Synthesis of 2‐ and 3‐Stannolenes via Addition of Trimethyltin Alkoxides to 3‐Diethylboryl‐4‐ethyl‐1,1‐dimethylstannole

Abstract

Trimethyltin alkoxides (2) react stereoselectively with 3-diethylboryl-4-ethyl-l,l-dimethylstannole (1) via addition of the Me3Sn group to C(2) to the C(2)=C(3) bond and a 1,2 shift of a n ethyl group from boron to C ( 3 ) to give the 2-stannolenes 3. The molecular structure of 3f' [R = (S)-2-Bu] was determined by single-crystal X-ray analysis, confirming the cis positions of the Et(R0)B and the Me3Sn group. These 2-stannolenes 3 undergo, upon heating to ca. 80°C, facile rearrangement by irreversible allylic migration of the Et(R0)B group to the 3-stannolenes 4 in which the cis positions of the boryl and the stannyl group are retained. All 2-stannolenes (in contrast to the 3-stannolenes) are readily deprotoborylated to give the 3-stannolene 5. The structures of 3, 4, and 5 follow conclusively from lH-, llB-, 13C-, and llgSn-NMR spectra. The negative sign of the geminal coupling constants 2J(SnSn) was determined in the case of 3, 4, and 5 by 2D lgSn/lH heteronuclear shift correlations.There are only a few methods available for the synthesis of stannolenes['X2], and in general, these routes are limited to a particular pattern of substituents. Considering the synthetic potential of cyclic organotin compounds in organic or organo-metallic synthesi~ [~~~], the straightforward synthesis of 2-or 3-stannolenes is an attractive object. Since we have found a convenient route to stannoles via 1,l-organoboration of diethynyltin c o m p o~n d s [~~~] , these compounds could serve as starting materials. Recently, we have shown that the reactivity of alkenes is greatly modified if stannyl and boryl substituents are pre~ent[~-~I. The boryl group in A [eq. (l)] functions as an electrophilic anchor, attracting the nucleophilic site of a suitable reagent Nuc--El such as MeO-SnMe3 (B), and then either the =CSnMe3 group or the C=C bond can be readily attacked by the electrophilic site. In this context, the reaction shown in eq. (1) is of particular interest, since in hexane addition to the C=C bond takes place exclusively to give @I. This attack at the C=C bond should place the boryl group and the entering Me3Sn group in cis positions, and it should also work when the stannyl and boryl group in A are in trans positions.In this paper we report on the reaction of the stannole 1 with a series of trimethyltin alkoxides 2 in order to shed some light on the reaction mechanism outlined in eq. (1). 'The stannole 1 and the trimethyltin alkoxides 2 are readily available according to eq. (2) and (3). Results and DiscussionThe first step of the reaction between the stannole 1 and various trimethyltin alkoxides is essentially complete when the reaction mixtures, prepared at -78" in hexane, are allowed to reach ambient temperature [eq. (4a)l. The tert-bu-Me tyl derivative 2k does not react, even after prolonged heating at reflux. In all other cases, lIySn NMR data for the reaction solutions indicate the presence of organometallicsubstituted stannolenes which were subsequently identified as the 2-stannolenes 3a-i. Therefore, we assume that the mec...

1,1‐Carboboration through Activation of Silicon–Carbon and Tin–Carbon Bonds

“…The search for intermediates to explain the formation of stannoles or 1‐stanna‐4‐bora‐cyclohexa‐2,5‐dienes started as early as 1977 and took several years, until a zwitterionic Pb derivative proved to be isolable at low temperature and structurally characterisable 65. Then, it proved possible to crystallise various zwitterionic tin derivatives (Figure 5), all of which supported the proposed mechanism 61,64,66–68. In every case the sum of bond angles at tin relating to the three σ‐bonded substituents approaches 360°, indicating the cationic character of the almost trigonal‐planar surrounded tin atom, stabilized by "side‐on" coordination to the corresponding C≡C bond of an alkynylborate unit.…”

“…Kinetic and electronic effects, together with steric repulsion, decide which product is preferred. Fortunately, mixtures are rarely obtained, and both types of products are extremely attractive for further chemistry (e.g.,68–70).…”

Activation of Silicon- and Tin-Carbon Bonds. 1,1-Carboboration of Alkynylsilanes and -stannanes

“…Fortunately, mixtures are rarely obtained, and both types of products are extremely attractive for further chemistry (e.g. Wrackmeyer et al, 1995b;Wrackmeyer and Klaus, 1996;Wrackmeyer et al, 2009). Tetraalkynylsilanes,and 1, Following the protocol for the synthesis of siloles (Scheme 4), treatment of tetraalkynylsilanes with triethylborane in boiling toluene for prolonged periods of time gives 1,1'-spirobisiloles, which can be readily protodeborylated, using acetic acid (Scheme 7).…”

Tin Organometallic Compounds: Classification and Analysis of Crystallographic and Structural Data: Part Ii. Dimeric Derivatives

“…Seven different donor atoms serve as bridges. Nine derivatives [41][42][43][44][45][46][47] utilize the carbon donor atom of a ligand to bridge tin moieties. The Sn-O-Sn bridge angle ranges from 125.0(5)° to 180°. Five examples with tetrahedral SnC 3 N chromphores are linked by a nitrogen donor atom of a ligand.…”

“…An oxygen atom links tin atoms in seven derivatives, tetrahedral SnC 3 0 chromophores [31,32,34,35], SnC 2 CIO [33a], and SnC 2 0 2 chromophores [30,33b]. Here there is more variation in the stereochemistry about tin; tetrahedral [41,43,44] and trigonal-bipyramidal [42,46,47], The other three examples have non equivalent tin atoms, SnC 2 CI 2 with SnN 2 C 2 CI 2 [42], SnC 3 CI with SnC 3 NCI [42] and SnCI 3 C with SnCI 3 OC [45]. The Sn-N-Sn bridge angle varies from 114.9(3)° to 125.2(1)°.…”