Oxetanes alkylation with

Recently, Hu and Neckers reported that triplet excited states of alkyl phenyl glyoxylates react rapidly and with high chemical yields with electron-rich alkenes forming oxetanes 24 with high regio and stereoselectivity (Sch. 6) [28]. The intramolecular y-hydrogen abstraction (Norrish type II) cannot compete with intermolecular reactions in most cases. When less electron-rich alkenes were used, the Norrish type II reaction became competitive. 

Aluminum alkyl initiators usually need water to be converted into their active form (alumoxanes). This has clearly been shown in the polymerization of 3,3-bis-(chloromethyl)-oxetane initiated with (i-C4H9)3Al and When the overall... 

Examples of C(sp )-0 reductive elimination of olefin oxides from Pt center have been reported recently (Fig. 12) [33]. In the case of Pt oxetanes derived from strained cycloolefins, norbomene and cyclooctene, the elimination reactions can proceed readily at 40 60°C in a number of solvents and even in the solid state. The reaction is highly stereoselective formation of a single isomer of an olefin oxide is observed for norbomene. Importantly, as for complexes 9 and 14 (Fig. 8), the reactivity of the Pt oxetane 19 with an alkyl ligand trans- to the sulfonate is much higher than that of an isomeric compound 18 where the pyridine ligand, a poorer leaving group is trans- to the alkyl. 

Hb polyethers based on 3-alkyl-3-(hydroxymethyl)oxetane have also been widely applied as core molecules for the preparation of multiarm star polymers. Carlmark et al. synthesized PMA star polymers with a similar route as mentioned above by ATRP after modification of hb poly[3-ethyl-3-(hydroxymethyl) oxetane] (hbPEHO) with 2-bromoisobutyryl bromide. The star polymers were prepared with CuBr/Mce-TREN as the catalyst in ethyl acetate at room temjjerature. In order to prevent gelation, the polymerization was conducted at low concentra tions and stopped after low conversion or by deaeasing o t e amoimt of catalyst used. The authors reported that e ratio initiating sites/catalyst could not exceed 1 0.1, otherwise, ge a tion occurred or no control over polymerization was possi... 

Grignard reagents having bulky alkyl groups react with trialkyltin hydrides to give compounds having a Sn-Mg bond, and are synthetically useful as a source of nucleophilic RsSn in particular, they react with carbonyl compounds, oxiranes, and oxetanes to give the -, jS-, or... 

The quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

Turro69 has reported that several alkyl ketones react with 1,2-dicyano-ethylene (12) by nucleophilic attack of the ketone (rr, n) singlet state to give oxetanes stereospecifically. Addition of diene quenchers which are known to deactivate ketone triplets do not affect the rate of oxetane formation. This... 

Studies of hydrogen bonding and iodine complexation on alkyl-substituted oxetanes have shown that 2-alkyl and 2,2-dialkyl substitution enhances the electron donor ability, while 3,3-dialkyl substitution substantially diminishes it. This has been attributed to a decrease in the bond angle strain at the oxygen atom, due to a Thorpe-Ingold effect at C-3, since the effect increases with the size of the alkyl substituent at that position (71BSF4576). 

The Friedel-Crafts alkylation of aromatic compounds by oxetanes in the presence of aluminum chloride is mechanistically similar to the solvolyses above, since the first step is electrophilic attack on the ring oxygen by aluminum chloride, followed by a nucleophilic attack on an a-carbon atom by the aromatic compound present. The reaction of 2-methyloxetane and 2-phenyloxetane with benzene, toluene and mesitylene gave 3-aryl-3 -methyl-1-propanols and 3-aryl-3-phenyl-l-propanols as the main products and in good yields (equation 27). Minor amounts of 3-chloro-l-butanol and 4-chloro-2-butanol are formed as by-products from 2-methyloxetane, and of 3-phenyl-l-propanol from 2-phenyloxetane (73ACS3944). 

Several methods for cleaving oxetanes by reaction with alkyl carbonium ions have been discovered. It is reported that 2-methyloxetane reacts with a-chloroethers in ether solution containing zinc chloride to form a mixture of two isomeric chloro-substituted acetals (equation 28) (72IZV125). 

Oxetanes are much less susceptible to cleavage by nucleophiles than oxiranes, except in the presence of acids. Several types of acid-catalyzed nucleophilic reactions are described in the previous section, such as reaction of hydrogen halides to give 3-halogeno-1-propanols and various acid-catalyzed solvolysis reactions. Another example of this type is the reaction of thiourea with 2-alkyloxetanes in the presence of hydrochloric or perchloric acid to give excellent yields of 3-alkyl-3-hydroxybutylisothiouronium salts (equation 39) <67CR(C)(264)1309>. 

Enol alkylation of cyclohexanone by oxetane has been achieved by the reaction of oxetane with an enamine salt, bromomagnesium Af-cyclohexyliminocyclohexane, in THF. An 80%... 

A rapid one-pot method for converting 1,3-diols into oxetanes by the intramolecular Williamson reaction has recently been described. The monolithium salt is generated by treatment of the diol with one equivalent of butyllithium in cold THF, followed by addition of one equivalent of tosyl chloride to give a monotosylate, which is cyclized by addition of a second equivalent of butyllithium (equation 83). Yields of 70-90% are reported for a variety of alkyl- and aryl-substituted oxetanes (81S550). Another simple method for converting 1,3-diols into oxetanes consists of converting them to cyclic carbonate esters by ester... 

The above synthetic methods for oxetane all involve formation of a new C—O bond. Cyclization by formation of a new C—C bond has been applied with compounds having benzylic or alkylic CH groups. Recent examples of this type of ring closure are the rearrangement of trans- 2,3-epoxycyclohexyl allyl ether by means of s-butyllithium and the dehydrochlorination of a-cyanobenzyl 2-chloroethyl ether with aqueous base and phase transfer catalyst (equation 86). Both reactions probably involve carbanion intermediates (76TL2115, 75MIP51300). 

R" may be alkyl or aryl. For dialkyl ethers, the reaction does not end as indicated above, since R OH is rapidly converted to R OR by the sulfonic acid (reaction 0-16), which in turn is further cleaved to R 0S02R" so that the product is a mixture of the two sulfonates. For aryl alkyl ethers, cleavage always takes place to give the phenol, which is not converted to the aryl ether under these conditions. Ethers can also be cleaved in a similar manner by mixed anhydrides of sulfonic and carboxylic acids733 (prepared as in 0-33). p-Hydroxy alkyl perchlorates734 and sulfonates can be obtained from epoxides.735 Epoxides and oxetanes give dinitrates when treated with N2Os,736 e.g.,... 

The formation of 3-pyrrolylcarbinols (280) from the photochemically induced reaction of pyrrole, or its 1-alkyl derivatives, with aliphatic aldehydes and ketones is thought to proceed via an oxetane intermediate (279) (79JOC2949). In contrast, the analogous reaction of 1 -phenylpyrrole with benzophenone leads to the formation of the diphenyl(2-pyrrolyl)car-binol, whilst the oxetane (281) has been isolated from the photoaddition of 1-benzoylpyrrole and benzophenone (76JHC1037, B-77MI30500). 2-Benzoyl-1-methylpyrrole undergoes a normal Paterno-Buchi photocyclization with 2,3-dimethylbut-2-ene, via the n -> v triplet... 

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