The discovery and use of transition metal-catalyzed reactions for the construction of C-H, C-C, and C-hetero-atom bonds for synthesis are important areas of current research in organic chemistry. One aspect within this area involves the use of transition metals and main-group organometallic compounds in catalytic systems by allowing access to highly chemo-, regio-, and diastereoselectivity processes. More importantly, the development of tandem reactions to construct a highly functionalized framework from a relatively simple organic precursors followed by further synthetic applications is a major goal in methodology.
Our group has paid increasing attention to the use of transition-metal-catalyzed heterobismetallic functionalization reactions of unsaturated organic compounds as tools for the construction of cyclic compounds. Among the palladium-catalyzed reactions, the simultaneous introduction of heterobimetallic reagents (X-Y), such as a borostannane (R2B-SnR′3), onto an unsaturated system occurs with high regio- and stereoselectivity. Bismetallative cyclization between two unsaturated systems in a tether can be seen as useful in synthetic organic chemistry, leading to cyclized substrates containing two different metal functionalities allowing for numerous further synthetic applications.
A key aspect of the cyclization of diynes is that the resulting 1,3-diene adopts a non-planar, helically chiral configuration, due to the steric demands of the X- and Y -substituents, which is fluxional in nature. These appealing systems can exhibit a low activation barrier at room temperature. However, if this atropisomerization can be frozen, the helical chirality can be exploited to influence chirality of other centers in the molecule.
We have shown the reactivity and use of a borylstannane [-N(Me)CH2CH2(Me)N-]B–SnMe3 in the cyclization to be far more superior than the corresponding silylstannane reactions of 1,n-diynes such as the 1,2-dipropargylbenzenes, 2,2'-dipropargylbiphenyls, 4,5-dipropargyldioxolanes and 1,4-dipropargyl-β-lactams. Analysis by NMR spectroscopy of those substrates which underwent facile cyclization has indicated the cyclization event to be atropselective for selected substrates due to the formation and observation of only one of the atropisomeric products in the spectra. Introduction of steric strain within the backbone of the diyne precursor has effectively increased the barrier of rotation between the non-planar atropisomeric (Z,Z)-1,3-diene products, resulting in the bicyclic products being static in nature at room temperature, as seen in the VT-NMR studies.
Studies also indicate that further applications of this cyclization reaction have to depend on finding proper derivatization reaction procedures. Though hydrolytically sensitive, the diazaborolidine substrates can be converted into the corresponding more stable dioxaborolidine derivatives; or be used in the tin-halide exchange reaction in the formation of the haloborolidine substrates. Further work in the cross-coupling transformation reactions is also imperative.