The (IPr)CuO
t
Bu catalysed reduction of 11 aryl and alkyl isocyanates with pinacolborane gave only the boraformamides, pinBN(R)C(O)H, in most cases. Overreduction, which hampers almost all ...isocyanate hydroborations, was restricted to electron poor aryl isocyanates (4-NC-C
6
H
4
NCO, 4-F
3
C-C
6
H
4
NCO, 3-O
2
N-C
6
H
4
NCO). Computational analysis showed stability of
(IPr)CuH
2
, which was proposed to be the catalyst resting state, drives selectivity, suggesting an approach to prevent overreduction in future work. In the case of iPrNCO, formation of this species renders overreduction kinetically inaccessible. For 4-NC-C
6
H
4
NCO, however, the barrier height for the first step of over-reduction is much lower, even relative to
(IPr)CuH
2
, resulting in unselective reduction.
The (IPr)CuO
t
Bu catalysed reduction of 11 aryl and alkyl isocyanates with pinacolborane gave only the boraformamides, pinBN(R)C(O)H, in the case of electron rich isocyanates. A computational study provided insight into this selectivity.
The β-diketiminato aluminum dihydrides, HC{(Me)CNAr}2AlH2 4: Ar = 2,6-di-isopropylphenyl (Dipp), 5: 2,4,6-trimethylphenyl (Mes) react directly with N-aryl-substituted N-heterocyclic carbenes ...(NHCs) by C–N bond activation to afford aluminum amido-alkyl derivatives of the form HC{(Me)CNAr}2AlCH2(N(Ar′)CH)2. The more sterically congested alane (4), bearing N-Dipp substitution, does not react with either 1,3-bis(2,6-di-isopropylphenyl)imidazol-2-ylidene (IPr) or 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), even under forcing conditions. In contrast, in situ generation of 1,3-bis(phenyl)imidazol-2-ylidene through deprotonation of the corresponding imidazolium tetrafluoroborate by KN(SiMe3)2 in the presence of compound 4 provides facile ring opening of the NHC at room temperature to yield HC{(Me)CNDipp}2AlCH2(N(Ph)CH)2. Although compound 5 also does not react with IPr, relaxation of the steric demands of the supporting β-diketiminate ligand to N-mesityl substitution enables analogous ring opening of IMes, with the formation of HC{(Me)CNMes}2AlCH2(N(Mes)CH)2 (7), when the reaction is heated to 80 °C. DFT calculations performed on model systems suggest that in comparison to the parent alane (AlH3) the enhanced propensity of these systems to induce NHC ring cleavage is a consequence of the relative stability of the initially formed five- and four-coordinate adducts as well as the augmented hydridic character of the Al–H bonds within the β-diketiminate-supported molecules.
Thermolysis of a 1,3‐dioxa‐2‐phospholane supported by the terphenyl ligand AriPr4 (AriPr4=C6H3‐2,6‐(C6H3‐2,6‐iPr2)) at 150 °C gives AriPr4PO22 via loss of ethene. AriPr4PO22 was characterised by ...X‐ray crystallography and NMR spectroscopy; it contains a 4‐membered P−O−P−O ring and is the isostructural oxygen analogue of Lawesson's and Woollins’ reagents. The dimeric structure of AriPr4PO22 was found to persist in solution through VT NMR spectroscopy and DOSY, supported by DFT calculations. The addition of DMAP to the 1,3‐dioxa‐2‐phospholane facilitates the loss of ethene to give AriPr4(DMAP)PO2 after days at room temperature, with this product also characterised by X‐ray crystallography and NMR spectroscopy. Replacement of the DMAP with pyridine induces ethene loss from the 1,3‐dioxa‐2‐phospholane to provide gram‐scale samples of AriPr4PO22 in 75 % yield in 2 days at only 100 °C.
