Hydrozirconation thermodynamics

Schwartz s reasoning for optimizing these thermodynamic considerations led to the development of hydrozirconation. Hydride complexes of the late transition metals do not in general exhibit the hydrometallation reaction, probably because the alkene complexes are too stable. This may be understood from the Dewar-Chatt-Duncanson model for alkene bonding, wherein back donation of metal d-elec-trons to the alkene Tr -orbital is a major contributor. For metal centers with d -electron configurations, there should be substantial stabilization of (3) with respect to (2). Such metals are only found towards the left end of the Periodic Table, particularly Groups III A to VA. 

Wipf, P., Takahashi, H., Zhuang, N. Kinetic vs. thermodynamic control in hydrozirconation reactions. Pure Appl. Chem. 1998, 70, 1077-1082. 

Assuming for Eq. 6.4 T AS 9 kcal/mol, the small value for AGreac —4 kcal/mol permits facile reversibility of the alkene insertion, double bond migration (by successive yS-H elimination/readdition. Scheme 6.7), and the easy thermodynamic manipulation to drive the reaction in either sense, in the so called hydrozirconation of alkenes (Eq. 6.5) [29]. 

In hydrozirconation with Schwartz s reagent, Cp2ZrHCl, addition to al-kenes leads to the anti-Markovnikov alkyl (Eq. 14.40). Remarkably, 1-, 2-, and 3-hexene all give the same n-hexyl product. The reason must be that the initially formed alkyls -eliminate. This moves the C=C bond along the chain in an alkene isomerisation reaction (Section 9.1), until the least hindered and thermodynamically most stable n-hexyl complex is formed (Eq. 14.41) ... 

In hydrozirconation of alkenes by Cp2ZrHCl, terminal alkenes insert in the anti-Markovnikov direction to give a stable 1° alkyl. Internal alkenes, such as 2-butene, insert to give an unstable 2° alkyl, that -eliminates to give 1- and 2-butene. The 1-butene can now give a stable 1° alkyl that is the final product. This is particularly noteworthy because the free terminal alkene is less stable than the internal alkene. Tlie outcome arises because the 1° alkyl is thermodynamically more stable than a 2° alkyl for steric reasons. The 1° alkyl, R, can subsequently be functionalized in a number of ways to give a variety of RX derivatives. Hydrozirconation is also effective with less reactive substrates, such as nitriles, where addition of Zr-H across the C=N bond is possible. ... 

One of the major sources of access to alkenylzirconocene intermediates [2] is through the hydrozirconation of alkynes with the Schwartz reagent Cp2Zr(H)Cl. Kinetically and thermodynamically favored syn-addition of this complex onto a terminal or internal alkyne followed by in situ treatment with electrophiles affords polysubstituted alkenes in high stereochemical purity (Scheme 12.1). 

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