Showing papers by "George M. Whitesides published in 1970"

Mechanism of formation of 1,2,3,4-tetramethylnaphthalene from 2-butyne and triphenyltris(tetrahydrofuran) chromium(III)

Abstract: \\bstract: Reacrion of triphenyltris(tetrahydrofuran)chromium(III) (f) with 2-butyne y'ields, inter alia, 1,2,3,4' tetremerhylnaphthalene (2). A free or metal-complexed tetramethylcyclobutadiene has been exclr-rded as an inter-mediare rn rhis reaction by the observation that 1,2-dimethyl-3,4-di(methyl-rir)naphthalene (6) is not a product of I and 2-burl ne-1,1,1-d3 (3). Examination of the relative yields of 1,2,3,4,5-pentamethylnaphthalene (15) and 1,2,3,' .1.6-pentamerhylnaphthalene (16) tbrmed in reaction of 2-butyne with trio -tolyl-, tri'nr-tolyl-, and trip -tolyltris-(terrahldroiuren)chromium(III) demonstrates thiit a iong-lived aryne-chromium complex is also not an intermedi-atc in fhe:e cyclizations. Labeling and kineric isotope experiments using partially deuterated derivatives of 1 fur-rhcr drse rcdir bcnzyne complexes as intermediates in the tbrmation of 2. The reaction of trimesityltris(tetrahydro-iurrn)chronrrumtlll) with 2-butyne to torm cis-2-mesitylbut-2-ene suggests that an initial step in these reactions may be rnserrron ot'2-butyne into a carbon-chromium bond rvirh formation of an intermediate vinylic chromium reagent (2{r. The additional observations that 2-biphenylmagnesium bromide and trichlorotris(tetrahydrofuran)chro-mrum(lll) relct to form 5-(2-biphenyl)dibenzchromole (29), and that this chromocyclc reacts with 2-butyne to yield 9.lr.t-dimcthylphenanrhrene, impliclte an analogous chromocycle 3,4-dimethyt-5-phenylbenzchromole (34) in the l'ormarron of 2. Taken togethei with further deuterium exchange and labeling experiments. thcse obscrvations lead ro rhc proposal thar the reaction of I with 2-buryne proceeds by initial insertion of one molccule ol'2-butyne into a phcnll-chromium bond of l, cyclization of thc resulting vinylic chromium contpound rvith elimination of benzene to iorm chromocycle 34, anci reaction of the chromocycle with a second molecule of 2-butyne to yield 2 (Scheme VII). rclooliuonrerizetion of acetylene by transition ntetal clitllvsts pror ides the basis for a number of useiul orslnic sl'ntheses.3 Attenrpts to establish mechanisms ior this class of reactions have been hindered by' dif'hculties in identifying the active organometallic catalvsts. Although a variety of structures have been postulated tor these catalysts, it has proved possible to isollte and characterize mechanistically significant reection intermcdiates or by-products in only a small number oi reactions.{'; Precedcnts derived from the ertensir elv explored chemistry of transition-metal acety-lenc conrplexes suggest that these cyclooligomerization reactions are initiated by coordination of one or more molecules of acetylenic substrate to a metal atom or metal atonr cluster of the catalyst. s'6 The process(es) b1' which the coordinated acetylenes are subsequently conr erted to product has been variously suggested to involr e metallocyclic, r'z's ?r-allylic,s\"'e'r0 or cyclo-butadienes' r0-r3 intermediates. or to proceed by a con-certecl \" n-ntulticenter\" rcaction pat[. t t' t3 Of these suggested and not necessarily exclusive mechanistns, only the …