Ethers intramolecular complexation

The ether-catalyst complex (II) splits into a complex anion (III) and a carbonium ion (IV), which rearranges to the configuration of maximum stability (V). This carbonium ion (V) could itself initiate polymerisation, but it is more likely that it attacks the double bond of the closely associated anion (III), giving the double ion (VI) in equilibrium with the aldehyde (VII). Rearrangements of the type (I)-(VII) have been observed for vinyl ethers [7], and a closely parallel isomerisation is that of isobutyl phenyl ether into para-tertiary butyl phenol under the influence of A1C13 [8]. It is unlikely that the steps from (II) to (VI) take place in a well defined succession. The process probably proceeds by a single intramolecular transformation. 

If the enthalpy of formation of 4-lithiobutyl methyl ether is interpolated between the values for the lithiopropyl and the lithiopentyl ethers to be —285 kJ moP, then the enthalpy of isomerization to the less stable 3-lithiobutyl methyl ether is - -10 klmoP, which is about half that of isomerization of n-butyl lithium to 5ec-butyl lithium (-1-21.3 kJmol ). However, a linear interpolation assumes the same strain energy for the 6-membered 4-lithiobutyl ether as for the above 5- and 7-membered cu-lithioalkyl methyl ethers. If it is less strained, then the isomerization enthalpy would be larger. How much of the isomerization enthalpy difference is due to other differences, such as intramolecular complexation and/or aggregation among the various species, is not known. Unfortunately, there is no enthalpy of formation measurement for the delithiated 7-methoxynorbornane. 

Interestingly, the protodelithiation enthalpy of 2-lithio-l,3-dimethoxybenzene is very nearly the same as that for the single methoxy species o-lithioanisole. If the stabilization of lithium by an ortho ether group is due mainly to intramolecular complexation or... 

The case of benzoin alkyl ethers illustrated in Figure 8.15 is a remarkable example of the effect of complexation with cyclodextrins. Such molecules normally undergo homolytic dissociation in solution (the Norrish type 1 process described in section 4.4) and there is practically no intramolecular hydrogen atom abstraction (Figure 8.15). When the benzoin alkyl ethers are complexed with a cyclodextrin to form a 1 1 association, it can be shown that one of the phenyl rings fits inside the cyclodextrin cavity in aqueous solution. When the solid complex is irradiated only the photoproducts resulting from hydrogen atom transfer are detected the opposite behaviour from irradiation of the crystal of benzoin alkyl ether as well as of solutions in benzene. 

A second, less used, strategy encompasses the Lewis acid catalyzed intramolecular reaction of a silyl enol ether with a propargyl cation. The latter can be conveniently generated by a cobalt complexed propargyl ether. This complexation strongly helps the carbocation formation. By using cobalt complexation, intramolecular aldol type reactions (for R = OR ) have been accomplished. ... 

The importance of intramolecular chelation is nicely demonstrated in the lithiation of the glutamic acid derivative 16. With s BuLi (-)-3 an enantioselective deprotonation is observed at position 5 leading after the car-bonylation-esterification sequence to the ester 17, whereas with s BuLi in ether, an intramolecular complexation favors an intramolecular enantio-selective deprotonation at position 1 leading to the ester 18 (Eq. 9). [17]... 

The samarium and ytterbium" species have been characterized crystallographically and were found to contain discrete anions and cations, whereas in the closely related unfunctionalized ytterbium complex Cp 2Yb(OC)Co(CO)3(THF) an Yb-OC-Co interaction (i.e., an isocarbonyl bridge) is observed. This reflects nicely the coordinative saturation effected by the intramolecular coordination in the ether-functionalized complexes. Both cations adopt Cz symmetry (approximate for Sm. crystallographically exact for Yb) and contain two chelating ether groups [Sm-0 249.6(10) and 249.9(10) pm. O-Sm-0 150.5-... 

In the lithiated dicarbamate 111 of (S)-2-(dibenzylamino)-l,4-butanediol (derived from L-aspartic acid) the 4-carbamoyloxy group also possesses a high tendency for intramolecular complexation [Eq. (31)] [75, 76]. The favorable equatorial positions of the dibenzylamino and 1-carbamate groups are displayed in the transition state of the deprotonation. When treated with sec-butyl-lithium in ether or THE, the bicyclic chelate complex 111 is formed exclusively by removal of the pro-S-IH atom. Trapping of 111 by many types of electrophiles gives stereohomogeneous substitution products 112 [Eq. (31), Table 3]. Since deprotection proceeds easily by the usual means, anion 111 constitutes a synthetic equivalent of the synthon 114. No deprotonation in the 4-position was detected, however this can be achieved by protecting the pro-S-lH by conversion to deuterium (see below and Sect. 2.5). 

This lowering can be attributed to the intermolecular binding between the ammonium group and the crown ether ring. One may suppose, therefore, that the trans-lH+ s would exist (at least partially) as cyclic dimers or polymers. On the other hand, the further decrease observed for cis-lH+ s can be rationalized in terms of intramolecular complexation of the ammonium tail by the crown ether ring. Conceivably, the intramolecular complexation in the cis-configuration would displace K+ ion from the crown ether ring. 

A three-component coupling involving three alkenes was employed using a stoichiometric amount of palladium to generate a bicyclic acetal 6.91, which could be converted to an epimer of the prostaglandin, PGF2 (Scheme 6.31). The three-component coupling involves nucleophilic attack of the alcohol 6.89 onto an ethyl vinyl ether-palladium complex 6.93, intramolecular alkene insertion, intermolecular insertion of... 

The reagent was prepared from l,r-dichlorodimethyl ether by treatment with lithium iodide and methyl-lithium. Intramolecular complexation between the lithium atom and the ether oxygen may facilitate formation of... 

The complexation of cobalt-60 radiation-crosslinked POE and of linear POE with molybdenum-VI salts indicated that, in the linear form, complexation is extremely weak and of low stability—40 to 80 times weaker than that with sodium salts (134). This was related to vacant d-orbitals in molybdenum being involved in the interaction process. This resulted in greater polarizability of the cations and steric restriction in coordination with the ether oxygen atoms of the polymer chains. However, when the salts were complexed with the crosslinked POE, stability constants two orders of magnitude higher than those obtained with linear POE were found. This was interpreted as being related to intermolecular rather than intramolecular complexation. Stoichiometry for the crosslinked POE system was 4.5 ethylene oxide units per molybdenum cation. 

The stereoselectivity of these intermolecular reactions between 1-alkoxyallylstannanes and aldehydes induced by boron trifluoride-diethyl ether complex is consistent with an open-chain, antiperiplanar transition state. However, for intramolecular reactions, this transition state is inaccessible, and either (Z)-.yyn-products are formed, possibly from a synclinal process105, or 1,3-isomerization competes113. Remote substituents can influence the stereoselectivity of the intramolecular reaction114. 

In the first example of an intramolecular Sakurai reaction, a six-membered ring was formed in the presence of boron trifluoride-diethyl ether complex starting from an acyclic enone36. 

Closely related to the crown ether adducts are the two intramolecular arenediazonium ion-crown ether compounds 11.6 and 11.7 which were synthesized by Gokel s group (Beadle et al., 1984b). Infrared and lH NMR spectra are consistent with the insertion of the diazonio group into the 21-crown-7 cavity. The complex 11.6 can therefore be described not in an anthropomorphic, but in a zoomorphic way, as an ostrich complex reflecting the common belief that an ostrich hides its head in a hole when endangered. For the complex 11.7 the spectra correspond to... 

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