The products were isolated by silica gel chromatography to give 46% yield of mono-allyl and 27% yield of diallyl phosphonate esters.
Finally, THP-vinylphosphonate 5 was subjected to transesterification by reaction with 20 mol % of TBAI and 5 equivalents of allyl bromide in toluene solution and heating in a microwave reactor for 5.5 hours.
In particular, carbonate derivatives 1 (phosphono allylic carbonates) of allylic hydroxy phosphonates undergo palladium-catalyzed addition of nucleophiles to give γ-substituted vinylphosphonates 2 in high yield (Scheme 1).
The nucleophile adds exclusively to the 3-position, with migration of the double bond into “conjugation” with phosphoryl group.
However, we have observed that β-substituted vinylphosphonates are unreactive towards cross metathesis and are therefore type IV substrates.
Since alkene cross metathesis is a powerful method of combing organic fragments in natural product synthesis, the value of vinylphosphonates as synthetic intermediates would increase if their reactivity could be enhanced to a level where they would participate in cross metathesis reactions.Therefore, given the propensity for vinylphosphonates to undergo RCM, it was proposed that an allyl phosphonate ester 14 would act as an initial site of metathesis, which would lead to a relay cross metathesis and thus render vinylphosphonates reactive.A series of cross metathesis reactions were performed to establish the baseline reactivity of vinylphosphonates (Scheme 4).Similarly, Hansen and Lee employed an allyl ether to activate enynes toward cross metathesis .Furthermore, there are several examples of vinylphosphonates participating in ring closing metathesis (RCM) reactions [23-25].As expected, the reactions generally proceed with complete chirality transfer.Various carbon, nitrogen, and oxygen nucleophiles participate in the palladium-catalyzed substitution reactions of phosphono allylic carbonates 1.Grubbs and co-workers classified terminal vinylphosphonates as type III substrates .Type III alkenes do not homodimerize, but will engage in alkene cross metathesis reactions.Not surprisingly, the terminal vinylphosphonate 15 underwent smooth cross metathesis with either 1-hexene or 1-heptene using our standard conditions (2% Grubbs II, 4% Cu I, CH reflux) [2,26,27] to give the substituted vinylphosphonates 12a or 12b in good yield.In contrast, when vinylphosphonate 12a was subjected to a cross metathesis reaction with methyl acrylate, the cross metathesis product 16a was formed in low yield (~11%) as part of a complex mixture.