Dihydrofuran complexes

This is further indicated in the reactions of 3-butyn-l-ol with [Fe( /2-CH2=CMe2)(CO)2( -C5H5)]+, which afford a mixture of dihydrofuran complex (64) and the oxacyclopentylidene complex (65) (84). The formation of these two derivatives involves a common tp-alkyne intermediate, which either forms 64 directly by internal nucleophilic attack of the oxygen on the complexed C=C triple bond, or rearranges to the vinylidene. This forms 65 by a similar attack of the hydroxy group on the a-carbon, followed... 

Re(Tp)(CO)( BuNC)(2,3-ri2-5-R-furan)] (R = H, Me), obtained as a mixture of coordination diastereomers, reacts with aldehydes RC(=0)H (R = Me, Ph, 3-furyl) in the presence of BF3 OEt2 to yield dihydrofuran complexes. Dihapto-coordinated trans-2-alkyl-3-acyl-2,3-dihydrofuran, tr[Pg.131]

Reduction of 3,5,5-tris-aryl-2(5// )-furanones 115 (R, R, R = aryl) with dimethyl sulfide-borane led to the formation of the 2,5-dihydrofurans 116 in high yields. However, in the case of 3,4-diaryl-2(5//)-furanones 115 (R, R = aryl R = H or r = H R, R = aryl), the reduction led to a complicated mixture of products of which only the diarylfurans 117 could be characterized (Scheme 36) (88S68). It was concluded that the smooth conversion of the tris-aryl-2(5//)-furanones to the corresponding furan derivatives with the dimethylsulfide-borane complex in high yields could be due to the presence of bulky aryl substituents which prevent addition reaction across the double bond (88S68). 

In 1996, the first examples of intermolecular microwave-assisted Heck reactions were published [85]. Among these, the successful coupling of iodoben-zene with 2,3-dihydrofuran in only 6 min was reported (Scheme 75). Interestingly, thermal heating procedures (125-150 °C) resulted in the formation of complex product mixtures affording less than 20% of the expected 2-phenyl-2,3-dihydrofuran. The authors hypothesize that this difference is the result of well-known advantages of microwave irradiation, e.g., elimination of wall effects and low thermal gradients in the reaction mixture. 

In 2004, Molander et al. developed another type of chiral sulfur-containing ligands for the intermolecular Heck reaction. Thus, their corresponding novel cyclopropane-based phosphorus/sulfur palladium complexes proved to be active as catalysts for the reaction between phenyltriflate and dihydrofuran, providing at high temperature a mixture of the expected product and its iso-merised analogue (Scheme 7.7). The major isomer C was obtained with a maximum enantioseleetivity of 63% ee. 

Fig. 8 The n-pair models of 2,5-dihydrofuran, oxetane and oxirane (first column) and the experimental geometries of their complexes with HC1 (second column) and C1F (third column), each drawn to scale. The angle 0 is almost identical in B- HC1 and B- ClF for a given B but increases from 2,5-dihydrofuran, through oxetane, to oxirane, as expected from the model (see text). The non-linearity of the hydrogen bond increases monotoni-cally from 2,5-dihydrofuran to oxirane. See Fig. 1 for key to the colour coding of atoms...

Table 1 The angles and 9 (in degrees see Fig. 8 for definitions) in complexes B- HC1 and B- ClF, where B is one of the cyclic ethers 2,5-dihydrofuran, oxetane or oxirane...

A novel phosphinito dipeptido ligand series were prepared, and fully characterized. These ligands readily form metal complexes with Pd(H) and Pt(II) precursors. The Pd(II) complexes were investigated for their suitability in asymmetric Heck reaction using 3,4-dihydrofuran as a substrate. 

Pd complexes 9-12 were tested for their catalytic behavior in the asymmetric Heck reaction involving the phenylation of 2,3-dihydrofuran (Scheme 3). The results are summarized in Table 2. The two isomeric products of 2-phenyl-2,5-dihydrofuran are formed with varying yields from 80% to 0%. The obtained ee s are high. Complex 12 is shown to be catalytically inactive. The lack of catalysis in complex 12 is rationalized by differences in the steric requirements between the diphenylphosphinites 1-3 (cone angle >140°) and the more sterically hindered cyclohexyl-phosphinite 4 (cone angle >170°) and the resulting stereochemistry on the Pd center. The ligands in complex 12 adopt a... 

Scheme 3 Asymmetric Heck coupling of 3,4-dihydrofuran and iodobenzene in the presence of Pd complexes 9-12 giving a mixture of regio and stereo isomers a) 2(R)-phenyl-2,3-dihydrofuran, b) 2(R)-phenyl-2,5-dihydrofuran c) 2(S)-phenyl-2,3-dihydrofuran d) 2(S)-phenyl-2,5-dihydrofuran.

General procedure for the Heck reaction. A mixture of iodobenzene (5 mol equ.), diisopropylethylamine (3 mol equ.), 2,3-dihydrofuran (1 mol eq.), and the complexes 9-12 (3 mol %) in degassed benzene was stirred at 70 °C. The progress of the reaction was monitored by GC. Upon completion, the mixture was filtered through basic alumina (58 mesh, Aldrich) to remove Pd and the products were identified using GCMS. 

Heterocyclic compounds are frequently used as hydrogen donors in the reduction of C-C double and triple bonds catalyzed by complexes of transition metals. Cyclic ethers such as [l,4]dioxane (39) and 2,3-dihydrofuran are known to donate a pair of hydrogen atoms to this type of compound. 2,3-Dihydro-[l,4]di-oxine (41), the product of dioxane (39), is not able to donate another pair of hydrogen atoms [46, 60, 73, 74]. These heterocyclic compounds are in general also very good solvents for both the catalyst and the substrates. 

