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Baldwin's rules in organic chemistry are a series of guidelines outlining the relative favourabilities of ring closure reactions in alicyclic compounds. They were first proposed by Jack Baldwin in 1976.〔Baldwin, J. E., Rules for Ring Closure, J. Chem. Soc., Chem. Commun. 1976, 734. (Open access)〕 Baldwin's rules discuss the relative rates of ring closures of these various types. These terms are not meant to describe the absolute probability that a reaction will or will not take place, rather they are used in a relative sense. A reaction that is disfavoured (slow) does not have a rate that is able to compete effectively with an alternative reaction that is favoured (fast). However, the disfavoured product may be observed, if no alternate reactions are more favoured. The rules classify ring closures in three ways: *the number of atoms in the newly formed ring *into ''exo'' and ''endo'' ring closures, depending whether the bond broken during the ring closure is inside (''endo'') or outside (''exo'') the ring that is being formed *into ''tet'', ''trig'' and ''dig'' geometry of the atom being attacked, depending on whether this electrophilic carbon is ''tet''rahedral (sp3 hybridised), ''trig''onal (sp2 hybridised) or ''dig''onal (sp hybridised). Thus, a ring closure reaction could be classified as, for example, a 5-''exo-trig''. Baldwin discovered that orbital overlap requirements for the formation of bonds favour only certain combinations of ring size and the ''exo/endo/dig/trig/tet'' parameters. Interactive 3D models of several of these transition states can be seen (here ) (java required). There are sometimes exceptions to Baldwin's rules. For example, cations often disobey Baldwin's rules, as do reactions in which a third-row atom is included in the ring. The rules apply when the nucleophile can attack the bond in question in an ideal angle. These angles are 180° (Walden inversion) for ''exo-tet'' reactions, 109° (Bürgi–Dunitz angle) for ''exo-trig'' reaction and 120° for ''endo-dig'' reactions. Angles for nucleophilic attack on alkynes were reviewed and redefined recently.〔Gilmore, K.; Alabugin,I. V. Cyclizations of Alkynes: Revisiting Baldwin's Rules for Ring Closure. Chem. Rev. 2011. 111, 6513–6556. 〕 The "acute angle" of attack postulated by Baldwin was replaced with a trajectory similar to the Bürgi–Dunitz angle.〔Alabugin, I. Gilmore, K.;Manoharan, M. Rules for Anionic and Radical Ring Closure of Alkynes. J. Am. Chem.Soc. 2011, 133, 12608-12623, 〕 : ==Applications== In one study, seven-membered rings were constructed in a tandem 5-''exo-dig'' addition reaction / Claisen rearrangement:〔Li, X.; Kyne, R. E.; Ovaska, T. V. Synthesis of Seven-Membered Carbocyclic Rings via a Microwave-Assisted Tandem Oxyanionic 5-exo dig Cyclization−Claisen Rearrangement Process, J. Org. Chem., 2007, ''72'', 6624 〕 : A 6-''endo-dig'' pattern was observed in an allene - alkyne 1,2-addition / Nazarov cyclization tandem catalysed by a gold compound:〔Guan-You Lin, Chun-Yao Yang, and Rai-Shung Liu. Gold-Catalyzed Synthesis of Bicyclo()nonadiene Derivatives via Tandem 6-''endo-dig''/Nazarov Cyclization of 1,6-Allenynes J. Org. Chem. 2007, 72, 6753-6757 〕 A 5-''endo-dig'' ring closing reaction was part of a synthesis of (+)-Preussin: : 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Baldwin's rules」の詳細全文を読む スポンサード リンク
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