-hexyne and trimethylsilylacetylene. When Sn(Ar iPr4 ) 2 was treated with three molar equivalents of 1-hexyne in benzene at 80 1C for 48 h, the deep blue color of the Sn(Ar iPr4 ) 2 gradually became dark purple. Removal of the solvent under reduced pressure afforded a purple powder. This was re-dissolved in diethyl ether (50 mL) and stored in a ca. À38 1C freezer to yield the pure compound 3 (68%) as a purple solid. However, attempts to grow crystals of 3 from a variety of other solvents were unsuccessful. A probable structure for 3 based on spectroscopic data is given in Scheme 1. The mechanism (Scheme 2) of the formation of compound 1 is based on known literature precedent. 6,23,30 Homolytic cleavage of a Sn-C bond was initiated upon heating, followed by the formation of a : : SnAr iPr4 radical and a terphenyl carbon radical Ar iPr4 , which can be contrasted with the generation of the radical Sn{CH(SiMe 3 ) 2 } 3 by heating hydrocarbon solutions of {Sn{CH(SiMe 3 ) 2 } 2 } 2 . 10,11,39 The reaction of the : : SnAr iPr4 / Ar iPr4 radical pair with phenylacetylene yielded the product 1. In the synthesis of 2, the Ar iPr4 is not added to the alkyne substrate, likely for steric reasons. We have previously shown that proton abstraction 23 from the most readily available source (solvent) is likely the radical termination step for the generated Ar iPr4 radical. Interestingly, the addition of Sn(Ar iPr4 ) 2 to trimethylsilylacetylene yielded the known distannene [(Ar iPr4 )Sn(CCSiMe 3 )] 2 (complex 4) arising from the dimerization of the stannylene monomer [(Ar iPr4 )Sn(CCSiMe 3 )] which was previously synthesized via the salt metathesis route between [SnCl(Ar iPr4 )] and LiCCSiMe 3 (Fig. 4). 36 The yield obtained for complex 4 was significantly improved using the arylstannylation route reported here (84% compared to 56% obtained for the salt metathesis route). Complex 4 has previously shown to undergo dynamic solution behavior, dissociating from the distannene dimer to the stannylene monomer at room temperature. These fast processes in solution may play a role in the preferential formation of the distannene, rather than the anticipated arylstannylated product, which was not observed.

In summary, the thermal homolysis of an Sn-C bond in Sn(Ar iPr4 ) 2 to generate the one-coordinate Sn(I) radical, :

: SnAr iPr4 and a terphenyl Ar iPr4 radical has been applied for the first time in the arylstannylations of alkynes at elevated temperature, to afford stable vinylstannylenes (products 1-3). For the case of trimethylsilylacetylene, the known distannene product (4) was generated, rather than an arylstannylated alkene product. The structures of 1, 2 and 4 were confirmed by X-ray crystallography and multinuclear NMR spectroscopy, while 4 was confirmed by 1 H, 13 C and 119 Sn NMR spectroscopy. Further mechanistic investigations are in hand.

There are no conflicts to declare.