Zirconocene-catalyzed kinetic resolution of dihydrofurans is also possible, as illustrated in Scheme 6.8 [18]. Unlike their six-membered ring counterparts, both of the heterocycle enantiomers react readily, albeit through distinctly different reaction pathways, to afford — with high diastereomeric and enantiomeric purities — constitutional isomers that are readily separable (the first example of parallel kinetic resolution involving an organome-tallic agent). A plausible reason for the difference in the reactivity pattern of pyrans and furans is that, in the latter class of compounds, both olefmic carbons are adjacent to a C—O bond C—Zr bond formation can take place at either end of the C—C 7T-system. The furan substrate and the (ebthi)Zr-alkene complex (R)-3 interact such that unfavorable... 

In 1985, Tsuji s group carried out a Pd-catalyzed reaction of propargyl carbonate with methyl acetoacetate as a soft carbonucleophile under neutral conditions to afford 4,5-dihydrofuran 109 [89-91]. The resulting unstable 109 readily isomerized to furan 110 under acidic conditions. In addition, they also reported formation of disubstituted furan 112 via a Pd-catalyzed heteroannulation of hydroxy propargylic carbonate 111 [92], Presumably, an allenylpalladium complex (c/. 114) was the key intermediate. 

Allenyltrimethylsilanes add to ethyl glyoxalate in the presence of a chiral pybox scandium triflate catalyst to afford highly enantioenriched homopropargylic alcohols or dihydrofurans, depending on the nature of the silyl substituent (Tables 9.39 and 9.40) [62]. The trimethylsilyl-substituted silanes give rise to the alcohol products whereas the bulkier t-butyldiphenylsilyl (DPS)-substituted silanes yield only the [3 + 2] cycloadducts. A bidentate complex of the glyoxalate with the scandium metal center in which the aldehyde carbonyl adopts an axial orientation accounts for the observed facial preference ofboth additions. 

However, considerable amounts of 2,3-dihydrofuran 50 and tetrahydro-furan-2-carbaldehyde 53 were present because of an isomerization process. The isomerization takes place simultaneously with the hydroformylation reaction. When the 2,5-dihydrofuran 46 reacts with the rhodium hydride complex, the 3-alkyl intermediate 48 is formed. This can evolve to the 2,3-dihydrofuran 50 via /3-hydride elimination reaction. This new substrate can also give both 2- and 3-alkyl intermediates 52 and 48, respectively. Although the formation of the 3-alkyl intermediate 48 is thermodynamically favored, the acylation occurs faster in the 2-alkyl intermediates 52. Regio-selectivity is therefore dominated by the rate of formation of the acyl complexes. The modification of the phosphorus ligand and the conditions of the reaction make it possible to control the regioselectivity and prepare the 2- or 3-substituted aldehyde as the major product [78]. As far as we know, only two... 

In particular the synthetic approach to dihydrofurans (first equation in Figure 4.23) represents a useful alternative to other syntheses of these valuable intermediates, and has been used for the preparation of substituted pyrroles [1417], aflatoxin derivatives [1418], and other natural products [1419]. The reaction of vinylcarbene complexes with dienes can lead to the formation of cycloheptadienes by a formal [3 + 4] cycloaddition [1367] (Entries 9-12, Table 4.25). High asymmetric induction (up to 98% ee [1420]) can be attained using enantiomerically pure rhodium(II) carboxylates as catalysts. This observation suggests the reaction to proceed via divinylcyclopropanes, which undergo (concerted) Cope rearrangement to yield cycloheptadienes. 

Complex [RuCl(=C=C=CPh2)( 7 -p-cymene)(PCy3)][PF6] (or its triflate salt) was also effective in the RCM of enynes. Thus, as shown in Scheme 38, the straightforward synthesis of 3-vinyl-2,5-dihydrofurans 104 from enynes 103... 

Inaba and coworkers reported that a Ti-BINOL complex is an effective catalyst for the desymmetrization of epoxide 44 using primary amines as nucleophiles. Of significant note is the efficiency of this reaction, with only 1 mol% catalyst necessary to attain high yields and selectivities [Eq. (10.11)]. Unfortunately, this epoxide is uniquely effective in this reaction. Cycloheptene oxide, dihydrofuran oxide, and an acyclic version of 44 each provided negligible yields under these reaction conditions ... 

Schmidt et al. reported similar reactions of 3-butynols with Cr(CO)5(L) and Mo(CO)5(L) [18]. In most cases, a similar tendency to that reported by McDonald was observed, that is, five-membered cyclic carbene complexes were obtained when Cr(CO)5(OEt2) was employed, while dihydrofurans were obtained when Mo(CO)5 (NEt3) was employed, however, in one specific case, a unique difference of the reaction pathway was observed. Thus, when hemiacetal 44 was treated with Cr (CO)5(OEt2), the corresponding carbene complex 45 was obtained, which was further converted to dihydrofuran by treatment with DM AP. On the other hand, when 44 was... 

Dihydrofuran reacts with /3,7-unsaturated a-keto esters with copper or zinc complex catalysts to generate furo[2,3-/ ]pyran derivatives in good yields with high stereoselectivity. The synthesis proceeds via an inverse electron demand hetero-Diels-Alder reaction <2000CC459>. 

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