Grignard reagent reaction with oxetanes

Grignard reagents react with oxetane, a four-membered cyclic ether, to yield primary alcohols, but the reaction is much slower than the corresponding reaction with ethylene oxide. Suggest a reason for the difference in reactivity between oxetane and ethylene oxide. 

Osmium tetroxide, reaction with alkenes, 235-236 toxicity of, 235 Oxalic add, structure of, 753 Oxaloacetic acid, structure of, 753 Oxetane, reaction with Grignard reagents, 680 Oxidation, 233, 348 alcohols, 623-626 aldehydes, 700-701 aldoses, 992-994 alkenes, 233-236 biological, 625-626 phenols, 631 sulfides, 670 thiols, 668... 

The reaction of oxetanes with Grignard reagents and organolithium compounds is well known and has been useful, particularly with oxetane itself. A recent application involves use of the reaction of isopentylmagnesium bromide with oxetane as the first step of a synthesis of an insect pheromone (equation 43) (76CB3375). The Grignard reaction with 2-methyloxetane and 2-phenyloxetane is less satisfactory, due to the extensive formation of by-products. 

Grignard reagents react slowly with oxetane to produce primary alcohols. Propose a mechanism for this reaction, and suggest why oxetane reacts with Grignard reagents even though most ethers do not. 

Reactions of Grignard reagents with oxetane lead to three-carbon homologation yields are often good, but 3-halogenopropanol may be a significant by-product [37] ... 

The a,(3-unsaturated ester (8), aldehyde (9), and nitro (10) compounds participate as electrophiles in a number of useful conjugate addition reactions. Copper-catalyzed addition of Grignard reagents provides access to aryl butanoic acid derivatives substituted with an oxetane (equation 1 in scheme 13.7). The ester, aldehyde, and nitro electrophiles also undergo mild Rh-catalyzed additions of aryl and vinyl boronic acids (equation 2 through equation 4 in scheme 13.7). Interestingly, the unsaturated aldehyde participates readily in an amine conjugate addition to afford oxetane substituted 3-amino-acetaldehyde derivatives. 

Amongst the few reported reactions of oxetans is their non-stereospecific conversion into oxazines when treated with nitriles and the reaction of the (R)-lactone (44) with a Grignard reagent as the key step in a novel synthesis of (i ,/ )-phytol. ... 

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