Ethyl-vinyl complexes

A further significant feature of the ethyl-vinyl complexes Tp Ir(C2H4R)(CH = CHR)(NCR ) (R = Me (292), (473), (390), R = Bu (387)) and, in acetonitrile, 198, is their propensity for intramolecular [3 + 2] cycloaddition of the alkenyl and nitrile ligands.This affords iridapyrrole complexes 474, 475, 545, 546 and 547 (Chart 9 discussed in Section II-E) for which fully delocalised aromatic structures have been assigned on the basis of NMR spectroscopic data. Besides this study the hydrides 201 and 204 were found to react with a series of aldehydes, resulting in formation of the alkoxide-alkene chelates 244-247 (Scheme 13, Section III-B.l). 

Ethyl /m s -2-butenyl sulfone (86) together with some ethyl vinyl sulfone are obtained by the reaction of ethylene and. SO2 in wet benzene using PdCl2. SO2 behaves mechanistically similarly to CO in this reaction[66]. Hydrosulfination of alkenes with SO2 and H2 is catalyzed by the Pd(dppp) complex. The sulfinic acid 87 is a primary product, which reacts further to give the. S-alkyl alkanethiosulfonates 88 as the major product, and 89 and the sulfonic acid 90 as minor products[67]. 

The chiral BOX-copper(ll) complexes, (S)-21a and (l )-21b (X=OTf, SbFg), were found by Evans et al. to catalyze the enantioselective cycloaddition reactions of the a,/ -unsaturated acyl phosphonates 49 with ethyl vinyl ether 46a and the cyclic enol ethers 50 giving the cycloaddition products 51 and 52, respectively, in very high yields and ee as outlined in Scheme 4.33 [38b]. It is notable that the acyclic and cyclic enol ethers react highly stereoselectively and that the same enantiomer is formed using (S)-21a and (J )-21b as the catalyst. It is, furthermore, of practical importance that the cycloaddition reaction can proceed in the presence of only 0.2 mol% (J )-21a (X=SbF6) with minimal reduction in the yield of the cycloaddition product and no loss of enantioselectivity (93% ee). 

The above described reaction has been extended to the application of the AlMe-BINOL catalyst to reactions of acyclic nitrones. A series chiral AlMe-3,3 -diaryl-BINOL complexes llb-f was investigated as catalysts for the 1,3-dipolar cycloaddition reaction between the cyclic nitrone 14a and ethyl vinyl ether 8a [34], Surprisingly, these catalysts were not sufficiently selective for the reactions of cyclic nitrones with ethyl vinyl ether. Use of the tetramethoxy-substituted derivative llg as the catalyst for the reaction significantly improved the results (Scheme 6.14). In the presence of 10 mol% llg the reaction proceeded in a mixture of CH2CI2 and petroleum ether to give the product 15a in 79% isolated yield. The diastereoselectiv-ity was the same as in the acyclic case giving an excellent ratio of exo-15a and endo-15a of >95 <5, and exo-15a was obtained with up to 82% ee. 

It was found that 2-propenyloxymagnesium bromide reacts much more readily with nitrile oxides than other known dipolarophiles of electron-deficient, electron-rich, and strained types, including 3-buten-2-one, ethyl vinyl ether, and norbomene, respectively (147). Therefore, this BrMg-alkoxide is highly effective in various nitrile oxide cycloaddition reactions, including those of nitrile oxide/Lewis acid complexes. 

A closer examination by ex situ analysis using NMR or gas chromatography illustrates that intrazeolite reaction mixtures can get complex. For example photooxygenation of 1-pentene leads to three major carbonyl products plus a mixture of saturated aldehydes (valeraldehyde, propionaldehyde, butyraldehyde, acetaldehyde)38 (Fig. 33). Ethyl vinyl ketone and 2-pentenal arise from addition of the hydroperoxy radical to the two different ends of the allylic radical (Fig. 33). The ketone, /i-3-penten-2-one, is formed by intrazeolite isomerization of 1-pentene followed by CT mediated photooxygenation of the 2-pentene isomer. Dioxetane cleavage, epoxide rearrangement, or presumably even Floch cleavage130,131 of the allylic hydroperoxides can lead to the mixture of saturated aldehydes. 

Mukai et al.85 reported an asymmetric 1,3-dipolar cycloaddition of chromium(0)-complexed benzaldehyde derivatives. As shown in Scheme 5 52, heating chiral nitrone 171a, derived from Cr(CO)3-complexed benzaldehyde, with electron-rich olefins such as styrene (173a) or ethyl vinyl ether (173b) generates the corresponding chiral a.v-3,5-disubstitutcd isoxazolidine adduct 174 or... 

