Rearrangements carbenes, rate

Photolysis of diazirine 75 in N2 at 9 K produced carbene 76, whose IR and UV/Vis spectra fit predictions by DFT calculations. The IR spectra were most consistent with a carbene conformation with Cl aligned 90° to the adjacent C-CH bond. The carbene was found, by IR spectroscopy, to rearrange to chloroadamantene 77 slowly at 9 K in the dark. The rate of rearrangement was somewhat faster in Ar matrices at 9 K or at higher temperatures in N2. 

Rate measurements are straightforward if the carbenes can be monitored directly. As a rule, the decay of carbene absorption is (pseudo) first-order, due to rearrangement and/or reaction with the solvent. In the presence of a quencher, the decay is accelerated (Eq. 1), and the rate constant kq is obtained from a plot of k0bs versus [Q], Curved plots were often observed with proton donors (HX) as quenchers, particularly for high concentrations of weakly acidic alcohols. Although these effects have been attributed to oligomerization of the alcohols,91 the interpretation of curved plots remains a matter of dispute.76 Therefore, the rate constants reported in Tables 2-4 are taken from linear (regions of) obs-HX plots, or refer to a specified concentration of HX. 

Although the RIES mechanism of Scheme 3 fits the overall kinetic results, and is strongly supported by spectroscopic and chemical evidence presented below, there are loose ends . For example, k /k, the Y-intercept of Eq. 13, gives the partition between rearrangement of the excited diazirine (1 ) and its loss of nitrogen to carbene 2. It is difficult to see why this should depend on alkene identity, yet small dependences have been observed.19,33-37 The behavior can be understood in terms of the CAC mechanism (Scheme 2, Eq. 11), where the Y-intercept is dependent on the rate of rearrangement of the CAC. On the other hand, there are reports that the Y-intercept does not vary in experiments with benzylchlorocarbene and (e.g.) 1-hexene, a-chloroacrylonitrile, or TME.23... 

One can correct the observed distribution of 1,2-C and 1,2-H products so as to reflect only the carbenic pathway.28 The corrected 1,2-C/1,2-H ratio (4.8), coupled with a pyridine ylide absolute rate constant for overall carbene rearrangement (kc + % = 6.8 x 107 s 1), gives the partitioned rate constants kc = 5.6 x 107 s 1 and ifeH = 1.2 x 107 s-1.28 The dominance of 1,2-C over 1,2-H in this system will be discussed below. 

Clearly, rearrangements do occur in the excited states of diazirine and diazo carbene precursors. Kinetic studies of carbenic rearrangements need to consider the possible intervention of RIES when absolute rate constants are partitioned between competitive rearrangement pathways on the basis of product distributions.28... 

The kinetics of alkylacetoxycarbene rearrangements provides another opportunity to observe the influence of bystander substituents on 1,2-H shifts. Carbenes 73-77 undergo the competitive rearrangements of Eq. 28 with the product distributions and associated LFP rate constants shown in Table 6 (with h and kAc partitioned according to the product distributions).88,92... 

Reaction with pyridine leads to the formation of a UV-active pyridinium ylide. Rate constants for the alkylcarbene reaction(s) can be extracted from the intercept of the linear correlation of feobs for ylide formation versus the pyridine concentration. Consider first the 1,2-H shift that converts chloromethylcarbene (48) into vinyl chloride (Scheme 7.17). The LFP experiments show that the H shift occurs with k= 1.2 — 3.0 X 10 s in isooctane, cyclohexane, or dichloroethane at 21-25 The rearrangement is fast, but not ultrafast carbene (48) has a lifetime... 

The facile isomerization of simple alkylcarbenes by alkyl or hydride shifts is so rapid that these species cannot be conveniently studied even by laser-flash photolysis.128- 130 a-Fluori-nation of alkylcarbenes dramatically reduces the rate of rearrangements.131-134 e.g. fluoro(methyl)carbeneis sufficiently stabilized that it can be trapped by cyclopenta-1,3-diene.135 This type of insertion reaction is quite typical of a-fluorinated carbenes. oc-Fluorinated earbenes are less prone to CH insertion, generally adding stereoselectively to alkenes.136... 

Tantalum carbene complexes such as 5 and Ta(=CHCMe3)(S—C6H2-i-Pr3-2,4,6)3 (py) are effective, provided the conditions are such as to allow the coordinated base (THF or py) to give way to monomer (M). In the first example the initially formed tantalacyclobutane complex has been isolated and shown to have a trigonal-bipyramidal structure, and to polymerize NBE at a rate that is independent of [M], In this case the rearrangement of the intermediate tantalacyclobutane complex, to form the tantalum carbene complex, controls the rate of polymerization. In contrast, in the second example the rate is first-order in monomer here the reaction of the tantalum carbene complex with the monomer is the slower step. In both cases the polymer, after termination by reaction with benzaldehyde, is nearly monodisperse93,94. 

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