Pyridines have been shown to mediate the release of ethene from a terphenyl supported 1,3‐dioxo‐2‐phospholane. This generates a dioxophosphorane dimer which is an oxygen analogue of Lawesson's and Woollins’ reagents. This compound was structurally characterised and its solution behaviour interrogated. Insight into the ethene release reaction is provided by the isolation of a DMAP‐coordinated dioxophosphorane
Three ring-expanded N-heterocyclic carbene-supported copper(
i
) triphenylstannyls have been synthesised by the reaction of (RE-NHC)CuO
t
Bu with triphenylstannane (RE-NHC = 6-Mes, 6-Dipp, 7-Dipp). ...The compounds were characterised by NMR spectroscopy and X-ray crystallography. Reaction of (6-Mes)CuSnPh
3
with di-
p
-tolyl carbodiimide, phenyl isocyanate and phenylisothiocyanate gives access to a copper(
i
) benzamidinate, benzamide and benzothiamide respectively
via
phenyl transfer from the triphenylstannyl anion with concomitant formation of (Ph
2
Sn)
n
. Attempts to exploit this reactivity under a catalytic regime were hindered by rapid copper(
i
)-catalysed dismutation of Ph
3
SnH to Ph
4
Sn, various perphenylated tin oligomers, H
2
and a metallic material thought to be Sn(0). Mechanistic insight was provided by reaction monitoring
via
NMR spectroscopy and mass spectrometry.
A copper(
i
) stannyl supported by a ring-expanded N-heterocyclic carbene has been synthesised and its reactivity towards heterocumulenes has been investigated.
Reactions between PhSiH3 and alkali‐metal diamidoalkylmagnesiates (M{N(SiMe3)2}2MgBu, M=Li, Na, K) provide either selective alkyl metathesis or the formation of polyhydride aggregates contingent upon ...the identity of the Group 1 metal. In the case of M{N(SiMe3)2}2MgBu, this reactivity results in a structurally unprecedented dodecametallic decahydride cluster species.
Double deca: Reactions between PhSiH3 and alkali‐metal diamidoalkylmagnesiates (M{N(SiMe3)2}2MgBu, M=Li, Na, K) provide either selective alkyl metathesis or the formation of polyhydride aggregates contingent upon the identity of the Group 1 metal (see figure).
Deprotonation of triphenyl germane with NHC-supported copper alkoxides afforded four novel (NHC)CuGePh
3
complexes. Of these, (IPr)CuGePh
3
(IPr = :C{N(2,6-iPr
2
C
6
H
3
)CH}
2
) was selected for ...further investigation. Analysis by EDA-NOCV indicates it to be a germyl nucleophile and its σ-bond metathesis reaction with a range of p-block halides confirmed it to be a convenient source of Ph
3
Ge
−
. The Cu-Ge bond of (IPr)CuGePh
3
underwent π-bond insertions with
t
BuNCS, CS
2
, and PhNCO to furnish a series of germyl substituted carboxylate derivatives, (IPr)CuXC(Y)GePh
3
(X = S, NPh; Y = S, N
t
Bu, O), which were structurally characterised. (IPr)CuGePh
3
inserted phenyl acetylene, providing both the Markovnikov and anti-Markovnikov products. The (NHC)CuGePh
3
compounds were validated as catalytic intermediates; addition of 10 mol% of NHC-copper(
i
) alkoxide to a mixture of triphenyl germane and a tin(
iv
) alkoxide resulted in a tin/germanium cross coupling with concomitant formation of alcohol. Moreover, a catalytic hydrogermylation of Michael acceptors was developed with Ph
3
GeH adding to 7 activated alkenes in good conversions and yields in the presence of 10 mol% of NHC-copper(
i
) alkoxide. In all cases, this reaction provided the β-germylated substrate implicating nucleophilicity at germanium.
Four N-heterocyclic carbene (NHC) supported copper(
i
) germyls have been synthesised containing both normal- and ring-expanded NHCs. Their π-insertion and σ-bond metathesis chemistry has been explored in stoichiometric and catalytic regimes.
