Cyclization Strategies to Polyenes Using Pd(II)-Catalyzed Couplings of Pinacol Vinylboronates

Abstract

As a complement to Pd(0)-catalyzed cyclizations, seven Pd(II)-catalyzed cyclization strategies are reported. α,ω-Diynes are selectively hydroborated to bis(boronate esters), which cyclize under Pd(II)-catalysis producing a diverse array of small, medium, and macrocyclic polyenes with controlled E,E, Z,Z, or E,Z stereochemistry. Various functional groups are tolerated including aryl bromides, and applications are illustrated.

Using Ring Strain to Control 4π-Electrocyclization Reactions: Torquoselectivity in Ring Closing of Medium-Ring Dienes and Ring Opening of Bicyclic Cyclobutenes

“…The unstrained macrocyclic (E,E)-diene 20 was formed readily with our palladium-(II)-catalyzed oxidative macrocyclization conditions (eq 1). 18 In contrast, cyclization of 13 (eq 2) resulted in dimerization, giving cyclic tetraene dimer 21 in low yield. We calculated the strain energy of the carbocyclic analogue for direct macro- cyclization to be +46 kcal/mol (1, Figure 2).…”

“…We anticipated that highly strained cyclic 1,3dienes could be easily prepared under mild conditions by employing our palladium(II)-catalyzed oxidative macrocyclization of bis(vinylboronate esters). 18 Strain could be incorporated by modifying either the ring size or the cis/trans configuration of the double bonds. The strained ring could be formed by a thermodynamically favored reductive elimination via a kinetically favored palladacycle intermediate.…”

Hydroelementation of diynes

“…The crude bis-alkynyl-Bpin was then added to a solution of Schwartz reagent Cp 2 ZrHCl 80, and the obtained zirconocene was then hydrolysed to the bisborylated diene 93 in 49% yield (Scheme 18). 134 Wang's and Srebrnik's procedures were also applied for the formation of other (E,E)-or (Z,Z)-vinyl boronate esters with moderate yields, which were further cyclised using PdCl 2 (PPh 3 ) 2 94 to various cyclic polyenes with controlled (E,E), (Z,Z) or (E,Z) selectivities 95-104 (Schemes 18 and 19). 134 Non-catalytic hydroboration of diyne 105 with sterically hindered di(iso-pinocampheyl)borane ( l Ipc 2 BH) 106 was carried out in THF at 0 1C yielded the desired selective hydroboration of the Me-substituted alkyne.…”

“…134 Wang's and Srebrnik's procedures were also applied for the formation of other (E,E)-or (Z,Z)-vinyl boronate esters with moderate yields, which were further cyclised using PdCl 2 (PPh 3 ) 2 94 to various cyclic polyenes with controlled (E,E), (Z,Z) or (E,Z) selectivities 95-104 (Schemes 18 and 19). 134 Non-catalytic hydroboration of diyne 105 with sterically hindered di(iso-pinocampheyl)borane ( l Ipc 2 BH) 106 was carried out in THF at 0 1C yielded the desired selective hydroboration of the Me-substituted alkyne. Addition of bromodienoate 107, Pd(PPh 3 ) 4 35, and TlOEt to the product provided the targeted cross-coupling product 109 in 83% yield with excellent regioselectivity (495 : 5) (Scheme 20).…”

Stereoselective Construction of (E,Z)‐1,3‐Dienes and Its Application in Natural Product Synthesis

“…The Pd-catalysed oxidative cyclisation of tethered terminal bis(vinyl boronic esters) was developed by Merlic et al, which accessed cyclic E,E-, Z,Z-, E,Z-or exodimethylene-dienes in a stereospecific manner. [72,73] Using this method, E,Z-bis(vinyl boronic ester) substrates 73 and 74 were cyclised to give E,Z-macrocycles 75 and 76, respectively, in good yields (Scheme 28).…”