Epoxides Lewis acid activation

Titanium-IV compounds with their Lewis acid activity may catalyze an interfering rearrangement of the starting allylic alcohol or the epoxy alcohol formed. In order to avoid such side-reactions, the epoxidation is usually carried out at room temperature or below. 

In addition to the above hydrolysis reactions, dinuclear approach to providing joint Lewis acid activation and nucleophile activation has been applied to other organic reactions (Figure 6.14) including stereoselective ring-opening of epoxides (18) [65, 66], stereoselective aldol condensation (19) [67, 68], and stereoselective reduction (20) reactions [69]. 

The mechanistic aspects of this Lewis acid promoted reaction have been examined by low temperature NMR studies, and reaction of the lithium alkynides with the Lewis acid activated epoxides is indicated. The order of the addition of the reagents does not affect the product yields provided that the reaction is carried out at -78 C addition of BF3-OEt2 to a mixture of an epoxide and an alkynide or addition of an epoxide to a mixture of an alkynide and BF3-OEt2 are both possible. The transmetalation between RLi and BF3 which produces unreactive organoboron compounds is shown to be very slow at this temperature. - ... 

Intramolecular vinylation of Lewis acid-activated carbon electrophiles with vinylsilanes is very valuable for construction of carbocycles and oxygen- or nitrogen-containing heterocycles [4]. In contrast, fhere are few reports of intermolecular vinylation [525]. Schaumann et al. recently reported TiCl4-promoted vinylation of epoxides wifh l,3-bis(trimethylsilyl)-l-propene (Scheme 10.200) [526]. 

Comparison of proposed ensembles for (c) asymmetric epoxidation and (d) Lewis-acid activation of epoxides for nucleophilic attack. 

Transition metal catalyzed ring expansions of cyclic ethers to lactones under pressures of CO [51, 52] have been reported for tetrahydrofuran [53], oxetanes, and epoxides [54—56]. Carbonylation of epoxides is particularly important since P-lactone products are challenging synthetic targets (see Section 2.2.5). Using Co(CO)4 in combination with a Lewis acidic Al-salen counterion, the reaction of (R)-propylene oxide and CO occurs with stereochemical retention (Scheme 2.23) [57]. The mechanism is believed to involve Lewis acid activation of the epoxide followed by nucleophilic ring opening with Co(CO)4 [58]. 

The EPR data therefore revealed that the starting Mn(ii) catalyst converted to the high-valence [LMn =0] complex (at g 4.4) upon interaction with a two-electron oxidant. This species was found to act as a Lewis acid activator of the oxidant, leading to the new third oxidant of composition [LMn" =0(0X)]"" ", which accounted for the predominant oxidation pathway in olefin epoxidations. This pathway was noted to be quite distinct from the alternative second oxidant scheme, that forms by pre-coordination of the oxidant to the starting Mn(ii) catalyst. The excellent enantioselectivity demonstrated with these catalysts was particularly encouraging. 

Displacement of activated chlorine atoms also proceeds with certain types of organic compounds, but only in the presence of Lewis acid catalysts. Particular examples include epoxides, polyhydric alcohols, trialkylphosphites (12), and P-aminocrotonates (13). These additives are commonly used in conjunction with metallic stabilizers to provide complete, high performance, commercial stabilizer packages. 

In the next step of the sequence, the authors sought to introduce a hydroxy-methylene substituent at the unsubstituted 7-position of the enone. This bond construction can be carried out by conducting a Baylis-Hillman reaction with formaldehyde. In this instance, the authors used a modification of the Baylis-Hillman reaction which involves the use of a Lewis acid to activate the enone [26]. Under these conditions, the enone 42 is treated with excess paraformaldehyde in the presence of triethylphosphine (1 equiv), lanthanum triflate (5 mol%), and triethanolamine (50 mol%). It is proposed that the lanthanum triflate forms a complex with the triethanolamine. This complex is able to activate the enone toward 1,4-addition of the nucleophilic catalysts (here, triethylphosphine). In the absence of triethanolamine, the Lewis acid catalyst undergoes nonproductive complexation with the nucleophilic catalyst, leading to diminution of catalysis. Under these conditions, the hydroxymethylene derivative 37 was formed in 70 % yield. In the next step of the sequence, the authors sought to conduct a stereoselective epoxidation of the allylic... 

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