Beceived inlISA 6 Jnly1970; received inI0Kfor~blicetiom 3 Septe&er IgO) Tbe 1,3-dipolar cycloadditioa reectioe has been the sobject of tiaz eabaustive ard elegant iaveatigationa of Eluiagen sad w-a~rkers.~ Tkse reactions shap all the cbarecteristics of coacerted reactions, iacluding stereospecificity, regioaelectivity frequently inccrpatible with diradical or dipolar iatermediatea, d-1 influeace of solvent polarity oa rate, -ate activatioe entbalpies, and large negative activation entropies.' Although a recent suggestion baa beea lade that these reactions proceed &rough diradical intermaliates,2 Euiagen bas wonted wnvimrieg evidence supporting the coacertedeeaa of the majority of 1.3-dipolar cycloaddition reactioas.lb Orbital a-try analysis of l,g-dipolar cycloaddition reactioea Iam revealed that these reactions are themally allowed by virtue of the presence in 1,3-dipoles of a km electron system isoelectronic vith tbe ally1 anion. s A simple eateaaimof this analysis reveals&at l.j-dipolea may add in a concerted manner to triema (6n), but sot to dieeea (km). Armed vith this prediction, the study of 1,3-dipolar cycloadditions to trieoes uaa begun. both to seek corroboration of this prediction, and to iaveatigate.furtber tbe steric, electroaic, and spatial factors which induce pariaelectivity in capeting cycloadditioas.4*5 Dipheaylaitrileidne, I, vaa generated ia tbe preseme of tropone by tbe slow additioa of triethylaeine to a beoaemz solution of u-chlorobeaaylideae pbenylhydraaiae, IIe, aed tropope in *