A systematic study of the catalyst structure and overall charge for the dehydropolymerization of HB·NMeH to form N-methyl polyaminoborane is reported using catalysts based upon neutral and cationic {Rh(Xantphos-R)} fragments in which PR groups are selected from Et, Pr, andBu. The most efficient systems are based upon {Rh(Xantphos-Pr)}, i.e., [Rh(κ-P,O,P-Xantphos-Pr)(H)(η-HB·NMe)][BAr], 6, and Rh(κ-P,O,P-Xantphos-Pr)H, 11. While H evolution kinetics show both are fast catalysts (ToF ≈ 1500 h) and polymer growth kinetics for dehydropolymerization suggest a classical chain growth process for both, neutral 11 (M = 28 000 g mol, Đ = 1.9) promotes significantly higher degrees of polymerization than cationic 6 (M = 9000 g mol, Đ = 2.9). For 6 isotopic labeling studies suggest a rate-determining NH activation, while speciation studies, coupled with DFT calculations, show the formation of a dimetalloborylene [{Rh(κ-P,O,P-Xantphos-Pr)}B] as the, likely dormant, end product of catalysis. A dual mechanism is proposed for dehydropolymerization in which neutral hydrides (formed by hydride transfer in cationic 6 to form a boronium coproduct) are the active catalysts for dehydrogenation to form aminoborane. Contemporaneous chain-growth polymer propagation is suggested to occur on a separate metal center via head-to-tail end chain B-N bond formation of the aminoborane monomer, templated by an aminoborohydride motif on the metal.
The μ‐amino–borane complexes [Rh2(LR)2(μ‐H)(μ‐H2B=NHR′)][BArF
4] (LR=R2P(CH2)3PR2; R=Ph, iPr; R′=H, Me) form by addition of H3B⋅NMeR′H2 to [Rh(LR)(η6‐C6H5F)][BArF
4]. DFT calculations demonstrate that the amino–borane interacts with the Rh centers through strong Rh‐H and Rh‐B interactions. Mechanistic investigations show that these dimers can form by a boronium‐mediated route, and are pre‐catalysts for amine‐borane dehydropolymerization, suggesting a possible role for bimetallic motifs in catalysis.
[Rh(κ
2
-PP-DPEphos){η
2
η
2
-H
2
B(NMe
3
)(CH
2
)
2
t
Bu}][BAr
F
4
]
acts as an effective precatalyst
for the dehydropolymerization of H
3
B·NMeH
2
to form
N
-methylpolyaminoborane (H
2
BNMeH)
n
. Control of polymer molecular weight is
achieved by variation of precatalyst loading (0.1–1 mol %,
an inverse relationship) and use of the chain-modifying agent H
2
: with
M
n
ranging between 5 500
and 34 900 g/mol and
Đ
between 1.5 and
1.8. H
2
evolution studies (1,2-F
2
C
6
H
4
solvent) reveal an induction period that gets longer
with higher precatalyst loading and complex kinetics with a noninteger
order in [Rh]
TOTAL
. Speciation studies at 10 mol % indicate
the initial formation of the amino–borane bridged dimer, [Rh
2
(κ
2
-PP-DPEphos)
2
(μ-H)(μ-H
2
BN=HMe)][BAr
F
4
], followed by the crystallographically
characterized amidodiboryl complex [Rh
2
(
cis
-κ
2
-PP-DPEphos)
2
(σ,μ-(H
2
B)
2
NHMe)][BAr
F
4
]. Adding
∼2 equiv of NMeH
2
in tetrahydrofuran (THF) solution
to the precatalyst removes this induction period, pseudo-first-order
kinetics are observed, a half-order relationship to [Rh]
TOTAL
is revealed with regard to dehydrogenation, and polymer molecular
weights are increased (e.g.,
M
n
= 40 000
g/mol). Speciation studies suggest that NMeH
2
acts to form
the precatalysts [Rh(κ
2
-DPEphos)(NMeH
2
)
2
][BAr
F
4
] and [Rh(κ
2
-DPEphos)(H)
2
(NMeH
2
)
2
][BAr
F
4
], which were independently synthesized and shown to
follow very similar dehydrogenation kinetics, and produce polymers
of molecular weight comparable with [Rh(κ
2
-PP-DPEphos){η
2
-H
2
B(NMe
3
)(CH
2
)
2
t
Bu}][BAr
F
4
], which has been doped
with amine. This promoting effect of added amine in situ is shown
to be genera...
The archetypal single
electron transfer reductant, samarium(II)
diiodide (SmI
2
, Kagan’s reagent), remains one of
the most important reducing agents and mediators of radical chemistry
after four decades of widespread use in synthesis. While the chemistry
of SmI
2
is very often unique, and thus the reagent is indispensable,
it is almost invariably used in superstoichiometric amounts, thus
raising issues of cost and waste. Of the few reports of the use of
catalytic SmI
2
, all require the use of superstoichiometric
amounts of a metal coreductant to regenerate Sm(II). Here, we describe
a SmI
2
-catalyzed intermolecular radical coupling of aryl
cyclopropyl ketones and alkynes. The process shows broad substrate
scope and delivers a library of decorated cyclopentenes with loadings
of SmI
2
as low as 15 mol %. The radical relay strategy
negates the need for a superstoichiometric coreductant and additives
to regenerate SmI
2
. Crucially, our study uncovers an intriguing
link between ketone conformation and efficient cross-coupling and
thus provides an insight into the mechanism of radical relays involving
SmI
2
. The study lays further groundwork for the future
use of the classical reagent SmI
2
in contemporary radical
catalysis.
Cationic σ-dihydrogen, σ-amine-borane and neutral hydride complexes, based upon {Rh(PONOP)}, are all shown play a role in the dehydrocoupling of H3B·NMe2H. Movement between the three is promoted by free amine, NMe2H.
Detailed experimental and computational studies are reported on the fundamental B–H and P–H bond activation steps involved in the dehydrocoupling/dehydropolymerization of primary and secondary phosphine–boranes, H3B·PPhR′H (R = Ph, H), using the [RhCp*(PMe3)Me(ClCH2Cl)][BArF4] catalyst.
Oxidative Heck couplings have been successfully developed for 2,2-disubstituted cyclopentene-1,3-diones. The direct coupling onto the 2,2-disubstituted cyclopentene-1,3-dione core provides a novel expedient way of enantioselectively desymmetrising all-carbon quaternary centres.
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