Prominent Ligand Types in Modern Cross-Coupling Reactions - New Trends in Cross-Coupling: Theory and Applications - page 36

Catalysts based on diadamantylalkylphosphines have also been demon-
strated to be useful in several related cross-coupling reactions. For example,
Molander and Gormisky used Ad 2 P(n-Bu) in SMC reactions of aryl and
heteroaryl chlorides with cyclopropyl- and cyclobutyltrifluoroborate
potassium salts. 58 Buchwald-Hartwig aryl amination reactions are success-
ful with a broad scope of electronically and sterically varied aryl chlorides
with Ad 2 P(n-Bu)/Pd systems as reported by Beller and co-workers. 59 They
additionally showed that the arylation of ketone enolates using the
Pd/Ad 2 P(n-Bu) system is successful with a usefully broad substrate scope, 60
while Hartwig's group utilized the related ligand Ad 2 P(t-Bu) in a-arylation
reactions of aryl bromides with aza-g-lactones towards the synthesis of
quaternary amino acid derivatives. 61
Plenio and co-workers were the first to develop general conditions for
Heck alkynylation reactions (sometimes referred to as the Cu-free Sonoga-
shira reaction) of aryl chlorides using Ad 2 PBn, 62 although improved systems
have been developed subsequently. 63 Reductive carbonylation (formylation)
reactions of aryl bromides with Pd/Ad 2 P(n-Bu) was also reported by Beller's
group. 64-66 with some useful mechanistic studies. 66 Specifically, through a
detailed mechanistic study, Beller and co-workers proposed that the cluster
complex (CO) m Pd n L n and the hydrobromide complex PdL 2 (H)(Br) act as ''res-
ervoirs'' of Pd and that the highly active monoligated PdL complex is slowly
released during the course of the reaction. The low level of catalyst concen-
tration maintained throughout the catalytic cycle allows oxidative addition to
outcompete palladium(0) aggregation with eventual formation of palladium
black, a process which is second or higher order in palladium 67 (Figure 2.16).
In 2006, Shaughnessy and co-workers introduced a new type of trialkyl-
phosphine for aryl amination reactions of aryl bromides and chlorides:
neopentyl-substituted phosphines (Figure 2.17). 12b These ligands had been
known in the context of coordination chemistry since the 1970s, 68 but their
use in catalysis was essentially non-existent. 69 A series of neopentyl-based
ligands, such as (di-tert-butyl)neopentylphosphine (DTBNpP), tert-butyldi-
neopentylphosphine (TBDNpP) and trineopentylphosphine (TNpP) have
been studied, with the motivation being their larger cone angles reflecting
the increased steric demand of the neopentyl-substituted phosphines.
Cone angles calculated from DFT-optimized structures revealed that
DTBNpP (y ¼ 1981) is more sterically encumbered than P(t-Bu) 3 (y ¼ 1941).
Substitution of tert-butyl groups for additional neopentyl groups gives rise to
even more sterically demanding ligands (TBDNpP, y ¼ 2101; TNpP, y ¼ 2271).
This ligand series was further evaluated for electron donation by measuring
the CO stretching frequencies of the ClRh(PR 3 ) 2 (CO) complexes; not un-
expectedly, DTBNpP is less electron rich than P(t-Bu) 3 as its CO stretch at
1946 cm 1 is 18 cm 1 higher than that of the P(t-Bu) 3 complex. Additional
substitution of neopentyl groups for t-Bu groups further increases
the CO stretch, but to a lesser extent with each substitution (TBDNpP,
n ¼ 1953 cm 1 ; TNpP, n ¼ 1957 cm 1 ). The neopentylphosphines were
compared along with P(t-Bu) 3

in the aminations of deactivated