σ‐Bond metathesis reactions between (6‐Dipp)CuOtBu (6‐Dipp=:C({Dipp}NCH2)2CH2, Dipp=2,6‐iPr2−C6H3) and three diboranes gave access to three new copper(I) boryl complexes (6‐Dipp)CuBcat, ...(6‐Dipp)CuBneop, and (6‐Dipp)CuBhex (cat=1,2‐O2C6H4; neop=(OCH2)2C(CH3)2; hex=OC(CH3)HCH2C(CH3)2O). Whilst (6‐Dipp)CuBcat and (6‐Dipp)CuBneop formed rapidly in toluene, access to (6‐Dipp)CuBhex required heating to 60 °C for days. The complexes were characterised by single‐crystal X‐ray crystallography which showed in all three cases that the systems were monomers and distorted‐linear at the copper atom. The stability of (6‐Dipp)CuBneop was found to be comparable to that of (IPr*)Cu‐Bneop (IPr*=1,3‐bis(2,6‐(diphenylmethyl)‐4‐methylphenyl)imidazol‐2‐ylidene); it persisted in solution for days with no sign of decomposition. (6‐Dipp)CuBhex is a rare crystallographically characterised example of a complex containing a boryl anion supported by the hexylene glycolato ligand.
A ring‐expanded NHC with flanking di‐iso‐propylphenyl groups has been shown to support 3 copper(I) boryl systems. This allows the structural characterisation of (6‐Dipp)CuBcat, (6‐Dipp)CuBneop, and (6‐Dipp)CuBhex (cat=1,2‐O2C6H4; neop=(OCH2)2CMe2; hex=OC(CH3)HCH2C(CH3)2O). All three compounds are reasonably stable, readily isolable sources of boryl nucleophiles, and (6‐Dipp)CuBhex is a rare example of a boryl anion bearing the hexylene glycol ester.
The homoleptic alkaline earth hexamethyldisilazides, M{N(SiMe sub(3)) sub(2)} sub(2) sub(2) (1: M = Mg; 2: M = Ca; 3: M = Sr), have been demonstrated as active pre-catalysts for the ...cross-dehydrocoupling of Si-H and N-H bonds under mild (25-60 degree C) conditions. The reactions are applicable to the coupling of a wide variety of amine and silane substrates and are proposed to occur viaa sequence of discrete Si-H/M-N and N-H/M-H metathesis steps. Whereas reactions of dialkyl group 2 species with 2,6-di-iso-propylaniline and phenylsilane delivered a series of well-defined compounds consistent with this rationale, kinetic analysis of the cross-coupling of diethylamine with diphenylsilane provided evidence for a more complex and subtly variable mechanistic landscape. Although reactions performed with all three pre-catalysts presented a number of common features, in every case the calcium species, 2, was found to provide notably superior catalytic activity, an order of magnitude higher than both 1 and 3 and in excess of many previously described benchmark transition metal- or f-element-mediated processes. Variations in overall reaction order, mode of pre-catalyst activation and the nature of the rate determining process are postulated to arise as a consequence of the marked change in M super(2+) radius and resultant charge density as group 2 is descended.
Reactions of β-diketiminato group 2 silylamides, HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)M(THF)(n){N(SiMe(3))(2)} (M = Mg, n = 0; M = Ca, Sr, n = 1), and an equimolar quantity of pyrrolidine borane, ...(CH(2))(4)NH·BH(3), were found to produce amidoborane derivatives of the form HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)MN(CH(2))(4)·BH(3). In reactivity reminiscent of analogous reactions performed with dimethylamine borane, addition of a second equivalent of (CH(2))(4)NH·BH(3) to the Mg derivative induced the formation of a species, HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)Mg{N(CH(2))(4) BH(2)NMe(2)BH(3)}, containing an anion in which two molecules of the amine borane substrate have been coupled together through the elimination of one molecule of H(2). Both this species and a calcium amidoborane derivative have been characterised by X-ray diffraction techniques and the coupled species is proposed as a key intermediate in catalytic amine borane dehydrocoupling, in reactivity dictated by the charge density of the group 2 centre involved. On the basis of further stoichiometric reactions of the homoleptic group 2 silylamides, M{N(SiMe(3))(2)}(2) (M = Mg, Ca, Sr, Ba), with (CH(3))(2)NH·BH(3) and (i)Pr(2)NH·BH(3) reactivity consistent with successive amidoborane β-hydride elimination and R(2)Ndouble bond, length as m-dashBH(2) insertion is described as a means to induce the B-N dehydrocoupling between amine borane substrates.
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