Showing papers by "George M. Whitesides published in 1972"

Mechanism of thermal decomposition of dibutylbis(triphenylphosphine)platinum(II)

Abstract: The thermal decomposition of di-n-butylbis(triphenylphosphine)platinum(Il) (l) in methylene chloride to n-butane, l-butene, and a complex of platinum(0) is proposed to take place by an intramolecular process involving an init ial dissociation of I equiv of triphenylphosphine from 1 to yield a three-coordinate intermediate 5, elimination of platinum hydride from one butyl group of 5 with concomitant ransfer of the resulting l-butene to the vacant coordination site, several cycles composed of rapid addition of platinum hydride to the coordinated butene and subsequent rapid reelimination of platinum hydride from the resulting butylplatinum complexes, and a final reductive elimination of n-butane from an intermediate having both hydride and butyl moieties bonded to platinum. Three significant conclusions concerning the mechanism of thermal decomposition of t have emerged lrom this study. First a vacant coordination site on platinum is a prerequisite for thermal decomposition under the conditions studied. Second, the identity of the rate-l imiting step for the overall decomposition reaction depends upon the concentration of triphenylphosphine added to the solution: in the absence of added triphenyiphosphine, the rate-l imiting step is the dissociation of I to 5; in the presence of ca. I equiv of added triphenylphoiphine, the rate-limiting step is the reductive elimination of butane. Third, the olefins participating in the platinum hydride addition-elimination sequence are coordinated in the intermediate platinum Complexes ufficiently firmly that they do not exchange with 1-butene free in solution. f nformat ion concerning the mechanisms of thermal r decomposi t ion of a lkyl der ivat ives of t ransi t ion metals is pert inent both to theoret ical d iscussions of the electronic structure of carbon-metal c bonds and pract ical appl icat ions of t ransi t ion metal organometal l ic compounds in organic synthesis and catalysis. ( l ) Supported' in part by The Nat ional Science Foundat ion, Grants GP-7266 and GP-142,17. (?) Nat ional Inst i tutes of Heal th Predoctoral Fel low, 1967-196g: E. B. Herschberg Fel low, 1966-1967. (3) Nat ional Inst i tutes of Heal th Predoctoral Fel low. 1967-1970, Journal ol'the Anterican Chemical Societ;' I 94:15 I Jult ' 26, 1972 Mechanisms involving both the homolytic scission of carbon-metal a bonds and the 6 elimination of metal hydrides have been proposed for these thermal decomposi t ions;{ '5 however, the lat ter course has been established as the more common for-n-alkyl derivatives (4) M. S. Kharasch and O. Reinmuth. \"Gr ignard React ions of Nonmetal l ic SubstancEs,\" Prent ice Hal l , New York, N. Y. , 1954, Chapter 5. (5) Reviews: (a) G. E. Coates, M. L. H. Creen, and K. Wadc, \"Organometal l ic Compounds,\" Vol . 2, 3rd ed, Methuen and Co., London, 1968, Chapter 7; (b) G. W. Parshal l and J. J. Mrowca, Adcan. Organometal . Chem.,7,157 (1968): (c) F. A. Cotton, Chem. Reo., 52 , 557 (1955) ; (d ) I . I . K r i t skaya , Usp . Kh im. ,35 , 167 (1966) . of platinum(Il),u rhodiur,n(I),7 and copper(I),E by ideniif ication of metal hydrides as products of the decompositions, and for derivatives of other metals by less direct techniques.s'e The nature of the factors determining both the rate of metal hydride elimination in the thermal decompositions of these classes of compounds and the reversibi l i ty of th is el iminat ion are unknown. The work reported in th is paper deals wi th an examinat ion of the mechanism of thermal decomposition of crs-di-n-butylbis(triphenylphosphine)platinum(Il) (1), a representative alkyl derivative of a dE transition metal ion, whose physical properties make it amenable to detailed mechanistic examination. The purpose of this study was to substantiate a pathway for itreimat decomposition of 1 involving platinum hydride elimination and to provide detail to the steps surrounding this elimination.

Free radical intermediates in the reaction of neophyllithium with oxygen

Abstract: Neophyllithium (l) has been oxidized in solution by reaction with molecular oxygen at very low, controlled , rates. When autoxidation is carried out at 25\" in n-pentane or n-heptane solution, a significant fraction of the reaction products contain the benzyldimethylcarbinyl moiety; these products strongly implicate neophyl free radical as an intermediate. [n hydrocarbon solutions containing tetrahydrofuran or triethylamine, and in pure diethyl ether solution, formation oIthese products is largely suppressed. Neophyllithium is tetrameric in n-pentane at concentrations of 0.07 M but dimeric in diethyt ether at similar concentrations (-0. l0 M); vapor pressure studies establish that I coordinates strongly with tetrahydrofuran, but weakly with triethylamine and diethyl ether. Correlation of product distributions with the composition of the aggregates of I inferred from these physical studies indicates that the unsolvated tetrameric aggregates of neophyllithium auto,xidize in major part by a path involving free neophyl radicals, while the solvated dimeric aggregates autoxidize predominantly by a path not involving free alkyl radicals. Th. reaction of organolithium and-magnesium com-I pounds with molecular oxygen is important as a synthetically useful method of converting these reagents to alcohols and as an almost ubiquitous side reaction in their preparation and handling.,r-; These autoxida

Nitrogen and boron spin-lattice relaxation in borazole

Abstract: The observetl l ine shlpes oi t i re rH nucie:rr nragnetic resort:l , t lce spectr& of nent borazole and of borazole trr toluene-r1c, dirnethyi