Diazoalkanes frontier orbitals

Diazoalkane addition to cyclopropenes having electron withdrawing substituents at 1- or 2-positions generally leads to the regioselectivity predicted on electronic or frontier orbital grounds ... 

Some 1,3-dipoles, such as azides and diazoalkanes, are relatively stable, isolable compounds however, most are prepared in situ in the presence of the dipolarophile. Cycloaddition is thought to occur by a concerted process, because the stereochemistry E or Z) of the alkene dipolarophile is maintained trans or cis) in the cycloadduct (a stereospecihc aspect). Unlike many other pericycUc reactions, the regio- and stereoselectivities of 1,3-dipolar cycloaddition reactions, although often very good, can vary considerably both steric and electronic factors influence the selectivity and it is difficult to make predictions using frontier orbital theory. 

The rates of 1,3-dipolar cycloadditions of diazoalkanes to alkenes and alkynes have been determined electron-attracting substituents in the latter increase the rate, in accordance with frontier molecular orbital theory, which predicts that these reactions are controlled by the interaction of the highest occupied molecular orbital of the diazo-compound with the lowest unoccupied molecular orbital of the dipolarophile " the kinetics of the reactions of methyl diazoacetate or phenyl diazomethanesulphonate, on the other hand, give rise to U-shaped activity functions, which is also explained by the theory. Diazomethane or... 

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