February 27

1740

Journal o f t h e American Chemical Society

Gollnick, K.; Schroeter, S.; Ohloff, G.; Schade, G.; Schenck, G. 0. Justus Liebigs Ann. Chem. 1965, 687, 14. The reported product ratio (Gollnick. K.; Schade, G. Tetrahedron Lett. 1966, 2335) contradicted our rule. Reinvestigationlo showed that this ratio must be inverted. Direct structural information is only available in the case of l-methylcy3-thujene (E), and 3-carene (13). In the other cases, we exciobutene (I), trapolate from the geometries of the stable conformers of the parent cycloalkenes which are all known. The stable conformers of cyclobutene and l-methylcyclobutenez' are planar. The pairs of allylic C-H bonds on each side of the ring have identical, mirror-symmetric orientations as in A. The stable conformers of cyclopentene, the envelope conformation.2z and of cycloheptene, the mirror-symmetric hair,*^.^^ both have pairs of allylic C-H bonds in identical, mirror-symmetric orientations on either side of the ring; one pair has quasi-axial orientations as in A and the other has quasi-equatorial orientations. The stable conformer of (2)-cyclooctene22,z3and both stable conformers of (Z)-cyclododeceneZ4have no symmetry, but have a pair of allylic C-H bonds in nonidentical, quasi-axial orientations on one side of the ring, approximately as in A, and a pair in quasi-equatorial orientations on the other side. In contrast, the stable conformer of cyclohexene. the h a l f - ~ h a i rhas . ~ ~pairs of allylic C-H bonds with different orientations, quasi-axial and quasi-equatorial, on each side of the ring, as in B. 3-Thujene (8) is essentially planarz6and has a pair of allylic C-H bonds in different orientations but approximately as in A on one side of the ring. The geometry of 3-carene (13) is not known with certainty; it is either one of the boats22c~27 or a planar form.22c,z8 The former has a local geometry similar to that of cyclopentene and -heptene, the latter one similar to that of cyclobutene. The geometry of 2-carene (14) is unknown. Bak, E.; Led, J. J.; Nygaard, L.; Rastrup-Anderson. J.; Sfirenson, G. 0. J. Mol. Struct. 1969, 3, 369. See also ref 23c. Huang, Y. S.; Beaudet, R. A. J. Chem. Phys. 1970, 52, 935. (a) Allinger, N. L.; Sprague, J. T. J. Am. Chem. SOC.1972, 94, 5734, and references cited therein. (b) Ermer, 0.; Lifson, S. /bid. 1973, 95,4121, and references cited therein. (c) Gunther, H.; Jikeli, G. Chem. Rev. 1977, 77, 599, and references therein. Review: Casanova, J.; Waegeli, B. Bull. SOC.Chim. Fr. 1975, 911. Anet, F. A. L.; Rawdah, T. N. TetrahedronLett. 1979, 1943. Eliel, E. L.; Aliinger, N. L.; Angyal, S. T.; Morrison, G. A. "Conformational Analysis"; Interscience: New York. 1965. Abraham, R. J.; Holden, C. M.; Loftus, P.; Whittaker, D. Org. Magn. Reson. 1974, 6, 184. (a) Acharya, S. P. Tetrahedron Lett. 1966, 41 17. (b) Naumov, V. A,; Bezzubov, V. M. Dokl. Akad. Nauk SSSR 1966, 771,634. Abraham, R. J.; Cooper, M. A,; Whittaker. D. Org. Magn. Reson. 1973, 5, 515. This hypothesis is reminiscent of, but different from, the previously established rule that an individual ene addition is favorable when the ground-state geometry of the substrate is such that the (single) C-H bond being broken is roughly perpendicular to the plane of the double bond.6 This rule is almost solely based on data on half-chair cyclohexenes, which have only one such C-H bond, and are exceptions to our rule. For recent work, see: (a) Jefford, C. W. Tetrahedron Lett. 1979, 985, and references cited therein. (b) Grdina, B.; Orfanopoulos, M.; Stephenson, L. M. /bid, 1979, 4351, and references cited therein. One of us has previously advanced this hypothesis,2aon the basis that the constitutional requirement for the PSEA to acyclic substrates appears to be the presence of the element HC,-C=C-CbH (Ca, Cb cis). 1.e.. the Curtin-Hammeti principle is applicable: the product composition depends on the relative energies of the transition states and not on the populations of the ground-state conformations. The Hammond postulate, however, may also be applicable: the transition states should resemble the reactants (provided that the rate-determining step is strongly exothermic; this may not be the case if a perepoxide or some related species is involved).

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February 27, I980

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Karl H. Schulte-Eke,* Valentin Rautenstrauch* Research Laboratories, Firmenich SA CH-121 I Geneca 8, Switzerland Receiced September 28, 1979

Isolation and Characterization of [ R ~ ~ ( C O ) I O ( ~ ~ - CA O Key ) ~ ] -Product : in Rhodium Carbonyl Chemistry Sir: The dianion' [Rhlz(CO)3o]*- is known to react reversibly a t room temperature with carbon monoxide. This reaction produces a labile species which previously was tentatively formulated as [Rh12(CO)-34l2- on the basis of volumetric measurements of absorbed CO in THF,2 and which has characteristic IR2 and I3C N M R 3 spectra. For several years 0002-7863/80/ 1502- 1740$01 .OO/O

Figure 1. Atomic numbering scheme and R h - R h distances. Estimated standard deviations in distances are