These meta selective transformations owe their selectivity to a mechanism involving a directing group assisted ortho cyclometalation, which activates the position para to the newly formed C–Ru bond. 36,37 In an alternative approach, ruthenium catalysis can be employed for the direct meta sulfonation, 38,39 secondary 40,41 and tertiary 41–43 alkylations, bromination 44–46 and nitration 47 of arenes. Utilizing non-covalent interactions between a directing group and a substrate can also lead to meta selective reactions.
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12–24 Transient directing groups can also be introduced using carboxylic acids 25–29 or norbornene, 30–35 effectively installing a temporary directing group to the ortho position, which can undergo a subsequent ortho C–H functionalization. In a conceptually similar approach, a range of extended templates containing coordinating pyridine or nitrile groups have been developed, overcoming issues of conformational stability by forming particularly stable metallacycles. Scheme 1 Ruthenium catalyzed alkylation of arenes. Because of this, a number of alternative strategies have been devised ( Scheme 1). However, generating the corresponding metallacycles for more remote C–H bonds such as the meta C–H bond becomes increasingly challenging as ring size increases and conformational stability decreases. 3–9 Key to the success of this strategy is the relatively facile formation and conformational stability of the intermediate 5 or 6 membered metallacycles associated with ortho C–H functionalization. In the field of aromatic sp 2 C–H functionalization, this strategy has enabled a wide range of functionality to be introduced to the ortho position of arenes, utilizing a diverse range of directing groups and transition metal catalysts. This can lead to the formation of a metallacycle which is sufficiently reactive to undergo subsequent functionalization. This method utilizes existing functionality within a molecule to coordinate a transition metal catalyst and position it in the vicinity of a C–H bond. 1,2 One of the prevailing methods is the directing group approach. There are however a number of strategies to overcome these challenges and research into this area has received great attention in the last decade. However, given the relatively low reactivity of C–H bonds and their ubiquity in organic molecules, intrinsic challenges arise when developing new methodologies with adequate reactivity and selectivity. Introduction The direct functionalization of C–H bonds is an attractive methodology for the atom economic and streamlined synthesis of organic molecules. A distinct second activation of the coupling partner allows site selective reaction between both components.
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Mechanistic analysis reveals an activation pathway whereby cyclometalation with a ruthenium( II) complex activates the substrate molecule and is responsible for the meta selectivity observed. Previously unreported primary alkylations with high meta selectivity have been enabled by this methodology whereas using straight chain alkyl halides affords ortho substituted products.
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HALO 2 PRODUCT KEY 2017 WINDOWS
Complete Community Freedom: Halo 2 for Windows Vista offers gamers the freedom to host their own dedicated servers for maximum multiplayer action.A catalytic meta selective C–H alkylation of arenes is described using a wide range of α-halo carbonyls as coupling partners. The Halo 2 community can now tap their endless creative potential to develop an unlimited number of user-created multiplayer levels. New Multiplayer Map Editor: The game also features the all-new new Map Editor, guaranteeing additional new, fan-generated content in the future. The service enables Windows Vista gamers all over the globe to connect via their PCs and play on an innovative and competitive online forum.
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HALO 2 PRODUCT KEY 2017 PC
Halo 2 for Windows Vista taps into the power of Live Anywhere, a service similar to Xbox Live, to deliver online matchmaking for PC gamers. Brandish dual-wield weapons Covenant arms and pilot human and non-human vehicles and aircraft to battle intelligent AI alien enemies. Exciting Single-Player Gameplay: Experience first hand the epic struggle between Master Chief and the Covenant armada. In this sequel, the battle comes to Earth, and Master Chief, a genetically enhanced super soldier, is the only thing standing between the relentless alien Covenant and the destruction of all humankind. The saga continues with Halo 2 the international award-winning sequel to the highly successful and critically acclaimed Halo: Combat Evolved.