Figure 26. Proposed stereochemical model for the hetero-Diels-Alder reaction of ethyl vinyl ether and acylphosphonate catalyzed by 55c-Cu(II) complex.

Jprgensen and co-workers (247) investigated the asymmetric 1,3-dipolar cycloaddition reaction catalyzed by bis(oxazoline)-copper(II) complexes. In the presence of 25 mol% 269c, nitrone (401) reacts with ethyl vinyl ether and methoxypropene to afford the [3 + 2] adducts in modest diastereoselectivity and high enantioselectivity, Eq. 217. Ethyl vinyl ether preferentially forms the exo adduct while methoxypropene prefers the endo mode for reasons that are unclear. 

Figure 7.3 Standard Gibbs energies of transfer for reactants and activated complex for the Diels-Alder reaction of cyclopentadiene ( , ) with ethyl vinyl ketone (2, A) from 1-PrOH to 1-PrOH-water as a function of the mole fraction of water initial state (1 + 2, ) activated complex (o).

Recently, the first examples of catalytic enantioselective preparations of chiral a-substituted allylic boronates have appeared. Cyclic dihydropyranylboronate 76 (Fig. 6) is prepared in very high enantiomeric purity by an inverse electron-demand hetero-Diels-Alder reaction between 3-boronoacrolein pinacolate (87) and ethyl vinyl ether catalyzed by chiral Cr(lll) complex 88 (Eq. 64). The resulting boronate 76 adds stereoselectively to aldehydes to give 2-hydroxyalkyl dihydropyran products 90 in a one-pot process.The diastereoselectiv-ity of the addition is explained by invoking transition structure 89. Key to this process is the fact that the possible self-allylboration between 76 and 87 does not take place at room temperature. Several applications of this three-component reaction to the synthesis of complex natural products have been described (see section on Applications to the Synthesis of Natural Products ). 

In order to control the stereochemistry of 1,3-dipolar cycloadditions involving this type of nitrone, the Cu(OTQ2-BOX complex 238 was found to be the most suitable catalyst (Scheme 12.81) (367). The 1,3-dipolar cycloaddition of 256 with the electron-rich ethyl vinyl ether 232a as the dipolarophile in the presence of 25 mol% of 258 proceeded at room temperature to give a high conversion, an exo/ endo ratio of 84 16, and exo-251 was obtained with up to 93% ee. 

Meyers and Shimano discovered the unusual deprotonation behavior of ethoxy-vinyllithium-HMPA complex (EVL-HMPA) for the deprotonation of the trans-oxazoline 366 and the cw-oxazoline 367. The EVL-HMPA complex is prepared by deprotonation of ethyl vinyl ether with ferf-butyllithium in THE followed by addition of HMPA. Reaction of the frani-oxazoline 366 with both the EVL-HMPA complex and conventional alkyllithium reagents (RLi) resulted in deprotonation at the benzylic 5-position. In contrast, deprotonation of 367 occurred at the 4-position with an alkyllithium reagent RLi, whereas benzylic deprotonation predominated with the EVL-HMPA complex (Scheme 8.117). ° The authors proposed that EVL-HMPA complexes with the 5-phenyl substituent prior to deprotonation. 

Diethyldiphenylurea (m.p. 71) Diphenylamine (m.p. 53) 2-Heptadecylbenzothiazole Nine higher Esters (in 95% EtOH) Ethyl vinyl ether Hydrogen bromide (forms crystalline complex) Hydroquinone (m.p. 170.5)... 

The kinetically controlled product in the reaction of 99a,b with a large excess of ethyl vinyl ether was the thermodynamically less stable diastereo-mer 137 (Ph and OR trans) (Scheme 37). Complex 137 was present in solution as two conformers that rapidly interconverted. At room temperature in CDC13 the complexes 137 isomerized completely within several hours to form the diastereomer 138. In 138 the substituents OEt and Ph occupy mutual cis positions, thus minimizing steric interaction with the bulky W(CO)5 fragment on the heteroatom.250... 

In addition to 99 and ethyl vinyl ether, other para-substituted thiobenzal-dehyde complexes [W(CO)5 S=C(H)C6H4R-p ] (R = H, Me, OMe, Cl, Br) and other open-chain vinyl ethers R (R2)C=C(R3)OR4 (R R2 = H, Me R3 = H, Me, Ph, C6H4Me-p, C6H4OMe-p R4 = Me, Et, Bu1) as well as cyclic vinyl ethers such as 2,3-dihydrofuran, 4,5-dihydro-2-methylfuran, and 3,4-dihydro-2//-pyran have also been employed successfully.224,251 Thio-and selenoketone complexes did not react with vinyl ethers. 

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