Olefin metathesis is arguably the most powerful carboncarbon bond breaking and making reaction in chemical synthesis.[1] Depending on the nature of the reacting partners, olefin metathesis can be used for ring-opening polymerization (ROMP), [2] to create advanced polymeric materials, [3] transformation of acyclic diene substrates into complex cyclic organic molecules (in ring-closing metathesis (RCM)) [4] or polymers (in acyclic diene metathesis (ADMET)) [5] or in cross metathesis (CM) to generate unsymmetrical olefins. [6,7] Although olefin metathesis is fully reversible, RCM, ADMET, and CM rely on the elimination of ethylene, the simplest olefin, as a thermodynamic driving force. Used by itself or in tandem with other synthetic transformations, [8,9] olefin metathesis is a versatile method for the modern synthetic chemist.It is generally acknowledged that a metal carbene species, {L n M=CRR'}, is required and that interaction with an olefin substrate leads to four-membered metallacyclobutane intermediates or transition states, {L n M(CRR') 3 }, by a 2+2 cycloaddition; cleavage of this intermediate in the opposite sense by which it was formed leads to olefin metathesis, creating a new carbon-carbon double bond and regenerating an active metal carbene. Metal carbenes are generally classified as being nucleophilic (electron rich) or electrophilic (electron poor) in character at the carbene carbon atom, but an effective olefin-metathesis catalyst exhibits behavior between these two extremes. Two carefully tuned classes of mediator have evolved into the catalysts of choice for olefin metathesis. Schrock catalysts [10,11] are molybdenum-or tungsten-based alkylidenes with a fairly specific ligand set designed to modulate the properties of the carbene (Figure 1). These catalysts display high activities and stabilities, but are sensitive to ambient air and moisture and are relatively intolerant of polar functionalities. The Grubbs-catalyst portfolio [12] consists of a variety of ruthenium-based systems of general formula [Cl 2 (L)(L')Ru = C(H)R] (compounds 1) which are significantly more functional-group tolerant, but do not exhibit the same levels of activity or longevity as the Schrock catalysts.

The 14-electron ruthenium phosphonium alkylidene complex [(IH2Mes)Cl2Ru=CH(PCy3)][B(C6F5)4], 1b, a highly active olefin metathesis catalyst, reacts with stoichiometric quantities of ethylene at -50 degrees C in CD2Cl2 to generate the ruthenacyclobutane complex [(IH2Mes)Cl2RuCH2CH2CH2], 2, and [CH2=CH(PCy3)][B(C6F5)4] in quantitative yield by NMR spectroscopy. 1H and 13C NMR spectroscopies on 2 and 2-13C3 are consistent with a symmetrical C2v structure, providing the first experimental information concerning this crucial intermediate in ruthenium-mediated olefin metathesis. At -50 degrees C, exchange with free ethylene takes place on the chemical time scale. Complex 2 decomposes in solution upon warming to room temperature, generating propene and unknown ruthenium product(s).

This paper provides an industry perspective on atomic layer etching (ALEt) process. Two process sequences representing two different methods of ALEt are described, followed by several examples where ALEt can be an enabling process technology in the semiconductor industry. The authors believe that there needs to be an increased understanding of surface functionalization, modification and chemistry-based material removal. We are confident that this review article will allow for increased scientific and technological solutions for enabling ALEt.

The phosphonium alkylidene [(NHC)Cl2Ru=CH(PCy3)]+[B(C6F5)4]-, 1, (NHC = N-heterocyclic carbene, Cy = cyclohexyl, C6H11) reacts with 2.2 equiv of ethylene at -50 degrees C to form the 14-electron ruthenacyclobutane (NHC)Cl2Ru(CH2CH2CH2), 2. NMR spectroscopic data indicates that 2 has a C2v symmetric structure with a flat, kite shaped ruthenacyclobutane ring with significant Calpha-Cbeta agostic interactions with the Ru center. Intramolecular exchange of Calpha and Cbeta is fast (14(2) s-1 at 223 K) as measured by EXSY spectroscopy. Intermolecular exchange of Calpha and Cbeta with the methylene groups of free ethylene is much slower and first order in both [Ru] and [H2C=CH2] (4.8(3) x 10-4 M-1 s-1). Activation parameters for this process are DeltaH++ = 13.2(5) kcal mol-1 and DeltaS++ = -15(2) cal mol-1 K-1, also consistent with a rate limiting associative substitution as the key step in this exchange process. On the basis of this observation, mechanisms for the intermolecular exchange process are proposed and the implications for the mechanism of the propagation steps in catalytic olefin metathesis as mediated by Grubbs catalysts are discussed.

