Kinetics of Hydrogen Atom Transfer from (η⁵-C₅H₅)Cr(CO)₃H to Various Olefins:  Influence of Olefin Structure

Abstract

Treating (etha(5)-C(5)H(5))Cr(CO)3H (1) or (etha(5)-C(5)H(5))Cr(CO)3D (1-d(1)) with an excess of olefin containing the opposite isotope generally leads to H/D exchange, although hydrogenation is also observed in some cases. Application of an appropriate statistical correction to the observed exchange rate gives kH and kD, the rate constants for H* (D*) transfer from (etha(5)-C(5)H(5))Cr(CO)(3)H (D) to various olefins. The values of kH and kD vary appreciably with the substituents on the double bond. Phenyl-substituted olefins accept H* more readily than do carbomethoxy-substituted olefins, although the latter accept H* more readily than do alkyl-substituted olefins. A methyl substituent on the incipient radical site increases k(H) at 323 K by a factor between 5 and 50. A methyl substituent on the carbon to which the H* is being transferred decreases kH substantially. On the whole, the rate constants for H* transfer reflect steric effects as well as the stability of the resulting carbon-centered radicals.

Mn-, Fe-, and Co-Catalyzed Radical Hydrofunctionalizations of Olefins

“…In 2007, the Norton group demonstrated reductive cyclization of diene 194 in a 5- exo -trig fashion to obtain the substituted cyclopentane 195 as a mixture of diastereomers in excellent yield by using the catalytically generated chromium hydride, CpCr(CO) 3 H, which the group had studied extensively 290,291,293,294,295,296,297,298,299,300,301 (Figure 97; see also Section 6.2 ). The geminal diesters of 195 were required to obtain this high yield by virtue of the accelerating Thorpe-Ingold effect, but reductive cyclization products could still be obtained as the major products, though in lower yield, without these present.…”

“…38 However, these and related reactions were thought to proceed through a hydrometallation/M-C bond homolysis mechanism. Notably, prior to Shenvi and Herzon, Norton 298 had noted in passing that these reactions probably 298 proceed through a HAT mechanism. Nojima 118,119,120 indicated similar possibilities, but depicted radical character at the transition state, not as a discrete step.…”

Catalysis Involving the H• Transfer Reactions of First‐Row Transition Metals

“…To date, the only isotope effect reported for Cp(CO) 3 CrH, by Bullock et al . [58] , is k H / k D = 0.45 ± 0.10 for its reaction with 2 -cyclopropylpropene at 68 ° C. We have used 0.45 (the effect of the temperature difference is negligible compared to the uncertainty) to estimate the k H values for hydrogen atom transfer to olefi ns; we have then calculated the relative rates from these k H values to deduce the chemoselectivity of this step [61] . Some of the olefi ns in Table 1 5) .…”

“…The two C = C in the diene must, however, perform different functions: one must accept H • quickly; the other must undergo facile intramolecular attack by the radical thus generated. Table 1.4 [61] predicts that the "a" double bond in 1 should be kinetically favored for H • transfer (25 times faster), yielding radical 2 rather than radical 3 (Scheme 1.9 ) . Scheme 1.9 H • transfer to diene 1 .…”

Activation of M—H bond upon the complexation of transition metal hydrides with acids and bases

“…For example, СрM(CO) 3 Н (М = Cr, Mo, W; Ср =  5 С 5 Н 5 ), НМ(СО) 5 (М = Mn, Re) and CpM(CO) 2 H (M = Fe, Ru, Os) exibit unique reactivity. [3][4][5][6][7] The hydride transfer from the metal complexes is the key stage of the ionic hydrogenation reactions both in stoichiometric and in catalytic variants. Hydride carbonyl complexes СрM(CO) 3 Н (М = Cr, Mo, W) are used as hydride donors in combination with proton donor, tri fluoromethyl sulfonic acid CF 3 SO 3 H, in reactions of hydrogenation of substituted alkenes, 8 and also aldehydes and ketones.…”