Stereospecific reactions hindered

The complexes will effect oxyamination reaction with alkenes in a stereospecific reaction (Scheme 8).290 After reductive cleavage of the intermediate alkanolaminato complex (I) (see below) vicinal amino alcohols (II) are formed. The reaction is unusual in that the new C—N bond is always formed at the least substituted terminal alkenic carbon atom, and there is a clear preference for the imido complex to use its NR group for coordination to the osmium despite the steric restraints of R.299,300 However, the least sterically hindered part of the alkene moiety is attached to. the nitrogen atom.290,291,300 The yields of amino alcohols in the reaction can be improved by addition of tertiary alkyl bulkhead amines (see below). 

A common misconception is that a stereospecific reaction is simply one that is 100% stereoselective. The two terms are not synonymous, however. A stereospecific reaction is one which, when carried out with stereoisomeric starting materials, gives a product from one reactant that is a stereoisomer of the product from the other. A stereoselective reaction is one in which a single starting material gives a predominance of a single stereoisomer when two or more are possible. Stereospecific is more closely connected with features of the reaction than with the reactant. Thus terms such as syn addition and anti elimination describe the stereospecificity of reactions. Stereoselective is more closely coimected with stractural effects in the reactant as expressed in terms such as addition to the less hindered side. For example, syn addition describes stereospecificity in the catalytic hydrogenation of alkenes, whereas the preference for addition to the less hindered face of the double bond describes stereoselectivity. 

The rearrangement proceeds from the Si-state of the 1,4-diene 1. The Ti-state would allow for different reactions like double bond isomerization. Rigid systems like cyclic dienes, where EfZ -isomerization of a double bond is hindered for steric reasons, can react through the Ti-state. When the rearrangement proceeds from the Si-state, it proves to be stereospecific at C-1 and C-5 no -isomerization is observed. Z-l,l-Diphenyl-3,3-dimethyl-l,4-hexadiene 5 rearranges to the Z-configured vinylcyclopropane 6. In this case the reaction also is regiospecific. Only the vinylcyclopropane 6 is formed, but not the alternative product 7. ... 

The hydroboration reaction occurs stereospecifically, and the boron attacks from the less-hindered side of the molecule by normal cis addition, owing to the presence of the bulky benzyloxy group in the axial position. 

In their synthesis of lankacidin antibiotics, an insertion reaction of an azidoformate-derived acyinitrene on the electron-rich dihydrofuran 290 to introduce the sterically hindered C-3 amino appendage in the macrocyclic framework. The oxazoline formation is totally stereospecific with the... 

The nucleophilic capture of tricyclane radical cations 115 " and 117 " supports the role of conventional steric hindrance 115 reacts at the 3° carbon ( 116 ), whereas the chiral isomer 117 + is captured by backside attack at the less hindered 3° carbon ( 118 ). " Both reactions are regio- and stereospecific and avoid attack at the neopentyl-type carbon (denoted by an asterisk). 

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