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CH, CH3 L' have nearly the same basicity and are small. Species I and I1 may be either transition states or intermediates H2C P -;h$' + phb',,C= . Y N 1X oN-Ph and are not necessarily identical. The overall mechanism is associative in nature as N% H' suggested by the following evidence. The isomerization CH3 CH, 0 0 rate decreases as L and L' increase in size and upon changing X- from C1- to N3-. lo ~ ~ S - [ C H ~ P ( C ~ H ~ ) ~ ] ~ PtC12 is converted instantaneously at 25" in CDCL to trans-[CH3P(CsHj)2]~PtC12 by (CH30)3P,and [(CHBO)BP][CH3P(C&15)2]PtC12is present in solution, whereas with (C6HL)3Pisomerization has not occurred for this complex within 48 hr to any measurable extent and [(C6H5)3P][CH3P(C6HS)2]Ptclz is not present in this solution even after 48 hr. Moreover, equilibrium thermodynamics support the steric importance and ?~~,~~ solvent dependence of the m e c h a n i ~ m . ~ This mechanism is also consistent with data obtained5 for [(C6H5)~PCH3]2Pd(5-CF3-tetrazolate)Z for which the pathway changes from (3) to (1) by addition of (C6H5)ZPCH3. Thus, a third pathway for cis-trans-isomerization does exist and does predominate in at least some cases. We conclude, therefore, that the general mechanism (+)(S)-4a, X = NS02C1;Y = 0 (+)$a, X = NS02C1;Y = 0 contains three separate pathways whose importance (+)( S)-4b, X = N H Y = 0 (+)-5b, X = 0; Y = NSOICl varies as a function of the metal, solvent, coordinated ( + ) - ( S > ~ C ,X = 0 Y = NS02C1 ligand, catalyst (which may be solvent), and anion. the signs of rotation and the attachment atoms at the Acknowledgment. The financial support of the chiral carbon atoms in 2 and 4a and 4b being the same. University of Nevada, Reno, Research Advisory Board The chirality of the dienes 3 and 5 are assigned as shown and the donors of the Petroleum Research Fund, adby comparison of the optical properties of the resulting ministered by the American Chemical Society, is gratePTAD adducts 6 and 7 (PTAD approaches the least fully acknowledged.
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(11) D. A. Redfield and J. H. Nelson, Inorg. Chem., 12,15 (1973).
David A. Redfield, John H. Nelson* Department of Chemistry, University of Nevada Reno, Nevada 89507 Received June 24, 1974
Chiral 1,2-Bisalkylidenecyclopentanes. Direct Formation via Cycloaddition Reactions of Chiral Substituted Alkenylidenecyclopropanes' Sir : 2-Phenylisobutenylidenecyclopropane(1) undergoes cycloaddition reactions with 4-phenyl-l,2,4-triazoline3,5-dione (PTAD) and chlorosulfonylisocyanate (CSI) to form 1,2-bisalkylidenecyclopentane derivatives. 2, In a study of the stereochemical aspects of these cycloaddition reactions, ( - ) - ( R ) - l has been prepared4 and reacted with PTAD and CSI at 0" in methylene chloride (+?.5 (-1-7 yielding adducts 2 and 3 and 4 and 5 respectively, all hindered face of the diene opposite the phenyl group)2 of which are optically active. which have opposite signs of rotation and configuraThe configuration of 4b, formed by the hydrolysis of tions compared to 2 and 4a and 4b. Nmr analysis of 2 4a, has been directly related to (+)-(S)-phenylglycine and 4b in the presence of the chiral shift reagent trisby ozonolysis and metaperiodate ~ x i d a t i o n . ~The (trifluoroacetylcamphorato)europium(III) [Eu(tfac),] absolute configuration of 2 is assigned the same as in 4, indicates that 2 and 4a are formed stereospecifically. (1) (a) Cycloaddition Reactions of Cyclopropane-Containing SysTo our knowledge 3 and 5 are the first examples of tems. VII. For the previous paper in this series see D. J. Pasto and A. chiral, skewed 1,2-bisalkylidenecycloalkanesto be preF.-T. Chen, TetruhedronLert., 713 (1973). pared. The chirality of the dienes arises from severe (2) D. J. Pasto and A . Chen, J . Amer. Chem. Sac., 93, 2562 (1971); D. J. Pasto, A. F.-T. Chen, a n d G . Binsch, ibid.,95,1553 (1973). steric interactions between the phenyl and the "inside" (3) D. J. Pasto, A. F.-T. Chen, G. Ciurdaru, and L. A. Paquette, methyl of the isopropylidene group forcing the diene J . O r g . Chem., 38,1015 (1973). (4) D. J. Pasto and J. K. Borchardt, TerrahedronLett., 2517 (1973). to assume a nonplanar configuration. The extent of ( 5 ) The details of the stereochemical transformations and correladistortion of the diene chromophore is substantial as tions will be reported in a future full article describing the mechanistic shown by X-ray structural studies carried out on an implications of the stereochemical studies. Journal of the American Chemical Society
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1 September 18, I974
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adduct similar to 3, the dihedral angle between the isopropylidene functions being 52.3 O q 6 Passage of the phenyl past the "inside" methyl results in racemization, a process that must possess a rather high activation energy barrier.' Adducts 3 and 5 show no tendency to undergo racemization over the course of several. weeks at room temperature. The ease of preparation of chiral alkenylidenecyclopropanes containing a wide variety of functions attached to the cyclopropane and terminal allene carbon will allow the facile preparation of variously substituted chiral 1,2-alkylidenecyclopentanesrelated to 3 for structural and racemization kinetic studies. The results of such studies will complement and extend the results derived with chiral substituted biphenyls. Such studies have been initiated in our laboratories. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. ( 6 ) D. J. Pasto and W. R. Scheidt, unpublished observation. (7) Kiefer and coworkers have measured the activation energies for the racemization of a 1,2-bisisopropylidenecyclobutane and -cyclohexane deriving values of
