Stereoselective Cyclization of Functionalized 1,n-Diynes Mediated by [X−Y] Reagents [X−Y = R₃Si−SnR′₃ or (R₂N)₂B−SnR′₃]: Synthesis and Properties of Atropisomeric 1,3-Dienes

Abstract

The borylstannane [-N(Me)CH(2)CH(2)(Me)N-]B-SnMe(3) is a superior reagent capable of effecting bisfunctionalization-cyclization in several highly functionalized 1,n-diynes, 1,n-enynes, and 1,n-allenynes (including 1,2-dipropargylbenzenes, 2,2'-dipropargylbiphenyls, 4,5-dipropargyldioxolanes, and 1,4-dipropargyl-β-lactams) where the more well-known silylstannanes fail. Variable-temperature NMR studies showed that conformational restraints imposed by selected backbones increase the activation barrier for the helical isomerization in (Z,Z)-dienes that are generated in the cyclization of the diynes. In the biphenyl and dioxolane systems, the reactions proceed with surprisingly good regio- and stereoselectivity. The resulting diazaborolidine derivatives are hydrolytically unstable but can be isolated by recrystallization or precipitation. For further synthetic applications, it is advantageous to convert these compounds in situ into the corresponding dioxaborolidines with either retention of the Me(3)Sn group or replacement of this group via halodestannylation. The configurations of the vinyl moieties are preserved in these reactions. Highly functionalized dibenzocyclooctadienes, which adorn the carbon frames of several important cytotoxic natural products, can be synthesized using this chemistry.

Silylboranes as Powerful Tools in Organic Synthesis: Stereo- and Regioselective Reactions with 1,n-Enynes

“…The barriers to inversion of the analogous compounds from addition of borylstannanes were studied by variable temperature NMR experiments. 74 As expected, the rate of helical isomerization is influenced by the structure of the backbone; whereas the barrier to inversion is low (ΔG # < 5 kcal/mol) for a product with a simple biphenyl backbone, a single diastereomer could be isolated from a chiral derivative obtained using 2-(dimethylphenylsilyl)-1,3-dimethyl-2-bora-1,3-diazacyclopentane as reagent (Figure 3).…”

“…73 One example is a borastannylative cyclization achieved by using a PdCl 2 (PPh 3 ) 2 as catalyst (Scheme 23). 74…”

Recent Advances in Boron‐Substituted 1,3‐Dienes Chemistry: Synthesis and Application

“…In the course of the reaction the difunctional (B and Sn functionalized) dienes 107 , 108 and 111 were formed. The overlapping of the C−B and C−Si bonds in these molecules was observed (Schemes and Scheme ) …”

Borylative Cyclization Reactions

“…The exceptional control of regio‐ and stereoselectivity, a consequence of the mechanism, resulted in the placement of the Si and Sn substituents in an “inside” orientation, thus creating a helical motif. In these reactions, only ( Z , Z )‐dienes are formed, and common functional groups such as silyl and alkyl ethers, esters, amides, nitriles, chlorides, and even free amines and alcohols were tolerated in the starting diynes …”

“…RajanBabu and co‐workers expanded this methodology to the use of Me 3 Sn–B[‐N(Me)CH 2 CH 2 (Me)N‐], a [B–Sn] reagent, finding this reagent had several advantages over the [Si–Sn] partners, including increased reactivity, better chemo‐ and regioselectivity in the reactions of several key substrates, and broad utility in the use of the adducts in complex molecule synthesis . For the borylstannylation/cyclization of 1,6‐diynes, several substrates were tested, including 1,2‐dipropargylbenzenes, 2,2′‐dipropargylbiphenyls, 4,5‐dipropargyldioxolanes, and 1,4‐dipropargyl‐β‐lactams, to prepare four‐, five‐, and six‐membered‐ring compounds in good to excellent yields (Scheme ).…”