1 Introduction
Carbon–hydrogen bond activation by transition metals has found its way to becoming an important aspect of organic synthesis. Metals have been found to break C–H
bonds and then participate in follow-up reactions, oftentimes insertions of olefins or
alkynes, that permit functionalization of a substrate or the formation of fused-ring
systems. As this chemistry is developed, it is clear that selectivity in C–H bond
activation is a critical issue that must be controlled to make a given functionalization reaction valuable.
In this chapter, we will present a summary of results that have been reported over
the last 25 years with a transition metal complex that activates a wide variety of C–
H bonds. As this chemistry developed, additional insight has been obtained that
permitted further extensions of the work that have led to a deeper understanding of
the factors that influence metal–carbon bond strengths. These bond strengths play
an important role in determining the selectivity in reactions such as regioselective
olefin insertions, so it is important to be able to predict how the formation of one
bond vs. another will affect the thermodynamics. The importance of these factors
will be revealed, and the effects of ancillary or “spectator” ligands on metal–carbon
bond strengths will also be quantitatively analyzed and interpreted. This is all
possible because the unsaturated metal fragment [Tp
0 RhL] where L ¼ CNR,
PMe 3 , or P(OMe) 3 has proven to be very reactive toward a wide variety of C–H
bonds, allowing the necessary comparisons to be made.
2 Hydrocarbon Activation by [Tp
0 Rh(CNR)]
We first reported that the 16-electron rhodium fragment [Tp
0 Rh(CNR)] where
CNR ¼ neopentyl isocyanide could activate hydrocarbon C–H bonds by irradiation
of the carbodiimide precursor in benzene [1]. 366 nm irradiation of the yellow
complex 1 led to the colorless phenyl hydride product in good yield. The quantum
yield was determined to be 1.0 Æ 0.3, which is higher than for many other organometallic photoprecursors [2–6]. Compound 1 is readily prepared by the reaction of
phenyl azide with the Tp
0 Rh(CNR) 2 .
68
W.D. Jones
Carbon–hydrogen bond activation by transition metals has found its way to becoming an important aspect of organic synthesis. Metals have been found to break C–H
bonds and then participate in follow-up reactions, oftentimes insertions of olefins or
alkynes, that permit functionalization of a substrate or the formation of fused-ring
systems. As this chemistry is developed, it is clear that selectivity in C–H bond
activation is a critical issue that must be controlled to make a given functionalization reaction valuable.
In this chapter, we will present a summary of results that have been reported over
the last 25 years with a transition metal complex that activates a wide variety of C–
H bonds. As this chemistry developed, additional insight has been obtained that
permitted further extensions of the work that have led to a deeper understanding of
the factors that influence metal–carbon bond strengths. These bond strengths play
an important role in determining the selectivity in reactions such as regioselective
olefin insertions, so it is important to be able to predict how the formation of one
bond vs. another will affect the thermodynamics. The importance of these factors
will be revealed, and the effects of ancillary or “spectator” ligands on metal–carbon
bond strengths will also be quantitatively analyzed and interpreted. This is all
possible because the unsaturated metal fragment [Tp
0 RhL] where L ¼ CNR,
PMe 3 , or P(OMe) 3 has proven to be very reactive toward a wide variety of C–H
bonds, allowing the necessary comparisons to be made.
2 Hydrocarbon Activation by [Tp
0 Rh(CNR)]
We first reported that the 16-electron rhodium fragment [Tp
0 Rh(CNR)] where
CNR ¼ neopentyl isocyanide could activate hydrocarbon C–H bonds by irradiation
of the carbodiimide precursor in benzene [1]. 366 nm irradiation of the yellow
complex 1 led to the colorless phenyl hydride product in good yield. The quantum
yield was determined to be 1.0 Æ 0.3, which is higher than for many other organometallic photoprecursors [2–6]. Compound 1 is readily prepared by the reaction of
phenyl azide with the Tp
0 Rh(CNR) 2 .
68
W.D. Jones