Grignard reagents react with epoxides but dissolve in THF, what happens in oxetane (four membered ether ring)? Ring strain causes the epoxides to react and THF does not react because the ring strain is not large enough. Does the four membered ring react or not? Does it favor reaction with the harder or softer nucleophile perhaps - thus only reacting with some Grignard reagents?
In a 1979 paper entitled Copper-catalysed reactions of Grignard reagents with epoxides and oxetane, Huynh, Derguini-Boumechal, and Linstrumelle report that the copper-catalysed ring opening of oxetane by various Grignard reagents gives the alcohols "in acceptable yields":



$$ \begin{array}{cccc}
\ce{R} &amp; \ce{X} &amp; \text{Solvent} &amp; \text{Yield (%)} \\
\hline
n\text{-}\ce{C4H9} &amp; \ce{Cl} &amp; \text{Ether} &amp; 75 \\
\ce{C6H5} &amp; \ce{Br} &amp; \text{THF} &amp; 52 \\
\ce{CH2=CH-CH2} &amp; \ce{Br} &amp; \text{Ether} &amp; 50 \\
\ce{(CH3)2C=CH-CH2} &amp; \ce{Cl} &amp; \text{THF} &amp; 50
\end{array} $$