Cyclohexenyl triflate

Scheme 7.5 Heck reactions of dihydrofuran with phenyl- or cyclohexenyl-triflates with (benzo)thiophene-containing phosphinooxazoline ligands.

Table 7) is substantially larger than that observed in acyclic derivatives (4b and 5a of Table 7) and could indicate anchimeric assistance effects. The authors, however, tend to exclude this possibility on the basis of the large rate enhancement (103 relative to cyclohexenyl triflate) observed in the case of compound 146 which gives mainly rearranged products formed by methyl shift and none by ring contraction. 

Because of the preferred linearity of vinyl cations, the stability of cyclic vinyl cations depends on the ring strain or the ring size. Rates of solvolysis of Inflates show that the 1-cyclohexenyl cation is 6 X 10 -fold less readily generated than the 1-methyl-1-propenyl cation. The 1-cyclopentenyl cation is not formed at all. On the other hand, 1-cyclobutenyl triflate is 3700 times more reactive than 1-cyclohexenyl triflate.The stability of the cyclobutenyl cation is due to resonance involving cyclopropyl-stabilization of the positive charge. ... 

Coupling of PhSiF3, Ph2SiF2, or Ph3SiF with 4-/er/-butyl-1 -cyclohexenyl triflate afforded 4-terAbutyl-l-phenylcyclohexene in 57% (2.5 h), 62% (2.5 h), or 0% (24 h) yield, respectively. Here again, the difluoro, i y group was the most effective [26]. 

Reaction of o-2-propynylphenol (112) with 2-iodothiophene (113) gave the 2-alkylidenedihydrobenzofuran 114 and the benzofuran 115 [36], The reaction of a nitrogen analog 116 afforded the pyrrolidine derivative 117 [37], The propargyl tosylcarbamate 118 underwent trans carboamination with cyclohexenyl triflate to give the oxazolidinone 119 in the presence of TFP as a ligand and benzyltriethy-lammonium chloride [38,39],... 

Ring-modified analogs have also been tailored using a Stille coupling, such as that formed from reaction between racemic dimethylated cyclohexenyl triflate 46 and tetraenylstannane 47 (Scheme 20). The decreased steric hinderance due to the 2,6-dimethyl-, rather than 2,6,6-trimethyl-, cyclohexenyl pattern in 46 allows for a very efficient route to the observed pentaene 48, which was ultimately parlayed to acid 49 in three additional operations. 

The diphosphine oxazoline ferrocene ligands 38-40 were applied to the asymmetric intermolecular Mizoroki-Heck reaction of 2,3-dihydrofuran (1) and cyclohexenyl triflate (35) (Scheme 11.17). With 1.5 mol% of catalyst prepared in situ the reaction was completed in 36 h, affording the two products 11 and 36. 

Hashimoto et al. [38] reported the synthesis of the P,N oxazoline-containing ligand 59 derived from ds-2-amino-3,3-dimethyl-l-indanol and its application to the intermolecu-lar Mizoroki-Heck reaction. Their best reaction conditions for arylation of various cy-cloalkenes (Scheme 11.23) showed conversions of up to 91% and enantiomeric excesses of up to 98% for the reaction of cyclohexenyl triflate (35) with 2,3-dihydrofuran (1). Reactions using 4,7-dihydro-l,3-dioxepin (46) as the substrate proved less successful, with low conversions of 37% achieved, albeit with high enantiomeric excesses of up to 90%. In all cases, low amounts of isomerization were observed. These results are consistent with, if not better than, many analogous phosphinooxazoline ligands. 

Guiry and coworkers [19] reported the application of ferrocene-based P,N ligands 60 and 61 to the Mizoroki-Heck reaction of phenyl triflate (2) and cyclohexenyl triflate (35)... 

They observed that the larger the aryl group the faster the reaction proceeded. For example, the palladium complex of 65d catalysed the reaction of cyclohexenyl triflate 35 with... 

A 10 3 2 mixture of butyl vinyl ether, EtjN, and cyclohexenyl triflate treated with a little Pd(OAc)2 in DMSO, and heated to 60-5° with stirring for 3h - 1-(1-butoxyethenyl)cyclohexene. Y 87%. Significantly, vinylation takes place at the a-position (whereas with aryl triflates a mixture of regioisomers is obtained). F.e. with both cyclic and alicyclic enol triflates, and conversion to methyl a,(3-ethyleneketones s. C.-M. Andersson, A. Hallberg, J. Org. Chem. 54, 1502-5 (1989). 

Decadiene and cyclohexenyl triflate added to a soln. of dimethyl sodiomalonate (generated from dimethyl malonate and NaH), ca. 2 eqs. LiCl, and a little Pd(PPh3)4 (freshly generated) in DMSO, and heated under argon at 65° for 48 h - product. Y 80% (90% E). Regio-defined enol triflates are more readily accessible than vinyl halides (cf. 40, 584). F.e.s. B. Friess et al.. Tetrahedron Letters 29, 4089-92 (1988). 

---