638
part responsible for the net decrease in the magnitudes above). By analogy to the angular dependency of 4J noted by Manatt, et al.' (for couplings of type 4J1,3i n l), of most of the vinylic couplings observed between the as well as by Barfield and Garbisch (four-bond allylic dienes and the diene complexes (Table I). The Jvalues obtained for the iron complexes do allow S S ) one ~ ~can~ estimate , ~ ~ that for the diene iron commeaningful discussion of the relative orientation of the plexes the value of 4J1,3will approach 0 Hz at an Hl-H3 Finally, the behavior of V and ?J twist angle of -30". vinyl C and H atoms, but only after several competing upon diene complexation to Fe(CO),, decrease in coupling mechanisms have been unraveled. It can 5J,,4by -80% and increase in 2J1,1t by -50%, are be assumed that the effect of (a) above will be very indicative of an extensive loss of t contribution to Jnet large for the three-bond J's and minimal for the fourupon complex formation. and five-bond J's. l 4 The results of several semiempiriBased upon the above considerations we feel that the cal calculations of the relative u and t contributions to best structural description of a diene-Fe(CO)s complex vinylic coupling have been published.'j There is is provided by structure A in which all the C and H reasonable agreement among these calculations and atoms are coplanar. In addition these t complexes in conjunction with experimental data the following seem to provide an excellent empirical test of pmr relationships have been identified: (i) u and t contributions to long range J's and u contributions to coupling theory. l8 vicinal J's are sensitive to H-H orientation;'4z15f(ii) 3J (17) M. Barfield, J . Amer. Chem. Soc., 93, 1066 (1971). is u dominant with T coupling estimated to contribute (18) This work was supported by a grant from the Petroleum Reca. 8-14z for cis coupling;15aje'g(iii) there is disagreesearch Fund, administered by the American Chemical Society. ment regarding the relative u and t contributions to Phillip Crews 4J;16however, the t component of 4J appears to be Division of Natural Sciences minimal for H-H atoms trans-coplanar ("W" arrangeUniversity of California, Santa Cruz ment) and it becomes dominant as coplanarity between Santa Cruz, California 95060 H atoms is lost;12bj15d-f and (iv) the tcontribution to ?I Receiued September 6, I972 is also sensitive to relative H-H orientation. In a cisoid diene 5J is predominantly t in character, as opposed to the trans-trans j J in a transoid diene which Evidence for a Unifying Mechanism of Nucleophilic has nearly equivalent t and u character. 12b,15d-f InSubstitution into Sulfonium Ions' formation of the relative signs of the u and t vinylic Sir : couplings is also important; both theory and experiment indicate that for a cisoid diene 2J,, ,J,, 3J,, and jJ, We wish to report that borderline behavior has now are plus in sign while ?J,, 4J, are minus and both 4Juand been observed in the competitive substitutions by sol"J, may vary in sign as a function of H-H orientation. vent water and added nucleophile into benzyldimethylAttachment of Fe(C0)3 to dienes 1, 3, and 4 results sulfonium ion (I-, 140", H 2 0 ) and into p-methoxyin a decrease in magnitude of the vicinal J's by -30benzyldimethylsulfonium ion (N3-, 60 ", HzO). Thus, 40% (3J's of 1 and 3 adjusted to reflect a planar confor example, in the presence of 0.1 M NaN,, an aqueous formation). The X-ray of 7 shows a stretching of the solution of p-methoxybenzyldimethylsulfonium nitrate spz-sp2 double bond for 4 + 7 of ca. 0.11 A,13 which (0.0066 M ) at 60" furnishes 82 % p-methoxybenzyl azide l4 according to Karplus would decrease Jl,2 by -32 Z. and 18 p-methoxybenzyl alcohol at a rate 1.82 times Assuming for complexes 5-7 that the t contribution to that observed in the absence of azide ion (Table I). It Jl,2 approaches 0, a maximum diminution of J1,Z = 4 6 x can readily be shown2 that an s N 2 model would predict can be estimated for conversion of a diene to a dienea", H.0 Fe(CO), complex in which the H1-H2 dihedral angle is CH~O - CH,S+(CHJ, ~ 0 ~ -Na",r held constant at 0". Significant torsion about the C1-C2 bond in the complexes would further decrease Jlz. Thus it would appear that the observed trends in the couplings (Table I) roughly favor a diene-Fe(CO), complex structure wherein H1 and H2are coplanar. a rate enhancement of a factor of (82/18) 1 = 5.55 More substantial evidence in support of this concluand that an SNI model would predict a factor of I.OO., sion is obtainable from inspection of the four-bond J'S While kinetic behavior is obviously borderline, the in the table. There is essentially no variation in 4J1,3 ratio of products, [ArCH2N3]/[ArCH20H],is a linear between the planar olefin models as compared to the function of azide ion concentration from [N3-] = 0.03 corresponding iron complexes: 4J1,3(2) = 1.0 US. to 0.12 M , with slope m = 48.0 f 1.0 (Table I). 4J1,3(5)= 1.4 Hz; (cor) = 1.1 US. 4J1,3(6)= 1.2 These data are of course reminiscent of those which and 4J1,3(7)= 1.1. The close similarity of the 4J1,3 led us to propose2 the ion-pair mechanism for substitucouplings among these five compounds is in accord with tion into alkyl halides and sulfonates and an analogous the notion that a four-bond J between H atoms in a scheme, in which an ion-dipole assemblage, [ArCiplanar "W" orientation is primarily u in character (iii
+
(14) M.I
