Ring opening with alkoxides

Sivaev, I. B., Kulikova, N. Yu., Nizhnik, E. A. et al. 2008. Practical synthesis of 1,4-dioxane derivative of the c/oio-dodecaborate anion and its ring opening with acetylenic alkoxides. J. Organomel Chem. 693 519-525. 

Nucleophilic Reactions. The strong electronegativity of fluorine results in the facile reaction of perfluoroepoxides with nucleophiles. These reactions comprise the majority of the reported reactions of this class of compounds. Nucleophilic attack on the epoxide ring takes place at the more highly substituted carbon atom to give ring-opened products. Fluorinated alkoxides are intermediates in these reactions and are in equiUbrium with fluoride ion and a perfluorocarbonyl compound. The process is illustrated by the reaction of methanol and HFPO to form methyl 2,3,3,3-tetrafluoro-2-methoxypropanoate (eq. 4). 

In contrast reaction with aprotic nucleophiles, e.g. alkoxides, LiAlILt and sulfur ylides (Z ), yields amino acid derivatives (341), resulting from sp C—N bond scission. The third possible way of ring opening, namely at the C—C bond, has also been observed in certain cases, i.e. (342) -> (343) (67TL5033). 

This scheme is remarkably close to the coordination insertion mechanism believed to operate in the metal alkoxide-catalyzed ring-opening polymerization of cyclic esters (see Section 2.3.6). It shares many features with the mechanism proposed above for the metal alkoxide-catalyzed direct polyesterification (Scheme 2.18), including the difficulty of defining reaction orders. 

The anionic Rh(I) porphyrin [Rh(OEP) induced ring-opening reactions with 4- and 5-membered ring lactones to give organometallic products with the rhodium bonded to the alkoxide carbon rather than the carbonyl carbon. 

Quite often in the ring-opening polymerization, the polymer is only the kinetic product and later is transformed to thermodynamically stable cycles. The cationic polymerization of ethylene oxide leads to a mixture of poly(ethylene oxide) and 1,4-dioxane. In the presence of a cationic initiator poly(ethylene oxide) can be almost quantitatively transformed to this cyclic dimer. On the other hand, anionic polymerization is not accompanied by cyclization due to the lower affinity of the alkoxide anion towards linear ethers only strained (and more electrophilic) monomers can react with the anion. 

Prior to the development of the aluminum initiators described above, a series of bimetallic /x-oxo alkoxides, such as (269) and (270) were examined as lactone polymerization initiators.811 812 At 10 °C, (269) polymerizes CL in a moderately controlled manner (60 equivalents, t1/2 = 23min), as shown by a linear DP vs. [M]0/[I]o plot and Mw/Mn values between 1.3 and 1.5. Mn data is consistent with only one of the terminal alkoxides initiating the ring opening, although in the presence of "BuOH, which is known to dissociate (269), all four alkoxides are active. 

NMR studies on the alkoxide initiators confirm that all the lactones polymerize via an acyl— oxygen scission, including /3-PL (which, by contrast, opens at the alkyl—oxygen bond with (251)). Monomer coordination and subsequent ring opening may be observed by 111 NMR spectroscopy. Coordination is also observed with 7-BL and 7-VL, although these adducts are stable to insertion and polymerization does not proceed. 

Addition of alkyllithium to cyclobutanones and transmetallation with VO(OEt)Cl2 is considered to give a similar alkoxide intermediates, which are converted to either the y-chloroketones 239 or the olefinic ketone 240 depending on the substituent of cyclobutanones. Deprotonation of the cationic species, formed by further oxidation of the radical intermediate, leads to 240. The oxovanadium compound also induces tandem nucleophilic addition of silyl enol ethers and oxidative ring-opening transformation to produce 6-chloro-l,3-diketones and 2-tetrahydrofurylidene ketones. (Scheme 95)... 

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