21 8
Although this research opens the way to the chemical determination of the hydrophobic binding sites of proteins, some caution in interpretation is required. The yield of radioactive glutamic acid is small (1-3 %) and although the identification of the glutamic acid is reasonably secure, its detailed mode of formation must still be proved. At most, the equations shown in this communication must be read as diagrams pertaining only to that portion of the photolyzed product that yields glutamic acid. (17) National Science Foundation Fellow, 1965-1969.
Ronald J. Vaughan,” F. H. Westheimer James B. Conant Laboratory, Haruard Unicersity Cambridge, Massachusetts 02138 Receiwd Nocember 15, 1968
Rhodium(1)-Catalyzed Valence Isomerization of exo-Tricyclo[3.2.1,0z~4]oct-6-ene Sir: Valence isomerizations are receiving continuing interest. Recently several examples have been reported of these reactions in the presence of catalytic amounts of transition metal complexes. It was concluded that reactions which are “forbidden” (energetically unfavorable) according to the Woodward-Hoffmann postulate under thermal conditions may occur under mild conditions in the presence of a suitable transition metal complex. z, In this communication we report on an example of a transition metal complex catalyzed valence isomerization in which a cyclopropane ring and a double bond are ( l ) , 4 p 5 either involved. exo-Tricyclo[3.2. 1.0z~4]oct-6-ene pure or dissolved in CDC13,was converted quantitatively (2) in the presence into tetracyclo[3.3.0.0z~8.04~6]octane of 10 mole % of Rhz(C0)4C12at room temperature. a
h
Product 2 had been reported earlier by Freeman6 and LeBel.’ Our spectral data (complex peaks centered (1) H. C. Volger and H. Hogeveen, J . Am. Chem. Soc., 89,2486(1967); Rec. Trac. Chim., 86,830(1967); Chem. Commun. 1133 (1967). (2) W. Merk and R . Pettit, J . Am. Chem. Soc., 89,4788 (1967). (3) F. D. Mango and J. H. Schachtschneider, ibid., 89, 2484 (1967). (4) S . D. Koch, R. M. Kliss, D. V. Lopiekes, and R . J. Wineman, J . Org. Chem., 26, 3122 (1961); H . E. Simmons and R. D. Smith, J . Am. Chem. Soc., 81,4256 (1959). ( 5 ) Nmr data of 1: 6 (ppm from TMS) H(1,5) 2.71, H(2,4) 1.03, H(3a) 0.79, H(3b) 1.45, H(6,7) 6.35, H(8a) 1.06, H(8b) 0.96; J(1,5-6,7) 1.72 cps (triplet), J(3a, 3b) = 6.3 cps; J(2,3b) = 3.0 cps J(2.3a) = 7.5 cps, J(8a,8b) = 8 cps (AB pattern). (6) P. K. Freeman, D. G. Kuper, and V. N . Mallikarjuna Rao, Tetrahedron Letters, 3303 (1965). (7) N. A . LeBel and R . N . Liesemer, J . A m . Chem. Soc., 87, 4301 (1965).
Journal of the American Chemical S o c i e t y
at 1.61 and 1.26 ppm in the ratio 6:4; parent peak at mje 106 and C-H stretching frequency at 3020 cm-I, characteristic of cyclopropane rings) are about identical with those reported by Freeman and LeBel. During the reaction, the red catalyst Rh2(C0)4C1zwas converted into a yellow, catalytically less active, complex. The elemental analysis and the infrared data (a terminal carbonyl group (2040 cm-l), an acyl group (1700 cm-l), and a bridged chloride (283 cm-l)) suggest a structure similar to that reported for the complex obtained from cyclopropane and Rhz(C0)4C1z,which contains a 1-rhodiacylopent-2-one ring.8 The exo arrangement of the fused cyclopropane ring in 1 is required for the occurrence of valence isomerization. This is demonstrated by the failure of the endo isomer 39 to undergo a similar reaction. Even at 100” 3 did not react. The results can be explained in terms of the geometry of the tricyclic systems. In the exo isomer 1, the T orbitals of the double bond, together with the orbitals forming the cyclopropyl bent bond between CZand C4, are ideally situated for interaction with the orbitals of the rhodium atom, considering the reported edgewise coordination of cyclopropane toward tetravalent platinum. lo It is of interest to note that valence isomerization of 1 under thermal (200’) and photochemical conditions has been reported to yield tricyclo[3.2.1 .0z,7]oct-3-ene (4)” and 2 (20736, respectively. Irradiation of the
-4 endo isomer 3 leads also to 2 (19 %).6 Apparently, the spatial arrangement of the fused cyclopropane is not critical under these conditions. The reaction pathways of the conversion of 1 into 2 under catalyzed as well as under photochemical conditions have in common that occupied antibonding orbitals-either by back-bonding from the metal or by excitation-are involved The conversion of 1 into 2 suggests a possible route for the preparation of tetracyclo[3.3. 1.0z~8.04~6]nonane (6) (triasterane) from exo,exo-tetracyclo[3.3.1.02~4.06~8]nonane (5).l2 All attempts to effect the conversion failed; the starting material was quantitatively recovered. Only the complex Rhz(C0)4C12was converted into a yellow complex, with presumably a structure similar to that of the complex obtained from 1. (8) R. M. Roundhill, D. N. Lawson, and G. Wilkinson,J. Chem. Soc., A , 845 (1968). (9) I