The reactivity of highly Lewis acidic perfluorinated borafluorenes C 12 F 8 BR (R ) C 6 F 5 , 1a; CH 3 , 1b) and the nonfluorinated 9-phenyl-9-borafluorene (2) toward [Cp*Al] 4 was investigated. The reaction of 1 with [Cp*Al] 4 leads to the formation of thermally robust η 1 Lewis acid-base adducts 3a,b as the thermodynamically favored products. Use of the less Lewis acidic 2 does not alter the mode of reactivity, with the η 1 Lewis acid-base 4 formed preferentially. Reduction of 2 to the 9-boratafluorene 2‚Li 2 (THF) n is readily accomplished in THF solution. However, reaction of 2‚Li 2 (THF) n with [Cp*AlCl 2 ] 2 or Cp*AlCl 2 (THF), 5, affords aluminum metal, 2‚THF, and Cp*H as the main identifiable products. Compounds 3a, 3b, 4, and 5 were fully characterized including their X-ray structures. A DFT computational study was conducted to probe the reason for the strong preference for η 1 bonding, which essentially stems from the localization of aromaticity in the flanking phenyl rings in the 9-borafluorene ring system.

The reaction of phosphonium alkylidenes [(H2IMes)RuCl2=CHPR3]+[A]- (R = C6H11, A = OTf or B(C6F5)4, 1-Cy; R = i-C3H7, A = ClB(C6F5)3 or OTf, 1-iPr) with 1 equiv of ethylene at -78 degrees C, in the presence of 2-3 equiv of a trapping olefin substrate, yields intermediates relevant to olefin metathesis catalytic cycles. Dimethyl cyclopent-3-ene-1,1-dicarboxylate gives solutions of a substituted ruthenacyclobutane 3 of relevance to ring closing metathesis catalysis. 1H and 13C NMR data are fully consistent with its assignment as a ruthenacyclobutane, but 1JCC values of 23 Hz for the CalphaH2-Cbeta bond and 8.5 Hz for the CalphaH-Cbeta bond point to an unsymmetrical structure in which the latter bond is more activated than the former. In contrast, trapping with acenaphthylene leads to an olefin carbene complex (6) in which the putative ruthenacyclobutane has opened; this species was also fully characterized by NMR spectroscopy and compared to related species reported previously.

The C-H activation of methyl tert-butyl ether (MTBE) mediated by pincer iridium complexes derived from PNP and PCP frameworks has been studied. Double C-H activation of MTBE by these complexes leads to formation of Ir(I) Fischer carbenes of the type (pincer)IrdC(H)O t Bu via elimination of H 2 . In one case, the structure of the Fischer carbene has been confirmed by single-crystal X-ray analysis. For both systems, the carbenes are obtained as kinetic products, and prolonged thermolysis leads to the formation of thermodynamically stable trans-(pincer)Ir(H) 2 (CO) complexes. A mechanism for this transformation is presented along with reactivity studies supporting the proposal.

Initiation processes in a family of ruthenium phosphonium alkylidene catalysts, some of which are commercially available, are presented. Seven 16-electron zwitterionic catalyst precursors of general formula (H(2)IMes)(Cl)(3)Ru=C(H)P(R(1))(2)R(2) (R(1) = R(2) = C(6)H(11), C(5)H(9), i-C(3)H(7), 1-Cy(3)-Cl, 1-Cyp(3)-Cl, 1-(i)Pr(3)-Cl; R(1) = C(6)H(11), R(2) = CH(2)CH(3), 1-EtCy(2)-Cl; R(1) = C(6)H(11), R(2) = CH(3), 1-MeCy(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(2)CH(3), 1-Et(i)Pr(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(3), 1-Me(i)Pr(2)-Cl) were prepared. These compounds can be converted to the metathesis active 14-electron phosphonium alkylidenes by chloride abstraction with B(C(6)F(5))(3). The examples with symmetrically substituted phosphonium groups exist as monomers in solution and are rapid initiators of olefin metathesis reactions. The unsymmetrically substituted phosphonium alkylidenes are observed to undergo reversible dimerization, the extent of which is dependent on the steric bulk of the phosphonium group. Kinetic and thermodynamic parameters of these equilibria are presented, as well as experiments that show that metathesis is only initiated through the monomers; thus dedimerization is required for initiation. In another detailed study, the series of catalysts 1-R(3) were reacted with o-isopropoxystyrene under pseudo-first-order conditions to quantify second-order olefin binding rates. A more complex initiation process was observed in that the rates were accelerated by catalytic amounts of ethylene produced in the reaction with o-isopropoxystyrene. The ability of the catalyst to generate ethylene is related to the nature of the phosphonium group, and initiation rates can be dramatically increased by the intentional addition of a catalytic amount of ethylene.